Holley Carb Tuning Problems --- Help

[CdnCutlass]

Hey Everyone,

For quite some time I have been having difficulty tuning my car. Last year I was plagued with ignition problems and carb issues. Now after having solved all the ignition issues with a new distributor, ignition box and coil, carb tuning remains a problem.

The biggest issue I have is that no matter what I do I cannot get the car to idle in gear unless I have my foot on the pedal or turn the idle way up. When you put the car in gear the rpms drop it sputters, coughs and eventually stalls out. Adjusting the idle mixture makes no difference when the car is in gear. In park it has about 15 inches of vacuum, in gear almost nothing as it hardly runs. Likely around 7 inches with your foot on the gas to keep it running.

Holley tech support didn't have much advice other then pointing to a vacuum leak. I used propane and quick start but couldn't find a vacuum leak.

The carb is 770 cfm street avenger. It has a 6.5 power valve, silver secondary spring, 31 squirter. 68/71 jetting, the original 72/75 seemed really rich according to the plugs and tailpipes are black with deposits.

The car has a bad bog off idle (wants to fall on its face, feels lean) but I figure I need to fix the idle circuit first.

The engine is a 408 small block chevy, 9.5:1 compression, Patriot 2121 heads, Edelbrock rpm air gap manifold. The cam is somewhat mild, its a Comp XE 274H.

If anyone has suggestions on where to look it would be appreciated. If you need more info on the engine etc. please let me know.

 

[CWPottenger]

Well, this is not a quick fix kinda question.

I do not know what you have done to this date, so don't take offense if I start from beginning.

On a Holley:

1st thing you Must verify/set/fix is the float levels of both primary and secondary. If they are not working nothing else will correctly.
2nd thing is verify choke is fully off (car warmed up before any adjustments can be made)

Once those two things are set you can then look at idle circuit.

 

[Fox80]

Like the above post said it can be many things causing this, like he stated you have to get the baseline of a Holley correct or it will never work, also you stated you fixed the ignition, more often than not the problem your having is ignition related not carb. Usually when a motor has a big rpm drop when you put it in gear it's because it is to lean, but not having enough timing advance at idle will cause a lean condition ( remember retard=lean advance=rich with no carb adjustments) a motor requires far less fuel to run when the timing is advanced hence the invention of vacuum advance for fuel economy, if your not using vacuum advance you will need a very rich idle circuit to make it stay running in gear, if you have vacuum advance the way I do it is to do the baseline setting on the whole carb including the idle screws then play around with timing until it starts to straighten out, then fine tune the carb. NOBODY can tell you what your motor will need for idle timing every motor is different, look at a GM service book almost every motor is set differently by GM, the fact you state you have a stumble off idle would make me think ting as well ie.. not enough advance at idle when the mechanical advance kicks in

 

[CdnCutlass]

The floats are set properly with the fuel level just at the bottom of the sight plug. The choke definitely comes off now, from factory setting it would not fully disengage until that was adjusted.

The old MSD Streetfire distributor died (no spark) shorty after I upgraded everything with a Pertronix Flamethrower kit. It was disappointing that it was so poorly made. You should have seen the play in the shaft, so much that the rotor would come in contact with the module hold down screws and take a chunk off the rotor body.

Base timing is 8 degrees, with 34 degrees total in at 3200 rpm. At idle I do have plenty of timing with 8 degrees plus vacuum advance which is plugged into a full manifold source. I spent a lot of time getting the timing right last year. I can go up another two degrees safely to 10 initial and 36 total but wouldn't want to run much more on a street motor.

 

[Fox80]

Ok sounds like you at least have checked the timing lol, do you know exactly what your vacuum advance gives you for additional timing? Your base timing and full mechanical advance sounds fine but if your motor is not pulling enough timing at idle because..... A it doesn't produce enough vacuum for the canister you have or.....B the canister doesn't have enough travel in it, some motors need a crazy amount of full idle timing to idle under load without a super rich carb setting, if you don't have an adjustable can try one maybe you can dial it in that way, not trying to say your troubles can't be carb related, but like the guys at C&S carbs tought me most all "carb" problems are unsolved ignition problems

 

[pontiacgp]

It might not be a vaccum leak around the carb that is causing the problem, I had a ripped grommet on my brake booster that caused a vacuum leak I couldn't find for a while

 

[crotchss]

I'm wandering if alot of your issue is the way you have the vacuum advance hooked up.Every engine I have built I hook the advance up to the timed vacuum port. If you're at idle in park with "blank" degrees of vacuum advance and drop it into gear your idle speed goes down as does your advance because of less vacuum pull. Your engine specs are close to what I run in my 1989 K1500 (406 SBC 10:1 compression, XE274, TrickFlow G2 Twisted Wedge heads, Eddy RPM Airgap, 770 Street Avenger, full length Hedmans) and I have my timing set at 18* initial-36* total @ 3000 rpm. Vacuum is hooked to the timed port and never hesitates, idles great and is very responsive. The way it looks to me you have all this timing as you're idling around, then jump on the throttle and all of that timing disappears and drops from say 30* to 8* for an example causing the studdering. I've worked on many cars that guys have had the timing hooked up this way reporting the same issues. I switch them to the timed port, reset the timing, and instantly problems are gone. Hope this works for you, it has never done me wrong over the past 20 years. Good luck

 

[CdnCutlass]

Thanks guys this is great information and a new perspective on the issue. I can try using the timed port and bump up the initial timing, and swap out the bushings on the distributor to limit mechanical advance. I have to be careful, any more then 12 degrees it will probably start killing the starter. Fortunately I should have some time this weekend to troubleshoot. I will try this first and see where it gets me.

In terms of vacuum leak I did check around the manifold and brake booster. Fortunately I have very little vacuum connections where a leak could spring up.

 

[CdnCutlass]

So I went back to the timing and set initial to 14 degrees. I hooked up the vacuum advance to the timed port and took out the 28 degree mechanical advance bushing and put in the 21 degree. Idle mixture screws are 1 turn out. I have 13-14" of vacuum in park/neutral. Float levels on the carb are spot on.

What a difference this made! The car has so much more drivability and will idle by itself in gear.Total timing might be 30-32 degrees. The bushings Mallory supplies are not really spot on compared with the reading on the timing gun. But I can change to a 25 degree bushing which should put me right around 34-36 degrees total timing.

Looks like cruising jetting is close but slightly on the lean side. Out of the box the carb was jetted 72/75 I am currently at 68/71.

High RPM once the car hits 5200 it wants to die, then the revs climb again, some backfiring as well I am thinking secondary jetting is too lean.

Should I up the jets or dial in a few degrees more timing first and reassess?

 

[CdnCutlass]

Car is running pretty decent right now. 16 degrees initial, 36 total timing. 72/75 jets with a silver secondary spring and 3.5 powervalve. Currently have a 28 squirter. Trying to determine if it is rich or lean off idle.

I can get to 5500rpm without issues before there is some popping. Less than before with the leaner jets. I just filled the tank with 89 octane instead of the 91 I typically run and it had no affect on the high rpm issue. It makes me think if it was detonation it would have gotten worse with lower octane fuel.