67 250cid, 1-barrel revs at idle, diesels shutoff

[Matt67C10]

My 1967 C-10 with the 250, inline six and 1-barrel carb has been revving at idle when warned up (I'm guessing around 2000rpm, no tach) and at shutdown, it carries on for 4 or 5 cycles of diesling. I'm no carb expert, can anybody point me in the right direction of what to look at? Thanks.

[LockDoc]

Originally Posted by Matt67C10

My 1967 C-10 with the 250, inline six and 1-barrel carb has been revving at idle when warned up (I'm guessing around 2000rpm, no tach) and at shutdown, it carries on for 4 or 5 cycles of diesling. I'm no carb expert, can anybody point me in the right direction of what to look at? Thanks.

Where is your idle adjusting screw set at as far as RPM? Sounds like you could have a vacuum leak or timing is off....

LockDoc

[LongBox]

Dieseling on shut off means enough air is getting into the cylinders to allow it to fire. The firing is caused by hot spots in the combustion chamber, perhaps deposits that have heated up during operation. Retarded timing can make this worse, as it causes more heat to be developed in the combustion chambers.

So, how does the air get into the cylinders after shut down? Either the idle speed is set too high, so the throttle plates are open a bit, or, as has been suggested above, there is a leak somewhere in the intake system (carb to manifold? manifold to cylinder head?)

The idle can be at normal speed, even with the throttle open too much, if the timing is retarded. To solve the problem, make sure your ignition timing is set correctly, and then adjust the idle speed to specs. If the problem has not gone away, check for leaks, and finally, make sure the plugs are the correct ones for the engine.

[Overdriven]

I have the same issue with my 250, idle is fast but isn't too high though. It doesn't diesel too often but it is embarrassing when it does a good backfire a few seconds after being shutoff. Feel like Uncle Buck! I have a broken head bolt and verified a vacuum leak spraying carb cleaner along the intake manifold/cylinder head. Not sure what previous owner did to the timing and such to help mask it, first time it dieseled I did the carb cleaner and stopped investigating. Runs good otherwise so just want it to get me through to my v8 swap, which will hopefully be this winter.

The above is advice is solid. I definitely would've done the same and installed new intake gasket if I didn't have a broken bolt in the manifold and was t doing a v8 swap so soon.

[demian5]

Get your basics correct
Adjust idle, set timing, adjust idle again, adjust idle mixture, adjust idle again, drive, adjust idle again, adjust idle mixture again.

If it isn't that simple, check for vacuum leaks.

[Matt67C10]

Originally Posted by demian5

Get your basics correct
Adjust idle, set timing, adjust idle again, adjust idle mixture, adjust idle again, drive, adjust idle again, adjust idle mixture again.

If it isn't that simple, check for vacuum leaks.

Thanks all, great advice. I'll start there and let you know how it works out.