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Discussion Starter · #1 ·
My Chevy engine backfires and diesels when I try to turn it off. Here is the set up. The car is a restored
1956 Chevy Belair that I recently completed. It has an inline 6 cyl. 250 motor in it (not a 235). I’m using a conventional distributor with points and an external coil. The motor was professionally rebuilt both the block and head at a machine shop. It has original intake and exhaust manifolds with a new NOS Carter YF carburetor. I have only starter it 5 or 6 times so far. It is nearly new in that regard.
The car starts without any problem. However, when I give it gas, it hesitates and sputters. As it warms up, it runs very rough and wants to die. I feather the gas petal and it will increase RPMs but when I back off, it starts to die. At low RPM I can hear some knocking. When I try to shut off the engine, it will not stop and begins to diesel. Sometimes it backfires through the carburetor.
What is most likely wrong? Is the timing off? Valves not adjusted correctly? Suggestions appreciated.
Thanks, Glenn
 

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My Chevy engine backfires and diesels when I try to turn it off. Here is the set up. The car is a restored
1956 Chevy Belair that I recently completed. It has an inline 6 cyl. 250 motor in it (not a 235). I’m using a conventional distributor with points and an external coil. The motor was professionally rebuilt both the block and head at a machine shop. It has original intake and exhaust manifolds with a new NOS Carter YF carburetor. I have only starter it 5 or 6 times so far. It is nearly new in that regard.
The car starts without any problem. However, when I give it gas, it hesitates and sputters. As it warms up, it runs very rough and wants to die. I feather the gas petal and it will increase RPMs but when I back off, it starts to die. At low RPM I can hear some knocking. When I try to shut off the engine, it will not stop and begins to diesel. Sometimes it backfires through the carburetor.
What is most likely wrong? Is the timing off? Valves not adjusted correctly? Suggestions appreciated.
Thanks, Glenn
I think you better pull the distributor and start from scratch--sounds like timing is way out of whack
 

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You asked this question over at Stovebolt too.

I think you need to start with the low hanging fruit. Check the plugs, compression, plug wires. Then check to see if your timing is accurate. That TDC is actually marked accurately as TDC on the timing tab and/or that you do not have the timing accidentlialy too far advanced. Might consider readjusting the valves. Also, see if any vacuum leaks. Spray some WD 40 or something aroung the carb base, vacuum advance lines. Check to see if the vacuum advance can is intact and not ruptured.

One other thought, might be a ruptured power diaphram. The YF uses a mechanical/vacuum rod arrangement where vacuum applied to the power diaphram over comes spring pressure keeping the rod in the jet at idle and part throttle. When the vacuum is over come by the spring, the thick part of the rod moves up and out of the main jet. If the diaphram is ruptured, which is a distinct possibility given the NOS status of your carb (i.e., not ethonol resistant) the thicker part of the rod will be out of the main jet feeding the motor fuel through the main jet that it may not be able to use, rich condition, which you are trying to compensate by making adjustments in idle speed, timing. So you may need to take it apart and check that the diaphram is not bad. I know its been rebuilt, does not mean a thing. Could also be the main jet is loose, or the rod got bent somehow. Its just a matter of pulling it apart and double checking
 

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Discussion Starter · #5 ·
I have the timing set at 10 degrees BTDC. I read somewhere that a I6 250 should be 4 degrees BTDC with not vacuum connected to the distributor. What is your recommendation?
 

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I think your issue is carb related but if you want to eliminate a few things first, I would deal with the low hanging fruit first, then the timing. If the distributor has say 12 degrees of mechanical, which is 24 at the motor, and the factory specification is 5 degrees advanced, that is a total of 29 degrees. You are currently at 34 which is not a an outrageous amount and may very well be fine but I might go back to the factory specification just so you can have a baseline for your diagnosis. Then, If the other low hanging fruit checks out, and the timing is right, I would head to the carb.
 
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