tuning a 500 inch stroker for 91 pump gas

[68charger440]

I have a 440 bored 40 over with a balanced 500 inch stroker kit installed
Stock fresh 906 heads. 
10.52 static compression.
edelbrock performer rpm intake
Edelbrock performer cam
Holley 750 carb
Stock converter
3.91 sure grip
18 initial timing
5280 foot altitude here in Denver
My problem is that I can not easily get anything better than 91 octane anymore by my house.  When I used to run 93 I was OK, but now I can not get rid of the ping when I get on it running 91.
Suggestions?

[firefighter3931]

Try backing the total timing off 3-4* and see how that works. 

Ron

[Cooter]

I'm at 10.8:1 and iron heads. Car won't hardly start with 93 when hot.
Good luck, but I see aluminum heads in your future, or one heluva solid cam with loads of duration.

[Kern Dog]

Even the biggest Edelbrock cam is a mild one. My guess is that your cranking compression is at or above 190. Iron heads with that compression isn't going to play well with a mild cam.
I have tried the advice of ...swap in a bigger cam with a later intake closing and it wasn't enough for me. I ended up switching to thicker head gaskets to get the compression down to around 10 to 1.

[fy469rtse]

yep i agree with cooter, i see aluminum heads in the future,
thats the next thing Ron is going to mention , 

[firefighter3931]

Guys, he's at 5000+ ft of elevation.....that makes a big difference   It ran good with 93 but detonates with 91 so he's not that far off   With the current configuration there are two options :

(1) back the timing off a few degrees
(2) spike the pump gas with some race fuel

Of course a set of closed chamber aluminum heads would make a big difference as well.....if that is an option.


Ron

[6spd68]

Is mixing lead an option?

[68charger440]

Thanks to everyone for the input.  I have played with the timing but no go.  Even when running the 93 octane, one tank would be fine and the next would have a slight ping when I got on it, so 93 had me right on the edge.  Aluminum heads might be possible down the rode but not right now. 
If I did go down that road wouldn't most closed chamber heads have a smaller cc and thus increase my CR more than the heat dissipation would help?  I don't know of any aluminum heads that have anything close to the 88 cc volume the 906 has.  Do any of you know of any opened or closed chambered heads that have anything close to the 88 cc volume?  Or is that not as important as I was led to believe it was?
As far as the head gasket goes... How thick of a head gasket can I use and/or maybe some sort of a spacer?
I am not opposed to fuel additives if cheap, but this car is 99% grocery getter so I don't want to bump my fuel costs to $5 a gallon just for running around town.
I am also not opposed to a cam change if the overlap would lower the CR enough, but I don't want anything to radical since I do drive it mostly around town. 

[Kern Dog]

The bigger cam doesn't reduce the static compression ratio but it does change the dynamiccompression ratio.
The standard & common Fel Pro head gasket is .039 compressed. I went with a .075 Cometic. It lowered my squeeze almost a full point and stopped the knocking. I can run 91 octane and hammer the car without knocking.

[68charger440]

The bigger cam doesn't reduce the static compression ratio but it does change the dynamiccompression ratio.
The standard & common Fel Pro head gasket is .039 compressed. I went with a .075 Cometic. It lowered my squeeze almost a full point and stopped the knocking. I can run 91 octane and hammer the car without knocking.

Wow, I didn't think I could get nearly a whole point with just the head gasket change. That sounds both inexpensive as well as effective.  Thanks!!! 
Just to help me understand, why wouldn't the cam change the static CR since the piston would be further up the cylinder before the valves are closed and would have less volume at the beginning of the compression stroke?

[justcruisin]

The static compression ratio doesn't change with cam timing, it is based on the swept volume of your cylinder in relation to the sum volume of the head chamber, piston compression height, piston valve relieves and head gasket without consideration to cam timing. The dynamic ratio factors in cam timing.

[firefighter3931]

I don't know of any aluminum heads that have anything close to the 88 cc volume the 906 has.  Do any of you know of any opened or closed chambered heads that have anything close to the 88 cc volume?  

