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 Post subject: MT250 Power Needed
PostPosted: Sun Jan 25, 2004 9:52 pm 
Okay so here is the deal. You have an MT250 74 vintage... it's dog slow. Not just slow, but Da-Hawg slow. Yet, you still want to play around and race it in an AHRMA race, knowing it's no techno wonder (even for a 74 model though it does have an UP pipe).

So tell me what you think about this --- what can I do to get some power out of it?

***Raise the exhaust port 2mm and clean the rest up.
***Mill the head 30,000ths for more compression and which will make up for the exhaust port being lifted.
***Find either a 2mm larger carb, or try to have this one bored 1.5-2mm for more top end and flow.
***Put the heavy flywheel on a lathe so it will lose some ounces and allow the bike to rev quicker.

The crank is going to stay, too much hassle to split the cases etc, so...

Questions:
***Is the above good for some serious HP, or would it just be easier to bolt on a certain year CR250 jug fit on this bike?
***Should I go to a CR250 style down pipe if if I go with the CR cylinder - as I'm sure this thing has a ton of baffles inside of it (most likely)...

Give me some suggestions guys. Basically this thing weighs about (guessing) after removing the steel airbox, light and other do-dads, probably 250lbs. It has probably terrible suspension, so I'm trying to get an engine which is realistically inexpensive to build, so I can find the smooth lines, and straights and try to make up time there -vs- spending a ton on suspension.

Anything you can think of, post away.

Is this bike just too far out into enduro (street legal) land to ever be semi-competitive on just a fun basis?

Thanks.
Mike


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 Post subject: Re: MT250 Power Needed
PostPosted: Sun Jan 25, 2004 9:53 pm 
Mike, I had a '74 MT250 that I used for woods and occasional MX. I milled the head a bit, raised the exhaust port (in the range of what you're thinking) and used an aftermarket down pipe. Take off the extra junk, including the battery, and put some good shocks on it and it'll perform OK against other 74 era equipment (but will never keep up with a CR or YZ in a straight drag race). Maybe go up 1mm on the carb (although I left mine stock to keep the low end for woods riding). With these mods my MT could easily outdrag a Husky 250. Not the fastest, but it was a pretty good all around bike until I could afford a real MXer. The biggest problem with the bike is the weight. The frame is mild steel vs chromoly on the CRs, and the MT came with steel rims and much heavier hubs.

Good luck.


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 Post subject: Re: MT250 Power Needed
PostPosted: Sun Jan 25, 2004 9:54 pm 
I'm not sure exactly on the MT250 but on other twostrokes you can increase the intake timing by shortening the piston skirt on the intake side by a couple millimeters. This is nice for road bases gearing because it keeps the same exhaust timing and power band. Try a little at a time, and keep a spare piston around in case you go too far. Make sure the exhaust port is open as much as possible at BDC. If you do raise the cylinder there's an opportunity to increase the port area by lowering the bottom edge of the port. You can also go a little wider on all the ports as long as the ends of the rings dont overlap a port. A larger carb (somewhere between CR nad MT size) and tuned pipe will do wonders but start to make tuning difficult. An aftermarket pipe will weigh much less than the stock MT one. Try to find some aluminum rims, that'll help, not only with weight but with flywheel mass as well.. Good luck.
Let us know what you did and how it works.
-Todd


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 Post subject: Re: MT250 Power Needed
PostPosted: Sun Jan 25, 2004 9:54 pm 
First off are you the same Mike Patrick who used to race for Norton and Yamaha in the So Cal desert and Baja?

The MT gear ratios are a bit high as Rick pointed out. They're fine for desert and other cross country events. I used the MT gears when I raced desert back in the 70's.

You might try a PVL ignition (http://www.pentonimports.com)or find a used CR ignition on Ebay. Heavy flywheel I think is just going to make it to slow.

I don't know a whole lot about the port timing on the MT but if you have access to a mill maybe machine a plate that will lift the cylinder 0.020"-0.030" and then machine the head down. That would increase your port timing and duration and get the compression ratio back. (Anybody have an opinion on this idea????)

