Super MD carb question

Dan MD · Dec 13, 2012

This message is a reply to an archived post by DanMD on August 09, 2012 at 09:53:42.
The original subject was "Super MD carb question".

Well I have been shutting the gas off and letting the carb run dry and then switch it over and it still revs up. I pulled the manifold off after running on gas and shutting down without going to diesel. I found gas on the walls of the manifold and in the head past the butterfly valves. It was wetter on the front two cylinders. I checked the aircleaner and it has no restriction. I checked the butterfly valves and they are closing fairly good. I put a shop vac on the head side of them and using a smoke stick, I have to have the smoke stick right at the intake for the vac to be able to pull any smoke in at all. My local service manager thought they looked good also.

Any thoughts on how I am getting gas in the diesel intake? I also believe the valves are all set correctly in the head.

If it is the buttefly valves leaking, how do you get them to seal up better? I let them find center and tightened them up already.
Dan

Dan MD · Dec 13, 2012

http://www.ytmag.com/cgi-bin/viewit.cgi?bd=farmall&th=879703

Well I have been shutting the gas off and letting the carb run dry and then switch it over and it still revs up. I pulled the manifold off after running on gas and shutting down without going to diesel. I found gas on the walls of the manifold and in the head past the butterfly valves. It was wetter on the front two cylinders. I checked the aircleaner and it has no restriction. I checked the butterfly valves and they are closing fairly good. I put a shop vac on the head side of them and using a smoke stick, I have to have the smoke stick right at the intake for the vac to be able to pull any smoke in at all. My local service manager thought they looked good also.

Any thoughts on how I am getting gas in the diesel intake? I also believe the valves are all set correctly in the head.

If it is the buttefly valves leaking, how do you get them to seal up better? I let them find center and tightened them up already.
Dan[/quote]

Janicholson · Dec 13, 2012

One condition might be an over rich mix for start operation. On mine, it was necessary to keep it rich to allow it to run, and that ment some choke for about 30 seconds. Then it seemed kinda rich even then. Maybe your carb is rich and fuel is condensing into the intake runners.
The butterflys are adjusted as you have done to get them to seal up. however if the linkage or mechanical stops are out of adjustment, they can be held open. One issue is that the stops must be set to """just""" shut, or, if too tight, the plates will stick shut in cold weather and not allow diesel. Jim

n9lhm · Dec 13, 2012

The valves close when starting to make the engine pull air through the carb. There is nothing to prevent the engine from still pulling air through the carb when on diesel mode, except that the diesel air-only passages are much bigger. That's why there is a valve on the carb to shut the gas off when on diesel. They all rev up on switchover when its cold outside, you will probably notice it's much less in warm weather.

Dan MD · Dec 13, 2012

Mine needs a lot of choke even when warm out. I need as much choke as you describe if not for longer. Does this indicate anything?

Interestingly, this tractor doesn't rev up on a warm switch over. I don't use the choke either on a warm start.

Thanks,
Dan

Janicholson · Dec 13, 2012

They are really lousy runners on gasoline. The combustion chamber in decompression mode must deal with incomplete scavenging of the exhaust, and the compression of enough fuel and air and mixture to ignite with those weird plugs. Then the burning must blast past the third valve to ignite the mix in the main cylinder. Fuel condenses out and pools in the manifold. then is picked up and combusted in the diesel mode making the rev situation. Jim

Super MD carb question

Dan MD

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