Edelbrock makes an 88cc open chamber RPM head. 

http://www.summitracing.com/int/parts/edl-60189/overview/

When the time comes to install the new heads you might want to consider upgrading the cam....the one you're using is small for a 500in build and is increasing the dynamic compression too much. 

Have you done a compression test ?

Do you have the cam specs handy ?


Ron

[68charger440]

I don't know of any aluminum heads that have anything close to the 88 cc volume the 906 has.  Do any of you know of any opened or closed chambered heads that have anything close to the 88 cc volume?  

Edelbrock makes an 88cc open chamber RPM head. 

http://www.summitracing.com/int/parts/edl-60189/overview/

When the time comes to install the new heads you might want to consider upgrading the cam....the one you're using is small for a 500in build and is increasing the dynamic compression too much. 

Have you done a compression test ?

Do you have the cam specs handy ?


Ron

Thanks everyone again
I haven't done a compression test in a couple of years and can't find the sheet I wrote them down on when I did.  I'll try to find some time to do that in the next few weeks.  I knew the performer cam was not good but I had it in the garage from a different motor I built years ago, and figured until I got around to the heads it would be better than stock.  I attached the cam specs below.

[BSB67]

You should first do a cranking cylinder pressure test.  This will help us where you are at relative to the altitude. 

Things that can help: Cooler plugs, cooler thermostat, make sure you don't have a lean condition, heat crossover blocked, slower timing curve.

Your cam is pretty small too.  Larger cam with a larger LSA will help your cranking cylinder pressure but maintain a very docile idle.

[Cooter]

You musta been after a 500 inch motor with a stock idle with a cam that short.

[68charger440]

You musta been after a 500 inch motor with a stock idle with a cam that short.

I will get that compression test as soon as I can, but I am recovering from neck surgery from c4 thru c7 that went wrong and left me pretty weak and with some partial paralysis so what was once a piece of cake is now a struggle.  Even something as simple as a compression test can wreck me for a couple of days.  Enough whining, I just wanted to let you know why it might take me a while to get it done.  Maybe I'll shame my son into coming over to do it.  As for the cam mismatch, like I said earlier it was in my garage when I built the motor and I threw it in there while I decided what I wanted to do with the top end.  Then my health went downhill and everything stalled out.

[firefighter3931]

Sorry to hear about your health issues....you have a lot on your plate to deal with. Best wishes for a full recovery. 

Based on the cam specs combined with the static compression it's clear why you're having these issues. That cam would be considered very mild for a 440 let alone a 500ci build. Realisticly, you should have at least another 30-40* of duration @ .050 valve lift with that combination.

The cranking compression numbers will tell the whole story.


Ron

[69white hat]

If this is a stupid question please forgive my ignorance.

Wouldn't a longer duration cam, which allows the piston to move up more before the valves shut and compression begins, just force a bunch of o2 and fuel out of the cylinder causing less power and poor mileage?

It makes sense in my brain, but I don't get complimented on my brain a lot, LOL. Please, someone explain how it really works, as I have the same problem.
I have 160 cranking compression, but not a clue whats in the 440. I have edelbrock aluminum rpm heads. The timing is turned way down to reduce pinging and when I decide to rebuild the motor I want to know what duration has to do with it.

Thanks.

[Kern Dog]

That is a REALLY mild cam!

