Heyas Folks....

By way of introduction, I am a DIY Corvette owner ... with little or no experience... (dangerous I know:yesnod:) part of the reason I purchased my sweet Vette is because I wanted to learn more about automobiles, and have some fun fixing up and modifying my car.

I hope you folks will bear with me while I learn and ask a fair number of dumb questions !... This forum seems to have some helpful folks !!

Right now I am having a problem with dieseling after shutoff when the car has warmed up. Also, for a short time after the choke has dis-engaged the idle is far too low, on the verge of stalling then after a few minutes the idle
seems to normalize around 1100,1200 rpm.. which is too quick I think.

If i start the car cold, and drive it for under 5 minutes, and then shut it off.. the engine does not diesel.

My engine is a stock L48 with stock carb, air intake...etc.

 

Check these things in order:

1:) Hot idle Speed (compare to the underhood emission label)
2:) Vacuum leak
3:) Ignition Timing

Good luck. Report back let us know your findings.
:flag

 

Thanks !!

Do I adjust the hot idle speed on the carb first and then check for vacumn leaks and re-adjust idle if I find some?... or would a vacumn leak create an abnormally false high idle speed ? I am still trying to learn how that system interoperates....

 

That problem is most often associated with the timing being too low. Put a light on it, and adjust it to factory specs. It's an easy thing to do, before exploring other possible causes.

Remember: When you see hoofprints, look for horses, not zebras.

:cheers:

 

Welcome NW79 :seeya

 

:withstupid:

Check your PCV valve too.

 

Here's a stupid question. What is dieseling after shutoff? See it in the repair manuals, but don't know what it means.

 

Dieseling means that it continues to run for a brief second after the ignition is shut off and then sputters off. A lot of times this occurs to motors that have big cams and need high idles to get them to run properly. My car diesels out on me after it gets up to temperature. Kind of embarassing to pull in somewhere and shut it down to have that happen. I normally turn the key and right when the engine is supposed to normally shut of (with the car in gear) I slowly let my clutch out to put a load on the motor and it shuts off normal.

 

mine used to do it. i corrected it simply by using a higher octane gas.

 

What octane are you using, I am use the 87.

I think it is the relationship between the Carb/Fuel system and the emission control system that has me intimidated. Ah well... have tools, will tinker.

Thanks to everyone for their responses.

 

Good set of responses so far.... here is another one...

As the throttle shaft & bore wears out, the clearance between the two is enough to allow air to pass between them. The engine gets too much air, and runs lean. This won't be a problem when the engine is cold and the choke is on, because the carb is providing extra fuel via the choke. When the choke goes off, the mixture leans out, and the engine tends to stall. So... the owner adjusts the idle set screw up higher to keep the engine from stalling. But.... when the engine is shut down, it is important that the throttle plates rotate all the way back to completely closed position to avoid "dieseling" or "run-on". Since the idle has been adjusted up too high, the plates don't close all the way, and the engine trys to stay running, assisted by carbon deposits & detonation in the combustion chambers (in lieu of spark).

This is probably not your problem, but it is possible. The solution in this case is to install bushings in the carb base to take up the excessive clearance.

Wiggle the throttle shaft front to back & top to bottom (not drivers side to passenger side). If it is extremely sloppy, you may need some machine work.

I'd vote for one of the other causes first though. Check the easy stuff first.

 

I went from the cheap stuff right up to 92. it's probably more than sufficient for a stock 78, but it definitely solved the dieseling problem.

 

Here is a little more to think about...
When an engine diesels its actually running backwards. Rotatig in the opposite direction.
Other factors are hot spots in the combustion chamber. ie...build up of carbon that glows hot enough to light the fuel just as the spark plug does.

Too low an octane also does it. When an engine "Pings" it's rather interesting....and...very harmful.

Fuel ignites when the plug fires and the optimum burn is a leading edge or flame front that travels across the piston top to the other side of the cylinder.

Pre-ignition (pinging) is fuel lighting from other directions...many flame fronts that burn over the piston top and actually collide with one another.

It's so violent that what you hear as ping is actually flame fronts hammering each other and your piston top. The fuel lights too soon with too low an octane.
The higher the octane the better the ability of the fuel resist ignition under pressure. Also the new fast burn heads all use flat top pistons. Domes are "old school"...the flame in a dome situation has to travel much farther....
Up...over and down the dome yielding less cylinder pressure with slower burns. Less an explosion....

Small chambers and flat tops are the hot set up.

Just a little tangent...sorry... :steering:


[Modified by 81ZZ4, 10:40 AM 3/24/2003]


[Modified by 81ZZ4, 10:43 AM 3/24/2003]

 

Ok.. so I took a look at the carb, and noticed that the choke was set to middle setting... looked in the shop manual and default setting for that is 2 notches lean, so I changed it... and now it works alot better..

I am still going to do a review of the vacumn hoses...etc.. also may have to goto the higher octane, but its pretty expensive these days...

 

Carbon buildup in the chambers could cause it. Worn timing chain causing late valve timing could do it to by making combustion temps higher. Untill you solve the problem, try shuttimg it off while in gear. Many times that puts enough extra load on the engine to make quit when you turn it off.