Need help from some of you old hotrodders or Circle Track guys (hint: Hotrod Lincoln).
I put a Holley 2 barrel (65-4412S) on the son's 66 GMC 305E V6 a while back, rather than rebuilding the old WW Stromberg. This carb worked really well out of the box for a long time....then...he changed the fuel pump.
I can’t see how that had anything to do with it, but since then, when it is cold, it won't idle, and has a ton of bog and hesitation on throttle tip-in, and will die unless you "make it run" with multiple quick jabs of throttle. Sometimes it even wants to backfire. If you can get it past the accelerator pump and into higher RPMs, it runs fine.
If you give it any choke when it is cold, it dies. It will start up and run, even when completely cold, but then dies. Seems too rich, but turning in idle screws ½ turn and it will die.
When it gets warmed up, it will rev fine, and will idle much better, for 5 minutes or more, but still eventually dies on idle. Seems to run and pull fine, and the bog, hesitation, backfire, disappears.
When warm, Idle is OK, but continues to be unsteady and sputters a bit--specs call for idle to be 400-450 RPM and it will quit if you try to get it to run that low, so idle is set about 550 RPM
To date, we have:
1. Checked fuel pressure and confirmed 5 PSI at idle.
2. Replaced all fuel lines between pump and carb intake
3. Set float level to just below the sight hole on the carb
4. Set idle adjustment screws with vacuum gauge to max vacuum. (The thing will pull over 20 inches of vacuum—no big lopey overlapping cam in this one)
5. Pulled plugs and noted way rich, dropped main jets 2 sizes
6. Drained fuel tank and replaced with fresh gas
7. Replaced the plugs
8. Checked the timing with a light and it is real close (again, can’t get the idle down to spec)
So I don’t know where to go from here. What I don’t get is how poorly it runs when cold, then when it warms up, it runs pretty good. I would expect some choke to make it tolerable until it warms up. I also do not understand why it will idle just fine for several minutes then eventually dies.
I am thinking it is too lean at throttle tip-in and my only thought is to explore the accelerator pump nozzle…
(I told my son when he gets a new exhaust put on the truck to have Oxygen sensor bungs welded in both banks. I am ready to buy a wide-band sensor…)
Any thoughts from you folks who have messed with these things (“magic” carbs) a lot more than I have?
