When I bought my '78 F100 / 300 it had been sitting in a driveway for almost two years unstarted. I got it started and it runs fine, except it diesels when I shut it off. The past owner said he put in a new carb (looks stock).

I assume the mixture is too rich? Or could it be dieseling from just sitting around so long without running? Any suggestions first or should I just get out my manual and start adjusting?

faype02(No Email Addresses In Posts!)
1978 F100 6-Cyl 300

 

Your timing may be too far advanced. Retard it a few degrees and see what happens. Timing is the usual cause for dieseling. I've never seen bad mixture ratios cause it but that doesn't mean it can't happen.

 

I had a 300/6 last summer that did the same thing. Try retarding the timing and make sure the idle speed on the carb is adequetly low enough. Another thing that was a problem for me was the kick down lever on the choke was set a bit too fast.


1971 F-350 1 ton dump, ground up fully restored w/ 360 motor

 

I'm not sure about the older 6's, but the 8's have an electric dashpot on the carb step cam. It has one wire to it. If the engine idle is too high when you turn the key off, many times it will diesel.

This dashpot allows the throttle to close all the way when key is turned off. Check to see if you have one, it's probably the culprit.

*********
94 E-150 High Top Conversion Van 4.9/C-6
89 Mustang LX 5.0 - 5 speed

 

Dieseling is generally caused by fresh fuel "dropping" onto the red-hot tops of pistons after the ignition has been cut-off. Find out why the throttle is not closing properly and you should be able to remedy your problem.

 

OK, I took your advise and checked the timing - it was 11 degrees BTDC, so I upped it to 6, which is posted on the valve cover label.

Seems to have done the trick, though it still did diesel once, so I'll let you know if it hasn't totally gone away. Can't find a tachometer yet, so can't check the idle speed. But thanks for the help!

By the way, the 330/6 doesn't have an electric dashpot - its mechanical. I took a look at it and it went BOOINGGG!! and spat out the shaft onto the ground. It was rusted out and non-functional already, but can't see how it would affect dieseling anyway, since it wasn't even touching the throttle. I'm waiting for a new one to come. Seems like it's only supposed to prevent stalling when you let off on the gas quickly.

faype02(No Email Addresses In Posts!)
1978 F100 6-Cyl 300

 

Also could be caused by excessive carbon build up on the valves and or pistons. Any pinging during acceleration or goin up hills? That would indicate a timing problem. Too much advance.

 

Some dieseling still, though definitely better.
No pinging going up hills, so I'll try lowering the idle. Guess I really should get a tach, but they're so damned expensive....

 

I fiddled with the idle and that did the trick! Thanks for the advice. The idle was cranked way up, the throttle was a little sticky too.
Problem solved.

 

Single barrel carb right? Some of the carbs have a dashpot and solenoid combination that mount together as a unit. The energized solenoid kicks the throttle to the idle position (ignition on). On mine, the solenoid is mounted seperately from the dashpot to a bracket on the front of the carb (rebuilt Carter YFA). The dashpot combo or {dashpot only} mounts to a bracket on the rear of the carb (towards the firewall). The solenoid is supposed to be a fail safe against dieseling. When the ignition is off the throttle spring forces the throttle closed with no resistance from the solenoid effectively choking the intake. The dashpot is in place to cushion the throttle plate as it returns to idle from higher RPM's. When I didn't have my dashpot installed (and had the mixture a little rich), if I hit the gas quickly and let off just as fast the engine would idle sporadically and sometimes stall (AT). This occurs because the carb injects a lot of gas, then Vac is high on decel in RPM's - engine chokes on to little O2 and too much gas. Good to hear your idle adjustment and timing have helped. If you continue to have problems, look into the dashpot and solenoid carb controls. The brackets are hard to come by and and the solenoid is pricey, but they work great when all is adjusted correctly. -Tetra ('79 300-6 C4 - whopee!)