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Discussion Starter · #1 ·
My '72 MAR OSSA has what looks like an older version of the VM 28 Mikuni carb with a bolt flange to mount. I just got the bike and it had sat for a long time. The carb was filthy and the bowl was siliconed on tightly. All is clean now but I am having a leak out of the bowl seam. The carb sits at an angle and the lowest bowl seam is where it is leaking. I don't know much history on this set up although it looks like the previous owner had a similar problem. My first thought was to lower the floats but they already seem lower than current specs for a VM 28. Just want to make sure it isn't something else before I start moving the float level. Not having a good source for gaskets, I made a paper gasket since I assumed this would be adequate. Any thoughts would be welcome.
 

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Or get some cork gasket material at an auto parts store and make a gasket out of that.

Finding a gasket for that old carb may be difficult.

If it still leaks after that, bolt the carb to the engine without the float bowl on it, turn on the gas, then use a screwdriver to gently lift the float and see it that shuts off the fuel flow completely. If it does, that means the needle valve is okay but the float level is too high.
 

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Discussion Starter · #4 ·
Thanks. Got some cork gasket and no more leak. Starts and runs but even after warm, only wants to idle with the choke on. The choke can be half on but if all the way off, it wants to die. What is the procedure and where do I start to get it to idle on its own? I have the adjustment on top of the carb at the cable, the fine adjust mid cable, the idle screw on the carb, and the air screw on the carb. Plug is new and gapped right. Fresh air cleaner in place.
Thanks for any assistance.

Jim in Folsom, CA
 

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Pilot jet is probably plugged. Take the carb apart, the pilot jet is located in a small cast-in cylinder just in front of the main jet. A small flat-bladed screwdriver is needed to remove it. Once it's out, look through it and see if it's clear. If not, strip a piece of wire and remove one strand of copper. Use that to remove the blockage. Make sure the holes on the side of the pilot jet are also clear.
The enrichener should only be used for starting and perhaps a few brief shots of enrichener may be needed as the engine warms. Leave it on too long and you'll foul the plug.
 

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A common problem with two strokes is that the crank seals go bad and let air into the motor. You need to leave the choke on to allow more fuel to mix with the un-metered air that's getting in. Put some new crank seals in and you should be fine. It could also be leaking air from the center cases, base gasket or head gasket.

Zilla!
 

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My '72 MAR OSSA has what looks like an older version of the VM 28 Mikuni carb with a bolt flange to mount. I just got the bike and it had sat for a long time. The carb was filthy and the bowl was siliconed on tightly. All is clean now but I am having a leak out of the bowl seam. The carb sits at an angle and the lowest bowl seam is where it is leaking. I don't know much history on this set up although it looks like the previous owner had a similar problem. My first thought was to lower the floats but they already seem lower than current specs for a VM 28. Just want to make sure it isn't something else before I start moving the float level. Not having a good source for gaskets, I made a paper gasket since I assumed this would be adequate. Any thoughts would be welcome.
Just did a Google search of "Mikuni vm28 gasket" and had plenty of sites pop up. Shouldn't have any problems finding one that way. Or, you can make one. Might also want to make sure the mating surfaces (ie the bowl and carb housing) are completely flat.
 

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Discussion Starter · #12 ·
Thanks for the info guys. Are crank seals manufacturer's parts only or are they just a certain size seal that is available from most bearing houses? Are there any tests simpler than a leak down test to see if the seals are leaking?
Does anyone know the torque for the cylinder head bolts? One similar OSSA motor is 12 ft lbs.
 

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Pop them out and measure them. Good news is that they are metric. With Bultacos I just go to the bearing house. Probably the same with Ossas.

No real tests, other than either oil blowing out of the side cases or excessive smoking from oil being drawn in past the seal. Or it just runs bad.

I recently changed the ones on my Pursang and they didn't really look bad but for $5ea I wasn't really worried. It ran better afterwards.

You might try Trials Central forum for Ossa stuff. PM me if you can't find it. Smart f'n people over there for those kinds of bikes.
 

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Cool bike. Post a pic!! Are you planning to compete on it? I just missed a Montessa of around the same vintage for $400. it had been sitting for a while but these 2S motors are so simple to work on. You should have no problems finding most parts. Wanna buy a '73 CZ-250 radial head project?

Dale
 
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