Pre-Ignition Running On on 68 2000 Engine

[sworxx]

After I spent big bucks and a year and a half waiting to have my Roadster's engine repaired, I drive it home from the shop, park it in the garage and shut the engine down. Only it does not shut down, it makes the horrendous noise and continues running to the point I have to stall it to shut it off. Since I don't want to drive it all the way back to have it repaired properly and wait another 6 months, anyone know what I need to look for to try and fix it myself?

Also, if you want the name of the shop to avoid, just e-mail me. I don't want to flame anyone.

Thanks.

[RoadsterYosh1]

well, what repairs did he do?

things I would start with is the fuel, fresh? using premium? the timing? spark plug wire in the right order, etc.

[RockFish]

How long would it run on on if you didn't stall it ? If it would continue running for a significant amount of time, I'd be asking myself how the fuel was continuing to be ignited after I've shut the key off. Combine this with what work you had done, and this might point you in the right general direction.

[sworxx]

Maybe 20-30 seconds.

[dbrick]

Same exact problem here with my 69 2000. Two biggest culprits are vacuum leaks (mine is the carb shafts) and/or a hot spot, usually glowing carbon on spark plug or cyl head or edge of a valve. Either the glowing carbon or simple compression ignited the mixture, that's why it is called dieseling, and if I'm not mistaken, the engine is actually running backwards. I keep forgetting to check which way the car lurches when I let the clutch up in reverse to stall it.

1. Check if you have the right spark plugs and see if they are all carboned up.
2. check for vacuum leaks at carburetors and intake manifold gasket. If you use gumout, be careful spraying it around hot exhaust it can flash and cause a fire. When the gumout hits the leak, the engine will slow down or stall.

3. don't let it run on for long, beats the engine up, especially if it really runs backward, the oil pump will not be working

[sworxx]

Thanks I will check it out, I figured it was a vacuum leak. I'm fairly certain that the spark plugs are newer and are correct. I'm really surprised that after all the engine work I had done that the mechanic did not catch this problem.

[nismou20]

Hi,

Use premium gas, make sure your carbs are not running too lean and you may need to retard the timing. With all that said, my 69 U20 still runs on at times so I've gotten in the habit of popping the clutch in first with my foot on the brake. Never diesels any more.

Tom

[SLOroadster]

My car would do that with the SUs on it, when I went to Mikunis the issue went away. U20s seem to do that even more if you are running a B cam with tired SUs. I would agree that hotspots will ignite fuel being sucked out of the carbs.

Will

[Ted928]

My engine would spin backwards - painful to hear.
I tried a few things to fix my problem. High octane did not solve the problem so I went back to regular octane and did a few things.

I checked to make sure the ignition timing through the RPM range was correct - it was.
Carb shaft bearings helped.

Using colortune enabled me to finally set both carb mixtures "properly" - it made a big improvement. One carb was too rich. If I turn the engine off when the rev's are still above idle (~900) it will still diesel but if a wait 20 seconds so the idle speed drops to 700 then it will not diesel.
I have not yet tried high octane fuel again to see if the problem is completely gone.

[dbrick]

I was pretty sure about the backward running, thanks for confirming. On further research, my carburetor shafts are leaking badly, causing a bunch of symptoms. Once those are repaired, I'm guessing the problem will be gone or 95% better.