Vintage Dirt Bike Q & A

what can be done about a pipe up grade. iv'e thought of using an old 73-74 cr250m downswept pipe because the frome from any mt250 is built the same as the 73-74 elsinore with minor changes. even the hangers for the pipe on the cr were used as the rear brake light holder. i believe honda took a cr250m frame and welded on a few different things. the pipe would be a direct bolt on i believe, the cylinder heads have the echaust port in the exact same angle. the bore and stroke are the same for both bikes as well but the cylinders are clearly different. the only thing is it's really hard finding a cr250m pipe that aint bashed to hell and i'd like upswept if possible. would an mr250 pipe work? how would it effect performance? i know it would need some modification.

Just as an example, this pipe should work:

http://cgi.ebay.com/ebaymotors/NOS-Hond ... ccessories

There's also the Bassani MT250 up pipe, which was offered as a factory option at one point. They come up on eBay fairly regularly.

The MR250 pipe is made to fit the later 250 cylinders, with the exhaust manifold on the left side. It would take a whole lot of work to fit one on an MT.

Your stock pipe is probably clogged with carbon (NOT in the removeable baffle, but in the spark arrestor where the tail section meets the main pipe body). Drilling this out with a 1/2" or 5/8" drill will make a big difference in mid and top end.

There's quite a few differences between the MT250 and CR250 frames, if you look closely - more than just brackets and mounts. Just two nearly invisible mods - changed steering tube angle and footpeg placement - can make for an entirely different-handling bike.

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

Ya the mt250 is way racked out more so than any other enduro I have from honda. I dont know but I dont think the cr250m was like that.

i know, i'm not restoring the bike, more making it into a racing enduro and i hat the stock exhaust. i thought, " to hell with it" and bought a dg pipe from a 73 elsinore for it. i Swear iv'e looked and looked and it should be a direct bolt up. the cr 250m an mt250 cylinders are exactly the same if seen from the front. the only difference i believe, is that they made the cr250m cylinder machined for reed valves. rejetting will be the next big thing. any tips? i'm definatly going with a biger carb. as far as ive read no ones ever tried this. worst case senerio i just sell the pipe in like new condition if it won't fit. wish me luck!

I've spent the last year or so converting my "CR175" to an up pipe - including competely redesigning the pipe when it worked like crap - so I'm probably not the guy to ask about up-to-down-pipe conversions. In my woods a down pipe would last maybe a week or two.

Putting a larger carb on the MT250 cylinder probably won't be easy, which could be why you don't see much mention of it on the Web. The problem is the intake manifold; it's a small 2-bolt unit with a banjo fitting for the oil injection. Perhaps you could get a 30mm carb to fit into the manifold by soaking it in boiling water for a while, as I did with my 175. You'll then have to carefully enlarge the manifold I.D. and cylinder opening for more flow (the intake port in the cylinder should be OK, but the transfers really should be widened for more flow). You may have to modify the throttle cable, and figure something out regarding the airbox boot.

Pretty much all the MT250 hop-ups I've heard about involve putting complete CR250 top ends on the MT cases.

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

Yeah, its definatley a project. and an experiment. i'm only 16 so iv'e got tons of time to mess with it on top of my oter two vintage bikes. i might source out a different intake boot all together and might end up with the cr250m cylinder on the bike. Down pipes do take a beating, but for me, i dont plan to ride it hard at all. more take it out as a camp bike to put around on and down old logging roads which happen to be plentiful where i live, not racing and jumps and stuff. not to mention it's registered for street use. we'll see how it works out. what research iv'e done shows a promising result though, also, iv'e read a book on expansion chamber theory and supposevly exhaust gasses naturally want to flow downward. this is probally why most GP racers dont have upswept, it just upswept because of rocks and such. we'll see. i did find a post on here about using a 34mm mikuni carb in which the author included jet sizes to start with, good stuff, but would that be just a mikuni VM series?

Alright so i bought a 73cr250m cylinder assembly. now lets talk jetting, and carbs. should i go with the stock cr250 carb or a bigger mikuni. what about jetting? i'm also having the cylinder bored out to probally the first oversize plus it'll be running a DG performance racing pipe.

The cr cylinder will bolt right on no problems, you can get the gasket kit for the cr250m from www.amsracing.com I know its a different bike but the gaskets are the same except for the cylinder but thats what you will need now as its a different cylinder. The cr cylinder has a different bolt setup on top so if using the mt head you will need to remove a stud from the cylinder no big deal, I would use the cr head as it has one more stud and will help with the head gasket. I would go with the cr manifold setup and a 36mm mikuni carb thats what most people run on the cr250m. Your not gonna be able to jet yours quite like that but it will be simmilar. The mt has a huge mag on it compared to the cr so even tho you have the cylinder its not gonna be a cr. Also another thing to keep in mind is your oil pump I took mine off and used a bigger manifold but now I have to use premix. I run mine at 36:1 and may take it out leaner once I get some miles on it. Good luck on your project those cr parts can be expensive.

yeah, it is a bit pricey. I will probally have the flywheel shaved down/ lightened up a bit. i could use the magneto from a 75-76 cr250 as it's a direct bolt up but i'd like to keep the ability to run lights. frame so far is primed and ready to paint with a quality PPG gloss black. I have after market bars for it i'll paint as well as the chrome is bad. next i'll be rebuilding some showa 360 shocks i got from the local scrap yard. they're pretty decent I.M.O. i ran the bike for a few months with them and for what i'm using the bike for, they're perfect. I think this bike will be pretty cool, and i might have something that will keep up with my friends and they're modern bikes. Thanks for all the input and this web site is awesome!

I am still using the stock flywheel on mine I think it helps with the low speed riding and it helps keep the power hit a little smoother. I also wanted to keep the lights so didnt use the cr mag and stator for that reason. There is a cdi kit for the cr from pvi and I talked to them and they can sell you one with a 12 volt coil added for the lights and then you could do it but I dont know of any other options. I have spent quite enough on mine for the time being so I am just using it as is for now. My pipe came from vintage elsinores before they went under and its an ok pipe it just didnt fit as well as I would like and that may be cause its for a cr and not an mt. I do have a airbox setup I bought and never used its for removal of the stock airbox. Its some sheet metal you can connect a 36mm mukuni to and its got a uni airfilter on the other side at an angle to clear the inner fender. It fit real nice untill I reed valved my engine and than I couldnt use it. If you think you can use it let me know as I have no use for it you can probably have it for shipping costs. Here is a link to my bike just remember its a work in progress and yes you can make them very fast lol full throttle and its hard to go straight

http://s201.photobucket.com/albums/aa26 ... buy/Mt250/