carburetion issue??

[Leif A.]

Good afternoon, All,
I need your collective advice, ideas, thoughts, etc. Got my car back from the shop yesterday...routine stuff with the exception of a carb rebuild on the original Holley (was leaking a bit but running fine). Took her out today for a nice Sunday afternoon drive and at a steady 65-70MPH there is an occasional stumble...just for a split second, then she continues to run fine for a few minutes and then a momentary stumble again. Probably happened 10 times during a 30 minute drive with no specific regularity. Outside temp around 80 degrees. My car is a '67 L79. A/C was not turned on during this trip. Temp gauge showed 210 degrees.

[Timothy B.]

Re: carburation issue??

That sounds like a electrical issue to me. Check and make sure the coil is oriented properly in the bracket and wiggle the plug to firewall connection.

[Leif A.]

Re: carburation issue??

I'll start there, Tim, thanks. While the car was in the shop for the carb rebuild, I had both the engine harness, headlight harness and dash harness replaced.

[Edward J.]

Re: carburation issue??

Leif, I like Tim's Idea, I would try wiggling the harness connections( firewall, ignition switch, dist. lead, etc.) with car running to see if you can duplicate the running problem. If your running with the Ignition shield on, try removing the shield and road test again. If all is well try removing the Dist. cap and checking the leads to points and the ground on base plate.

[Leif A.]

Re: carburation issue??

Leif, I like Tim's Idea, I would try wiggling the harness connections( firewall, ignition switch, dist. lead, etc.) with car running to see if you can duplicate the running problem. If your running with the Ignition shield on, try removing the shield and road test again. If all is well try removing the Dist. cap and checking the leads to points and the ground on base plate.

Thanks, Ed, will check all those, as well. I'll let y'all know what I find.

[Gerard F.]

Re: carburation issue??

Leif,

Just a few thoughts.

Maybe it is a distributor issue with too much advance. You are probably all in with centrifugal advance at 65-70 mph, and just cruising along you are starting to get some pre-ignition.
Maybe try setting the initial advance back 1-2 degrees and see if that helps.

It does sound like an electrical (spark) issue.

The only carb issue that I would see, would be that you are running too lean or there is a vacuum leak in the carb. 210 seems pretty hot for just cruising along, and that might be the culprit.
Try tightening down the carb to manifold nuts, and check the throttle base and shafts for vacuum leaks.

[Wayne W.]

Re: carburation issue??

My Holley does the same thing, a little stumble, mostly hot and in between throttle. It sits a lot and get worse with time, so rebuilding and adjustments usually take care of it temporarily. A good balance of the idle adjustments helps, and as suggested check for vacuum leaks and total timing.

[Duke W.]

Re: carburation issue??

Leif,

Just a few thoughts.

Maybe it is a distributor issue with too much advance. You are probably all in with centrifugal advance at 65-70 mph, and just cruising along you are starting to get some pre-ignition.
Maybe try setting the initial advance back 1-2 degrees and see if that helps.

Unless someone has installed really light springs in the centrifugal advance, I doubt it. The OE spec is 30 @ 5000, which is very lazy. The lightest springs I have found bring it all in at 3000-3500, but since the curve is non-linear, most is in before 3000. So cruising at 70 with a 3.70 axle should yield full centrifugal and full vacuum advance the sum of which along with initial should be in the range of 52-54 degrees. A recent L-79 I set up this way runs very well.

I also recommend a 12" B26 VAC since it meets the Two-Inch Rule. The OE 8" VAC is more aggressive than necessary, but unless there is transient detonation with an aggressive centrifugal curve, the OE VAC will probaly be okay, but not optimal.

On virtually all the cars I have analyzed, I found the idle mixture way off. The mixture screws were often half a turn out from the seat different side to side. In the 50 plus years I've been tweeking carburators, I don't recall ever setting one up with other than the same number of turns out on each side. The adjusting procedure is in the shop manuals. It's very simple and it takes about two minutes to accomplish. An optimized idle mixture is critical to obtaining a good, stable idle, stumble-free start up from a dead stop, and smooth low load cruise.

On a '65 L-76 and '65 L-79 I recently analyzed, both of which likely have OE design cams and both have the OE carburetors, which are the exact same part number, the L-76 needed 1.5 turns out from the seat (900 idle speed) and the L-79 1.0 turns (750 idle speed). The nominal initial setting specified in the shop manual is 1.0 turns for both engines.

Duke

[Leif A.]

Re: carburation issue??

'll look at all the above this weekend. Thank you all for your educated and kind remarks.

[Dennis O.]

Re: carburation issue??

Unless someone has installed really light springs in the centrifugal advance, I doubt it. The OE spec is 30 @ 5000, which is very lazy. The lightest springs I have found bring it all in at 3000-3500, but since the curve is non-linear, most is in before 3000. So cruising at 70 with a 3.70 axle should yield full centrifugal and full vacuum advance the sum of which along with initial should be in the range of 52-54 degrees. A recent L-79 I set up this way runs very well.