In short:
Static compression is easily measured with a guage. ALWAYS do this with all 8 spark plugs removed. Anything over 190 will be a struggle to avoid detonation. You can try colder spark plugs, back off the timing, remove the vacuum advance and/or only run the car in cool weather but you will always be dancing on the edge of the cliff.
Dynamic compression is a calculated number based on a few factors. The theory is that you can have 8, 10 or even 50 to one compression but that actual compression doesn't begin until the intake valve closes. As the piston passes Bottom dead center and rises, the LATER the intake valve closes, the LOWER the dynamic compression. There are several online calculators to determine the dynamic ratio. Again, the belief is that you need to be at 8.0 or less to run knock free on 91.
Without going into too much depth regarding camshaft science, the later intake closing does make the engine idle rougher BUT it is a tradeoff. IF the heads have the flow capability, the loss in low rpm idle quality is usually MORE than offset by higher rpm power.
White Hat:
With 160 cranking compression, the only reason I could imagine that you are knocking is that the spark timing is too far advanced or that you are using regular or mid grade fuel. Not sure of your ignition, but The Mopar Performance distributors included in their electronic conversion kits had a fast and long advance curve. If a man set his initial to 15 degrees, the 28 degrees of mechanical advance (Standard procedure with MP dist.) puts you at 43 degrees! I don't know of any Wedge that needs that much timing. I had this problem. If I set the initial to where the idle quality was good, the car knocked badly on full throttle. If I retarded the timing to stop the knock, it idled as if I pulled 5 plug wires off. I had to dig into the distributor to limit the amount of advance that the distributor would deliver. In my case that was 16 degrees.

[68charger440]

Ok, I found my cranking compression numbers from a couple of years ago.  Its roughly 160 across the board. 

On a side bar question...  I have my original Holley 3918 750 carb that I had gone through awhile back and never put it on the car.  I was thinking of putting it on since I have never been happy with my idle with the carter 750 afb that is on there now.  The original Holley has got 65 jets and a power valve is marked going clockwise E 8 5 2 that came with the rebuild kit. Is that an 8.5? The old PV is marked 7.4 7.  If I remember right my vacuum was in the mid teens when idling in drive.  Before I put it on the car do you have any suggestions on the jets and PV to start with or should I just start with these stock jets and PV for now?

[68charger440]

Guys... hold of on using the 160 cranking compression I posted earlier.  That sheet I found might have been from a different motor.

[68charger440]

OK I pulled ONE plug, not all of them and tested that cylinder with WOT.  It read 163 so I think the sheet I found was the correct one.  Adjusted for altitude it would get me to 185-190 range, so I think the 160 to 165 range is correct for me being over a mile high in altitude.

[firefighter3931]

The 160ish cranking pressure is in the ball park. It certainly could be a fuel delivery issue (too lean) under load. That carb you listed above is a 4160 style vac secondary holley which is basicly designed for a stock 440. You might be able to jet it up enough to make it work ; 72/80 with a 6.5PV would be a good starting point.

A wideband O2 would really help in this case. If it's going way lean on the AFR under load the readings will confirm this.



Ron

[firefighter3931]

OK I pulled ONE plug, not all of them and tested that cylinder with WOT.  It read 163 so I think the sheet I found was the correct one.  Adjusted for altitude it would get me to 185-190 range, so I think the 160 to 165 range is correct for me being over a mile high in altitude.

Your calculations appear to be spot on.    At 5000ft the correction factor is .8617

190 psi x .8617 = 163.72 psi

I'd start looking at the carb after you verify that your total timing is at 36*

What head gasket are you using ?


Ron

[68charger440]

OK I pulled ONE plug, not all of them and tested that cylinder with WOT.  It read 163 so I think the sheet I found was the correct one.  Adjusted for altitude it would get me to 185-190 range, so I think the 160 to 165 range is correct for me being over a mile high in altitude.

Your calculations appear to be spot on.    At 5000ft the correction factor is .8617

190 psi x .8617 = 163.72 psi

I'd start looking at the carb after you verify that your total timing is at 36*

What head gasket are you using ?


Ron

The head gasket is a Rol .044 compressed
The carb has always been an issue for me since I stink at dialing them in.  I have three choices.
The Carter competition 750 that is on the car right now.  Doesn't idle at low rpm very well.
A Holley Street Avenger 870 That probably needs a rebuild
The stock rebuilt 750 that I mentioned earlier in this thread.
I never could get the street avenger or the carter to idle smooth at low rpm.  That is why in desperation I am asking about the original Holley 750. 
Getting a smooth idle at 700 or 800 rpm should not be a problem with my current configuration on the motor?  Isn't that Right?

