408 not all there

[chargd72]

I recently dropped in a new 408 build and I'm getting some weird symptoms.  Here are the build basics:

Magnum block with MRL low compression stroker kit (compression 9-9.1 for boost)
Edelbrock Performer RPM heads with plenty of port work done
CSU blow through carb
Mallory boost reference control box
Unilite distributer locked out at 36*
Aeromotive A1000 fuel pump with 10 AN inlet and 8 AN return (fuel tank sumped)
NGK FR6 plugs gapped at .035

The motor is currently naturally aspirated until I get some brackets in from Paxton.  The same carb, ignition system and fuel system were running great on my 318 I just took out. Now the 408 idles great and had good low end throttle response (even though the power doesn't seem all there) and when I get to 2k+ rpm is starts to hesitate.  By the time I get to 4K rpm it's almost like I hit a rev limiter and the motor won't climb.  I thought I was maybe running lean but I checked my primary jets which were size .068.  Kevin at CSU said .068-.072 should be ideal.  He also said my spark plugs looked good even though I thought they looked a little lean.  It has to be somewhere in the fuel or spark delivery but I'm stuck.  Feel free to ask more about the build.  I just included the stuff that I thought applied to the problem.  

[firefighter3931]

Try a different carburetor. If the carb you're running on the engine is designed/calibrated for a blow thru application it will not work well naturally aspirated. 



Ron

[chargd72]

I would have thought the same thing.  But the blow through carb ran better on my N/A 318 than 3 different edelbrocks.  Plus, Kevin at CSU said a blow through would still be able to run fine, it just won't run optimally. 

[Challenger340]

Just a suggestion, if it feels like it hit a rev limiter ? maybe it has ?
Have you tried a different ignition box other than the current one just for sh*ts and giggles ?

[firefighter3931]

Looking at those plugs ; heat range is off and it's running rich. The ground strap line should be 1/2 way up the electrode and it looks like yours is close the base. The base ring is very dark which is an indication of too much fuel.

Here's a good tutorial on spark plug reading :

http://members.uia.net/pkelley2/sparkplugreading.html

I'm pretty sure that if you had a wideband O2 sensor to monitor the A/F ratio it would show fat/rich as the rpms start to climb. 



Ron

[firefighter3931]

Just a suggestion, if it feels like it hit a rev limiter ? maybe it has ?
Have you tried a different ignition box other than the current one just for sh*ts and giggles ?

Certainly something else to consider. If the box is designed for boost it may be pulling timing out of the engine as RPM's increase....that's what is supposed to happen with boost applications. 

Get a timing light on it and rev it up to see what's going on. 



Ron

[chargd72]

Just a suggestion, if it feels like it hit a rev limiter ? maybe it has ?
Have you tried a different ignition box other than the current one just for sh*ts and giggles ?

Certainly something else to consider. If the box is designed for boost it may be pulling timing out of the engine as RPM's increase....that's what is supposed to happen with boost applications. 

Get a timing light on it and rev it up to see what's going on. 



Ron

The control box is programmable.  It won't pull timing until I tell it to.  It also needs a positive pressure reading from the MAP sensor for timing to be pulled.  I have verified the timing is steady through RPMs. 

[chargd72]

Looking at those plugs ; heat range is off and it's running rich. The ground strap line should be 1/2 way up the electrode and it looks like yours is close the base. The base ring is very dark which is an indication of too much fuel.

Here's a good tutorial on spark plug reading :

http://members.uia.net/pkelley2/sparkplugreading.html

I'm pretty sure that if you had a wideband O2 sensor to monitor the A/F ratio it would show fat/rich as the rpms start to climb.  



Ron

Kevin at CSU said exactly what you did.  The base ring is too dark so running rich.  I'm waiting on TTI to custom weld in O2 bungs and is still about 3-4 weeks out.  They had a 6 week back up when I ordered them!  Then I can get a proper A/F reading.

The plugs are a 6 heat range which are a bit cold but I bought them for the blower.  Would that be causing that bad of a misfire if used N/A?

[cdr]

put a different type of spark plug in, like a plain autolite,   3924?

[firefighter3931]

Just a suggestion, if it feels like it hit a rev limiter ? maybe it has ?
Have you tried a different ignition box other than the current one just for sh*ts and giggles ?

Certainly something else to consider. If the box is designed for boost it may be pulling timing out of the engine as RPM's increase....that's what is supposed to happen with boost applications. 

Get a timing light on it and rev it up to see what's going on. 



Ron

The control box is programmable.  It won't pull timing until I tell it to.  It also needs a positive pressure reading from the MAP sensor for timing to be pulled.  I have verified the timing is steady through RPMs. 

Ok, sounds like the box is working fine and timing is steady 

Looking at those plugs ; heat range is off and it's running rich. The ground strap line should be 1/2 way up the electrode and it looks like yours is close the base. The base ring is very dark which is an indication of too much fuel.

Here's a good tutorial on spark plug reading :

http://members.uia.net/pkelley2/sparkplugreading.html

I'm pretty sure that if you had a wideband O2 sensor to monitor the A/F ratio it would show fat/rich as the rpms start to climb. 



Ron

Kevin at CSU said exactly what you did.  The base ring is too dark so running rich.  I'm waiting on TTI to custom weld in O2 bungs and is still about 3-4 weeks out.  They had a 6 week back up when I ordered them!  Then I can get a proper A/F reading.

The plugs are a 6 heat range which are a bit cold but I bought them for the blower.  Would that be causing that bad of a misfire if used N/A?

A 6 heat range is about right for a pump gas NA street type build but the electrode is indicating something is off  With boost you will needed a colder plug for sure. At least a 7 with 10psi or less. Colder if you plan to run more boost 

Running as rich as it appears to be a misfire is certainly a possibility. I would recommend trying another carb that's calibrated for a NA application and clean up the plugs and re-install. A Holley 750 would be a good option. 



Ron

[chargd72]

Found a buddy that has an old 750 DP.  I will need to do a rebuild on it but I will report back once I get on and running.

[firefighter3931]

Good plan....750DP is perfect for a healthy 408 ! 

Set the jetting up with 72/80 with a 6.5 power valve to start and see how that works 


Ron

[chargd72]

I put on a 20 year old 750 DP and to my surprise the car ran great!  It's weird how the CSU carb was flooding it as the power valves should not come in until they see positive pressure (boost).  Now I can start modifying the head bracket and get this thing supercharged.  As always, thanks for the help guys!

[firefighter3931]

I put on a 20 year old 750 DP and to my surprise the car ran great! 

I told you that would happen 

A blo-thru carb has a very aggressive fuel curve. A centrifugal blower makes power as the RPM's climb so a carb matched for that type of power curve (progressive boost) has an aggressive fuel curve. It's more than jetting....the metering blocks are also modified to produce increased fuel to compensate for the progressive boost curve that the centrifugal blower produces. 

That is why the CSU carb idled well (no boost) but got overly rich as the RPM's began to climb. The carb doesn't know that there is no blower on the engine and dumps fuel the way it was designed to do. I'm sure the blo-thru carb will work fine once the blower is installed 


Ron