Carb problem

[Leon]

Hey, I've got a carb problem I'm hoping someone can help me with. Recently rebuilt the 259 in my '64 Lark into a 289. Found an AFB manifold and got a 'rebuilt' AFB Carter from a Stude parts vendor who shall remain nameless. The carb is model 3088S. I have about 1500 miles on the engine and I've had a stubborn dieseling problem on shutdown. The first thing I tried was to retard the timing, and it is way back to just 4 degrees. I've also got the idle down to 650 RPM, which is what I believe Stude called for. I have a standard Delco window type distributor, and the dwell is correct. I have a standard mechanical fuel pump which has good pressure. I set the mixture at idle using a vacume guage. Every thing is more or less stock 289. I suspect something in the Carb, but I don't know what it could be. Any ideas?

[Sonny]

It sounds to me like you're right Leon, I'd be looking at the carb. too. Two things that I can think of, right off the top of my head. First, it could be a leaking accelerator pump. Second, check all of the linkage and make sure the secondaries are adjusted and closing completely, or no linkage binding, with your foot off the gas pedal.

Sonny

http://RacingStudebakers.com

[buddymander]

choke not opening all the way, idle mixture screws too far out, float too high, jets too big, metering rods too small; anything that can cause an over-rich condition causing a buildup of carbon in the chambers. Try the mixture screws in as far as possible without lowering the idle or vacuum reading and I'd check the plugs for a black, sooty condition. These symptoms won't disappear until the carbon is cleared from the chambers. Prolly gonna hafta go with larger metering rods for the simplest fix.

[Tbatson]

Dieseling is caused by an engine running too hot. You said that you rebuilt your 259 into a 289. Is the engine now running hotter than before? Possibly some break in problems causing extra heat? If the engine temp has remained the same then you have to cut the fuel. Make sure that the throttle plates are closed at idle and that you are getting as much of your 650 idle through the AFB idle circuit rather than having the throttle plate cracked open and drawing a fuel mixture from higher in the carburetor neck. Next check the float level and like Sonny said see if somebody forgot to put that check ball in the accelerator pump circuit. - Tom

[Roscomacaw]

Running too hot is one cause but so is having carbon buildup in the cylinder or on top of the piston. This carbon can glow around it's edges and particularly where it's flaked a bit and has a point to it. This point literally glows red - like a glow plug in a model airplane engine. Consequently, you can turn off the ignition and the engine will still run on because it's developed it's own ignitor.
An old and sometimes successful trick is to pour water down the carb while holding part throttle to keep the engine from stalling. The resultant steam can cause the the loose carbon to dislodge and go out the tailpipe. I've seen it work myself.
OF course.... if your problem only started with the installation of this carb, then you need to look hard at some function of that carb not being right![}]

Miscreant at large.

[GTtim]

I'm just wondering if loose/worn throttle shafts can contribute to/cause a problem like this?
Tim K.