1965 L-76 tune/adjustment question

[Philip A.]

As stated this is a 1965 L-76 (327/365hp). Recently had the carb rebuilt by a reputable rebuilder recommended here. Car starts, idles, and runs as expected at higher rpm. The issue is accelerating off-idle. If done slowly all is fine. If accelerating off-idle too quickly, the motor bogs/rpm drops/stalls and sometimes sneezes out the side-pipes. Is this a symptom of a vacuum leak at the carburetor or something else?

[Mike M.]

Re: 1965 L-76 tune/adjustment question

try backing off the adjustment for the accelerator pump. mike

[Larry M.]

Re: 1965 L-76 tune/adjustment question

I am not a carb expert, but just finished rebuilding my stock Holley (R3810) for my 67 327/350 HP car. Rebuild was very successful and car started and has run without issues in all driving modes.

According to Lars paper on rebuilding and tuning Holley carbs, what you describe he attributes to a lean condition. I would suspect either incorrect accelerator pump color cam on the throttle shafts, or a bad accelerator pump diaphragm, or smaller than OEM accelerator pump nozzles shooting into the primary bores, or incorrect accelerator pump lever adjustment.

I am assuming that your problem does not involve the secondary throttle shaft.........but you need to tell me if your issue is when doing 1/4 mile runs or "hole-shots", where the secondary can come into play.

I would talk with your carb rebuilder and get his advice, but specifically checkout the areas I mentioned. He should have this info, or it is available in the Chevrolet Overhaul Manual for your car, or available from the Holley technical website.

I went thru each of these areas and did all the checks and adjustments called for in the Overhaul Manual and Holley Rebuild Kit instructions. And all worked as claimed for my car.

Larry

PS: The first accelerator pump diaphragm I installed from the Holley rebuild kit, was hanging up occasionally when I was testing it. I installed another one and it performed much better and smoother. No hanging up. You might have a similar issue. As Mike says, the throttle shaft to accelerator pump lever adjustment screw (one with the small spring) is also important. If incorrect, accelerator pump operation will be erratic.

Also, backfiring can possibly blow the power valve unless a special "backfire ball" is installed into the carb body. Our early carbs did not have this feature. But many have installed it since. You can ask your carb builder if he did this.

[Leif A.]

Re: 1965 L-76 tune/adjustment question

Philip,
As Mike and Larry have alluded to, the accelerator pump adjustment comes to mind. I had a very similar situation and tightening the accelerator pump adjustment screw (about a quarter turn) fixed my problem. Sometimes they are adjusted just a tad too loose and when you come off idle/accelerate the car will bog on you for lack of proper air/fuel mixture. Just my personal experience.
Obviously, the other items Larry mentions could easily be contributing to your issue, as well.

[Duke W.]

Re: 1965 L-76 tune/adjustment question

As stated this is a 1965 L-76 (327/365hp). Recently had the carb rebuilt by a reputable rebuilder recommended here. Car starts, idles, and runs as expected at higher rpm. The issue is accelerating off-idle. If done slowly all is fine. If accelerating off-idle too quickly, the motor bogs/rpm drops/stalls and sometimes sneezes out the side-pipes. Is this a symptom of a vacuum leak at the carburetor or something else?

I'm not sure what you mean by "accelerating off-idle", but due to the very high overlap of the 30-30 cam (assuming that's what it was rebuilt with) it's tough to get going from a dead stop with minimal clutch slippage without a bit of stumble. I've found that at sea level, mild temperature the best idle mixture screw setting is 1.5 turns out from the seat and when going through the idle speed mixture adjustment procedure you should always turn the screws in equal increments - typically a quarter turn at a time and maybe 1/8 once you've got it dialed in pretty close from the initial 1 turn out from the seat recommended start point.

Also as previously suggested make sure the accelerator pump and other carb adjustments are IAW your 1965 Corvette Shop Manual Supplement. It also helps to run a fairly high idle speed... 900 minimum.

[Philip A.]

Re: 1965 L-76 tune/adjustment question

I'm not sure what you mean by "accelerating off-idle", but due to the very high overlap of the 30-30 cam (assuming that's what it was rebuilt with) it's tough to get going from a dead stop with minimal clutch slippage without a bit of stumble. I've found that at sea level, mild temperature the best idle mixture screw setting is 1.5 turns out from the seat and when going through the idle speed mixture adjustment procedure you should always turn the screws in equal increments - typically a quarter turn at a time and maybe 1/8 once you've got it dialed in pretty close from the initial 1 turn out from the seat recommended start point.

Also as previously suggested make sure the accelerator pump and other carb adjustments are IAW your 1965 Corvette Shop Manual Supplement. It also helps to run a fairly high idle speed... 900 minimum.

Duke
It does have the 30-30 cam. What I mean by "accelerating off-idle" is with the car idling in neutral you cannot rapidly depress the gas pedal without the described symptoms. In neutral it will increase RPMs without symptoms if the gas pedal is depressed slowly. Moving from a dead stop it is exactly as you describe, but I still need to be careful about slowly depressing the gas pedal to avoid the symptoms and stalling.

[Duke W.]

Re: 1965 L-76 tune/adjustment question

That's more clear and the accelerator pump adjustment is a likely culprit, but you should check and adjust as required all the "adjustments" on the carb that are detailed in the '65 Corvette Shop Manual Supplement.

Prior to this you should check the spark advance map. Details are in the above manual. The VAC should be a 236 16 to B28, start at 4", all in at 8". With the VAC disconnected check that centrifugal is all in at 2350, then with a dial back timing light set it in the range of 38-39 a few hundred revs above the point of max centrifugal. Then connect the VAC and check total idle advance. It should be in the low 30s. Test drive and check for detonation. If evident reduce total WOT advance in 2 degree increments until it goes away.

Whenever dealing with a drivability issues, it's always best to check the ignition system first. There's an old adage among knowledgeable FI owners: "90 percent of "fuel injection problems" are the ignition system.

Duke

[Tim S.]

Re: 1965 L-76 tune/adjustment question

As stated this is a 1965 L-76 (327/365hp). Recently had the carb rebuilt by a reputable rebuilder recommended here. Car starts, idles, and runs as expected at higher rpm. The issue is accelerating off-idle. If done slowly all is fine. If accelerating off-idle too quickly, the motor bogs/rpm drops/stalls and sometimes sneezes out the side-pipes. Is this a symptom of a vacuum leak at the carburetor or something else?

I am assuming it did not do this before the carb rebuild?

[Philip A.]

Re: 1965 L-76 tune/adjustment question

I am assuming it did not do this before the carb rebuild?

I have no memory of it doing this prior; however, the car sat for several years unused which led to the need for the rebuild and my memory may be faulty.

[Tim S.]

Re: 1965 L-76 tune/adjustment question

As some of the others have mentioned, I would check the adjustment of the accelerator pump arm. Check to see that you are indeed getting a shot out of the nozzles for the pump. Don't over think it..........these are simple engines. Do you know what a lean car smells like? If you are that lean (vacuum leak) you are going to smell it.

I would refer back to Duke's comments on ignition and timing. Do you feel certain that the timing marks on the balancer are accurate? Many a SBC have slipped a balancer ring. At that point, you can either remark the balancer, exchange the balancer, or try to time the engine by vacuum. We are also assuming the voltage going into the ballast resistor is up to spec and the voltage coming out is as well.

Feel free to message me for my phone # if you would like to talk or want some help.

Tim