So for my rebuild in another thread...I replaced the stock 70 350 with a rebuilt 455. On the old 350 I use to run into the odd dieseling when shut off.

I dont have the car on the road yet but I do run it occassionally and the 455 diesels occassionly when I turn it off. It does rev a little high at idle (1200) on a new 670 Holly but I am skeptical that it is an ignition issue that was present before and still is present. I am using the same distributor as before but I got rid of the points and installed one of those electronic ignighters. I have the same volt regulator and altenator....

Any ideas....I was thinking about trying a higher octane.

 

Yes. Reduce your idle, or switch it off while in gear.

- Eric

 

1200 seems a little high.
I run my BB at 7 to 750.
I know it's annoying, but you
can shut it off in gear to make
the dieseling stop.

Beat me to it Eric!

 

Your idle is too high, drop it down at least to 650, if it idle well.

 

The ignition system has nothing to do with a dieseling problem. It can't, the ignition is already off when the dieseling occurs.
I also agree that 1200 RPM idle is too high, although with a radical cam it may be necessary.
On a lot of stock setups there was an "anti dieseling solenoid". It was activated by the ignition circuit and would keep the throttle slightly open so it could idle . When the ignition was switched off the solenoid would drop down and allow the throttles to close tight. Denying the engine any air or fuel.
I have a feeling that this solenoid was eliminated when the Holley carb was installed.

 

On European cars the idle system was separate from the rest of the carburettor, the throttle blade would be closed completely at idle. The shut off solenoid closed down the idle system.


Roger.

 

So there was no idle circuit in the carb?
Dieseling is almost alway a byproduct of poor fuel, or combustion chambers that are coked up from burning oil.

Get the engine warmed up and then squirt water down the carb at about 2500rpm. See how much crap comes out the tail pipes. Just might cure you ills.

Just as a side note. Radical cams needent have a high idle. Its all in how the package is set up. I've got 279 @ .050 and it'll idle at 850 in gear and 900 out of gear.

 

Well dumb question I assume but what is dieseling

 

Its when the ignition is turned off and the engine continues to run on from self ignition. Usually only lasts a few seconds. Often called "run on".

 

Dieseling is when you shut the
car off and it keeps running on.

Beat me to it Smitty.
That's twice in the same thread, ha ha.

 

Gotcha my friends cab over does that lol but I guess that's where it gets the name dieseling

 

Check the float lever and the needle may be stuck open in the bowl or have a problem with the fuel line being too close to the block and getting hot, boil over will also cause dieseling.

 

thanks for the tips....not a carb guy by no means. I had the idle screw all the way back and 1200 is the low I can go. There is the adjusment screw under the electric choke that I am going to try next.

 

CLcutlass...are you referring to the curb idle (warmed up) adjustment, or the hi idle (cold start) setting? They'll be set with different screws.

 

Disconnect your throttle linkage from the carb and see if it will idle lower, your linkage may be too short.

 

I wasn't very clear in my description, the carbs had an idle circuit, what I meant was it wasn't affected by the rest of the system.
BTW most SU carburettors DIDN'T have a separate idling system. It was a very simple design that worked remarkably well for years until emissions regs killed it off. Nobody made a carburettor better at cold starting ever.


Roger.

 

I have the curb idle back as far as it can go......I never touched the hi idles yet.

 

I dont think that is it....I cant move it any more closed by hand

 

You probably have a vacuum leak, that will cause it to idle to fast and running to lean which causes excessively high combustion chamber temps and resulting in dieseling.....
That's my guess..........

 

Is there any chance your throttle linkage is hanging up on something? Like part of the casting on the intake?

 

Quite possible too, how about put a vacuum guage on it, tell us what you got.

 

I did have one on there but took it off...if I recall it was 15-16"

 

Smitty, his is not meant as a correction to what you said, just to add a comment.
I had "Run on" in which case the MSD electronic ignition (if you have one) gets an alternate current path thru the voltage reg. when the ignition is turned off.
This term "Run on" is used by MSD and the symptom is; the engine continues to run normally after the key is switched off and will continue to run. The Deiseling thing does not sound normal especially when it finally chokes and dies.
Ref. the MSD site for this problem which is entirely different.

George

 

If you have the idle screw out as far as it will go and the idle is still 1200 you have a problem somewhere. The throttle blades should be closed at that point. I would check for binding linkage and vacuum leaks.

 

That's pretty low. I should be higher, more like 20"

 

I will check it out for sure....this has a medium cam so will that make much of a difference. Dont ask me the specs', I dont have them. I recall the builder said it was a stage 2 cam

 

what is the best way to check for vac leaks....the engine sounds real smooth. i thought with vac leaks it would be rough ?

 

The reason you may not be able to close it anymore by hand may be because the throttle linkage is not allowing it. Are the butterflies all the way closed at the base of the carb? Is the choke completely disengaged?

 

I'll check it out. I am using the stock cable with the gm adaptor for the Holley. I guess I can just look down the carb to see the butterflys

 

hooked up a gauge....reading 14 -16 " at idle , checked the vacuum to the breaks for leaks, seems ok. Looked down the carb at the inside blades, they seems closed but not perfectly flat, i touched the screwdrive down there to see if they had any more movement....seems ok. The choke does not open completely vertical but moving by hand did not make much of a difference if any. I bucked my adaptor bracket to my cable in a little to see if that would help, not much of a difference. But whatever I done I got the thing to idle at 750-800 now, sounds a little choppy but I actually like the way it sounds. I am going to continue to look for vac leaks but it should run good enought to get to the paint booth.

 

Disconnect the throttle linkage and see how it idles. Also check to see that the tang on the linkage that the idle screw touches isn't bent. If you back out the idle screw all the way until you can't see it the tang should be resting on the body of the carb.
I've enclosed three pics of a Holley. This is actually a BG 2BBL but the linkage is basically the same if not exactly like yours. The idle adjustment screw is on the drivers side behind the throttle shaft. You can clearly see the tang I'm talking about and how if I backed that screw off all the way it would disappear into the base plate and the tang would follow it. The last pic is bad but shows the throttle blade opening as it came out of the box. Hard to tell because of the shadow but it is somewhere between a 1/32 and a 1/16 open max.

Just to be clear the fast idle adjustment for the choke is behind the choke housing on the passenger side. There's no way you can mistake the two.

 

if it dropped after bumping the linkage mount I believe that would be it. if you have it set too far back towards the firewall adjusting the screw won't do a thing to lower it if the throttle is still pulling on it. I'd disconnect it completely and let the carb close all the way then connect the linkage. you may have to adjust the pedal and linkage. I know on mine after switching to a holley on mine I had the exact opposite problem and would only get about 3/4 throttle and couldn't hit WOT. Linkage was fine just the throw in the pedal had too much slack.