Lt-1 Stalls at idle

[Bill C.]

Re: Lt-1 Stalls at idle

A few things to think about.
2. Have you checked the advance weights in the distrbutor? Are they hanging partially open? (advancing timing thus idle speed) and then relaxing to the point of letting the idle down and dying. Make sure they weights are lubed properly and free of corosion.

This happened to a good friend here in Tallahassee.
Drove him nuts trying to figure out --

[Jeff M.]

Re: Lt-1 Stalls at idle

If you are adjusting the idlle mixture screws to gain the highest vacumm method, you are still a little fat, turn in 1/8 turn (LEANER)of each screw and you will be real close to perfect. I have verified this with my lm1 air fuel meter. It is amazing how sensitive the mixture screws get when they are right on

[James G.]

Re: Lt-1 Stalls at idle

Tim you brought up a good point. I will retighten the bowl screws also. The weights were operating correctly a week ago when I timed the car and had the cap off. I never thought about a vacuum leak elsewhere, sometimes you have to think out of the box as they say. If anyone thinks of something else please post it. Thanks to all who replied.

[Bill M.]

Re: Lt-1 Stalls at idle

I had a similar problem on a brand F. I fixed it by venting the float bowls to atmosphere instead of the carbon cannister.

[James G.]

Re: Lt-1 Stalls at idle

Bill, I thought my carb was vented to atmosphere?

[Paul O.]

Re: Lt-1 Stalls at idle

James you are correct the carb is vented to the atmosphere. When was the carb last rebuilt or cleaned and new gaskets used. When you replaced the bowls did you also replace the metering block to body gaskets and the bowl gaskets. They could be the incorrect ones what I mean is the porting holes maybe not be aligned correctly or they are installed incorrect it just a thought. Has the carb had a recent backfire through it, if it has and still has the old style power valves they could have been damaged by that. Its just a thought and when you fix this problem please post the fix a lot of us pass on info and most of the time we never hear if we helped at all or if there is something new we need to learn. Thanks Paul 18046

[Bill M.]

Re: Lt-1 Stalls at idle

Bill, I thought my carb was vented to atmosphere?

James:

I'm not suggesting a fix for your specific car. I'm relating how I fixed a similar problem on my car. I try to relate facts when helping with a diagnosis. (I haven't had your problem with the 780 Holley on my 396...yet.)

My suggestion is that you consider hot fuel handling as one potential source of your problem. Consider using race gas, see what happens. If that fixes it, maybe a phenolic spacer would fix it when using today's pump gas?

Good luck!

Bill

[Jim T.]

Re: Lt-1 Stalls at idle

James my 70 350/300 bought new started something new after about 10 years of driving. It ran fine around town but taking a trip the engine would die when exiting an interstate stopping on the off ramp.
Could not figure it out. Removed the distributor to check clearance of the distributor gear. The clearance was excessive. I used the GM GR.2.379 #1837617 shims.
The engine did not die coming off the interstate anymore.
Just this summer had a idle problem the ZZ1 crate engine in my 70 using a Holley. Removed the fuel block and metering block and gave them a good cleaning with Gumont spraying through the applicator tip into the metering block passages. The cleaning corrected the problem.

[Duke W.]

Re: Lt-1 Stalls at idle

I had similar idle stability problems with my '63 L-76 when new, and in my case it was a vacuum advance issue. The engine only pulls 12" at typical idle speed, but the VAC required 15-16" to fully advance. The result was a "dithering" VAC diaphragm,which ultimately lost advance and idle speed, sometimes resulting in stalling. Installing the 64/65 spec 8" VAC solved the problem.

Since all LT-1s have ported vacuum advance this isn't likely and issue, so my best guess is that your idle stability problems are due to fuel percolation, assuming the carburetor is functioning properly.

Fuel percolation can be mitigated by reducing heat rejection at idle and reducing heat transfer to the carb. Heat rejection at idle can be reduced by converting to full time vacuum advance, but you must also change the VAC to a B28 8" unit or you will suffer the same problems I described above on my car. You should also wire the heat riser valve full open.

If the above doesn't sufficiently mitigate the problems, block the heat riser passage. I believe LT-1 has a "divorced" type choke i.e. the choke coil is above the heat riser passage on the manifold. In this case, block just one side - the side opposite the choke coil, and you must also securely wire the heat riser open. This will create a dead end passage for exhaust gas to migrate to the base of the choke coil and carb. It should allow proper operation of the choke coil, but considerably reduce heat transfer to the base of the carb, which should mitigate percolation issues.

