Stumble under hard acceleration. Timing? New carb, GM HEI, eCore coil

[DrewTX]

I have a 1971 Dodge Charger the previous owner built the 318, bored 0.40, 8.5 pistons, heads polished &bowl blended cut 0.20 (?), Edelbrock Performer Intake and Edelbrock Performer Cam 0.42 lift, 270 duration. It had the 90's Holley FI system, which I replaced with a Demon 625 new carb and GM HEI. I runs great, fires right up with initial timing at approximately 18°, not sure what the complete timing is with vacuum advance, but doesnt changes little when hooked up to manifold vacuum advance or progressive-front port.

I have a serious stumble on hard acceleration that just doesnt seem to go away, tweeking timing. Brand new carb, new Summit e-core coil, new GM module. Any ideas out there?
thanks, DRew

 

[DrewTX]

I have an A1 cardone new distributor. Solid 12V to coil and module. Do you think I would benefit from a distributor limiter plate? I'm somewhat guessing at this point.

 

[furryfrog]

Some initial thoughts,

Did these problems begin only after you swapped to the carb and gm HEI system?

If your not on a full stomp on the gas pedal, say 3/4, does it rev out without misfiring? Does it miss in all gears?

Do you think its just one cylinder missing, or most of them missing?

What is your total timing with the vacuum advance unplugged when the centrifugal advance is all in. Have you tried disconnecting the vacuum advance for some testing? The vacuum advance should not be active when your foot is on the floor, only under light cruise conditions. Don't hook the vacuum advance up to manifold vacuum.

Do you have a timing light?

Do you have a wideband O2?

Do you have a picture of your charger?









/

 

[threewood]

Does it stumble immediately or a few seconds into wot? Can you put a pressure gauge on your fuel line to see if it is dropping off at wot?

I went through a wot stumble a while back and it turned out that the fuel pump couldn't keep up and dropped to zero pressure at wot. Worth a look if you can get a pressure gauge in-line and watch it while driving.

 

[DrewTX]

Does it stumble immediately or a few seconds into wot? Can you put a pressure gauge on your fuel line to see if it is dropping off at wot?

I went through a wot stumble a while back and it turned out that the fuel pump couldn't keep up and dropped to zero pressure at wot. Worth a look if you can get a pressure gauge in-line and watch it while driving.

thanks on this, I will definitely check this out.

 

[RT K_line]

When I put my Demon on I had to play with jetting and power valves. The power valve helped the most. It had been a while since I messed with carb tuning but here's what I did. I put a vacuum gauge on the intake and while its idling snap the throttle open and watch the gauge to sere how far it drops. Then put that number valve in. There is probably more to it than I can remember like I said It's been awhile. I'm sure someone can chime in and correct me if I'm wrong, Id like to know too! my stumble is 95% gone but could use a little tweaking still.

 

[aeon280]

If its a street demon 625 than it wont have a power valve like the Holley based Demon carbs. Its more like tuning a Carter carb. You can start by adjusting the accelerator pump location (There are 3 holes) and also adjust the secondary air valve opening rate before spending money on the jet and meter rod kit. If it still chugs and you have eliminated a miss or timing issue, you may need to adjust the secondary jet size or metering valve and springs for full throttle to compensate for the up-sized cam you are running

 

[BeepBeepRR]

This guy is having the same issue.

 

[PurpleBeeper]

These guys are "firing on all cylinders" and giving good advice. I recommend trying answer ALL of these questions and also start with the basics before you start throwing parts at it.
a. You NEED to know what your timing is doing.... initial, full mechanical advance, what is the vacuum advance doing... at idle, part throttle, full throttle, etc
b. A fuel pressure gage would be a big help
c. Do a full tuneup - plugs, wires, cap, rotor, PCV valve
d. check/replace all your vacuum lines going to the carb, air cleaner, vacuum advance

AFTER ALL THIS IS DONE/REPLACED/CHECKED

e. take really good notes on your carburetor about how it's set up now. Start playing with some of the stuff that Aeon280 talks about. In general, "more squirt" from the accelerator pump, having the vacuum secondaries (if you have them) come in slower and a bad power valve (Holley, not Demon, so see what Aeon280 suggests) would be thing to look at.
f. IF ALL the above checks out, then "maybe" replace the timing chain.... last thing to try.... since it can do some really, really weird things with your timing once it gets sloppy & they don't cost too much anyway

Hope This Helps

 

[JimKueneman]

