Backfire at Shut Down

[Telly]

78 FJ40 with 94,000 miles and California emissions. Bought it about a year ago and the PO had the carb rebuilt and tuned (valves, timing, replaced emission hoses). The 40 runs great but has a backfire only at shut down. "Dieseling" is a common occurance too. After warm-up, the exhaust note is not smooth at idle...it stumbles a bit. The PO told me the 40 has always done this during his ownership and he simply forces the engine to stop turning with the transmission/brake to prevent the backfire.

I'd rather fix it than jocky around with dropping the clutch at shutdown. Suggestions on where to look first? The carb and timing are set-up per the PO but I have not verified it (my timing light broke). I suspect maybe the exhaust? There is no cat nor do I think they came with one originally.

Thanks.

 

[BigRed40]

If it is actually a true backfire, as in a loud pop, then I would suspect timing and/or an air/fuel ratio that is too rich.

 

[wombat]

Sometimes it can be as simple as the idle too high.

 

[JohnnyC]

start by checking the timing and idle speed (see post above)

PO tuned.... well when was the aircleaner replaced? spark plugs? tune-up just before you got it or a year prior?

 

[Fast Eddy]

Backfire requires some oxygen too. Exhaust leak?

Read here too: https://forum.ih8mud.com/40-55-series-tech/770976-40-backfires-only-between-shifts.html

 

[inkpot]

If it is dieseling, then check the idle cutoff solenoid. Lots of info on this board if you search. This could also allow enough fuel into the system to cause some backfire. John

 

[Telly]

All good advice. I will checking the following in this order: timing, idle cutoff solenoid, idle mixture, idle speed, and finally exhaust leak. I need to get myself up to speed on the workings of the carb anyway. I also need to spend the money for a good digital timing gun.

 

[edwjmcgrath]

I would look at the end of the idle solenoid to see if someone cut it off. Some people have been known to do this if they have a bad solenoid or problem with electrical feed to the solenoid. The only downside to cutting the end off the solenoid is that it can cause dieseling and backfiring on shut down, particularly if the idle is set high....

 

[Fast Eddy]

I reread your OP. I think you're placing too much weight on what the PO told you. Sounds to me like it needs a proper tune-up and carb adjust. There are a bunch of little emissions components that could have failed. The emissions manual has fairly simple tests for most (all?) of them.

I do agree with your PO that you shouldn't let it diesel. I'd put it in 2nd before shutting it down and pop the clutch with a foot on the brake. Dieseling is probably spinning the engine backwards, and it's not good for the bearings.