Also set the idle speed at about 900. The lower the idle speed the more tendency there is for the idle to become unstable. I was never able to idle my '63 L-76 (Duntov cam, which has about the same effective overlap as the LT-1 cam) much lower than 850 and achieve decent idle stability, even after I reworked the spark advance map to yield fixed total idle timing. These high overlap cams need high 20s to mid 30s total idle timing to minimize idle fuel flow and heat rejection to the cooling system. The severe exhaust dilution at idle results in slow combustion speed, which increased the timing requirement for best thermal efficiency, and combustion instability, which is what causes the "lope".

Duke

[James G.]

Re: Lt-1 Stalls at idle

Duke and Jim thanks for your input. Duke as a side note, I adjusted my lifters according to your specs and procedure and the engine sounds great. I am assuming you are talking about the exhaust heat riser? Also how would I block off the heat riser passage on the manifold? Different gasket? You described exactly what I thought might be a fuel issue. I did have my idle @ 900 when I left my house and when I returned it dropped and stalled after about 10 seconds.

[Duke W.]

Re: Lt-1 Stalls at idle

In rereading your OP it appears that the problem started immediately after installing the accelerator pump, so I believe that bears some attention.

The changes I suggested will help the engine run better, but at long as idle timing and idle fuel mixture are stable, the engine should idle. Something about the new accelerator pump may be causing excess fuel (leakage?), which can cause the idle to destabilize and engine stall due to an over-rich mixture. Excess fuel percolation can cause the same problem.

The most common way to block the head riser to remove the inlet manifold and fab some stainless steel shims - say .005" stock to block the passage when the manifold is installed. Install them between the manifold and gasket and leave a bit sticking out. That way they can usually be removed just by loosening the manifold bolts and grabbing the shim with a pair of needle nose pliers.

I also referred to blocking the heat riser valve on the RH exhaust manifold. Does it function properly or is it stuck? In any event try wiring it full open.

Duke

[James G.]

Re: Lt-1 Stalls at idle

Duke, the heat riser valve does work. When you mention idle timing do you mean the timing which the manual states, which in my case is 4 degrees btdc.?

[Duke W.]

Re: Lt-1 Stalls at idle

Total idle timing is the spark advance at idle with the engine in normal configuration. On pre-emission engines it is usually the sum of initial plus full vacuum advance.

On emission-controlled engines it is usually equal to just the initial timing value because of ported vacuum advance.

And some OE engines (like SBs with mechanical lifter cams) may need to idle higher that the beginning of the centrifugal curve, so their total idle timing might include a few degrees centrifugal.

Initial timing is just that - an "initial value" that is typically added to by vacuum and centrifugal advance, and the spark at any engine operating condition, from idle through part throttle to WOT is determined by a combination of all three.

Duke

[James G.]

Re: Lt-1 Stalls at idle

Duke, this car has always been a little soft on take off after it was good and warm, also hard starting if you shut it off and try to restart ,say like after getting gas. Could this also be linked to fuel percolation? What happens when gas percolates? does it gasify?

[Joe C.]

Re: Lt-1 Stalls at idle

After this long, convoluted thread, I am surprised that nobody asked you whether the stalling was happening to you before you changed the accel pump diaphragm.

OH, YEAH......your first post seems to state that it DID NOT.........is that right? ".......just fixed one problem and seem to have created another........"

So, then the question becomes: What did you do, EXACTLY, when you replaced the accel pump diaphragm. NEVER MIND THE GOSHDARN TIMING...unless you can state unequivocally that you messed with it at the same time that you "fixed" your accelerator pump AND ONLY IF the problem did not present before you "fixed" the pump. This thread will go on forever chasing white rabbits, unless you tell us how you "fixed" your accel pump.

There are 2 ways to replace the pri pump diaphragm:

1. Remove pri float bowl from carb, while carb is installed. Replace diaphragm.
2. Remove carb from intake manifold and replace diaphragm while pri float bowl is affixed to Holley.

Which did you do. Because each method involves its own "can of worms".

No need to re-state Einstein's Theory of Relativity here.

Joe