MD Questions

Looking at a 1948 MD locally owned and am attempting to eventually get it to run on the gas side. The owner last ran it about a year ago, but started it on diesel by towing it. From what I know about MD's , which isn;t much, they should start on gas and shut down on gas to [protect the cylinder head. The linkage from gas to diesel seems to work fine, the radiator is full of antifreeze, and oil doesn't appear to have any antifreeze in it.
There is a wire from the Mag to the manifold that has been broken, but is still there and could be spliced together. This is to kill the mag during diesel operation, I think. This tractor has no ignition switch, and has an alternator. Shouldn't it have a push/pull switch, otherwise, how do you shut it off after switching back to gas??
Not sure what's going on here, nor whether to tackle this project, but could sure use some advice. I might end up owning this tractor if things work out. Thanks, and sorry for long post.

There should be a switch to ground the mag (as does the switch on the butterfly shaft. The starting system is complex, but mot obscene. I would say if it runs on Diesel, it will be possible to make a go of the gas stsrt system.
If it runs well, buy it. It needs to be operating with a bit of load for an hour or more to be sure things are OK.
Opinions vary, but the method of shutdown after work is best done on diesel at idle for no less than one minute if operated for ten minutes, and no less than 3 minutes if well warmed up. Diesel operation is far cooler than running on gasoline, and has the advantage of not heating the compression release third valves as well. I believe this as fact. It is worth more than 1500 if the tires are good. Less proportionally if it can't be started on Diesel, or has bad tires. I would like another one! JimN

Bruce you are right about the wire going to the mag it should be the on off switch in the manifold operated by the switching from gas to diesel linkages. Dont bother switching back to gas to shut off as that leaves the starting valves hanging open in the cylinders and wont provide contact for proper cooling for them. Loosen the drain plug on the oil pan and keep loosening till something drips out to see if there is any coolant leak.With a clean crb and good spark they will start easy by hand on gas and warm up properly to switch to diesel. Just let them cool off for 5 or 10 minutes before shutting them off after running.

The wire that runs from the mag to the manifold is a cutout wire that grounds the mag out to prevent constant spark to the plugs.
No ignition switch is necessary as on gas side and throttle down. engage starter and play with choke and it should run. (giving clean fuel, new points, clean carb, etc.)
To shut it off, idle down on diesel and switch back to gas. with throttle lever in gas mode (diesel off) pull the fuel switch lever back to ground out the mag.
LEAVE THE LEVER TO SWITCH FUELS IN DIESEL TO PREVENT WARPING OF THE VALVES AND SEATS.

Thanks for your reply. I'm very new to this, but the MD seems to be a very intriguing machine. Question:: If the tractor is idled for several minutes on the diesel side, as you said, and the throttle lever moved to it's lowest position, would that shut the tractor off without the need of a push/pull ignition switch?? This tractor hasn't had a lot of TLC, but has been stored inside, kind of grimey, but no rust, tin is OK except for grill which could be straighted, front tires ok, rear tires pretty well gone and mismatched, but do hold air. No belt pulley, does have hydraulics, and not dripping anything. Just thought I'd fill you in. Regards, Bruce Simmons

Thanks to all who replied. I'll give it a go and see what happens.

With a Mag it will not need a switch, it just had one when new. It is "needed" to stop the engine when a decision is made to stop it when running on gas to start it. JimN

Yes it should die when the throttle is closed when in diesel operation. That model didn't come with a on off switch, just the one in manifold to ground mag. Back in the day, IH instructions for shuting tractor down were. Switch to running engine on gas and shut gas off at starting tank or fuel bowl, wait until engine stops, after engine stops switch lever back to diesel operation.

Jim I'm not claiming I know whitch is the best way to shut a gas start diesel down but I have some questions and thoughts about it and will probably get flamed on them. Lots of times I see someone say they run hotter on gas. I,m having a hard time with that. Heat is what fires diesel fuel in the cylinder. If you let one of those engines run unloaded until temp. drops and stabilizes then switch to gas for 15 to 20 seconds until engine clears up on one of the later models that has a switch then shut it off and switch back to diesel, I can't see where it would heat things enough or any to matter whitch way its shut off in above case. Also I wonder how hot the starting chamber is with intake butterflies and starting valves closed. If switched to gas I would think the intake air would be cooler than inside engine and would offset any heating of the head if just run a short time on gas. My own opinion is if allowed to cool on diesel first and if switched to gas for 30 seconds or less it doesn't matter a lot which way is used. But I don't claim thats the correct answer.

I have a 49 MD that was our power tractor before my time and after that was on a feed mill most of the time. Even when nothing else would start, if you could get it to turn over it would start. The proper warm up and cool down is "VERY" important to avoid a cracked head....ours never did. It also made it start much easier if after cooldown on diesel, we would switch it over to gas for just a few seconds and then back to diesel to shut it down. I don"t know why but thats the way it was. Also when it was cold (southern Minn) I would take the muffler off and that would help for starting too. It was on a 815 Farmhand feedmill for many years and that is how I found all this out. One half gallon of fuel for every two ton of hog feed.

I have no flame, but it is fun to discuss.
The mixed air and fuel on the Gasoline operation mode run through the same intake ports in the head that the air enters in the diesel mode. The Third valve is always open, in gasoline mode, without "seat" time to cool it. The gasoline compression relief chamber and spark plug are cooled to engine operating temp for the casting during operation such that the hotest element in the chamber would be the back of the compression relief valve.
A gasoline operation has flame fronts extending all the way to the edges of the entire combustion area including the compression relief area. This flame front burns until it is quenched near the boundary of the cold (relative) combustion chamber and prechamber and compression relief chamber. The diesel flame is concentrated in the area of the prechamber and the hole leading to the top of the piston/valves.
My true impression includes the idea that a post diesel run of a few seconds hurts nothing, and might (might) clean the spark plugs a bit from the start cycle carbon that could be there?
I was told by the man I bought my 48 MD (with a shutoff button, but distributor coil ign 12 volt.) that he ran it at max output for its entire life on a irrigation pump, and that it had litteraly no wear on the drive train. (in 1997 and 98, it still had 1/2 worn original tires on it. It was shut off after as much as 75 hours of full output operation by running it at idle diesel for 5 minutes, then just closing the diesel speed control. It never had the head off of it as far as I know. Good discussion, JimN

The whole premise of "cool down on gas," is based on the fact that the engine is WORKING on diesel and IDLING on gas.

A diesel engine that has been working at full capacity all day long will be hotter than a gas engine that has been sitting idling all day long.

By idling on gas for a few minutes before shutdown, as the original operator's manual describes, you are not only allowing the engine to cool, but are purging the cylinders of diesel fuel, making the engine easier to start tomorrow.

Thanks for the input, I never thought you would flame. I always wondered if the dead air in decompression chamber would stay hotter or cooler depending if starting valves were open or closed at shut down on a engine thats already cooled from work. Most heads I see are cracked at main valves so the above may be a mute point.