Vintage Dirt Bike Q & A

I have completed the rebuilding of my 1976 MR175 and am embarking on tuning it. It has taken a couple of years to complete and regrettably, I don't remember how much this one is over-bored. I replaced the rings and possibly the piston, bottom seals and bearings. I also has a 1976 or 1977 CR125 ignition. It has the original carb, but doubtfully the original or oem jetting.

The only other changes or adjustments I have made is replacing the clutch with another, and tightening the adjusting screw -- I still need to fine tune the cable. Since the rebuild I have not reset the timing, needle height, idle or air screw, etc.

I am looking for suggestions on the order and steps I should take to tune it and eliminate its performance flaws. It starts and runs. It does not stay idling and it "four-strokes"/"machine-guns" up to 5000 or so RPMs even after warmed up. This is consistent through all throttle positions; although there is some relief after winding it out and shifting it to the next gear at around 1/2 - 3/4 throttle. Even then it isn't running smooth. It does suggest it has good power available, even with the MR175 gearing. My understanding is that this sputtering is an over-rich condition.

So, I am guessing I should start with checking the timing: given the ignition type and potential over boring - 1,2,3 or 4 over, where and how should I set it? What order and steps should I take to adjusting or diagnosing the carburation?

Unrelated, but of also of concern, it is difficult to find neutral, and when it is in neutral, it slips into gear easily. Also, sometimes the kick starter will crack and slip and will not catch again until the bike is put into gear and rolled back and forth a little. Any ideas what might have been done wrong when assembling? Could the neutral be caused by too thick or too thin gasket material?

If you're running the stock pipe and compression ratio, I'd recommend a 118 main and 48 pilot jet. If the bike is four-stroking at half throttle, your needle jet is worn (moving the needle clip to the top position and a 3.5 slide might help a bit, but replacement is the only actual fix). Make sure the center of the hollow MR air screw is clear and the filter is clean, obviously. I always mention the keeping the pipe clean too, as this can mimic rich jetting (I'm not talking about the removable baffle, I mean the six 'turbine core' passages up ahead of the tailpipe; you'll need heavy-duty pipe cleaners and a torch, it's not that easy to do).

The only way to set the CR ignition is with a timing gauge, as the CR marks are useless for the 109mm MR rod length.

The kickstarter pawl retaining spring may have shifted and/or the detents are worn. It also sounds like the neutral stopper arm spring may have popped off, especially if it's staying in gear OK.

Did you use an OEM center case gasket?

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

I did not use the oem gasket, I traced and cut some using slightly thicker material. I believe the original is .018 and the material I used was .021, while the only other thickeness I could find was .014.

I ran it again tonight to get the mail, and it idled for me. I think before I do too much, I may want to run a tank though it since I don't think I cleaned the carb since working on it last. I will also switch the pipes to see if there is a difference. I am also going to burn some brush, so I think I will throw a couple of pipes in with it to burn out the gunk.

I don't like the sound of the neutral spring being off, I don't want to take this apart again. The shift arm springs up and down when shifting, it just doesn't like staying in neutral. I don't think I have gotten it into neutral while running.

It probably won't idle consistently with the CR ignition, as there's insufficient crank weight for decent idling or even proper crank balancing. Both my CR-ignition motors vibrated, and a buddy's does too; I had one of my cranks balanced by Crank Works and added a flywheel weight to the other, now things are much better.

Throwing the pipes into the fire won't clean out the spark arrestor much at all. The 'ring of fire' technique with an oxy/acetylene rig will, though.

The neutral stopper is accessible under the clutch cover. I doubt you'll have to split the cases again.

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