Lt-1 Stalls at idle

[James G.]

Re: Lt-1 Stalls at idle

Joe, I had the pump professionally replaced. I did take the carb off to have it done. It did not stall at idle before the pump was replaced but it had a bog when starting from a standing stop.

[Joe C.]

Re: Lt-1 Stalls at idle

Joe, I had the pump professionally replaced. I did take the carb off to have it done. It did not stall at idle before the pump was replaced but it had a bog when starting from a standing stop.

In that case, I'd take it up with the man who worked on it.
It is POSSIBLE, however improbable it may seem, that a small vacuum leak was created when the carb was R&R'd from the intake manifold........check the gasket and be sure that the carb stud nuts are evenly tightened. Ask if the repair was done by removing the bowl, or not. If the bowl was removed, the possibility exists that the needle/seat may have been disturbed. There is also the possibility that one or both needle/seats are passing fuel intermittently. If you can observe the venturis during the time that the engine "loads up", and you see fuel dripping from the boosters, then that indicates that the float level is too high..........a possible indicator of a foreign object or needle & seat issue.
I would concentrate on what may/may not have been disturbed during the diaphragm transplant before going any further. Again........that is only if you know with absolute certainty that the prob did not exist before.

Joe

[James G.]

Re: Lt-1 Stalls at idle

Joe, thanks for your reply. I will double check my carb bolts for tightness. I will ask my carb guy what he did exactly to the carb. I have already mentioned it to him about the stalling and he wants me to bring the car in. I would like to fix it myself but it seems like a lot of could be this could be that going on. Whats your opinion of the fuel percolation theory?

[Joe C.]

Re: Lt-1 Stalls at idle

Joe, thanks for your reply. I will double check my carb bolts for tightness. I will ask my carb guy what he did exactly to the carb. I have already mentioned it to him about the stalling and he wants me to bring the car in. I would like to fix it myself but it seems like a lot of could be this could be that going on. Whats your opinion of the fuel percolation theory?

James,

In rereading your fourth post above, this snippet is very important:
".............Idle always seemed to go soft when it had been running for a while.........."

I'll take that to mean that the problem WAS present before your carburetor surgery.

In that case, then the possibility of fuel percolation is very real. Lots of great remedies were mentioned by others, including spark advance, heat riser, exhaust crossover passage, insulating spacer for carburetor flange.

You could also rig up a temporary "cool can" to see if this solves the problem. Fuel percolation effectively raises the fuel level in the bowls, and will present itself by dumping fuel out the boosters, so, when the engine loads up, you should be able to see raw gas dripping from the venturi boosters during idling.

Using manifold vacuum into the distributor rather than ported is a "must", as well as jamming/tying your heat riser valve wide open. Blocking the exhaust crossover passage is good for across-the-board torque, but involves quite a bit of disassembly. I would ABSOLUTELY install a wood-composite or phenolic spacer under the carb if I had hood clearance. I'll guess that you'd be able to fit a 1/2" spacer without probs. Of course, the taller the better. Again, this is also good for power increase as it will not only keep the carb cooler, but will increase the size of your plenum without affecting runner diameter...............a win-win for torque and power production.

Joe

[Jack H.]

Re: Lt-1 Stalls at idle

Remember, when you check your carb mounting fasteners for tightness, there IS a torque spec! It's called out in your copy of the AIM and it isn't very tight...

That's because you want the carb mounting gasket to do its thing--absorb the thermal expansion/contraction movement of the carb vs. intake. When you over-torque, you risk deforming the gasket and actually creating the vacuum leaks the gasket is intended to thwart!

[Terry M.]

Re: Lt-1 Stalls at idle

Be very careful tightening the carburetor mounting screws. Too tight will break an ear off of the base. And don't ask how I know.

Someone once posted some elaborate carburetor mounting tightening instructions from one of the early C3 AIMs, but I can't now recall which year. If no one else can find it, I'll look for it at home later tonight.

[Jack H.]

Re: Lt-1 Stalls at idle

'Someone' can post again... It's in the AIM.

Two step criss-cross sequence. Pass one, take 'em up to 24-48 inch pounds. Yes, that's INCH not foot!

Pass two, final torque to 120-168 inch pounds.

So, we're talking 10-14 ft-lbs when done and that's NOT very tight for the reason I cited (allow gasket to absorb thermal expansion/contraction).

[Paul O.]

Re: Lt-1 Stalls at idle

James how thick is you carb to intake gasket on LT-1's that gasket was originally about 1/2 thick in a greenish to gray color. I think DocRebuild sell one but it is black in color but should help with heat insulation. Paul 18046

[James G.]

Re: Lt-1 Stalls at idle

Thanks again for all your replies. Terry, I went to the carb base school a long time ago, with my 68 Chevelle. I do not have an inch pound torque wrench, so I take a guess at the torque. ( an educated guess at that). I am going to try wiring the heat riser open and if I can fit a spacer in I will. What puzzles me is why a stock car with 62000 miles would get fuel perc.? Could this have been an issue from day one?

[Joe C.]

Re: Lt-1 Stalls at idle

Thanks again for all your replies. Terry, I went to the carb base school a long time ago, with my 68 Chevelle. I do not have an inch pound torque wrench, so I take a guess at the torque. ( an educated guess at that). I am going to try wiring the heat riser open and if I can fit a spacer in I will. What puzzles me is why a stock car with 62000 miles would get fuel perc.? Could this have been an issue from day one?

No.
The ethanol contained in today's gasoline raises its vapor pressure. So, it boils at a lower temperature for any given ambient pressure. Therefore, it is more volatile than good olde fashioned Sunoco 260.

Newer cars with EFI have their fuel under higher pressure all the way up to the injectors, and so are not subject to percolation problems.

[James G.]

Re: Lt-1 Stalls at idle

Joe, would an octane boost or similar product help or would I be wasting my money?

[Joe C.]

Re: Lt-1 Stalls at idle

Joe, would an octane boost or similar product help or would I be wasting my money?

Waste of money.

Read the third post at this link for an excellent explanation of why this is true:

https://www.forums.ncrs.org/showthread.php?t=73150&uid=9764

[Jack H.]

Re: Lt-1 Stalls at idle

Octane rating is a measure of the CONSISTENCY of a fuel's burn profile. You're not complaining about 'pinging' or detonation, so I don't see why you'd be exploring octane boost agents...

[James G.]

Re: Lt-1 Stalls at idle

Jack, I never had any pinging or detonation problems with the car and I run the highest octane I can get at the pump which is 93 around here. I hope to get some time to work on it this weekend and hopefully fix the problem. Thanks again to all that replied.