[firefighter3931]

The 870 avenger should be fine. With the rough idle it is usually related to the idle air bleeds. Some guys have luck restricting the size of the IAB using a small wire inserted in the hole to limit air flow into the idle circuit. That's something you can experiment with 

The felpro 1105 head gasket is a large bore (4.59 diameter) gasket with .051 compressed thickness and 13.7cc volume. Do you have an idea what the specs are on your current head gasket ? Maybe the 1105 would pull enough static compression out of it to make a difference 



Ron

[Cooter]

My 446 is @180 psi and will not stop pinging. Tune and attempt it, but I gotta feeling it's too much and not a carb problem.
I could be wrong as well. If you tune your way outta this problem, I need your ass here to tune me out of mine.

[Challenger340]

My 446 is @180 psi and will not stop pinging. Tune and attempt it, but I gotta feeling it's too much and not a carb problem.
I could be wrong as well. If you tune your way outta this problem, I need your ass here to tune me out of mine.

Trying to cheat the Dynamic Cylinder pressure thing with the Open Irons.... is alot tougher game with a Carb, better with the closed irons... but still tough.

Not like the supposedly "Pump Gas" Nitrous Clowns who can DROWN the flamefront with a solenoid until it ain't got time to detonate at higher rpm's, or rather keep it "just into" the detonation range which is big power on high ratio wedges.

Reverse flowing the coolant to dump the head temp will help a bit also ? .... like maybe -10-15 psi equivalent for a given timing curve ?
I forget the exact ratios... ?
But isn't like each 1* F Head/Coolant Temp good for like 3*F off the fuels peak self ignition temp ?
I better go back and bone-up on our data from long long ago in a land far, far away.

[68charger440]

The 870 avenger should be fine. With the rough idle it is usually related to the idle air bleeds. Some guys have luck restricting the size of the IAB using a small wire inserted in the hole to limit air flow into the idle circuit. That's something you can experiment with 

The felpro 1105 head gasket is a large bore (4.59 diameter) gasket with .051 compressed thickness and 13.7cc volume. Do you have an idea what the specs are on your current head gasket ? Maybe the 1105 would pull enough static compression out of it to make a difference 



Ron

I believe these are the head gasket specs - 4.505" bore and .045" thick. 11.8cc volume.  The total timing is 36*.  Since it doesn't ping unless I get on it, I guess I can live with it cruising around town for a little while if I have to. 
But my idle is still rough and that should be fixable with my setup, shouldn't it? 
Can you guys think of a reason my setup would prevent a smooth 750 rpm idle.  It's not a lumpy idle, its more of a misfire low idle.

[firefighter3931]

With that cam you should be able to idle at 600 rpm and have it purring like a cat. I don't recommend a 600 rpm idle, though 

I'm thinking your poor idle may be related to ignition and not carburation. 


Ron

[68charger440]

With that cam you should be able to idle at 600 rpm and have it purring like a cat. I don't recommend a 600 rpm idle, though 

I'm thinking your poor idle may be related to ignition and not carburation. 


Ron

Funny you suspect ignition...  I have gone back and forth on carburetor vs ignition on this problem forever. I had a Mallory distributor that I never got to put in the car.  I had a "FRIEND" that was going to go through it when I was building my motor and set it up for me.  He disappeared off the face of the earth along with my distributor, so I put in one of those cheapo Ebay all in one distributors thinking I would get the Mallory back eventually and disconnected my Mallory Hifire ignition box.  Maybe I should just bite the bullet and by another Mallory distributor.   
So you don't think the fact that I have too high a compression for 91 pump gas is keeping me from getting the idle smooth on 91?

[firefighter3931]

So you don't think the fact that I have too high a compression for 91 pump gas is keeping me from getting the idle smooth on 91?

Not at all...lots of compression with a smallish cam produces plenty of idle vacuum and smooth street manners/clean idle. 

You really need to upgrade that ebay distributor....they are garbage. A fellow member was running one of these and the adjustments for mechanical advance were non existant and the spark was jumping around with the timing light on it. 

