Is it carb or timing? Dieseling

The engine is running darn good EXCEPT it is dieseling a bit when I turn off the engine. The RPM’s does not seem to be set to fast. Is it carb or timing?

Larry,

As I recall, you have a ZS carb, which may be running rich and contributing to the problem. I would also check the run-on control valve. You can test it by pulling and grounding the slate/yellow wire. You should hear the valve operate. When its working, it will put vacuum over the float and kill the engine immediately. One last thought: are you running premium gas? Higher octane might help, too. Good luck.

Craig Kubiak
'79B

Carig, thanks for the response, you are right I have a ZS, I just tried leaning it a ¼ of a turn…started starving for fuel so I put it back. I messed with the run-on control, I am not sure but 1 wire may not have been pressed in all the way making contact. I ran the engine a few minutes to warm it up and had no dieseling. I will need to try it a few more times to see if it has been corrected for sure, but it is way to hot in the garage to play car tonight. Thanks for the help.

Dieseling is caused usually by overheating or carbon in the cylinders. I run my 79 on regular unleaded and I have never had a dieseling problem. An easy check of the anti-run-on vavle is to plug the end of the hose that runs out of the bottom of the valve. With the engine idling, plugging the hose should cause the engine to die immediately. If not, you have an air leak somewhere in the vapor extraction system. The valve opens when oil pressure is lost, causing the carb to quit pulling fuel. No fuel, no run-on. My question is...if they need a valve to stop it, is there a basic design flaw that causes it in the first place?

A design flaw? On a ZS carb? Is that possible? (Please note tongue planted firmly in cheek.)

Craig Kubiak
'79B

That's always been my opinion too. Starting in about 1971 there were a lot of band-aid "solutions" applied to cars - foreign and domestic - until the engineers got the emission problems pretty well sorted out (thanks mainly to EFI). Unfortunately, our favorite cars are no exception - and the late RB cars are probably among the worst, since BL spent as little cash as possible on engineering near the end of the run.
Maybe someone can explain why my DCOE equipped '78 (with timing advanced well beyond factory specs) never, ever shows a hint of dieseling with the anti run-on valve in a box on a shelf in the garage - but that same valve seems to be such a necessary piece of equopment on a stock ZS equipped car.??

Good chance it's just carbon buildup. When you get enough and it gets hot enough it will continue to ignite fuel. My Bea had just started that run-on thing when I drove her to MG2003 in St. Louis, about 110 miles form home. When I got there she didn't do it and hasn't since. I dumped a can of Sea Foam in the gas tank before I left that morning.

If I ever have a doctor tell me I need bypass surgery, I'm gonna tell her to try a Seafoam injection first....

Probably carbon build up. However, my 71B will diesel if I run straight 87 octane regular unleaded. If I mix the fuel with super unleaded, the dieseling goes away. My engine has been sitting idle for 14 years and I have only run the car some 200 miles now and it just started dieseling. I probably have carbon build up on this engine too.