72 LT1 Issue

[Dan A.]

Abut 2 week ago I picked up an all original 72 LT1 with 54,000 miles. It ran well well at first with the exception of a stumble at throttle tip in. It has begun to act like it is starving for gas starting at about 30-40 MPH. It loses power and sounds like it is only running on 4 cylinders. It will start and idle fine when it begins running like this but acts like it is going to die when you try to accelerate. The fuel pump had a slight leak so I replaced it along with the S shaped rubber hose between the pump and the hard line but the symptoms remain. I also replaced the fuel filters in the carb and the sock filter in the tank looks clean. I checked the rubber line between the tank and the hard line and it seemed OK. The timing is set up properly and the symptoms are the same if the timing control mechanism on the intake manifold is bypassed or not. The car received a top flight award back in 2010 and I suspect was not driven much since. I might also mention that the previous owner installed a Lectric Limited pointless ignition system at some point.
I am ready to start looking at the carburetor but figured I would see if anyone had any better ideas before i start that process. Thanks in advance for any ideas.

[Leif A.]

Re: 72 LT1 Issue

Check the ignition coil.

[Timothy B.]

Re: 72 LT1 Issue

Mike,

I would be thinking about replacing the wires, when I replaced the wires on my 67 I seem to remember the new short wires at approx 4-5000ohms. How old are the wires?

Dan,

I would be surprised if there is a problem with the Breakerless SE ignition, if the car sat for some time it's possible there is fuel gum up inside the carburetor. It's really difficult for anyone to tell you exactly where to look except to say if the engine acts like it is starving for gas you should start at the carburetor first. Did the engine run OK when you initially drove it?

The Holley could have a air leak around one of the gasket surfaces so if you are carburetor savvy get some Holley gaskets and take it apart to clean and install gaskets, be careful not to over torque the screws.

[Roy V.]

Re: 72 LT1 Issue

Dan, it does sound to me like a fuel delivery issue, as others have suggested, I would look at the carburetor and check the level of the float assembly, it could have got gummed up from old gas and from sitting.
Regards
Roy

[Dan A.]

Re: 72 LT1 Issue

Thanks to all that have replied. I tried a different coil but the symptoms remain. I guess I will take a look at the carburetor.

[Dan A.]

Re: 72 LT1 Issue

Update: I found that the primary jets were clogged with some kind of gunk. I cleaned them out as well as the secondary jets which looked OK. I also replaced both power valves and the accelerator pump, which I believe is adjusted right.Car runs strong and no longer exhibits the previous starvation problem. It does still have what feels like a lean stumble when you roll onto the throttle, especially in the higher gears. I have been searching the archives but haven't come up with anything definite. Any suggestions would be appreciated. I know I need to go through the whole carburetor but may send it off to Lars this winter for that.

[Jim T.]

Re: 72 LT1 Issue

When I had issues with my Holleys I always used a spray can of Gumont with the tube type applicator tip directly applied to all openings of the metering block for a good cleaning. Results were great.

[Monte M.]

Re: 72 LT1 Issue

I am glad you figured it out. I was going to suggest the power valve. Let's hope the problem does not return, but remember that a backfire through the carburetor can blow the power valve causing the condition you explained.

Best of luck,
M

[Terry M.]

Re: 72 LT1 Issue

Update: I found that the primary jets were clogged with some kind of gunk. I cleaned them out as well as the secondary jets which looked OK. I also replaced both power valves and the accelerator pump, which I believe is adjusted right.Car runs strong and no longer exhibits the previous starvation problem. It does still have what feels like a lean stumble when you roll onto the throttle, especially in the higher gears. I have been searching the archives but haven't come up with anything definite. Any suggestions would be appreciated. I know I need to go through the whole carburetor but may send it off to Lars this winter for that.

Dan,
Were the jets clogged with a white residue?
I have been told that such a residue is a common result of allowing modern pump gasoline to evaporate inside the carburetor.

[Dan A.]

Re: 72 LT1 Issue

Terry,
It was some sort of black gunk that was easily removed. For some reason the secondary jets were not affected. It continues to run well with the exception of the slight hesitation sometimes.

[Terry M.]

Re: 72 LT1 Issue

Terry,
It was some sort of black gunk that was easily removed. For some reason the secondary jets were not affected. It continues to run well with the exception of the slight hesitation sometimes.

Different stuff than what I heard about.
Good luck.

[Duke W.]

Re: 72 LT1 Issue

Update: It does still have what feels like a lean stumble when you roll onto the throttle, especially in the higher gears.

That may be somewhat the nature of the beast since the carb was calibrated on the ragged edge of lean surge at cruise for emission control. Make sure the accelerator pump adjustment is correct per the CSM.

Did you install the correct power valve? There are several versions that open at different manifold vacuum levels, and I recall most pre-emission types open at 6.5" Hg manifold vacuum.

Duke

[Dan A.]

Re: 72 LT1 Issue

Duke,
Thanks for the input The power valves I installed were the 6.5" Hg ones. I am beginning to believe that it might be about as good as it gets, which is not all that bad compared to some I have heard about

[Ken A.]

Re: 72 LT1 Issue

Actually, if you increase the squirter two sizes & make sure you get the tube squirter plus as recommended adjust the accelerator pump correctly you'll get rid of that off idle stumble.