tuning a 500 inch stroker for 91 pump gas

[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.