Vintage Dirt Bike Q & A

I picked up a 74 MT250 last week from elgueche (Richard), and have been browsing this forum since I first saw the bike earlier this summer. Very informative, Thanks! He said it would need some work to run but after about 20 minutes of looking 'er over I gave a kick or two and it fired right up!

Anyway, it smokes badly and has oil dripping from the exhaust manifold so after researching here I guess the crankcase seal is a good place to start. The manual describes putting the bike in gear and applying the rear brake to remove the nut holding the rotor, but the rotor still spins as freely as when in neutral. Am I missing something?

Thanks again,
-John

I've noticed the same thing. There is so much slop in the drive train that you can spin the flywheel around 2-3 times before the slack is taken out of it, then it still goes around another turn or two before it builds up enough tension to even think about loosening the nut.

I gave up and bought an air powered impact wrench. I bought a 1/2" drive because I work on cars and trucks too, but a cheap-O 3/8" drive probably would have been enough for motorcycles. They even make electric impact wrenches now that sell for less then $70.

In a bind I have fitted a socket to an impact driver. The head on these can take 1/2" sockets. You probably already have one or will get one in the near future if you are going to work on a 31 year old bike. They are available everywhere at less then $20. One of those Newtonian Physics things, sharp rap with an impact driver and the flywheel tends to stay stationary.

One other thing is you may want to pick up is one of those flywheel removal tools from a local bike shop or online shop like Dennis Kirk. Less then $20 and well worth it. I have mangled flywheels trying to use a conventional puller but the one specifically built for the Honda flywheel is fool proof.

Cool. I started spinning the flywheel to see what would happen but started to get nervous so I'll try the impact wrench route, been wanting air tools anyway!

But, now the clutch won't disengage (engage?) when the bike is in gear (sorry if the terminology isn't correct), meaning when in gear if I pull in the clutch nothing happens, it stays in gear...

Now what have I done?!

The clutch plates on old bikes, if they haven't deteriorated to mush, have a tendancy to stick. I just put the bike in 2nd gear and rock it back and forth violently to free them. I imagine if I rode them more then once a month they may stop sticking.

I've had some old Honda clutches that were so badly stuck that I had to use a screw driver to break them free.

I've replaced several clutches with either Parts Unlimited cheap-Os or Vesrah and had good luck with both.

Yeah, I'm assuming the clutch needs work. It was just odd how attempting to loosen the flywheel nut locked things up, it was shifting/clutching fine before that.

While compiling a list of things to do to the engine I removed the front-end, re-built some spare MR250 forks that came with the bike, and should have the chassis back together today so I can a) try your second gear trick b) warm up the engine to drain the gear oil, check the plates, etc. This is the first bike I've had that I even thought would start in the winter, Im looking forward to finally riding / fun-racing with friends on the lakes!

Based on what I've read here I know I'll need to re-build the engine. If the left crankcase seal is leaking shouldn't I assume they're all leaking? The previous owner provided a gasket kit and some of the parts for a 73/74 CR top-end hop-up which I am interested in doing. I need a cylinder. The first one I've seen for sale is on ebay now but is described as having a 71.25 bore, half way between Wiseco 2nd and 3rd over(?). Are there other over-size piston kits? Faster is better, but how much would 1.5mm over effect put-around-in-the-woods-ability?
Thanks so much!

You can only have one seal go out. Bearings get loose or water gets into the flywheel side and rusts the crank, then that side goes.

The clutch side is bathed in oil so it has a different set of problems but they do go out. Just me but I have had more problems with center case gaskets, base gaskets, intake manifold, leaks then clutch side seals.

I have two MTs and one was stripped down for dirt use and blown up by a previous owner. Even though I only fitted with a slightly oversized piston, that with a thin base gasket and a Bassani pipe gives a bit more compression and pretty good performance. I probably mixed a few CR parts in there, maybe head and piston but nothing dramatic. Figure it took at least 3 engines to get enough parts to build one good one so... I am a bit of wuss rider, even the stock one has enough HP to keep my knuckles white.

