The carb is an AFB. Starting from a stop, at low engine speed the engine will sometimes buck and jerk before it catches and accelerates away. Accelerator pump shot seems to be ok. There doesn’t appear to be any vacuum leaks.

From roughly 1500 rpm and lower the engine tends to run a bit rough. With a flat and cupped hand placed over the primary air horn to vary airflow, choke open, at about 1500 rpm, the engine smooths out and gains about 100-150 rpm.

I’m suspecting a lean air fuel mixture might be in play. Given the air bleeds are fixed in the AFB could raising the float level slightly possibly help low speed operation? .. The cam is the solid lifter LT-1. Might exhaust gas dilution be a possibility?

Thinking to also try a coil change out first to eliminate that possibility. Thanks for any replies.

John

 

John,
Your AFB should have 2 mixture screws in the front. Hook up a vacuum gauge and adjust them for peak manifold vac. Generally if your carb is working properly and in good condition they will end up 1 1/2 - 2 turns out from closed. That's the first and easiest thing to try. After that, I would be checking timing and advance before throwing any parts at it.

 

John, you are thinking correctly. Your engine is running too lean..it should slow down when you choke it with your hand at 1500 rpm. And you are correct about the alcohol-diluted gas as well, it causes engines to run lean. So if you have no vacuum leaks, you need to go a bit richer. Raising the float levels may help. Google the carb you have and you may get pointers on line on how to dial it in for your engine. Also a good call on the timing advance, but for sure it is running too lean. I would fix that first, since it is a known problem here.

 

Didn't someone here recently drop (and secure) a small wire into the idle air bleed to richen things up? If I remember it was a Holley, but same theory.

 

I corrected a slightly "wooden" tip in on an AFB equipped 440 by changing the metering rod springs, polished up the throttle response, no idea if it can help but it was easy.

Dan

 

I think that was me and my Holley and was at the suggestion of tbarb. Seemed to help but my engine does have a pretty sizeable cam and low vacuum signal (7-1/2 to 8"). If this car idles with a low vacuum signal due to the cam then it might help.

 

Is the carb a real AFB or a clone... what model number?

The LT-1 cam has about the same effective overlap at the Duntov cam, so OE AFBs like 3461S and 3721S/SA/SB should work okay. The 3721 models actually have leaner main jets and rods but a larger idle jet that's built into the primary venturi cluster. Float level has no significant effect on idle since idle fuel flow is a function of manifold vacuum.

The initial setting for OE AFB idle mixture screws is 1.5 turn out from the seat.

You should also look at your spark advance map. It should have full time vacuum advance and the LT-1 cam needs a 8" B28 VAC to pass the Two-Inch Rule. Total idle advance, the sum of initial, full vacuum with maybe a few degrees centrifugal if the curve starts below idle speed should be at least 30 degrees.

Typical LT-1 cam idle behavior when set up properly is 12" @ 900 with a noticeable lope.

Duke

 

A wipe was used to clean off some oil residue from the lower right main body base to view carb model number.

The rubber end plug was found lying on the intake manifold.




It appears the wipe knocked off the loose and cracked end plug from the vacuum nipple. It was only noted when the shiny vacuum nipple on the carb came into view.
A vacuum leak was suspected in the beginning when the problem first arose, but none were obvious or found at the time. The lean running condition makes sense now. .. A new installed end plug took care of the fast idle engine stalling out when cold.

The air horn was removed. Float level was checked and ok. This AFB is a model #9625 with electric choke. The carb was rebuilt a few years ago. Needles and seats looked good. The accelerator pump plunger was replaced with a new plunger that was in the parts stash.

The car was out again this morning to check out the buck/jerk problem. Still there. It appears to have gotten slightly worse. For some strange reason the engine buck and jerk only happens when starting out from a stop. After that, engine runs fine. Multiple sharp engine bucks and some missing starting off from stop now........perhaps the coil is on the way out.
The coil will be replaced.

The centrifugal timing was set at 36 degrees @3K rpm, vacuum disconnected. Total idle timing with vacuum comes in at 32 degrees. All in vac and centrifugal timing is 54 degrees.

I need to recheck this again. Intake manifold vacuum is 13.5. Vac can is a B26. Slightly below the two inch rule, the idea was to try and limit low engine speed vacuum advance.

The engine idle seems run best with the idle adjustment screws set at 2 1/2 turns out.

I’ll check back in after the coil is replaced to update the thread. Thanks for the replies.

John

 

John,

Try the engine with the initial timing set at approx 6-8* BTDC with the V/A connected just to see if it gets any better, 32* seems way to much. I think the B-26 control is all in at 12" so that is OK if I got that spec right.

Two and a half turns out on the emulsion screws is a indicator the circuit is two lean but try the timing first. Problem with the AFB's is they are not easy to tune like the Holley. Post #5 is a good suggestion to try also but it seems like the idle circuit is lean where as changing the metering rod springs affects the main circuit.

 

The buck and jerk when starting off from a stop was the ignition coil. Turns out the coil was in imminent failure mode and on its way out. A new replacement coil was installed.

Need to recheck ignition advance curve map and idle mixture setting. Low speed engine operation fuel mixture seems a bit lean still.

John

 

I would study and map out the VAC as mentioned by Duke. On my 327/365 with the 30-30 and LT-1 if idle is set at a lazy 850-900 (or lower) and you let RPM drop below that on clutch release, the centrifugal will fall off, which drops idle a tad, which drops vacuum, which drops the VAC and it will either be herky jerky, or the engine will just die.

I fiddled with lightening the centrifugal weights, and played with springs so the centrifugal stayed parked a few hundred more RPM. I also idle my car at 1100-1200 rpm, which ensures the VAC can does not fall off if I get sloppy with a clutch release, or if someone else drives my car.

 

Good point. .. I have a B28 vac can that was used previous to the B26 now in use. From I remember low speed engine operation was about the same between both vac cans. Thinking to reinstall the B28 can and revisit low speed engine operation using
the B28.

Along with using the B26 vac can the advance springs were juggled for tension and the centrifugal weights also lightened to limit low engine speed advance. A sheet metal tab was used to limit vacuum advance travel.

As a test yesterday the vacuum line was removed and plugged at the vac can to take some timing out. The engine ran way worse at low speed with vacuum disconnected. An engine with a stouter than stock cam seems to like a lot of ignition advance.

John