72 LT-1 Idle bog down FINAL REPORT

[Gary S.]

Last week, I posted this and got good replies - https://www.forums.ncrs.org/showthre...-Idle-bog-down. Based on a search here, I decided it wouldn't hurt to lower the fuel float level as described by some to see if this was related. To my surprise the car ran rougher, almost to the point of not idling at all. I pulled the bowl sight plugs and couldn't see fuel in the rear bowl whereas the front was right where I like it - a gentle rock from side to side will cause the fuel to gently spill out. Nothing on the rear. So I started increasing the float level and got 1 full rotation of the wrench before the fuel came up to where it should be. That thought caused me absolute astonishment so I thought maybe I had a stuck float or something. In addition, as soon as the rear float level was raised the idle started to smooth out. I am not the brightest guy in many areas but I still don't understand that relationship. I guess I need to do some more reading on Holleys.

I decided to start at square one and re-adjust everything that I knew HAD been in good order. I set the floats to a hair or two below the hole, played with both idle mixture screws and found that the driver's side was out 2 full turns whereas the passenger side was 1 1/2. I played with that and got them both the same at around 1 1/2+. I also checked the power valve and found that it took several seconds for the engine to bog down so I knew the PV was good. While I was at it, I decided to check the idle speed using an electronic gage versus the mechanical tach. Low and behold but my dwell/tach shows 850 rpm to be at around 950 on the tachometer. Btw, the dwell was 30, idle at 850 and vacuum was at 13.5" which is what I remembered, more or less.

I just got back from a 20 minute drive and the low speed driving, from around 1000 to 2000 is much improved. In addition, prolonged sitting no longer causes the engine to die.

Thank you all for the great advice. I love this board! My next project is to go to full time vacuum advance as so many of you suggested.

Gary

[Duke W.]

Re: 72 LT-1 Idle bog down FINAL REPORT

Idle/off idle fuel is metered by manifold vacuum which is way more than a fraction of an inch of fuel head in the bowls, so one would think that float level has little effect at idle. But carburetors are cantankerous beasts and it's a wonder they work at all, but as long as they are rich enough. OE carb calibration is usually good (at least on pre-exhaust emission control engines), but at the cost of higher fuel consumption than modern EFI with digital control.

Also, I believe there is some idle fuel flowing from the secondary bowl in order to keep the fuel somewhat fresh.

All the service manuals have several pages on carburetor adjustments - all the levers and linkages, so if there is a problem, systematically going through all the adjustment to ensure they are correct is a good idea.

I have always cautioned that idle mixture screws should be changed in equal amounts, but it's amazing how many are not.

I always recommend the use of an electric test tach with a fine scale to do the idle mixture speed adjustment. The in-car tach is not sufficiently granular or accurate for this task.

Duke