Vintage Dirt Bike Q & A

Anybody out there try to hop-up the MT250?

What modifications did you perform & what were the results?

Thank you
~JM~

_________________
WWW.CADDY500.COM
The High Performance
Big Block Cadillac Forum

Ya I got really carried away with my mt250 spent way too much money on it and still have to get the carb dialed in. I did the usual cr ad ons then had rich gagnon port the cylinder(a nos cr cylinder) and cases he added a boost port,he also adapted a reed valve setup he made for a hodaka to the cylinder. The front forks were replaced with cr forks with a cr lower clamp and mr upper clamp I installed a nos al baker fork kit in the forks that made them an inch and a half longer. The rear shocks were replaced with 79 xl250 shocks as they were a bit longer and fit(I have quite a few xl's). Its got a cr tank one with paint one without lol neither have a petcock. Mr front wheel cr rear wheel, I had the tranny back cut by John astleford company. I found a sandcast dg head for it and its got goki air caps on the forks, I am still gonna get a swingarm for it just have not done it yet and its got a down pipe from vintage elsinores who are no longer in business. I am in the process of jetting a 36mm mikuni vm for it and have a small kehin flat slide on it now and it is really fast like that so I am sure its gonna fly when I get it dialed in.

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

Excellent info! Thanks.

Could you share your jetting specs? I have a 32mm flat-slide on my bike right now, but it needs more.

_________________
WWW.CADDY500.COM
The High Performance
Big Block Cadillac Forum

I can give you the specs for my 36mm mikuni but the 32 mm is on the bike and I dont remember whats in it. I have to dig up the paper work for the port job its on that. You do know the jetting for my bike is not right yet and its not gonna work on your bike as the port work is not the same.

I'm just curious what the various jetting specs are as compared to mine.

Thanks
~JM~

_________________
WWW.CADDY500.COM
The High Performance
Big Block Cadillac Forum

All right I found the reciept the jetting for the 36mm mikuni roundslide is as follows
Q5 Needle jet
6F5 needle
410 main
20 pilot

Theese were all on the rich side of things as you should start rich and work your way down.

Thanks for the info. How much gain did you notice by swapping to a CR cylinder vs. the MT cylinder?

Anyone else have anything to add?

All that I've done so far, is a DG pipe & a 32mm flat-slide Mikuni. I've been mostly trying to remove excess weight as much as possible. I removed the turn signals, heavy up-pipe, air box & replaced the battery with a very compact AGM unit. That pulled about 15 to 20lbs. out of the bike.

_________________
WWW.CADDY500.COM
The High Performance
Big Block Cadillac Forum

It had a bad tranny in it, so I never ran it with a stock cylinder.

Here's a pic of my new battery. The 6volt system limits you somewhat as far as battery eliminator kits, trick lights, etc. I tried a large capacitor in place of the battery at first, but that kept blowing out the taillights. This new 6V battery is a sealed AGM (absorbed glass mat?) lead/acid type battery originally intended for emergency lighting panels & alarm systems. So far it is working great & weighs a fraction of the original.

~JM~

_________________
WWW.CADDY500.COM
The High Performance
Big Block Cadillac Forum

Does the AGM Charge while on the bike or do you ahve to recharge it with a charger? also could you run your winker off of it or do you just choose not to have them?

thanks,
Matt

The battery charges off the bike like the OEM battery did. I picked up the new battery at my local http://www.batteriesplus.com

I had removed the blinkers prior to this modification & have no idea if they would function or not.

_________________
WWW.CADDY500.COM
The High Performance
Big Block Cadillac Forum

The stock battery was about 5mah and the one he has there says its 1.3 ah so if its really 1.3 ah and not 1.3mah it would run the blinkers just fine. I am basing all of this on what I think to be tru that is ten mili amp hours equal one amp hour. I really dont see how such a small pack could be 1.3 ah but I dont really know much about electrical. I put a battery pack on mine meant for rc car remotes and it works everything just fine.

Anyone else out there working on an MT?

Any home porting tips, etc. Anyone figure out how to mount a 36/38mm carb to an MT cylinder?

Let's here all about it.

Thanks
~JM~

_________________
WWW.CADDY500.COM
The High Performance
Big Block Cadillac Forum

JM,

Your bike looks lean, mean, and ready to go! The MT gearbox should work great for street riding, and the MT front brake is actually pretty decent.

As you may know, I've been hopping up two MR175-powered bikes for what seems like forever. One thing I've learned is that the MT125, MR175, and MT250 cylinders are virtual clones, differing mainly in dimensions, not design.

Unless you've opened up that MT intake manifold to where the rubber is paper-thin (NOT recommended, obviously), you won't get any more than 29mm or so of poor-quality flow out of any >28mm carb. I fit a 30mm in my MR175 manifold (extremely similar to the MT250's manifold) and it worked like crap despite many jet / needle / slide changes. I couldn't believe how much better it ran when I put the stock 28mm carb (identical to the MT250's) and manifold back on. Fixing the 30mm setup took a lot of work which I won't bore you with, but it can be done.

Long story short: if you want max performance, minimum down time, and plenty of online support, I would go with the CR250 (or MR250?) cylinder transplant. Your carb-mounting problems will vanish, your existing pipe will match the porting, and your bike will fly (you may be surprised at how well the 32mm carb works, especially down low). If a bolt-on top-end solution like that was available for my MR, I would have done it in a hearbeat.

If you decide to stick with the MT250 cylinder there's still plenty that can be done, but it will take a lot of work to get to maybe 75-80% of the CR250's output.

Ray

_________________
'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

Ray,

Thank you for the compliment on my bike. There is a still more "cleaning up" that I have in mind for it.

I'm absorbing the rest of your message...

The MT intake manifold is the real performance choke. I have often thought of trying to fabricate a larger & straighter intake, etc.

~JM~

_________________
WWW.CADDY500.COM
The High Performance
Big Block Cadillac Forum