Recently completed a valve job and cam swap. Car back together and running very nicely ('67 L79). When warmed up, vacuum shows to be 15" Hg, B26 canister all in at 12.5" Hg. A/F screws both out from seat approximately 1 full turn...vac stays constant there at 15". Carb is a new Holley 4610 List 3810 (5 years ago). Choke sets on start up along with the fast idle cam. Car starts up just fine. Drive away, all is great. Car runs at a pretty steady 185* whether on the highway or from stop sign to stop sign. Here's the issue. When leaving a stop LIGHT (sitting and idling just fine for over a minute or so), when leaving the light the car bogs down terribly and wants to stall. Push the clutch in and feather the throttle to clear what feels like a fuel load up, release the clutch and the car drives off just fine. Runs terrific otherwise. If I clear the carb by revving the engine a couple of times before leaving the stop light, everything seems to be just fine. I am by no means a carburetor guy, so before I attempt to mess with this issue, I would appreciate y'alls professional opinions as to what might be going on. BTW, this issue was not present prior to the major work just completed.

She looks pretty enough, now I just need to fix this one final issue. Thanx.
Thanks to all in advance.

 

I would double check your timing

 

Timing 10* BTDC. When I pulled in the garage, I opened the hood, removed the air cleaner lid...the choke butterfly was completely open and when shut off, actuating the throttle gave good, strong squirts of fuel...don't think it's a percolation issue. Fuel lines are all at least 1/2" off the block and intake manifold.

 

What I have to go on suggests float level or possible needle/seat need examination.

Dan

 

Again, not being a carb guru, float level had crossed my mind. Wondering how it could have changed just sitting on the bench for a few weeks while the engine work was being done.

 

Leif - go to this web page and click on "Stumble", there are two types and the one you're discussing is often falsely blamed on the accelerator pump:

http://www.thecarburetorshop.com/Troubleshooting.htm

Might not solve your issue, but a stumble after the changes you say you made doesn't make sense, something else is amiss.

 

I agree, Frankie, and that's what has me scratching what's left of my hair. Thanks for the article.

 

Suggest making sure the accelerator pump lever has slight preload when the throttle is at curb hot idle position so that it immediately squirts fuel and doesn't lag. Possibly when you rev the motor a few times without load before leaving, it loads up the intake with enough fuel to overcome the lean condition by a slight accelerator pump delay.

But that adjustment shouldn't normally change over time so it the carb wasn't apart it wouldn't seem the adjustment should have changed.

Do you know your total static and centrifugal timing? If it's under 36 deg then you can try increasing it a couple of degrees and see if that helps. But even at 10 deg it should not bog and more timing would just mask the real problem.

Checking the float levels is a good idea too.

 

I think what's important is to determine if the off idle bog is from a lean condition or rich condition. I am thinking lean when you open the throttle blades powering off from a stop if it was rich the engine would have a hard time idling. With the choke partially on the car powers of fine, correct?

It's a classic symptom of accelerator pump adjustment, make sure a small throttle movement there is some liquid fuel from the pump squirters. Double check the pump cam is in the #1 position on the throttle arm and adjusted where there is no gap at the pump lever at idle position, loosen the spring adjustment so it's slightly preloading the lever, don't worry about the .015 WOT adjustment.

Is the b-26 a new control, you are very detail oriented but make sure to check the vacuum hoses and timing, dwell etc just so you don't miss the simple stuff.

Get some carburetor cleaner and carefully spray the air bleeds in the air horn. If the engine will tolerate it, open the side screws another 1/2 turn each and see if it's any better. The throttle blade adjustment may not be correct and maybe not enough transfer slot on primary and to much air in secondary.

 

You failed to mention idle speed. Quoting idle vacuum without stating the speed is meaningless. My experience with L-79s with OE equivalent cam is 14-15" @ 750 with the idle mixture screws one turn out from the seat on the OE Holley.

Your problem could be either lean or rich, but if there's no stumble after a couple of throttle blips, it's probably rich, and the first suspect is too high float level.

Remove the air cleaner, do a cold start and shut off after about 30 seconds. Then quickly use a strong flashlight to look down the venturis for fuel oozing out of any of the four discharge nozzles. If so the fuel level in the bowl is too high and needs to be lowered. There should be zero fuel flowing through the venturi discharge nozzles at idle, and after the engine is shut off, fully warmed up. Also do this test after engine shutoff with the engine partially warmed up - at least to the point where the fast idle is totally disengaged.

One of my driveability tests is to ease out the clutch very slowly from a dead stop without adding any throttle. The more overlap the cam has the tougher to do, but most should slowly accelerate to idle speed with full clutch engagement without a bad stumble, but it also depends on clutch engagement behavior. It can be difficult if the clutch is "grabby". The last one I did was a totally OE configured L-71 (still has the OE ported vacuum advance), and it passed this test with flying colors from 900 idle speed.

