BBC Accel Gen7 tuning problem

[Denvervet]

I've got a GM 572 thats got annoying low rpm stumble that I can't seem to tune out. Background...stock single plane manifold drilled for EFI using 48lb injectors and accel throttle body that I think is about 1350cfm with one throat and two large blades.
Originally tuned with MPFI and a dual sync distributor but that one lasted only a few months before it went out and had same manners as when I changed out to batch fire and standard MSD distributor.
Ive to a TKO manual so able to cheat around the stumbling but getting tired of having to baby it. Pretty much up until about 2200 rpm it does this but accelerates just fine to redline. I had a accel tuner help me tuning while I drove about 2 yrs ago. He had to dump lots of fuel in and it did better (about 12 AFR). Problem with that is plugs get fouled pretty quick and and fuel scrubbing cylinders so oil gets dark fast. On top of that sitting at a light will about burn your eyes with unburned fuel.
I use this car just to cruise and play with ...not a track car.
Cam Specs 0.632/0.632-inch lift and 254°/264°

Question is where to go with this? I've leaned up the fuel map to make it not run as rich as well as played with timing and hasn't helped significantly. Do I need a different manifold, go back to MPFI so I can tune individual cylinders, live with it, or have to change to a cam with less duration ?

[Ron in SoCal]

D-Vet there's an Accel expert on this forum, SuperMEFI or something like that? I bet Jody knows him.

Good luck man,

[PTAddict]

Originally Posted by Ron in SoCal

D-Vet there's an Accel expert on this forum, SuperMEFI or something like that? I bet Jody knows him.

Good luck man,

I think his forum handle is "supremeefi". And he is by all evidence on this forum a very experienced Accel tuner.

What do you mean by "stumble"? Do you mean surging/bucking at light throttle at 1500 RPM? That would be quite typical of a cam with that much overlap. As far as I can tell from the Googles, that cam has a 112 LSA. Doing the math, it has 35 degrees overlap @.050, which is about 20 degrees more than I typically recommend as the limit for reasonably smooth street driving. That's not a hard and fast number, it will definitely vary with engine and tuning, but it's a starting point.

[Denvervet]

Bucking until about 2000 rpm and getter better about 2200 rpm. And yes I assume the overlap duration as the cause and just wondering if there's any tuning tricks with accel efi to best deal with it. Understand it maybe the nature of the beast ...literally. I haven't spoken with anyone with this motor that's carbureted to see if same issue. I 'm sure being a mile high altitude doesn't help either.

[supremeefi]

Didn't I send you a cal or something a couple of years ago?
Send me a datalog. Make sure you include timing, O2 actual and correction, TPS and MAP. My email is [email protected]
Have you verified your timing? It may not run real smooth at low rpm due to camshaft size but typically you shouldn't have to run it that fat either.

Happy to help.

[GregWeld]

Originally Posted by Denvervet

Bucking until about 2000 rpm and getter better about 2200 rpm. And yes I assume the overlap duration as the cause and just wondering if there's any tuning tricks with accel efi to best deal with it. Understand it maybe the nature of the beast ...literally. I haven't spoken with anyone with this motor that's carbureted to see if same issue. I 'm sure being a mile high altitude doesn't help either.

I hate to bring this up -- but many times I've found that there is a simple MECHANICAL issue with the motor or install - which confounds the electronics. Simple as in - an exhaust leak. Bad weld on the O2 bung. Valves not lashed correctly. Valves not seating.

Sometimes it takes a leak down test to discover the issue, or a cylinder pressure check or both (leak down should find it).

I've also found that what SMELLS like unburned fuel -- is actually caused by a LEAN condition where the cylinders are too lean to light... thus the unburned fuel. This happens usually when the untrained tuner attempts to "make" some number rather than looking at what the engine wants. What it wants is an idle fuel ratio that gives the motor the highest RPMS and VACUUM --- no different than when adjusting a carburetor! For whatever reason people tend to forget this simple basic when tickling the ivory on the keyboard.

So on top of having Supremeefi look at your tuning -- I'd be also checking for some of the most basic engine health issues.