Dieseling after MC2100 install

[lah2420]

Just installed an MC2100 this afternoon. When I took it for a drive, after running a while it will diesel on shutdown. I also noticed my temp is about 10 to 15 degrees hotter than with the Weber. Here's everything I know right now---
Carb came off of a 90 model with 4.2. Previous owner said he had no problems with it. It only has about 700 miles on it and was purchased from a carb rebuilding shop. I have it on an 87 4.2.

Motor has TFI upgrade, but I think the plugs are still gapped at the stock gap. I don't remember regapping them and the motor is too hot right now to mess with.

Timing is about 9*BTDC at 1500 rpm.

I am running an RV cam 260 duration.

I did not try readjusting carb yet as it was already running good on another motor. I do not know if the other motor had any mods done to it.

Idle is at about 650 rpm.

I don't know if this has anything to do with the problem, but before I got the MC2100 I tried to do the Nutter bypass. With the computer out of the circuit, I could not adjust timing past 6*BTDC and it ran like crap. Nutter has been reversed.

Any good guesses?

 

[droopey]

Here's my guess, running to rich and Maybe if you are running non factory plugs they are too hot for the new set up. If the timing is right and the idle is correct then check too see if the carb is closing all the way.

 

[redrooster]

Timing issue. You have it too far advanced. Probably runs great at mid to upper range. Will also make the engine run hot and diesel on shutoff...idles like crap too though you may not notice it with a loping cam.

 

[lah2420]

Timing issue. You have it too far advanced. Probably runs great at mid to upper range. Will also make the engine run hot and diesel on shutoff...idles like crap too though you may not notice it with a loping cam.

Have you got a recommendation on the timing? I thought it was supposed to be 8 to 10*? Makes sense. I don't want to make it run like crap with not enough advance, but I do want it running right.

 

[redrooster]

I'm not a six cylinder expert, but I'll try and help:
In your post about not being able to adjust the timing past 6 degrees....that doesn't make sense. You say you have a computer, are you talking about the little yellow or purple plastic box? I thought those only controlled the choke. Computer controlled engines have an electronic timing advance that would have allowed minimal adjustment if not disconnected (the computer would counter whatever move you made). If that yellow box has any timing control, then you need to reference a manual to make sure you disconnect the right wire. I would think that unplugging the whole box wouldn't be correct, but not sure.
Disconnecting the timing circuit in a computer is the same as disconnecting the vacuum hose on a distributor. Gotta disconnect before you can time it.
I have no idea what your timing should be with the mods you have. You will probably have to time by ear and then make adjustements after you drive it.

 

[lah2420]

I checked the timing again and it was actually about 10* BTDC. I adjusted it to 8* and my heat problem went away. Now to fix the dieseling. I'll be checking for vacuum leaks as soon as possible. It idles about 750. Could that be too high?

 

[lah2420]

No vacuum leaks that I can find. I adjusted the idle mixture screws a little to drop the idle to between 600 and 650. I had originally set it using a vacuum gauge and it idled 750. On the first test run, it shuts down fine now. I'll check it more to be sure. Will using the mixture screws affect anything else? I couldn't use the idle speed screw because it isn't even touching anything.

 

[lah2420]

I think everything is working now. I have a friend who was an old carb guy before specializing in diesels. He said it was definitely one of three things. First was a possible leak around the throttle shaft, second was a pin hole in the EGR valve, and third was a leak around the EGR gasket. It ended up being the third. There wasn't even a gasket on the EGR. It's running much better now.