67 3020 gas issues. Carb related?

[Randy G]

I have a 67 3020 gas w/4575 hrs with a Marvel Shebler carb with the slanted right side when facing it. I've read on this forum they're junk but mine seems to run fairly well but not as well as my 2510 gas. The tractor smokes and uses oil, hard to start in cold weather, is a real gas guzzler, has yet to foul a plug(new 8 hrs ago) but they get VERY dirty with black soot with in less than 10 hours. Today I did a compression check and all four cylinders were 110 psi but # 3 was 108 psi. Manual says 160 psi is the starting point. Are the problems with the carb? Would a new Zenith carb be a solution to most of the issues? If so, who sells the best choice. We only use the tractor 10- 20 hrs yearly. Oil is cheaper than an overhaul considering the use it gets. Should I replace the carburetor? I know they're not cheap. Thanks for any suggestions.

 

[wore out]

It seems odd all four cylinders would be so "low" compared to the info in the book, but yet so close to each other.

Did you have the throttle set so the throttle butterfly in the carb was wide open for the compression test?

Is your compression tester known to be accurate?

If those questions check out the next thing I'd do would be to squirt a half-teaspoon of engine oil in the cylinders and check it again and let us know what you find.

Updating the carburetor is going to be $$$, this very site has both new and rebuilt, new is $655, rebuilt is $290 plus a $200 core charge, and I don't know if they will take yours back as a core toward the other style.

 

[tomstractorsandtoys]

I have parted several 3020's and the first thing to sell is the Zenith carbs. No one has ever said they wished for their old carb. We once had two 3020 gas tractors one ran good the other fair. One had the Marvel carb the other had the Zenith. Switched the carbs and the fair tractor was the good running one. I do have a neighbor that has a 3020 with the Marvel carb and I will say his does run very good. I send my carbs to Mark's Carb in Rice MN. He does a great job and he tells me he can rebuild the Marvel and make them right. Tom

 

[Randy G]

The tractor was at operating temperature when tested but I did not open the throttle. Should I re test? The tester is new from Harbor Freight.

 

[Roger in Iowa]

I think you need to replace the diaphram in the carburetor, AR10108. It acts as an accelerator pump when you or the governor really opens up the throttle. They develop leaks letting excess fuel into the engine. Way you can check is run the tractor in road gear full throttle and then slow down. Does the engine blurble, throw black smoke, cough and then run okay as rpms stabilize. Replace the diaphram. Did my 3020 30 years ago and still runs well. Have to have the throttle dead idle to start.

 

[J.Wondergem]

(quoted from post at 12:26:08 11/09/20) The tractor was at operating temperature when tested but I did not open the throttle. Should I re test? The tester is new from Harbor Freight.

Yes, retest it with the throttle wide open.

 

[miangus]

You may not actually know where the actual throttle setting was when you tested it as the governor may have opened the throttle. But yes tat with wide open throttle. I have learned that not all tester are correctly calibrated and there are other factors which can effect the reading. I would consider your uniform test results good. Maybe time for a carb rebuild, at least change that accelerator pump diaphragm. I have that carb on my gas 4020 and it preforms pretty well, actually cold starts excellent. I have found that idling the tractor for long period below about 800 RPM will load the plugs, I believe with no actual proof that idling at low RPM causes the accelerator pump to pulse and load up the engine.

 

[buckeye al]

The Marvel carb can have an AWFUL lot wrong with it & the tractor will still run. Mine was hard to start & tjirsty. Accelerator pump diaphragm was hard as a plastic mud flap. Both check valves for accelerator pump were missing and the float was so maladjusted it would not move under its own weight if you turned the carb upside down. Once all those were fixed it made a new tractor of it! Adjusting idle mix is a bit touchy. Some had 2 main metering orifices, others only 1. One saw one where one orifice had unscrewed & was rolling 'round in the fuel bowl. Smoked like a Baldwin Lima Hamilton but it ran. Just putting that back where it belonged and adjusting made it work great again.

Yes they're picky but once tackled I've had good luck with 'em. In the past parts were hard to find but they can be found. I'd take him apart.

Hope that helps. Good luck.