Vintage Dirt Bike Q & A

JM,

You really don't need to fabricate an intake manifold if you don't want to - a Suzuki 13110-16500 manifold should work fine for 32mm to 36mm carbs, and is still available from Suzuki for less than $50 (I have one here if you need measurements etc.). The problem is the oblong cylinder intake opening and gasket surface, same as the MR175. If you hog out the opening to match a round manifold, you end up with almost no upper and lower gasket surfaces. You can build out the surfaces by welding, or using JB Weld epoxy (my preference).

Then... you'll need to open up the intake port & passage to get the maximum benefit from the larger carb and let more mixture into the crankcase, widen the transfers & passages to let that added mixture into the cylinder where it can actually do some good (and preferably adjust the transfer timing to work with your CR pipe), and finally you'll need to widen and probably raise the exhaust port / passage to get rid of all the extra gases you've created. The CR cylinder - with all this work and more already done - just looks better and better.

I ended up building out my MR175 manifold with JB Weld and then opening it up to about 32mm; it looks and works fine, but was a lot of work. This wouldn't work on the MT250 manifold, though; there's just too much material to be removed. Pic below:

http://i399.photobucket.com/albums/pp74 ... ifolds.jpg

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

What all is involved in swapping a CR top-end onto the MT lower-end?

Isn't there a bolt pattern difference? Any draw-backs like a loss to low end throttle response, etc? Any case to transfer port misalignment issues?

Any idea what a reasonable price is for a CR cylinder, head, etc?

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JM,

Here's a thread right here at VDB that's pretty much on point - there are others as well:

viewtopic.php?f=21&t=131&view=next

From the above thread, it seems that a '73-'74 CR250M top end will fit and work about the same on MT250 cases as it would on CR cases, ignition and gearbox ratios aside. You'll also get the extra cooling of the early low-pipe CR250 head, which I've modified to use on my MR175 (it's a great head).

A comprehensive part-number comparison of top-end components like base gaskets, cylinder studs & bolts, etc. would be a good idea, but one thing's for sure; a lot of people have done this swap.

I don't know about a power-characteristics comparison, as I've never ridden a CR250M. I know my buddy's MT250 is just plain weak, so I have little doubt that a good-running CR250 cylinder will kick its ass at every rpm.

Perhaps woosh or others could give you a better idea about prices. There's not much on eBay right now, but that could change tomorrow:

http://shop.ebay.com/i.html?_nkw=%28197 ... m270.l1313

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

Top end bolts right on you just need to use cr gasket for the head and you will lose the oil pump as you will need a cr manifold. I paid 300 plus shipping for my cylinder but it was nos and from amsterdam, I did this to get one with all the fins intact and then the sob packed it really bad and it had a broken fin when I got it. I think the used ones go for 50 and up depending on the condition and the heads are not expensive either. I have already sold my extra cr parts but like ray said just watch ebay. You may need a cr gasket for the cylinder base as well but I dont think its different than the mt. I only use cr gaskets in my mt now they all work in it they just wont give you one for the oil pump well the manifold gasket will be different than the mt too. I would just go with the cr top end its really easy to do and will give you more power if you use a cr pipe and a mikuni carb.

You guys have me sold on the CR top-end swap, just need to find the required parts now. I run pre-mix already.

I have this 38mm carb off of a CR powered shifter Kart.

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Well I finally found & bought a CR cylinder & piston on eBay. I can't hardly wait till it arrives. Now I just need to locate an intake manifold & a head which I think I may have a lead on already.

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JM,

Just a reminder about the '73-'74 head - it's the way to go with a down pipe!

For that huge 38mm carb, you probably want the later 16221-381-013 or 16222-381-000 manifold, for the 36mm Keihin (43mm spigot O.D.).

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 shouldn't be reading this... Now I want to get a CR cylinder for mine...

I'm running stock everything... carb, airbox, filter, and have a larger expansion chamber on it... I'm currently trying to jet the carb up to match the pipe...

But... 36mm carb... CR head/cylinder... hmm... the gears are turning.

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I drink your milkshake!

How big is your fuel tank? I'm running pre-mix in a CR250 tank & it doesn't go far. I'm expecting my range to be even less once I swap a CR top-end on. My bike is more of a toy than a commuter of any kind. Changing my pipe provided the most bang for the buck so far. This was also a wise investment. http://trailtech.net/tto_temperature.html
Supposedly an exhaust gas temperature gauge is more helpful for tuning, but the cylinder/head temp gauge is super easy to install & read.

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I probably get close to 40 miles from a tank of premix gas in a 73 cr tank on my mt250. I would love an oil pump as I drive it on the street, but its just not gonna happen.

My bike is strictly off-road... not a street ride what-so-ever.

Did you have that exhaust installed on the stock cylinder? If so, what jet were you running in the carb?

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I drink your milkshake!

Yes. I needed to change the MT exhaust manifold to a CR manifold then the DG downpipe fit right up. At first I used the stock carb & jetting. I don't know what those specs are. It ran fairly well.
The specs on the 32mm carb are:
Main 210
Pilot 27.5
Needle P2

These specs are from SUDCO. I believe the top-end needs some richening as there is still a ping at hiway cruise. The bike is not well suited for the freeway right now. I'd like to add another tooth to the countershaft sprocket & get some more top-end out of it.

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