1970 LT-1 Stalls When Hot

[George C.]

I am having problems with my rebuilt Holley 4555 carburetor on my 1970 LT-1. The carburetor works fine on cold start up and idles fine at 800 to 900 RPMs in the garage for 15 to 20 minutes during the warm up.

When I drive the car on the highway at speeds of 65 to 70 MPH and come to a stop the RPMs fall off and the car stalls out.

Has anyone else had the problem?

[Mark E.]

Re: 1970 LT-1 Stalls When Hot

I don't mean to sound flippant, but has the idle speed been set with the engine at operating temperature? The "idles fine at 800 to 900 RPM" when cold may be while the fast idle cam is engaged. When warm, the choke opens, the fast idle cam disengages, and the curb idle setting comes into play. Check the CSM; it has a procedure for setting fast and curb idle. But I believe fast idle during warm up should be over 1,000 RPM; curb idle may be similarly set too low.

[George C.]

Re: 1970 LT-1 Stalls When Hot

No problem with cold starts and warms up okay on fast idle. The only time I am having this issue is when it is driven on the road and then you come to a stop sign or parking lot and it wants to stall.

[Timothy B.]

Re: 1970 LT-1 Stalls When Hot

George,

There is a post like this on the Corvette Forum right now with a 1963 owner experiencing the same problem. My suggestion to him was to make sure the idle speed screw was not turned in so far that the transfer slot was not over exposed. What may happen is that you adjust the A/F screws at idle but when you are driving and coast to a stop the coast vacuum is higher and pulls a rich mixture into the engine through the over exposed transfer slot.

Are you using the correct pcv valve.

[Edward J.]

Re: 1970 LT-1 Stalls When Hot

George, once the car stalls does it re-fire right away? or crank for a while?

[Duke W.]

Re: 1970 LT-1 Stalls When Hot

Does it still have the OE ported vacuum advance or has it been converted to full time?

[Mark E.]

Re: 1970 LT-1 Stalls When Hot

No problem with cold starts and warms up okay on fast idle. The only time I am having this issue is when it is driven on the road and then you come to a stop sign or parking lot and it wants to stall.

Let me ask this differently: Have you checked and set the curb idle and fast idle using the procedure in the CSM?

It's worthwhile to make this basic check before chasing other possibilities. The symptoms shared in your first post suggest both the fast idle (cold) and curb idle (warm) may not be properly set. For example, the fast idle speed spec for a '70 LT-1 in the CSM is 2200 RPM.

[Duke W.]

Re: 1970 LT-1 Stalls When Hot

For example, the fast idle speed spec for a '70 LT-1 in the CSM is 2200 RPM.

2200 is with the engine fully warmed up. On a cold start the fast idle should stabilize at about 1900-2000 within a few seconds of firing.

George - you need to thoroughly review the CSM to make sure all the various carb adjustments are correct including normal hot idle and idle mixture adjustment. Since the '70 LT-1 is an emission controlled engine it may have a different idle mixture adjustment procedure than pre-emission controlled engines. On the latter you adjust to obtain highest idle speed/vacuum. On some of the former you continue with the "lean drop" procedure, which is to lean the mixture to achieve a 50 RPM drop.

As long as your car doesn't have to be field emission tested I recommend you dispense with the lean drop if that's what the CSM calls for.

Duke