I also recommend a 12" B26 VAC since it meets the Two-Inch Rule. The OE 8" VAC is more aggressive than necessary, but unless there is transient detonation with an aggressive centrifugal curve, the OE VAC will probaly be okay, but not optimal.

On virtually all the cars I have analyzed, I found the idle mixture way off. The mixture screws were often half a turn out from the seat different side to side. In the 50 plus years I've been tweeking carburators, I don't recall ever setting one up with other than the same number of turns out on each side. The adjusting procedure is in the shop manuals. It's very simple and it takes about two minutes to accomplish. An optimized idle mixture is critical to obtaining a good, stable idle, stumble-free start up from a dead stop, and smooth low load cruise.

On a '65 L-76 and '65 L-79 I recently analyzed, both of which likely have OE design cams and both have the OE carburetors, which are the exact same part number, the L-76 needed 1.5 turns out from the seat (900 idle speed) and the L-79 1.0 turns (750 idle speed). The nominal initial setting specified in the shop manual is 1.0 turns for both engines.

Duke

I'll back Duke up on this. I have been fighting a very occasional stumble at cruise (like once every 2-3 miles) for the past two years. The car is a '67 L79 and I had the idle set at 750 per the specs. I finally took the time to set it correctly with a vacuum guage and no more problem. I believe the idle screws were screwed too far in, and to get the idle up to spec, the idle speed adjustment screw was set tighter than it should be, allowing more of the transfer slots to be exposed. I thought a 750 RPM idle was a 750 RPM idle, but evidently, not in these cars.

[Leif A.]

Re: carburation issue??

In the process of installing a new DeWitts 941 radiator. When installed, I'm going to tackle this issue with gusto. Again, thanks to all for your input. I have purchased a Mityvac and a timing light. Already have a dwell meter. The points have been eliminated with an electronic setup from Lectric Limited. Carb was rebuilt by a fellow with "mad carb skills" (his words). All new harnesses, new vac can (which I will check with the Mityvac), new thermostat and hoses going on with the new radiator. I have read dozens of articles on both forums over the past couple of months, as well as articles from other places. I believe I now have all the info at my fingertips to check timing, vacuum, advances, carb settings and all else with a level of confidence. This is going to be a bit of an adventure as I haven't done this in many years but after all the input and reading I feel comfortable tackling this issue. I will definitely report back when finished.

[Stuart F.]

Re: carburation issue??

Leif;

Above all, use common sense. It also, in the age of digital cameras, take pictures of how things were connected or anything else that you just might want to refer to. I never used to that, but even at my old age I can learn that it is helpful.

Good luck. There's nothing more satisfying than to do a project that is new for you and have it turn out well. I just got that feeling this past week end when I finally (after about three years) got a lean surge condition sorted out in a unique model Carter AFB 3461s carburetor. It had me stumped, as everything I tried would not make a difference. Then one day, about six months ago, I was looking the unit over carefully trying to figure out what I missed. It had a well worn primary shaft bore in the main body about which I consulted the experts. They told me to test for a vacuum leak around the shaft using starting spray. If the engine speeds up you have a leak, if not, you don't so forget about it. But, on close examination, I noticed the shaft was real sloppy at part throttle opening. As such, the transfer slots were exposed unequally between the two throttle bores. I thought about it for a long time, then finally I called our carb rebuild shop here in Florida that did special small jobs for me in the past and asked them if they could re-bush the primary shaft bores only. They agreed, so I sent the main body to them and had it back in a couple days. Even then, I had to summon some more enthusiasm to put the carb back together for I feared another disappointment. I promised myself that if it didn't help, that carb would go back on the shelf for good, or until my son's sold the car after I'm gone. To my wonderful surprise, the car ran better than ever before-even better than when I bought the car new in 63! So, once again; good luck.

Stu Fox

[Dan B.]

Re: carburetion issue??

If you just had the carb rebuilt, I would take a good look at the float settings. A low float setting at high speed could also cause a stumble like you describe. Easy enough to check and adjust on a Holley. Dan

[Leif A.]

Re: carburetion issue??

Stu,

Thanks for the encouragement. I had been following your trevails on this forum...glad to hear you were able to finally resolve your issue.

Dan,

Thanks for your input, as well. In all of my readings I've been doing I had read about the float levels. I've never done more than adjust the idle speed and A/F screws but I'm willing to open up those bowls and take a look. I am going to whip this thing.

[Dan B.]

Re: carburetion issue??

Leif, Unless I am mistaken, you should be able to remove the sight plugs while the engine is running and see the float level. The fuel should be right up to the bottom of the inspection plugs as I recall. I used to adjust the floats while it was running and that was that. No disassembly needed. All my stuff is AFB WCFB Carter now, but I played with a bunch of Holley's back in the day. Take a look and report back.