Ported vacuum advance may pass this test more easily than full time. As you ease out the clutch, speed and vacuum go down which looses vacuum advance, and may cause a stumble. With ported vacuum advance, total advance doesn't change as long as the centrifugal doesn't start until above idle speed.

On a normal "start" from a dead stop around town a L-79 should be able to achieve a smooth start to full clutch engagement without exceeding 1000-1200 revs.

Duke

 

Great stuff. As the car is warming up, during initial driving when choke is partially involved, the car leaves a stop sign with no issues. The B26 is new and check with my MityVac prior to install. Vacuum on the gauge is rock steady at 15" Hg when idling at 850 RPM (a little high, but I have A/C). The adjustments you suggest are where I am a neophyte. I either need a more detailed explanation as to what to look for and how to accomplish both of your suggestions. In all the reading I've done on this, the verbiage "transfer slot" comes up and I have zero idea how or where to adjust this feature. Your patience and advice are very much appreciated.

 

Prior to the work performed, this car pulled away from a dead stop exactly as you describe, Duke. Easing out the clutch with barely any throttle and the car would pull away as smooth as glass...never a stumble. Current warmed up RPM is around 850 to compensate for the pull down when the A/C is switched on. I'm going to go back over a few things this afternoon as you and tbarb have suggested. Most who have worked around these old Holley's for a long time tell me they're pretty simple...I've never indulged so it's pretty much like rocket science to me.

 

Leif, I have a F/A meter that you can borrow. I'm not to far from you in Grapevine TX. Send me a message if your interested, Tim

 

Appreciate the offer, Tim. I'll advise if that becomes a need. Tuning the F/A screws currently using a vacuum gauge and the good ol' "by ear". Thanks again.

 

Leif,

To Duke's point if it is a high float level to the point where fuel is starting to drip from the booster nozzles in the venturi's you should be able to put your finger over the primary vent tube and look for fuel to drip from the venturi. If your front bowl has that vapor vent you will need to seal that also so pressure from the bowl cavity will push fuel out the booster nozzle. If no fuel is obvious I think the float level is probably ok.

Try the side screws another 1/2 turn out on each side as this will allow more fuel into the engine at idle and see if it's any better.

 

Just came inside a few minutes ago from the garage. After reading some more about the intricacies of these carburetors, I checked some of the things that you and Duke suggested. First I figured out what you were talking about here (It's a classic symptom of accelerator pump adjustment, make sure a small throttle movement there is some liquid fuel from the pump squirters. Double check the pump cam is in the #1 position on the throttle arm and adjusted where there is no gap at the pump lever at idle position) There is no gap at that location and the slightest movement of the linkage produces fuel from the nozzles...cam adjustment on #1. To Duke's point, I ran the car for 30 seconds and then shut it off...no fuel dripping from the nozzles at all. Ran the car up to normal operating temperature and shut off, no dripping from the nozzles at all. Actuating the throttle linkage when warm and shut off, strong streams of fuel from both nozzles. I'm going to go back out and richen the A/F screws 1/2 turn and take her for a nice drive after traffic dies down a bit...around 7 this evening. I'll report back with results. Thanks for your help and patience.

 

One other thing to think about if the pump is adjusted correctly is to increase the accelerator pump shooter size from the stock .025 to the next size up (.028).

Why now and not before? The new valve job and camshaft replacement could require a slightly richer mixture, when you motor away from a stop and open the throttle blades the vacuum drops and the fuel falls out of suspension inside the intake manifold. The pump shot covers up the time needed for the related circuits to catch up and feed fuel to satisfy the engine.

It does not take long to replace the shooter nozzle but be very careful working over the manifold so nothing falls into the engine, it may be best to remove the carburetor.

 

Since the intake has been off, if nothing else works immediately you might check real closely for a possible vacuum leak somewhere. Some have used an unlit propane torch slightly opened up and move the head around the gaskets, hoses, etc, watching for any sign the engine suddenly picks up rpm.

 

Dan,
That was the first thing I did about a week ago...should have mentioned that earlier. I ran that propane everywhere, intake to head, china walls, carb to intake, all around every vacuum hose, and just about everywhere else. No increase in RPM at all. I appreciate the thought.
Just got back from a 40 mile drive. Plenty of stop signs, didn't catch too many red lights...seems to run pretty good with the A/F screws turned out another half turn (now 1 1/2 turns out). The lone red light that I had to spend more than 30 seconds at was at the entrance to my development...she wanted to stumble a bit, so out of an abundance of caution (not wanting to get rear ended), I stabbed the clutch, blipped the throttle, released the clutch and drove her home. The weather here is in the 90's with 66% humidity, so working for any amount of time on her is taxing, to say the least.

 

That sure sounds like, well what I said in post #4. Best to look at all the simple easy to correct causes first as 9 out of 10 are. Float level.

Dan