I got him a nice ready to run FireCore unit with a set of Firecore wires and Firecore HP coil....like night and day. 

How much initial timing are you running ? It should have at least 14-16* at idle 

Have you checked the wires for resistance ? If more than 250ohm/ft they are suspect 

Lots of ignition gremlins will cause poor idle/performance. 


Ron

[68charger440]

So you don't think the fact that I have too high a compression for 91 pump gas is keeping me from getting the idle smooth on 91?

Not at all...lots of compression with a smallish cam produces plenty of idle vacuum and smooth street manners/clean idle. 

You really need to upgrade that ebay distributor....they are garbage. A fellow member was running one of these and the adjustments for mechanical advance were non existant and the spark was jumping around with the timing light on it. 

I got him a nice ready to run FireCore unit with a set of Firecore wires and Firecore HP coil....like night and day. 

How much initial timing are you running ? It should have at least 14-16* at idle 

Have you checked the wires for resistance ? If more than 250ohm/ft they are suspect 

Lots of ignition gremlins will cause poor idle/performance. 


Ron

I'm running 17* at idle.  I can't check my wires.  It is snowing like crazy right now and I hate working in the cold.  I think I will change out that distributor.  Its supposed to be cold and wet for the next few days, but after that I will check the wires and post the results.

[firefighter3931]

So you don't think the fact that I have too high a compression for 91 pump gas is keeping me from getting the idle smooth on 91?

Not at all...lots of compression with a smallish cam produces plenty of idle vacuum and smooth street manners/clean idle. 

You really need to upgrade that ebay distributor....they are garbage. A fellow member was running one of these and the adjustments for mechanical advance were non existant and the spark was jumping around with the timing light on it. 

I got him a nice ready to run FireCore unit with a set of Firecore wires and Firecore HP coil....like night and day. 

How much initial timing are you running ? It should have at least 14-16* at idle 

Have you checked the wires for resistance ? If more than 250ohm/ft they are suspect 

Lots of ignition gremlins will cause poor idle/performance. 


Ron

I'm running 17* at idle.  I can't check my wires.  It is snowing like crazy right now and I hate working in the cold.  I think I will change out that distributor.  Its supposed to be cold and wet for the next few days, but after that I will check the wires and post the results.

Snow in May ? 

The base timing is in line with what i would use.  I'm wondering if the plugs or wires might be the issue ? Check the wires for continuity when you get a chance and inspect them closely for cracks and possible arcing/misfiring. You can also try running the engine in the dark looking for arcing...it'll be easier to spot. Seen a few "underhood lightening shows" over the years from arcing ignition wires. 

What plugs are you running ?


Ron

[68charger440]

The snow is breaking branches all over since the leaves are all out.

I'm running Champion rj12yc, but I want to change out the plugs before I try to swap out the carb and tune it.  Any suggestions on plugs?

[firefighter3931]

The snow is breaking branches all over since the leaves are all out.

I'm running Champion rj12yc, but I want to change out the plugs before I try to swap out the carb and tune it.  Any suggestions on plugs?

Normally I recommend an NGK XR5 plug for builds like yours but with the higher static compression i'm inclined to go with a slighly colder plug.

That being said, i would go with the NGK  R5670-6 and see how they work for you. 


Ron

[68charger440]

The snow is breaking branches all over since the leaves are all out.

I'm running Champion rj12yc, but I want to change out the plugs before I try to swap out the carb and tune it.  Any suggestions on plugs?

Normally I recommend an NGK XR5 plug for builds like yours but with the higher static compression i'm inclined to go with a slighly colder plug.

That being said, i would go with the NGK  R5670-6 and see how they work for you. 


Ron

I'll give them a shot. 