I have seen one or two MT's at AHRMA races but only looked at them in the pits. If your buddy is a good rider it may not matter. Unless he's going to ride in the expert class.

Good luck,

Loren


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 Post subject: Re: MT250 Power Needed
PostPosted: Sun Jan 25, 2004 9:56 pm 
Mikeeep,
I suggest you don't do this. The CR and MT are totally different. Granted, the cyclinder can be ported... but no matter what you do it her, it will still be a MT. The biggest problem outside of what you are planning to address is the complete bottom end. Cases, gears and crank. They are all different. The cranks are waaayyyy different and so are the cases for the crank area. The CR's crank and cases have tighter tolerences so they can generate higher bottom compression to push the air/gasses uphill into the cyclinder faster...
Also, when you port the cyclinder to generate more power you will produce it at high rpm's and your wide spaced gearbox may be spaced too far apart and have "holes" between gears. Especially second to third. A 50t rear sprocket may help this.
Buy an aftermarket pipe for a CR if it will fit. The stock CR pipes are pricey and the pickel silencers never worked good even when new. Too loud.
Forks should be allright but run 30 weight oil. Nothing thinner.
Shocks aren't shocks in my opinion, just dampners. There are replacements out there for around $170.00, I think. Progressive makes them. Your shop should be able to get these. If not, email me. My friend just put some on his '74 Husky MAG and they seem to work as good as my $325.00 Works shocks. But, with only 4" travel, who really knows. LOL
Frames break unless they are gussetted on CR and MT models.
Right foot peg breaks the bolt off so weld it before hitting a jump.
If I were in your shoes, I would do as much as affordable but never expect more than fun. But, most of us in vintage racing do it for fun. (till the gate drops)

Rick :D


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 Post subject: Re: MT250 Power Needed
PostPosted: Sun Jan 25, 2004 9:57 pm 
Thanks for the replies - first to the guy that asked if I'm the "Mike Patrick" that was the famous dez racer... nopers that's not me. Good memory though!

Regarding the bike itself and what I'm expecting out of it - not much. The bike is a slug in stock form and in a drag race with a current 80cc bike would get SMOKED big time.

I figured some mild porting, the head mods to return the mid-range and add about 3-4 teeth to the rear sprocket (or actually just drop down one on the front) might tighten the gearing up. THOUGH, it is for street use, HOOKER makes a header for this thing (as they were called) and though not as good as a down pipe expansion chamber, it has to be better than stock - the problem is, this cylinder has the exhaust port on the right hand side and is an up pipe, even if I found a CR down pipe they are all left side exhaust ports from what I have seen - or I'd have to just do some cutting/welding to make one work - so I found the Hooker Header and thought it would be cool to polish the aluminum and break out the red paint and re-paint the red hearts that came on them LOL (actually laughing my butt off thinking about me polishing this old pipes aluminum guards).

Anyhow, if I can get this thing just fast enough to keep from being dead last into the first turn, I'd be stoked. I found a local company with a milling machine, and looking at the flywheel, that sucker is big and heavy enough to anchor the Queen Mary in hurricane gusts - so I figure after looking at it, I can actually lose 50% of it's weight, without affecting the flywheel, other than quicker revs - it'll still be plenty heavy LOL

Rick thanks for the tip on the fork oil. The rear shocks are just what you said - they basically hold the back of the bike up. I'm going to see if they are or can be made removable, and then break out the Parts Unlimited catalogue to see if I can get a heavier shock spring, and then measure the fork springs and see what MM they are and see if a modern bike has something similar that I can pirate.

This is all in fun. If I can make a fun "play" bike out of it and then have fun at a vintage race... well I have a YZ465 also for that but that's a diff story. BTW, I just found a (?) 1975 YZ500 - I asked the guy if it was an SC500 and he said "No, it's a YZ500" and he's pretty sure it's a 1975 model (wish it were 74 to make the vintage class). I haven't seen it yet, but have been promised that it's really clean - any idea on what one of these things might be worth? I've got him talked down to $800 sight unseen...

Mike


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