Causes of dieseling on shutdown?

[Luke_Healey]

Just for curiosity's sake, I'd like to ask about the cause of my 1098 to prefer to keep kicking and bucking a couple of seconds after I turn off the key.

I've read that timing problems can cause it. Carbon buildup can cause it. High idle can cause it.


In my own case, my car does idle quite high. I do that so it won't die on me while it's taking its time to warm up. I think 1500rpm is where it idles.

I thought I had my timing set correctly. Does dieseling happen from too advanced or too retarded?

Carbon buildup? Well, I have been running rich all this time, but my plugs themselves are nice and beige/brown, rather than caked with soot. So, again, it seems like it's not tuned up way out of whack.

I remedy the situation by letting out the clutch in 2nd gear as I shut off the car, which kills it in its tracks. But I'd love to have the car work normally, like all of my others.

Any pointers or things to check out??

 

[jlaird]

Carbs sound good with brown plugs. Might be carbon but I would expect timeing. Set it to spec for your car and check again.

 

[Billm]

What octane fuel are you running? Lower octane ratings will cause this everytime.

 

[Luke_Healey]

I'm running 87 in it, since the other option around here is 90 with ethanol. I'm anti-ethanol in any old engine.

 

[regularman]

Its the high idle, I think. They will all do it if you keep the throttle cocked open enough. When its cold, you should get some fast idle along with the choke until you push it off. You should not have to keep it permanently idled high to warm up.

 

[Jim_Gruber]

That's your problem. Find some Sunoco 240 94 Octane. When I first got Bugsy my '68 with 1098 I figured small car, small engine, low octane gas. WRONG. I even diesel with 93 Octane Shell. Put some Octane Boosted in there.

 

[Luke_Healey]

I'll try readjusting it that way. Last year, my choke was binding up, such that when I pushed the knob in, it would stay engaged. The springs weren't strong enough, or something was binding up. Therefore, I didn't use the choke unless I knew I was going to have the hood up and could use my hand to pop the jets back up to normal position.

Do octane booster chemicals work well in this situation, or is it a waste of money?

 

[regularman]

Fix the choke before high octane gas and fancy additives. You know that is not right so start there and then idle it down and see what that gets you. I am far from an SU expert, I have the weber DGV but someone here probably knows how to fix that choke problem. Take a pic or a video and post it.

 

[jlaird]

Center the jets, the spring is supose to help a bit but centering is very important.

 

[Pythias]

I don't know how many times i've heard someone trying to blame this on TIMING.. but that just CANNOT be the problem. The circuit is OFF, so there is NO SPARK. Paul A. suggests two possible solutions (but not a cure).

1) give the car a slight rev just before/as you turn it off.
2) leave it in 4th gear and when you turn off the car, let out the clutch if it starts dieseling.

my 1098 does it occasionally even with the best of fuels.

 

[regularman]

I don't know how many times i've heard someone trying to blame this on TIMING.. but that just CANNOT be the problem. The circuit is OFF, so there is NO SPARK. Paul A. suggests two possible solutions (but not a cure).

1) give the car a slight rev just before/as you turn it off.
2) leave it in 4th gear and when you turn off the car, let out the clutch if it starts dieseling.

my 1098 does it occasionally even with the best of fuels.

Is your idle at the factory 700-800 rpms?

 

[Trevor Jessie]

Bill, If the timing is off can you end up with plugs too hot after the ignition is shut off? Not saying it is the problem here, but just asking.

 

[Luke_Healey]

That's why I don't buy timing either, other than that 'too advance' means it'll run hot and possibly cause the head to be hot enough to allow compression ignition (sort of like predetonation/pinging.)

The choke? I had it working UNTIL I centered the jets, hahaha.

I'm working on the car again tonight since my shocks arrived :wink:

I'll report back.

I think it's most likely due to my above spec idle.

 

[regularman]

I don't know how many times i've heard someone trying to blame this on TIMING.. but that just CANNOT be the problem. The circuit is OFF, so there is NO SPARK. Paul A. suggests two possible solutions (but not a cure).

1) give the car a slight rev just before/as you turn it off.
2) leave it in 4th gear and when you turn off the car, let out the clutch if it starts dieseling.

my 1098 does it occasionally even with the best of fuels.

Is your idle at the factory 700-800 rpms?

One thing I have noticed about a lot of engines is that as they get hotter they tend to idle a bit faster. The good news about dieseling is that it almost always means that you have good compression. That was an quick and dirty compression check for a used car when I was a kid. leave the gas down a bit to a real fast idle and then shut it off. If the compression is low then it will quit right away, if its good it should diesel a bit.

 

[Pythias]

My particular engine/carb combination idles slower when its warmed up and slower yet when its H-O-T. Ambient seems to make the same difference, that is it will idle slower in hot weather. I must say i almost never dieseled on the way home from LotO and that got it really hot. coming over the Rockies when i had to fill up, the car would simply shut off. but the engine needed goosing just to keep it running. don't know if it was the heat, altitude, or gasohol, but it idled like crap when hot AND had barely had the power to climb the hills at times...

 

[Billm]

At LOTO Glen Byrns was saying that if you mix 1/2 gallon of tolulene (paint thinner from the hardware store) to 6+ gallons of fuel then the octane will be the equivalent of (was it 92 octane that he said?). If Glen said it then I tend to believe it.
Bill

 

[Pythias]

Keep in mind that tolulene IS rocket fuel. i'm surprised that you can get it without a license from the ATF!

 

[Luke_Healey]

Yeah, I have a jug of acetone from years ago.. I use it to clean parts. But I don't know where to buy any pure tolulene around here. Also, I heard it gives you genital cancer, so I haven't been motivated to go out and find any :wink:

Now that I've got the carb jets centered, I can work on lowering my idle. The car starts up right away now.

If I let out in 2nd gear when I'm parking the car to bring the idle down, it shuts right off. I guess it is good to know that compression is good. My engine was rebuilt by the previous owner. Maybe he milled the head and bumped up the compression ratio at the time. He ran an engine shop.

 

[Luke_Healey]

I thought I'd bump this for a conclusion post:

I've got my idle down. I don't know what the RPMs are since I don't trust the tach in my car, but probably under 1000rpm now. The car doesn't diesel as badly, and sometimes not at all now.

High idle was definitely part of the problem. I've lubed all the choke springs and whatnots and it sort of works more smoothly now, so I can use the choke as normal and let it out as the car warms up.

 

[jlaird]

I should think it will take some time to burn the carbon out of the cylinders so the runon will go away entirely.