[68charger440]

OK, I,m back.  My brother gave me an msd 8387 all in one distributer and 8.8 mm accel race wires for my birthday.  So I installed them along with the new plugs and put on the original Holley 3918 carb which I had rebuilt. 
It seemed to run pretty well, but since I am running a edelbroçk performer rpm intake I have no choke with that setup and it is also a bit small for 500" motor.  Since it is mostly a street car that is not going to work.  I need a choke and I Don,t want to leave too much HP on the table with that carb.
I then switched to the Holley 870 street avenger, and it seems to run à little rougher, and I can,t get the mixture screws to do anything.  Both transfer slots in the primary and secondaries look square and my idle screw is backed all of the way out and it still idles at 1000 rpms and I can close off the mixture all the way with no effect. 
65 power valve front and back, 73 primary 80 secondary jets, and pink secondary  spring. 13" vac at idle in gear.
suggestions?

[justcruisin]

Presumably you have dialed in your ignition. If you can't drop the revs any further I would be checking for vacuum leaks, if that is OK maybe check your power valves for a hole in the diaphragm. If you can close the idle valve completely the fuel must be coming through the transfer slots or somewhere it's not meant to at idle.

[68charger440]

The initial timing is 17* and I used the two heavy silver springs.  My thinking there is that a slow curve will help with the pinging.  But the pinging is on the back burner until I get the carb to at least give me some response from the idle mixture screws. 
I sucked on the power valves before I put them in.  They seemed to hold fine.  I didn't have a better way to check them.  I have checked for vacuum leaks and found none, and I do have 13" of vacuum at idle in gear.  What else can cause fuel to get in there when I have the idle screw all of the way out and the transfer slots are all squared up?

[Paul G]

How are the floats set? Check the fuel level in the bowls. Then try closing off any flow through the secondary's. With the engine running cup your hands over them or find some other way of blocking off any air from getting in through the secondary's.   

[68charger440]

How are the floats set? Check the fuel level in the bowls. Then try closing off any flow through the secondary's. With the engine running cup your hands over them or find some other way of blocking off any air from getting in through the secondary's.   

I,ll recheck the floats tomorrow and cup my hands over the secondaries too. I'll let you know what I find.

[68charger440]

How are the floats set? Check the fuel level in the bowls. Then try closing off any flow through the secondary's. With the engine running cup your hands over them or find some other way of blocking off any air from getting in through the secondary's.   

I,ll recheck the floats tomorrow and cup my hands over the secondaries too. I'll let you know what I find.

The floats are set right, and when I cup my hands over the secondaries, it kills the motor.  Does that mean I have to close off the secondary butterfly all of the way at idle covering up the transfer slots to get the mixture screws to work?

[Paul G]

The engine is idling on the secondaries.  Check the throttle linkage to the secondaries, check the vacuum linkages to it too, I assume it is a vacuum secondary . Make sure they aren't holding the secondary butterfly open slightly. Do this before you try to adjust the secondary throttle position screw.

[68charger440]

The engine is idling on the secondaries.  Check the throttle linkage to the secondaries, check the vacuum linkages to it too, I assume it is a vacuum secondary . Make sure they aren't holding the secondary butterfly open slightly. Do this before you try to adjust the secondary throttle position screw.

It is a vacuum secondary.  The linkage looks right and moves freely.  I set the secondary to expose a square on the transfer slot before I installed it.  The only thing I can think of is to try closing the secondaries altogether using the secondary screw, but I have always been taught to leave the transition slots square on both primary and secondaries.  With a 500" stroker does that rule no longer apply?  It's a very mild cam.

[Paul G]

If your mixture screws are all the way in, and your idle adjust screw is all the way out, no vacuum leaks, then I would try closing the secondary idle butterfly just until the engine stumbles and dies. Then go back and bring out your mixture screws 1 1/2 turns, run the idle screw in a couple turns and start there.

Is this a used carb?

[68charger440]

If your mixture screws are all the way in, and your idle adjust screw is all the way out, no vacuum leaks, then I would try closing the secondary idle butterfly just until the engine stumbles and dies. Then go back and bring out your mixture screws 1 1/2 turns, run the idle screw in a couple turns and start there.

Is this a used carb?

