Super MD carb question

Ever since I inherited this tractor a year ago, it has run away for a second or two when I switch it to diesel. It only seems to do it the first start of the day or at least after being shut off for several hours. Since I've owned the tractor, I've overhauled the motor and the fuel injection system - no change. When I switch it over, I pull the compression lever and don't have to start the inj. pump until the motor starts to spin down after this initial runaway. After this, it runs good.



What I think is happening is that the motor sucks the carb dry and is dieseling on the gasoline from the carb bowl. I know that the needle valve is working, but it may not being closing fast enough. The gas tank doesn't go down at all while running on diesel either, just the carb is emptied out. With this information, I went through all the settings on the shift over linkage and it was way off. Do you think that this may have been my whole problem or is something else going on? With the compression valves closed in the head, is there another way for the gas from the bowl to be sucked into the motor? Is it possible the gas could be sucked out past the choke valve? I haven't fired it up yet since making these adjustments and I thought maybe I am forgetting to check something before I fire it up again. It scares me enough that something is going to break that I hate even starting it. Is there such a thing that I need to give it some diesel injection before I switch it over to dilute the gas somewhat? As far as the rebuild, everything is new in the head and block, so I don't think I have an issue in the motor. It acts the same today as it did before the rebuild and the injection work.



Thanks for any input,

Dan

I think you are on to something with adjusting your linkage. It should have been done after the rebuild :wink:

The book says to throw the decompression lever and throttle up at the same time.

If the intake butterfly valves are remaining open, even a little, the fuel will contaminate the intake stream. This will indeed cause the event. a clean serviced aircleaner is also important! Jim

I thought the butterflies opened when going to diesel mode and had to be shut for the gas side to create a vacuum through the gas side of the manifold? I also believe the butterflies are and have been working correctly, but not 100%.

The air cleaner is clean.

Thanks,
Dan

I was told by the mechanic that the linkage wasn't adjustable because it had been cobbled. It took me two hours to read the manual and adjust all of the intact adjustments with the manifold on. I'm thinking the mechanic didn't try real hard to perform the adjustments.

Dan

Maybe sticking, I was just thinking of how gas could get there. Mine was usually very rich on gasoline, and didn't runaway when switched. Jim

If this is any clue, mine likes a lot of choke to get going even when it is 100 degrees. I have had the carb apart and cleaned it. I think everything looks good in the carb.

Dan

Here is a thought, Start it on gas, and shut the gas off at the little tank, as it dies, switch to diesel. If the issue continues I would be suprized. Jim

I was thinking the same thing. Do you think I will have enough time to get around the tractor and up onto the platform before it runs out? I"ll probably trip and break something.lol

Thanks for your help,
Dan

From the time you turn the gas off to the time the tractor runs out of gas will be at least 60 seconds. You could get on the seat twice before it dies. :lol: That is how I turn mine off.

You can forget to turn the diesel on and run the tractor for over 25 minutes before it runs out of fuel. Trust me on that one.

The adjustments are a PITA, but they are necessary. My carb lever thing (the one behind the carb) was welded on and a little rigged up. It took me a good 30 min to get the weld ground off and retapped/hole died (is that a word?) and back on. Adjusting the starting valve clearance was the biggest pain IMO.

I think the starting valves work both ways. Not sure if I said it, but mine will suck the carb dry while I am running on diesel. It DOES NOT run away like yours. Mine will just keep sucking if you leave the gas on. I have not had the covers off the intake manifold. Mine is a little hard starting and that may fix the issue. Mine takes 3/4 choke even in summer.

When on diesel the butterflies in the manifold are wide open, and they close on gas cycle to make air travel through the carburetor part of the intake manifold. Does it smoke quite a lot of blue smoke when running on gas. You might not have total diesel fuel shut off when running on the gas cycle and the unburned diesel fuel makes it rev up when throwing decompression lever. Two ways to test. Open the bleeders on nozzles when running on gas to see if it changes exhaust, if pump is adjusted properly it will not. Other thing is to shut it down on diesel and immediately check for gas in bowl of carburetor. You may not be able to get gas out of carb drain with engine running on diesel due to pressure differential. Also, if it is using the gas out of carb it will die when trying to switch back to gas when running on diesel.

It runs perfect on gas, no blue diesel exhaust. I have a rebuilt pump and injectors from Central Iowa. I believe that the diesel side is in good shape.
It is as you describe, it will not restart on gas after being run on diesel.

Can you tell me how to keep the carb from being sucked dry? Do you think the proper linkage adjustments may have fixed it? They were way off.

Thanks,
Dan

Being as there are no more butterflies except in the intake manifold, and like I said they are wide open when on diesel, I just don't have an answer for you. I just happen to have a carb and manifold laying out in the garage as I am planning on putting that carb on a 450 to replace the carb that had been reworked at one time. I went out and looked over the manifold just to be sure how it layed out but I would want to take a look at the passages in the head also but can't do that. Don't work on them much anymore so kind of rusty. I know this tractor has an extra head laying somewhere also but don't know when I will get a chance to look it over. If the fuel tank stays full when on diesel that means you are getting good shutoff of the needle and seat when on diesel. Are you sure you have sufficient clearance on all the decompression valves so they fully close but that don't make sense either. I just don't know. Will think about it some more.

There is linkage for when lever is pushed to diesel position closes needle valve in carb. check that and see if adjusted properly. That way on my MTA anyway.

(quoted from post at 04:22:40 08/10/12) Being as there are no more butterflies except in the intake manifold, and like I said they are wide open when on diesel, I just don't have an answer for you. I just happen to have a carb and manifold laying out in the garage as I am planning on putting that carb on a 450 to replace the carb that had been reworked at one time. I went out and looked over the manifold just to be sure how it layed out but I would want to take a look at the passages in the head also but can't do that. Don't work on them much anymore so kind of rusty. I know this tractor has an extra head laying somewhere also but don't know when I will get a chance to look it over. If the fuel tank stays full when on diesel that means you are getting good shutoff of the needle and seat when on diesel. Are you sure you have sufficient clearance on all the decompression valves so they fully close but that don't make sense either. I just don't know. Will think about it some more.

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My starting valves have been set to spec. since I got the tractor. I just never did the rest of the linkage for some reason. I can't think of any other way for the gas to get into the combustion chamber but through the starting valves or maybe being sucked back through the choke somehow. I compression checked the starting valves, they all look good. One other clue is that it only seems to do it after it has sat for atleast a few hours and it is worse if it sets overnight. Maybe as others have said, the linkage adjustments will have fixed it.

Thanks again,
Dan