Vintage Dirt Bike Q & A

Hi, I wondered if any of you fellow Elsinore fans could point me in the right direction regarding jetting a VM36 Mikuni for a standard 1974 CR250M? I'm basically looking for a ballpark set-up to use as a starting point which I can then fine tune to suit my bike, here in the UK.
Any help would be greatly appreciated as I don't have a Keihin to use for cross reference and I as can buy a VM36 with my choice of needles and jets, it would be nice to be somewhere near from the start.
Many thanks,
Grae.

P.S. Does the 75 and 76 CR250 or MR250 rear wheel fit a 74?
Cheers!

I ran mine around 300 main 35 air

I finally went to a 40 pilot on my MR. No matter what needle I tried, there was a point about 1/3-1/2 throttle under heavy load where I got slight detonation. Still testing to see if this will give it just an itty-bitty-tad-bit more fuel to correct it.

Thank you very much for the info guys, that will give me a good starting point.

- Simiont, I think I bought your Mossbarger reed valve? If so, does it matter which way up it is installed? The reason I ask is that the reeds are different colours - one is cream, the other is black.

The 75 76 rear wheels had a dished sprocket on them so as far as I know they can not be used on the 73-74 bikes but the mt250 can although its heavier.

no it was just replaced any way up will do

I don't know if you can use one or not but the reason the newer ones have the dish sprocket is because the engine was moved in the newer frames. Is it possible that the sprocket was changed so that the same wheel assembly could be used?

Thanks again to everyone for their input.

Simiont - thanks for that, much appreciated!

Interesting comment Gary, I'll try and research that one. I wondered why the 75 and 76 CR and MR ran a dished sprocket. Perhaps, if the engine was moved to the left, (maybe to accomodate the LHS exhaust port exit and up pipe?) a dished sprocket re-aligns the chain wheels. If so, it may be possible to fit a flat sprocket and make up new spacers to suit the 74... Hmmm, food for thought!

Does anyone know how wide the 75, 76 CR rear hub is compared to the 74?

Cheers,

Grae.

I believe the rear wheels are interchangeable. Probably not the spacers. And obviously, you have to use the flat sprocket on 73-74 and dished on the 75-76. There used to be a spacer kit available to run the flat sprocket on the 75-76, but don't think you'll ever find one today.

Thanks for that mitchie, I reckon I'll risk it and buy one and see how I get on.. I'm guessing that the MR250 rear wheel is the same as the 75/76 CR250? It looks the same from what I can see in pictures. The reason I ask is they seem to come up for sale more often.

MR and CR rear wheels are the same, although I seem to see more cracks in the aluminum rims of the MRs than on the CRs. Don't know why, or I'm just imagining that. Both of my MR rear rims have a significant crack, but I run them anyway with no harm.

The '73 to '76 CR250 and MR250 all share the same 42601-357-030 rear wheel hub, if that's any help.

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

Nice one Ray, that makes things easy. I'll start looking for a nice example now with confidence. Thanks!
I guess cracking must be quite a common thing with these old aluminium rims as aluminium hardens with age but it seems odd that the MR rims are more prone to it than the CR's? I'll bear it in mind though mitchie.

Hey, this is a really friendly forum. That's helped me along tremendously with my winter project, so thanks to everyone for their input. And Thank you Nathan for making this possible!

Grae.

MR rims were lighter AND weaker. You can tell them by the extra lip at edge.