I'll try that, and yes I'm sorry to say it is used.  I rebuilt it before I put it on...  and I know you never know what stupid things people have done to it before you got it.  The guy said he bought it new 6 months before he sold it to me, and he also said it was just too big for his motor and that he didn't modify anything.
People can say a lot of things that aren't true though. 
I didn't see any signs of modifications, and I inspected it very thoroughly inside and out when I rebuilt it.

[Paul G]

Check for excessive wear around the throttle shaft at the base plate. When the carb gets old that wear can become excessive alowing air to get sucked in. If this carb is really only six months old that should not be an issue.

[68charger440]

Check for excessive wear around the throttle shaft at the base plate. When the carb gets old that wear can become excessive alowing air to get sucked in. If this carb is really only six months old that should not be an issue.

The shaft does seem too loose.  Can that shaft  play be corrected?  If not then is there a list of base plates that can be interchanged?

[Paul G]

The play in the shaft can be corrected but not cost effective to have it bushed. New baseplates are available too. Just not cheap. I would get the engine running as best as possible, spray some carb cleaner around the baseplate and shaft and see if the engine rpm changes significantly.

[68charger440]

The play in the shaft can be corrected but not cost effective to have it bushed. New baseplates are available too. Just not cheap. I would get the engine running as best as possible, spray some carb cleaner around the baseplate and shaft and see if the engine rpm changes significantly.

I'll try that tomorrow.

[68charger440]

Would the leakage around the secondary shaft cause the idle mixture screws to be ineffective whether closed all of the way or opened?

[c00nhunterjoe]

Spray carb and choke cleaner on it while idling. If its leaking the rpm will change.

[68charger440]

Spray carb and choke cleaner on it while idling. If its leaking the rpm will change.

It does change the idle, but the Holley tech I have been emailing back and forth with said that a small amount of vacuum leak at the shaft is built into the design.  I just Don,t know if mine is at an acceptable level or not.

[68charger440]

OK, I took a detour to deal with a couple of problems that were preventing me from tuning the motor. 
One was a low voltage to the coil  problem that was giving such a low voltage that the spark was weak and would cause a misfire.  After much frustration and digging I bypassed the ammeter which was fried to the point where it was acting as an resistor and giving low voltage to everything downstream including the ignition switch, coil etc. 
The second issue was the carburetor itself and trying to get it to respond to the mixture screws.  For that I ended up having to drill the throttle blades to regain control of the mixture.  The carb has very loose secondary throttle shaft and the blades don't seem to seat as well as they should, so that may have played into my need to drill the blades.  I still may replace the base plate or the carb entirely if I have to.
I addressed both of those issues through separate threads to keep from getting too far into the weeds on this thread.

Right now it has a fairly good 700 rpm idle, but I still want to try to tune out the detonation if possible.  If I can't tune it out then I will either put a thicker head gasket or aluminum heads on it.  Fixing the electrical problems above helped, but upon further review it is still there. 
I want to try backing off the timing even at the loss of power as long as it won't damage the motor.  I need some advice on how to go about this., and probably understanding in more depth how it all comes together.  I am thinking if I go to manifold vacuum instead of the carb ported vac then I will be able to set the initial more retarded and let the idle vacuum bring it to where it needs to be at idle, whereas with the ported vac it has little or no effect at idle.  I am thinking of backing the initial off from 17* now to 12 with the vac disconnected, and then  when it starts and has full vacuum at idle through the manifold it will get the the vac advance from the MSD dizzy..Am I way off base or what?

[firefighter3931]

Here's what i would try ;

Disconnect the vacuum advance and plug it off. Set the total timing to 34* and let the initial timing fall where it does. 

I'm assuming that you are using the black bushing in your MSD RTR distributor ?


note: If it's sucking air from the secondary throttle shaft, that will create a lean AFR and contribute to your detonation headaches. A wideband O2 sensor and air/fuel guage would tell the whole story. 



Ron

[68charger440]

Here's what i would try ;

Disconnect the vacuum advance and plug it off. Set the total timing to 34* and let the initial timing fall where it does. 

