Seemed to have developed a dieseling or run on problem after I shut the motor off, turned the idle down to 700 - 750 RPM from 800 - 850 RPM and it still does it.

Any suggestions on how to fix this ?

 

Wayne, try turning the idle down another 50 to about 650rpm. Trial and error but assume your idle quality will be ok (little lumpy, no doubt due to your setup).

 

Check this out Wayne:

"- Idle Stop Solenoid (ISS): 71-76. Useedd to set the idle higher than the
basic curb idle screw for emissions reduction and to allow further
closure of the throttle blades at engine shutdown to prevent 'deiseling'
or 'run-on'. Mounts on a bracket retained by base mounting bolt and
screw on the right side for 72-76. 71 mounted to the left side of the
intake under the carb linkage."

Believe you still have the quadrajet and although not exactly your carb it might point towards some things to look at.


http://www.geocities.com/tv_larsen/v...quad_guide.txt


[Modified by Fevre, 9:41 AM 6/10/2003]

 

Some other reasons can be: a lot of carbon in the chambers (glows red & ignites the fuel), sharp edges in the chambers (collects carbon, which flows red...), plugs too hot a grade (have you just changed grade/make of plugs?), incorrect idle mixture or engine running hotter than normal (timing recently advanced, different grade of fuel, cooling system problems?).
Good luck, it's embarassing when your engine is still running as you walk off!

 

Thanks guys, Paul a lot to think about there, could be carbon the motor has about 45,000 KM's or 30,000 miles. I have changed gas recently but it was because I thought the SUnoco 94 was causing the run on .

I run a cold plug to begin with and nothing else has changed, if anything the motor is running cooler ~170 deg since I filled in all the gaps around the radiator. Will try turning the idle down a little more, and check the idle mixture circuit.

 

650 is too low. How are you adjusting the idle speed? Auto or manual trans? Do you have the idle solenoid described above?


[Modified by BB2B, 2:06 PM 6/10/2003]

 

I'm adjusting the idle speed with the screw on the side. Maybe I could retard the timing a little to slow it down ?

No solenoid and manual tansmission.

 

I had the same problem with my ZZ4 engine after I installed the GM "hot cam" and ported the heads. Come to find out it was my idle speed. When I installed the new 650 Speed Demon, I set the idle to 900 RPM. After leaning the idle out, the engine still deiseled. I ended up having to go all the way down to 650 rpm (as GM recommends) to get the engine to shut off properly. Good luck. :cheers:

 

Motorhead,
The solenoid on my '81 was to up the idle speed when the A/C was on. My manual mentions another solenoid, under computer control, that set the idle speed. On shut off this other solenoid is supposed to drop the idle right down to prevent run on. My '81 didn't have that solenoid so, either it had been removed (unlikely, there was nothing obviously missing), or it was something for a later model (the '82)?
Retarding the timing is something we have to do here on engines that were designed to run on leaded fuel. They run a lot hotter on unleaded so retarding the timing, as well as preventing the pinging, lets them run a bit cooler. When the engine is shut down then it should be a bit cooler & not as prone to run on. In theory. That doesn't apply to old MG's :( I guess it wouldn't hurt to try retarding your timing a bit to see what happens, but if it never had the problem with the timing set as it is then it's probably something else.
I've had another thought on run on (I'm battling with the same problem with an old MG at the moment). I've never taken a Holley apart, but one engine that had run on problems turned out to be, after checking absolutely everything, an incorrect fuel level. The float needle & seat weren't sealing perfectly so the level was to high. On shut off the excess fuel was causing the engine to run on &, as the engine jumped about due to it, this was causing the fuel to slosh about (we guessed) which made it worse, as once the run on started it got worse & worse :( It could be worth checking if all else fails.
The worse case of run on I ever saw was with a Chieftain Tank. The engine was switched off but got a bad case of run on. It didn't stop & the revs got higher & higher, the crew bailed out & ran (along with everybody else!!!) & in the end the engine was revving it's nuts off (the mechs said that they start burning the sump oil when that happens). Eventually it blew up. Forget top fuel engine explosions, when a tank engine lets go it's spectacular!!!
If you find the problem could you post it (I'm having trouble getting this old MG to stop doing it & am running out of ideas!)?
:cheers:
edit: Something else! Years ago I found a run on problem was caused by a lot of wear in a carb. When the throttle was closed there was so much air still getting through that run on occured, no matter how low the idle speed was set (or tried to be set - it was all over the place).


[Modified by UKPaul, 5:34 AM 6/11/2003]

 

I think it is a combo of 3 things, idle speed, timing and check that Holley, when we were together at Santo's with Scot Dixon I thought that it was leaking a bit. What are you running 34 or 36 degrees timing?


[Modified by gdh, 8:24 AM 6/11/2003]

 

lots of good tips already.
a slightly richer idle will lower combustion temps lots!(if there were hot spots they will be gone)
i always shut any car off in gear(as i let the clutch out for sticks). they don't run on.

 

Now there's a novel idea :D

 

Went to the Doctor, said
IT ONLY HURTS WHEN I LAUGH
Dr. said
SO, DONT LAUGH :jester
I dont smoke when filling the gas tank, i dont shut a car off in neutral.

 

I had the dieseling problem. Embarrassing when I would shut it off. I would feather the clutch to keep it from doing it. If you used too much clutch the car would jump and make you look even stupider than the dieseling. I figured mine was carbon build up. So, I got the engine up to temp, ran it about 2500 rpm and would spray a little water down the carb to remove the carbon. I did this a few times after I would drive it and it cleaned it up. No more dieseling.

 

I shut mine off in gear too as I know it will run on. I just put it in first and lightly let out the clutch until I get a slight load w/rpm drop, (while holding the brakes) then turn off the key.
I've tried playing with the idle & timing, but the motor ends up with less overall. I like where the timing & idle speed are now and can deal with the trade off.


[Modified by 71coupe, 5:28 PM 6/12/2003]

 

SAC,
Good post :thumbs: Proof that the water spray trick does actually work :)

The suggestions about shutting off with it in gear etc are all good (I wish my GF would do that!), but in Motorhead's case it never used to run on & now it does, so there's got to be a simple cure for it - it's just finding out what it is that's the hard part :yesnod:
:cheers:

 

:iagree: