Carburator Dieseling

[jwhite]

72 Spider with a 2 ltr. engine. New 32/36 DFEV carb. I installed the carb right out of the box and was never able to get it to run right. Idol was rough, backfired real good through the carb when first started up, and it would diesel when I would shut it off. I took it to the carb shop and was told the power pump was bad. Well, the shop replaced the power pump and adjusted the carb and everything was going real good for about a week, then it started to diesel again. I have just read about dieseling a don't think it is carbon built upon the top of the pistons ( rebuilt engind has less than 7000 miles on it ). I am using regular gas but maybe should use the highest octane. ( I read that that could be one of the reasons for dieseling ). Other than that I have no idea what could be causing it. The car has an electronic ignition with a vacumme advance and mechanical fuel pump. I had tried an electric fuel pump when I was getting vapor lock and dieseling last summer runnning a 32ADFA on the car and that didn't help at all. Any ideas that I could try here at the house before I take it back to the shop?

[majicwrench]

Engines diesel generally cause the idle is too high. SHould be idling smoothy well under 1000rpm, say 700-800rpm. Does it idle smoothly?? What is the idle speed?? Is common to see the idle circuit plugged, then people run the idle speed screw up too far, then then fuel starts getting pulled out of the main circuit instead of the idle circuit. Do you have a way to remove the idle mixture screw and blow compressed air thru the idle passage?? Can you simply lower the idle??
Getting the combustion chambers too hot also contributes to the problem. Retarded timing makes the chambers hot, and also causes you to turn idle up, both bad. Odd things, like head gaskets hanging into combustion chamber, can also cause problems, but for the most part, it is a carb/idole speed/mixture problem.
Keith

[jwhite]

The idle speed is at 700-800 and real smooth. I can remove the idle mixture screw and blow compressed air thru the idle passage and maybe try to advance the timing. I will need to read up a bit on retarding and advancing the timing as I am not familiar with that at all, but know where the vacume advance is located. Thanks for your input and will get back to you with the results.

[majicwrench]

If car idling smooth no need to blow out idle mixture. And really, a degree or two of timing not going to make a big diff either. If car idling smooth, I would simply turn idle down a ways and see what happens. I would have to go see what my '72 idles at, but I would bet about 700rpm, and it never diesels. Is your engine original, or all jazzed up?? Does it simply chug a couple times after you shut the key off or is it one of those that darn near keeps running when shut off??
Again, first thing I would do is turn idle down and see how it works.
Keith

[So Cal Mark]

if the timing is retarded, you may have the throttle open even if the idle speed is 800. For the engine to diesel, it has to be pulling fuel in either through the main circuit or idle circuit. Does the engine diesel even before it's completely warmed up?

[mbouse]

swap the main and secondary idle jets.

[jwhite]

The engine is original ...nothing extra done to it. I drove it about 4-5 miles, temp guage at around 190 but can't remember if the fan was on... shut it off and NO problem. Drove it 7-8 miles and made two stops ...engine was at running temperature and the electric fan was on..... it would diesel every time. It will chug once, maybe twice but does not try to keep running.

Does the top part of the carb have to come off to remove the main and secondary jets? Looking down at the two jets can they just be unscrewed and pulled out?

[So Cal Mark]

jets have nothing to do with dieseling

[mbouse]

jets have nothing to do with dieseling

you are correct, mark. i was hunching that maybe other things were in the mix causing run-on. the 32/36 is not always an "out of the box" fix for every engine; though when properly installed works wonders for a two litre engine. main idle jet can run a little large, and swapping the jets has been known to rectify the poor idling conditions. as the original post states "Idol was rough, backfired real good through the carb when first started up " which have little or nothing to do with dieseling as well. swapping the jets on this carburetor has been known to rectify these stated issues.

the original post states that the writer just read about dieseling... i read that as possibly a mistaken use of terms.

having read the post now for a second time, i see that the issues predate the carburetor change, so monkeying with the carburetor at all prolly isn't the cure for the symptoms.

do we know what was done with the "Extra" fuel and vaccum lines that the 32/36 does not use? did the stock intake manifold remain on the car, or was it replaced with something less complicated? was the electric fuel pump used last year a low pressure or high pressure pump?

[jwhite]

The return fuel line has been capped off and I know that the intake manifold is NOT from a 2 ltr. Engine. It is the manifold that came off my 1800 engine. There again, I am not sure if that is the original manifold that came with the 1800 engine. The electric fuel pump I took off my car after it started to diesel was a Faucet pump I bought new from one of our vendors and was putting out 3 psi at the time I was using the 32adfa carb. I left it on when I installed the 32/36 (never checked the psi) but took it off last week to see if that was the problem ( still pumping fuel when I shut off the engine ). I guess it wasn't. I will try and swap the jets since it can't hurt and see what happens. Does the electric choke come into play at all with this issue?

[mbouse]

the choke controls the air volume into the air/fuel mixture. sure, it can have a great impact on the performance at idle if it is not open properly for the engine to get the mixture it needs.

the choke, if the ambient air is warm enough for shirt sleeves, should be wide open. especially if the intake manifold is at least warm to the touch.

[jwhite]

Looking at my choke now it is about 1/4 open. If I move the linkage it will shut almost closed, I mean just a very small gap which I understand is the way it should be. My question is.....since the engine is cold I would like to try to adjust the choke and want to know should I adjust it from it's current position (about 1/4 inch open ) or should I release the choke by moving the linkage then adjust? If I move the linkage to release the choke it looks to me there is no adjustment to be made because the choke butterflys will go to almost closed. I noticed on one of your posts in flu.org that the brass part on your choke is in a different position than mine. since yours is running right does that mean that mine is not adjusted correctly or are they all different?


http://www.flickr.com/photos/31164559@N07/?saved=1

[majicwrench]

I would not swap jets! Why would you??
I would not try to adjust choke unless you have a good reason to do so. To be frank, it does not sound like you know how the choke should operate, if that is the case you really shouldn't try adjusting it.
With the engine dead cold, ambient air temp under 60F, open throttle and choke plate should snap shut. Start engine, plate should be pulled open slightly by some sort of pull-off. As engine runs, choke plate should gradually open. WIthin 3-5 minutes of starting, plate should be all the way open. At this point if you open throttle a little, should come off of fast idle, and when you let off throttle should return to idle.
If idling smoothy, and just barely dieseling, again, begin by turning idle down slightly and see if that takes care of it. Choke has nothing to do with it UNLESS it is holding the idle up on fast idle.
Keith

[So Cal Mark]

well, the choke plate could be contributing to the dieseling even without affecting the fast idle. The choke richens the mixture by changing the venturi vacuum, so a partially closed choke could cause fuel to enter the venturi through the discharge nozzle even without the fast idle

[majicwrench]

If we are gonna be picky, extra fuel entering thru the "discharge nozzle" is not going to contribute to the dieseling. If there was fuel entering some way other than the idle circuit, would not be idling smoothy as owner noted above.
Choke is not causing dieseling unless it is holding idle up.
Keith