Vintage Dirt Bike Q & A

I have finally gotten decent tires on my 75 MT250, but the carb is flat worn out. I have a VM34 I am going to put on, complete with remote choke lever, but I need to make a new inlet tube to mount it on. I don't know if I should keep the oil injection or make it a pre-mix bike. The current pump over-oils like crazy, it smokes bad on smokeless oil and sucks it down at about 12:1. The adjustment seems ok on it, though. I am planning a rack behind the seat to carry a 2 gallon gas can, so pre-mixing won't be bad out on the road. Before I just rip all the stuff off, though, is there something I might be missing on the oil injection? If I do go pre-mix, the pump will stay in place with a piece of tube between the inlet and outlet and a little oil in it so I don't lose the tach drive.

You can pull the pump without loosing the tach drive, I did it on mine when I put a vm36 on it. I just made a small block off plate for the hole that was left behind to keep stuff out of the engine. I just carry a ratio right cup and some oil with me so I can still fill at the gas station.

It seems to me the MT250 oil pump meters out more and more oil the older it gets - probably clearances opening up from wear. I adjusted my buddy's pump cable down so the mark on the lever was well to the left of the mark on the pump body (less oil), it seemed to reduce the exhaust smoke quite a bit. I found out later he was also running 40:1 premix as backup, so it probably had a greater effect than I could see. According to the manual it always supplies a minimum quantity of oil regardless of adjustment, so I really wasn't worried about oil starvation at idle.

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

Could the one-way bolt be contributing to the excessive oil? It smokes like crazy at lower speeds, but on the highway it's not to bad. I am currently putting about 80miles a day on the bike, most of it 60mph freeway, and it doesn't consume nearly as much oil as it does in town.

Does anyone have a tank they want to get rid of for cheap? I want to build an extended range tank, but having a tank to start from would make the project much easier. I am planning to add about an inch down the middle, and at least an inch in height, probably two inches. I want at least three gallons by the time it's done.

How did you do this? The only marks I see are two lines, one on the lever and one on the pump body that line up. With the lever at the hard stop the lines are lined up and I cannot go any further to the left. Do you happen to have any pics of how his is adjusted? My plug is getting pretty wet with oil and it smokes quite a bit so I am trying to reduce pump output.

It's been a while, but on that bike all I remember doing was loosening the cable adjuster, and the lever mark (it's on a separate metal piece on the same shaft, is that correct?) moved to the left of the pump-body mark. I probably should have written "slightly to the left", as it wasn't off the left edge of the pump marking boss. Also, before we put the cable back on, the cable end of the lever was way down against the crankcase opening, lower than it was after the cable was connected, at which point I believe it may have looked somewhat similar to the picture 3-2 on page 19 of the MT250 shop manual except with the marks misaligned as mentioned above (again, from memory).

Is it possible your lever or indicator piece has gotten moved on the shaft somehow? I never touched the shaft nut, but I assumed everything on it was keyed to the shaft in only one position - maybe it isn't, and could have moved around if the nut was loosened in the past (just a guess).

If you're stuck with the hard limit, maybe you can just add in some slack with the adjuster; you'll still have too much oil at idle and low throttle openings, but maybe less at 1/4 throttle and up. I wonder if modern oils really require the throttle variation anyway, as they probably provide more than enough lubrication at high rpm with just the pump rpm increase.

The pump description on pages 4 and 5 is interesting. It seems to me there's only a narrow range of lever motion where the lever adjustment / valve has any effect even when the pump is new and tight, and if it's worn it seems like the plunger shouldn't have much trouble forcing oil past the valve under pressure. It also would be helpful to figure out the equivalent premix ratios at any given rpm/throttle opening from figure 1-13 on page 4... why do I seem to remember reading this somewhere...

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

Well, I have made the switch. I made a little 1/8" aluminum block off plate to cover the opening, it seems to be working well. I am running about 40:1 with Shell Nautilus oil, since it smokes a little less than the Lucas semi-synthetic I was using before, and smells better. I had to go this route because the only new throttle cable I could find didn't have the takeoff for the oil pump cable. It does still smoke at idle since the thing idles so rich. I haven't been able to correct this, so it's Mikuni time.
On mine, the marks aligned at the hard stop, so no way to go lower on the oil.