The 'HOT' ticket for an MT is to get a 1973-1974 CR250 cylinder and use that with its 4 bolt intake manifold and a CR or Mikuni carb. That is probably where the previous owner was taking it. You still have the wide ratio gear box and heavy flywheel of the MT so rumor is it becomes something still much less suited for a MX then a CR but better everywhere else.

Yep, that's where he was going but I still have to find a cylinder, likely have it bored over, piston kit, and pipe. I'll keep an eye out but I'd like to get the motor running well with the current set-up. If I can do that without ending up with a basket-case, then maybe I'll go for the 73/4 CR top-end hop-up.

Everything under the clutch cover looks really clean, the friction/plates measure little to no wear, the basket is excellent etc., but the plates really are sticking together. It doesn't take much effort to separate them and when evenly spaced there is still tremendous resistance when spinning the wheel. Has the oil permeated the friction material? Could the freezing temps make the oil sticky, should I use a thinner oil? I'll try cleaning them and see what happens.

After figuring out the clutch I think the engine is coming apart. There is a leak somewhere and tuning has not helped/solved the black oil dripping from the exhaust manifold and the white/blue smoke. While that's apart I'll dial-in the chassis with all the CR / MR / replacement parts elgueche was kind enough to collect and supply with the bike! Im sure I'll be starting a new topic soon...

Before you go to extremes with pulling the motor apart, are you still using the original oil injection system? If not what ratio premix are you using? Maybe too much oil is being used.

As to your clutch sticking, I have had trouble on several of my bikes also. My TL 125 would stick real bad after sitting and was cured by using Honda oil instead of using regular off the shelf name brand oil. But lately the clutch on my CT70H is sticking and I'm using Valvoline motorcycle oil, so I'll have to switch back to Honda and see if that makes a diffrence. My MR 250 seems to like any oil. Both the TL & MR have the original clutch plates in them the CT has Vesrah plates in it.

The oil-pump was the first thing to go and I pre-mixed at 35:1 with Belray non-synth 2 stroke oil. Too much oil is being used, but I don't think it's coming from the gas tank ...an ounce or two of black oily stuff will drip from the exhaust if the bike is run for just 5 minutes! The pipe was cleaned out last summer. The carb has been re-built and while I am no expert the bike starts, idles, and accelerates to around 5K fairly smoothly so I think it's close. The plug hasn't fouled but is wet and has a bit of oily residue on the electrode. So, other than the oil refinery and the clouds of smoke the bike is running great!?

Believe me, I have no interest in opening the bottom-end but it doesn't seem too unreasonable for a 30 year old ride. While sourcing replacement parts I will try leaning the fuel mix, a hotter plug and maybe some octane booster and see what happens. If you have any other suggestions please suggest away!

I put in some fancy Motul 10w-30 gear oil and it didn't seem to change anything. Ricortes 2nd gear trick worked once, but just once. Long enough to make a run down the alley in 3" of snow! Im going to clean the plates up as prescribed by the manual, and other posts here, and see what happens before I order new ones.

Thanks much, -John

Sounds like you have a bad seal on the clutch side, I'm running 32:1 on my MR and it hardly smokes. I don't know if you have to take the motor all the way apart, maybe someone here can tell you if you can just change the seal. I have an extra MR engine and the Honda shop manual, I'll give them a look and see what needs to be done, should be about the same for an MT as a MR

Yes, likely the clutch side seal and or the center crankcase gasket. Im pretty sure the clutch side seal has to be removed from the inside.

The only way to access the Right Side seal on the 74 MT-250

_________________
74 MT-250
06 Husky TE-450

Jake is right I looked it up also and is the same for this vintage of CR,MR, or MT. It's a good example of Murphys Law

Hey, that doesn't look too difficult.

But, what are all those round things with teeth on 'em?!

Wish me luck.