Vintage Dirt Bike Q & A

Hi Guys,
Has anyone on this forum lightened an MT250 or MR250 flywheel? And if so, how much did you shave off?

I weighed my spare MT250 flywheel at 1885 grams on the kitchen scales (so might not be very accurate) and would like to remove some weight from it but don't really have much idea as to how much.

Could anyone tell me the weight of the 73/74 CR250M rotor please as I'm guessing that if I lighten my MT flywheel by half the difference, I will probably be somewhere near what I'm after.

Thanks for any help or advice you can offer!

Cheers,
Grae.

Grae,

Before my MR175's points ignition mercifully self-destructed I considered lightening the stock flywheel, but I couldn't see any way to remove enough metal to make a difference with 100% safety on such a heavy unit. Thankfully the thing died quickly and sent me in a better direction.

As many here know, I'm a huge fan of using CR igntions on the MT and MR motors. Big gains in low-end power from the CDI box, instant revs, total reliability - what's not to like?

No, my lights don't work - they were only there for show anyway, as I never ride at night. There are plenty of low-power-consumption LED lighting options nowadays that are much brighter than the stock lighting, using just small gel-cell rechargeable batteries.

I realize you're on a budget, but you might consider checking eBay for pricing/availability on the '75-'77 CR250M CDI igntions just for grins.

I'd say the CR125M CDI rotor weighs maybe 700g - 900g max, and is only 58mm in diameter vs. 110mm for the MR175 flywheel. The CR/MT situation is proportionally similar, I'm sure. Also, the CDI ignitions have far less magnetic 'detent effect' on crank rotation than the points units do.

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 wouldn't try it on the MR250 flywheel. You could get rid of an ounce or three, but not enough to make a difference.
Having said that, I love my barbell flywheel. I can ride it up hills in third or fourth gear grunting like a four stroke. Be nice to get rid of the points, but I'm a heavy-flywheel-loving guy!! (and I'm not about to drop the scratch to go to a pvl--the divorce would make that $400 unit cost about $40,000)

Ray, I thought about that first of all but was under the impression that the crankshaft for the MT is totally different to the CR? Do you think the rotor and CDI from a CR250M1 will fit the MT crank then? I was planning on using a small gell type battery as I need the space for a bigger airbox. If this ignition fits, this might be the best route as after studying my spare flywheel, like mitchie pointed out, there's not much there that can be shaved off really. Having said that, I'm not sure I want my MT to be as responsive as a CR as I plan to use it for Green Laning (riding ancient non-tarmac dirt roads) and maybe the odd vintage enduro just for fun. That's why I was thinking somewhere between the weight of the CR rotor and MT flywheel would be about right.

Mitchie, you have a CR top end on your MR don't you? How does that feel with the heavy MR flywheel? The reason I ask is I have a CR250M top end with a 36mm Mikuni for my MT and wondered what the engine response is like using the stock flywheel? Is the MR250 flywheel lighter than the MT's? And if so, do you know if it fits the MT crank?

This is all good fun. I'm getting a lot of pleasure building this bike on a small budget, learning a lot along the way from all you guys who have already trodden the path I'm on. I really appreciate all this help and advice, saving me a lot of money on the learning curve!

Cheers

Grae

I have a CR cylinder and an MR cylinder. Right now, I'm running the MR. There isn't that much difference that I notice between the two. CR hits powerband slightly harder, but really not much difference. I run the CR copper gasket on both and 36 mikuni.
It revs quick enough for me, but I've never ran anything other than the MR flywheel, so I have no comparison.
But like I said, I lug mine a lot. I kind of like the grunt and I seem to have a little more control at lower rpms than when it is screaming. Use Nippo Denso points if you can possible find them. You can get points flutter at high rpm using el cheapos.

I had tried an mr flywheel in my mt and they wont fit the crank the mr flywheel has a bigger hole in the middle of it.

Grae,

All I can really tell you for sure is that the MT250 and '75-'76 CR250 use the same 12mm flywheel retaining nut, and that my picture of the MT and CR stators side-by-side appears to show they're interchangeable. If you really want to try it, the usual Sherlock Holmes drill is in order; compare parts diagrams and numbers, contact eBay CR250M parts sellers and ask for dimensions (many are quite helpful!), search the Web for actual usable info instead of "tried it, worked good" sound bites, etc. I have a feeling that if any CR250 CDI ignitions will work on the MT, it will be the early '73-'75 (optional) ones, not the later units (which might work on the MR250, though).

I think heavy flywheels must work better on bigger motors. The ultimate setup in my opinion for the heavy-flywheel guys would be the CR CDI with the rotor holes tapped for a bolt-on flywheel weight; you'd get the actual low-end power boost of the CDI advance curve combined with the extra inertia of the added flywheel mass. This would be easy to do on the 125 ignition, but I don't know how far the 250 rotor extends out past the stator windings.

If you absolutely have to keep the MT250 points ignition, it won't be the end of the world; it appears to be much higher quality and better design than the junk MR175 unit.

I opened up my MR's airbox last week, using a grinder to remove the top lip and drilling some additional holes in the bottom. It breathes much better, and now the air filter drops right in without a fight.

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