I'm assuming that you are using the black bushing in your MSD RTR distributor ?


note: If it's sucking air from the secondary throttle shaft, that will create a lean AFR and contribute to your detonation headaches. A wideband O2 sensor and air/fuel guage would tell the whole story. 



Ron

I am running the black bushing.
Should I leave the vacuum advance plugged permanently, or reconnect it after setting the 34* total timing?

[firefighter3931]

Here's what i would try ;

Disconnect the vacuum advance and plug it off. Set the total timing to 34* and let the initial timing fall where it does. 

I'm assuming that you are using the black bushing in your MSD RTR distributor ?


note: If it's sucking air from the secondary throttle shaft, that will create a lean AFR and contribute to your detonation headaches. A wideband O2 sensor and air/fuel guage would tell the whole story. 



Ron

I am running the black bushing.
Should I leave the vacuum advance plugged permanently, or reconnect it after setting the 34* total timing?

For now I would leave it blocked off. Drive it for awhile and see if the detonation goes away. The black bushing gives you 18* of mechanical advance so setting the initial at 16* should put you right at 34* total.

Something else that may help is a heavier set of advance springs....to slow down the curve and reduce advance timing under heavy load. Try it first with the advance plugged and the timing at 16/34 and see what happens. 



Ron

[68charger440]

OK, I was already pretty close to 16* initial and 34 total timing, so that didn't get me anything, but I borrowed a friends narrow band o2 meter.  It only read lean or rich, but it was good enough to show me I was real lean.  I played with the jetting until I got it looking pretty good and a bit on the rich side.  When the smoke cleared I had jetted it up 6 sizes. The pinging is gone on light and medium load now.  I couldn't test heavy load because I had some slipping between 2nd and third and didn't want to push it.  I didn't hear any detonation at all so I think we've got it.   Thanks so much for the help everyone and Ron in particular.  Now I have to try to adjust the kickdown linkage and hope that is causing the rev between 2nd and third.  I didn't see a sticky on adjusting the kickdown on the 68 Charger 727.  Do you know of a thread that I missed that covers it?

[firefighter3931]

Good work....wow 6 jet sizes sure explains the detonation issue. Those A/F gauges are awesome for sorting this stuff out 

The way I've adjusted the KD linkage is to open the throttle all the way and snug up the adjustment so that the KD has a small amount of travel after the carb is at WOT. Basicly the throttle and KD are synchronized to be full open at the same time....hope that makes sense.

The trans might also need a band adjustment to firm up the shifts and reduce overlap.



Ron

[BSB67]

From May 8:

"You should first do a cranking cylinder pressure test.  This will help us where you are at relative to the altitude. 

Things that can help: Cooler plugs, cooler thermostat, make sure you don't have a lean condition, heat crossover blocked, slower timing curve.

Your cam is pretty small too.  Larger cam with a larger LSA will help your cranking cylinder pressure but maintain a very docile idle."

[Cooter]

From May 8:

"You should first do a cranking cylinder pressure test.  This will help us where you are at relative to the altitude. 

Things that can help: Cooler plugs, cooler thermostat, make sure you don't have a lean condition, heat crossover blocked, slower timing curve.

Your cam is pretty small too.  Larger cam with a larger LSA will help your cranking cylinder pressure but maintain a very docile idle."

He did say thanks to all.....dontcha hate that???

[68charger440]

From May 8:

"You should first do a cranking cylinder pressure test.  This will help us where you are at relative to the altitude. 

Things that can help: Cooler plugs, cooler thermostat, make sure you don't have a lean condition, heat crossover blocked, slower timing curve.

Your cam is pretty small too.  Larger cam with a larger LSA will help your cranking cylinder pressure but maintain a very docile idle."

He did say thanks to all.....dontcha hate that???

My apologies if I have offended anyone by not naming them specifically for contributing to my ultimate solution.  I really do appreciate all of you!  I just singled out Ron because he held my hand every step of the way.   I'll be better next time in acknowledging everyone!