My 318 is setup very similar but with the FiTech EFI... I have the same issue. I have the fast and slow accelerator shots dialed up as far as they go and it still needs a bit more. This is from a dead 700 RPM stop to WOT. Rolling it is enough. If I start ticking either accelerators shot down it will quickly get worse and worse. I just added the Ignitor III to it and have tried timing from 2 ATDC per the label to 21 BTDC with the FBO plate to limit the mechanical advance to 34 degrees and the vacuum in to about 52 degrees. NOTHING touches it besides full on accelerator fuel. Also the FiTech will NOT pump in enough gas on a cold start to make it light off immediately when cold either. It takes 5-6 revolutions to get it to light off. I am beginning to think the small valves create such a high velocity that it dries out the manifold for an instant. I am running out of things to try as well.

 

[PurpleBeeper]

My 318 is setup very similar but with the FiTech EFI... I have the same issue. I have the fast and slow accelerator shots dialed up as far as they go and it still needs a bit more. This is from a dead 700 RPM stop to WOT. Rolling it is enough. If I start ticking either accelerators shot down it will quickly get worse and worse. I just added the Ignitor III to it and have tried timing from 2 ATDC per the label to 21 BTDC with the FBO plate to limit the mechanical advance to 34 degrees and the vacuum in to about 52 degrees. NOTHING touches it besides full on accelerator fuel. Also the FiTech will NOT pump in enough gas on a cold start to make it light off immediately when cold either. It takes 5-6 revolutions to get it to light off. I am beginning to think the small valves create such a high velocity that it dries out the manifold for an instant. I am running out of things to try as well.

Fuel pressure too low or pump can't keep up???

 

[JimKueneman]

Fuel pressure too low or pump can't keep up???

Nope pump is way too big for the 318 and hold 48lbs (Go-Street) no problem.

 

[PurpleBeeper]

Nope pump is way too big for the 318 and hold 48lbs (Go-Street) no problem.

Maybe disconnect the vacuum advance & re-time? 52-degrees sounds like a lot. Also, maybe a restricted fuel filter or line? Does it have a mechanical gage on it so your SURE it has 48psi to the throttle body?

 

[threewood]

Nope pump is way too big for the 318 and hold 48lbs (Go-Street) no problem.

Is there any way (with hard parts) to up the fuel shot in the FiTech?

 

[JimKueneman]

Is there any way (with hard parts) to up the fuel shot in the FiTech?

Yea but it is across the board. If I go higher then it shoots too much when it is hot.

 

[JimKueneman]

Maybe disconnect the vacuum advance & re-time? 52-degrees sounds like a lot. Also, maybe a restricted fuel filter or line? Does it have a mechanical gage on it so your SURE it has 48psi to the throttle body?

It is 18 initial, 34 with all centrifugal in and 52 with initial+centrifugal+vacuum. Exactly with Don from FBO told me to run it at. Everything is brand new. Yes I have a gauge at the throttle body. Many people with the FiTech have the same complaint. I want it to start in less than on revolution like a modern EFI car, it takes 5 or 6. It is their algorithm. They have the prime shot then they shut off the injectors for 5-6 revolutions and if it does not fire then they turn on the injectors. At that point if fires right up. Others the prime shot is enough to get it going. It seems it is mainly us with small mostly stock V8s that have this complaint.

 

[moparnation74]

It is 18 initial, 34 with all centrifugal in and 52 with initial+centrifugal+vacuum. Exactly with Don from FBO told me to run it at. Everything is brand new. Yes I have a gauge at the throttle body.

No question.....

Does it seem that vacuum advance is functioning correctly? Maybe a "stiff" diaphram....

 

[JimKueneman]

No question.....

Does it seem that vacuum advance is functioning correctly? Maybe a "stiff" diaphram....

Yea it works as expected. I made shims to get it to not pull in so far to get the 52. It releases back to 0 degrees as expected as I was pumping it up and down getting that adjustment correct.

 

[moparnation74]

Yea it works as expected. I made shims to get it to not pull in so far to get the 52. It releases back to 0 degrees as expected as I was pumping it up and down getting that adjustment correct.

The issue is the initial "lean" condition and based on what you listed it is a hard answer. Is it a limitation of the fitech on a small block? Shouldn't be...Have you asked them?(I am sure you have already)

 

[JimKueneman]

The issue is the initial "lean" condition and based on what you listed it is a hard answer. Is it a limitation of the fitech on a small block? Shouldn't be...Have you asked them?(I am sure you have already)

They don’t answer me when I ask detailed questions about their implementation. Probably because only one guy there knows the answer and he’s not answering phones.