Embarrassing : engine keep running for a few seconds

[mikemerkury]

My engine keeps running for a few seconds after key switch is off. Any pointers ?

[FastWoman]

I assume this is a carbureted engine? Does it run normally when it does that, or does it sort of sputter and run in surges until it dies? If it's the latter, it's "dieseling," probably from carbon buildup in the cylinders.

[ktm]

What year car?

[five&dime]

Sometimes the iginition switches in the old Datsuns do funny things. It is fairly commin for that to happen on the 510's Something to do with the ignition contacts. Consensus is always to swap out the back of the ignition switch. Or take it apart and clean the contacts.

Or it is the famous "diesling" as Fastwoman wrote.

[mikemerkury]

Sorry for the missing details :

- 73 240z, L28 and 5 spd from an automatic 76 280z

- Weber downdraft carb

- 14 Months mechanical, body and interior restoration/refresh was completed 2 weeks ago.

FastWoman : Your description is the closest to my problem the engine does "sputter and run in surges until it dies?"

[Mike B]

FastWoman : Your description is the closest to my problem the engine does "sputter and run in surges until it dies?"

You might try premium gasoline if you aren't running it already. My 1972 dieseled a bit on regular when I first got it, so I switched to premium and never had a problem again.

-Mike

[IdahoKidd]

Does it seem to do it more when the engine is hot? Check the timing, I am betting it is a tad fast. When it is hot and the timing is fast, the gas will detonate like diesel fuel but won't be able to sustain itself and die. If youi watch the motor, on its last surge of life, does it rotate backwards just a bit at the very end? If so, clear indication the timing is too far advanced. Another indicator might be that just before it starts, the starter will struggle, that is, just barely seem like it can turn the motor over, then it will pop and fire up. These symptoms point to timing. Other symptoms point different directions. I have had no experience with key switch issues so can't say.

Let us know.

Leonard

[sleepyzzz]

mine was doing that, and the guy who helped set my carbs up claimed it was idleing too fast, and after adjusting the idle down, it quit.

goodluck on fixing your z.

[FastWoman]

Fixing a dieseling problem is somewhat mysterious. It happens when compression is too high (from a lot of carbon buildup) and/or when there are tiny parts in the combustion chamber that glow hot enough to keep igniting the fuel/air after the ignition is cut off. I *think* your choice of spark plugs can do that. There are 3 types of NGK plugs recommended for my '78 -- the B5ES-II, B6ES-11, and B7ES-11, being hot, medium, and cold plug types. You might try the colder plug rating (?). As I recall, the 240's use the projected plugs (e.g. BPE7-11).

There are also some products touted to burn/clean carbon out of your cylinders. I have no idea whether they work. I've also gotten advice from an old/experienced mechanic to mix 1/2 gallon of diesel fuel in with a full tank of gas to burn out the carbon. I was a kid at the time and did this without questioning it (on a Toyota Celica). I got some very funny looks and a strong warning from the service station guy. The car ran OK on the mixture, and it didn't seem to make any difference that I could tell with regard to the carbon. I've also heard of the old-timers throwing handfulls of rice into their flathead engines to knock out all the carbon. I DON'T THINK I'D TRY THAT!! But that's what they did.

I'm just guessing that this problem has a longer-range fix, seeing as it took a while to develop (assuming carbon). I think the most important thing is to determine why your engine is building up so much carbon and to make sure it is running as cleanly as possible. I think the engine might eventually burn off all the built up carbon that way. Perhaps it's running too rich? What do the plugs look like?

Good luck!