Help with dieseling or engine run on

[jwscarab 20]

Wondering if I can get a little advise. My car seems to have a dieseling or engine run on when I turn off the key. It happens a LOT. It sounds horrible and I cringe every time - thinking a rod is boing to go thru the block. Sounds like a horse when they flap their jaws, with bearing noise added in....lol.

 

So, I have replaced my ignition from Mallory to MSD to resolve other issues, I have messed with timing during tune ups - and one thing is consistent - it has done this all along and still does it. Although it does seem to do it more with my higher timing advance setting.

 

Would u guys lean towards carbon build up in the cylinders causing a hot spot? If so, would a can of BG44K sucked in thru the carb plate, let to soak, then run help me out?

 

As for timing, I originally had it at 12 initial and 33 total, but it was very doggy. I since changed it to 16 initial and 37 total and it has regained its power back w/o pinging. I'm debating changing it to 14/35.

 

Any advise appreciated!! Thanks!

[Fordrevhead 29]

Did you check that the coil doesnt have power with the key off? What carb are you running? That would be where I would go next. Ethanol eaten metering block gaskets, etc. do you know the CR and what gas are you running?

[miketyler 15]

What gas are you running in that beast? Maybe the most common issue I have read for "dieseling" is cheap gas. I have to run the higher octane fuels in my cars to prevent that.

[jwscarab 20]

I am running 93 octaine only - from my local Marathon station. No additives.

 

I'll check on the coil power - but I believe its off, but I will double check. The carb is an 850 Demon. Not sure of the compression ratio. I do know its a 390 block with the original 428CJ crank, and slightly oversized (.030 tops) pistons, and it has the original 428CJ heads too. So its kind of a 390 bored, and stroked with the 428 crank. Not sure if any of this helps!

 

Thanks guys!

[LoneWolf2U 136]

Bring eng up to temp, with aircleaner off and idle set approx 1500rpm.

Take water in a spray bottle and fan a few shots over the carb.

Let the idle return and repeat.

Reset idle to normal and take it out on an open road and run first and second gear hard to a high rpm range and blow out any carbon fouling.

 

If that doesn't cure the problem, look elsewhere.

[Fordrevhead 29]

It took me a bit to find it, but read this and buy a diode, it might be the cheapest fix ever!

 

http://www.msdignition.com/page.aspx?id=3296

 

Make sure your secondaries aren't dumping ANY fuel at idle. If your plugs look good, richen up your mixture and drop the idle. You can also put a throttle solenoid on it to close the throttle plate completely. Do you know if your cam was degreed? Your initial timing being so high could be doing it too. In the meantime shutting it down in gear might help.

Edited March 23, 2014 by Fordrevhead

[jwscarab 20]

Thanks guys! I am going to try all your suggestions. I like the diode idea too - thanks very much for the link!

 

Also, been doing a bit more reading on timing, I probably need to put my idle back at 12-13 degrees and then swap out the washer in the distributor to get a bigger advance, so it will still be at 12-13 at idle, and 37-38 at 3000.

 

I'm a bit slow on progress, but I'll do all these and report back! Thanks!

[rockhouse66 22]

What carb are you running? If it is a Holley, I'm betting that carb adjustment is your problem.

[jwscarab 20]

Hi RH, its a Demon 850. I probably need to do a rebuild on it - totally agree!

 

I hope to get some under the hood time this weekend!

[rockhouse66 22]

I don't know how similar that Demon is to a Holley, but the Holley has an adjustment screw for the secondary throttle butterflies. It should be adjusted so that these are always cracked open just enough to expose the transfer slots (very narrow slots in the venturi). If this is not done, then you have to open the primary further with the idle screw to get your idle speed. This open primary will cause run-on. You want the engine running on all 4 venturis at idle. Then the primary will be in its normal almost closed position and the engine will not run-on when switched off.

 

That has been my experience. Yes, carbon buildup/hot spots could do it too, but it is a lot easier to work on the carb first!

[SA69mach 39]

Bring eng up to temp, with aircleaner off and idle set approx 1500rpm.

Take water in a spray bottle and fan a few shots over the carb.

Let the idle return and repeat.

Reset idle to normal and take it out on an open road and run first and second gear hard to a high rpm range and blow out any carbon fouling.

 

If that doesn't cure the problem, look elsewhere.

 

+1 on this.

 

I would highly recommend you try a octane booster, for at least a tank of gas, and see if this cures the issue. I have had two engines that had persistent dieseling. The big block was totally rebuilt, with performance upgrades, new cam. All the good stuff. It dieseled before, and it dieseled after. Some octane booster cured it.

 

Second was a stock 351 W 4V. No problem until I ran out of gas, got 5 gallons of low grade garbage, went to the gas station and it dieseled for 30 seconds. Filled with 93, no problems after that. I think some engines need higher octane fuel.

Anyway, I have had dieseling problems with two engines (well, three if you count the 'stock' big block.) Both just hated low octane fuel. both were cured by higher octane. If you have your timing set how you like it, but the only issue is dieseling, it is worth the effort to try a few bottles of octane booster, and see what happens.

 

I know you said you are buying 93 from Marathon. It is possible their fuel is not to spec. Try the booster, it is a cheap tool.

 

Best of luck

[Fordrevhead 29]

Yeah water injection will rid of carbon for sure! I was given a 1968 Briggs push mower a few years back. Original metallic green deck with Kmart sticker on it. They don't make them like they used to! Anyway, I got it to run but it had no power and was knocking and wouldn't rev up. I yanked the muffler and pipe off as well as the breather. Misted the water hose over the carb and it started bucking and jumping across the driveway. Black chunks started shooting out the exhaust hole and I was able to increase the throttle with response as it went. Soon it cleared up and was smooth at full bore! Fresh plug and air filter and I used that mower for 3 or 4 years without fail until I mistakenly ran it over with my truck.

[jwscarab 20]

Well I may have it taken care of. I basically had to pull the distributor because I wanted to set the initial timing back to 13. Then I put in a different size bushing to let the advance go 25 for a total of 38. I tried and I couldn't get the nut/washer/bushing on without turning the distributor upside down.

 

Once that was set I lower the idle to 700. Then I did the water trick. Funny thing is when I sprayed the mist the engine didn't miss a beat. So I sprayed 3 shots, and I did it 3 times. Then I went out and put about 60 miles on it, turning it off maybe 5 times. It never once had run on.

 

I'm going to lower the idle a tad more now that its running better. I will also check the coil for power with key off yet. But I think its fixed! Thanks everybody - good advise, and very much appreciated!