'46 Coupe 59AB Rough Idle

[51woodie]

I need some suggestions on where to look for blockage in the carb on my stock ’46, 59AB with Model 59 carb. The engine started idling rough and these are the steps I’ve taken to troubleshoot the condition.
Vacuum at idle is steady at 20”, shorting individual plugs gives little change in RPM. Idle is 525 RPM
Compression is 120/125 on all cylinders. Plugs 1, 4, 6, 7 are a bit sooty, plugs 2, 3, 5 and 8 are dark tan.
Left idle mix screw in and out slows idle, right screw in slows idle but out makes no change.
I changed the carb out with a spare and the rough idle is gone.
My assumption is there is a blocked passage somewhere, and the question is where do I start to look as I disassemble the carb?

 

[petehoovie]

I need some suggestions on where to look for blockage in the carb on my stock ’46, 59AB with Model 59 carb. The engine started idling rough and these are the steps I’ve taken to troubleshoot the condition.
Vacuum at idle is steady at 20”, shorting individual plugs gives little change in RPM. Idle is 525 RPM
Compression is 120/125 on all cylinders. Plugs 1, 4, 6, 7 are a bit sooty, plugs 2, 3, 5 and 8 are dark tan.
Left idle mix screw in and out slows idle, right screw in slows idle but out makes no change.
I changed the carb out with a spare and the rough idle is gone.
My assumption is there is a blocked passage somewhere, and the question is where do I start to look as I disassemble the carb?

 

[Zeke3]

Well, you might try calling former member CharlieNY, Charlie Schwendler in Orchard Park, NY for advice. His contact information has been posted on here numerous times. Otherwise, I would take the bottom off the carb and try to determine where the passages go from the air mixture screws using a fine piece of copper wire for a start.

 

[51woodie]

Thank you for your response. Charlie rebuilt the carb for me about three years ago, and it has performed like new up until the other day. I'd like to sort the issue with the help of "Barners", as a learning exercise.

 

[Ziggster]

I’m no expert, and have only rebuilt a few Holley 94s. I think with today’s fuel there can be issues with it sitting for long periods of time which might result in the gumming up of passages. You’ll have to start to disassemble things to have a look to see if anything looks out of place. Order a rebuild kit with the bits you think you might need.
Could be a slight leak in a gasket, worn out idle mixture screws, float issue, etc. I used a micro drill set to clean out a bunch of the passages in the main jet towers.

 

[kurt v]

There is a spray cleaner that you can get at a marina, it's called MERCURY POWER TUNE, fill up the carb and let it sit, it eats up all the crud. Works on anything with a carb, great stuff.