I started the 70 today after 1 year storage. It ran well when put away last fall. It started fine but idles very rough after warm up and backfires through the exhaust while winding down from higher RPM's. The motor accelerates fine in mid range. These symptoms occur while car is parked and without any real load on the motor. Seems like a vacuum advance problem. Any troubleshooting help would be appreciated.
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Re: &0 LT-1 trouble shooting help
Did you put "fresh gas" in it? Did you put gas stabilzer in the tank before you stored it? Also check for water in the gas.....Stale gas and water will give these symptoms. Hope that helps.... -
Re: &0 LT-1 trouble shooting help
Do you have the A.I.R. connected? A shot diaphragm will cause these symptoms.
GeezerComment
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Re: 70 LT-1 trouble shooting help
I siphoned the gas tank dry prior to starting and put fresh gas in along with a can of dry gas so hopefully the gas situation is OK. Tha AIR is connected and has worked fine in the past. I will remove the AIR belt and see if that helps. any other thoughts???? ThanksComment
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Re: &0 LT-1 trouble shooting help
Interesting thought. How does one test the diaphram?
Gary
72 LT1Comment
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Re: &0 LT-1 trouble shooting help
If you are still using a Holley carb on your LT-1 I think you need to replace the power valve. Using new fuel bowl and metering block gaskets put in a new power could very well fix your problem. With the carb not being used in awhile the gas evaporated during storage and could of attributed to your problem. The parts for this may be available from you local Auto Zone. I can buy power valves, metering block gaskets, and fuel bowl gaskets at my local Auto Zone. It might help if you know the Holley part numbers for these parts when you go.Comment
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Re: &0 LT-1 trouble shooting help
Jim,
Your suggestion sounds like a good one. My car has the original 4555 Holley on it. Whats my best source of the Holley power valve part number ????Comment
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Re: &0 LT-1 trouble shooting help
Holley power valves will have different vacuum openings. The Holley power valves come as one in a blister pack. The last two numbers if I remember correctly will identify the power valves vacuum opening. I am not at home to look in my Holley parts book, but I think your power valve opening is 6.5, so the last two numbers on the Holley blister pack will be 65.Comment
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Re: &0 LT-1 trouble shooting help & part # help
My Carb gasket kit is GM part number 3979472. I want to be sure it is the proper kit. Thanks for the help guys.Comment
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Re: &0 LT-1 trouble shooting help
Jim....I don't think you will get the symptoms he is talking about with a blown PV. I've not seen a PV diagnosed in this manner. Just my .02c worth.Comment
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Re: &0 LT-1 trouble shooting help
Could it be checked with a hand held (Mityvac) vacuum pump? Chas Kingston is probably the resident expert on this...........Comment
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you can check for a blown PV
by putting your finger over the bowl vent with the engine at idle. since the vacuum passage that controls the PV runs straight to a hole on the base plate the added pressure caused by your finger over the bowl vent will force gas into the engine causing it to stall quickly. if the PV is good it takes a while for the added pressure to cause the fuel level to raise in the float bowl and forcing fuel out of the boster nozzles causing the engine to stall.Comment
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Re: 70 LT-1 trouble shooting help
The potential issue with the A.I.R. is a failed diaphragm in the diverter valve, which fails the spool in the open position so the exhaust manifolds see air all the time - this causes "popping" on overrun, plus a significant vacuum leak. Connect your vacuum gauge to manifold vacuum and note the reading at idle; then pinch the signal hose from the carb base to the diverter valve with your finger - if vacuum increases, the diaphragm has failed.Comment
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Re: 70 LT-1 trouble shooting help
Hi Dave, Sounds like a gas problem to me. I hope fresh gas does the trick. Save the Wave.>GeorgeComment
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Re: you can check for a blown PV
Clem I am glad you posted this method of checking the power valve. I discovered this method years ago messing around with my Holley and have used it a few times after discovery, always worked for me. It is a very quick and easy check.Comment
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