Vintage Dirt Bike Q & A

I have had to put my MT250 into highway service for a bit, which includes about 60miles daily at 60-65mph. It's spinning 6200rpm at this speed, and after some tuning is only giving 27mpg, which is unfortunately an improvement over the 23mpg it got when I started making this commute. I would like to get over 30 at least, and I'm thinking getting the RPMs down would help a lot. It currently has a 40 tooth rear sprocket, but I have found one for an XL70 that has 37 teeth:
http://www.ebay.com/itm/JT-Rear-Steel-S ... %26ps%3D54
Would this work on the bike? Or is there another smaller rear sprocket option, or a larger drive sprocket?

The rear wheel is also showing it's age with broken spokes. It's not a huge deal since a previous owner twisted wire around them where they cross each other, but rather than putting used ones out of my old wheel on as these break, I'd like to replace the whole lot with new and set the wheel up right. It doesn't help that most of them are rusted into the nuts, and snap off when I try to unthread them. Breaking the rear tire down daily isn't fun. I have seen plenty of new ones for the front, but none for the rear. Is there a source for these as well?

The XL70 used a much smaller 420 chain, so that eBay sprocket won't work on your MT.

This is the sprocket I'd recommend - you may be able to find it cheaper elsewhere:

https://www.denniskirk.com/sprocket.p82 ... 825215.sku

Just pop it on and adjust the chain, rather than pulling the wheel / removing chain links / etc. This sprocket should bring your revs down to 5,800 or so.

You can also try raising the needle clip one notch, running a leaner main jet, and removing the tailpipe muffler baffle for testing; pipe restriction doesn't really affect mileage at low speeds but can make a big difference on the highway, and the Honda MT pipes are about as restrictive as they get. Ignition timing can also make a difference, and opening up that submarine-style air box a little couldn't hurt either.

I just clip out rusted spokes with a small bolt cutter, then replace with OEM. I've seen cheap spoke sets on eBay (maybe from Thailand?), but have never tried them.

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 ordered rear spokes from http://www.buchananspokes.net/ highly recommend them. Got some front ones on ebay from Taiwan and wish i hadn't they are thinner than stock and very cheaply made.

I sent an email off to Buchanan since they don't list the MT250 on their site. Looking at the price estimates, though, I'm going to be $100 into this at least. But then I will be able to actually center and true the wheel too-
I saw that larger front sprocket on Epay, it's the same price there. I'll have to check if I have the 15 or 14 on it now.
It's running very nice on the highway, so I don't want to lean it out. The plug is a nice honey color and I don't want to melt the piston. I got my mileage increase by putting an exhaust ring in it, I thought it had one but that was just the tube the ring mounts over. Now no more greasy residue on the jug and it'll actually idle sort-of. It still needs a carb, the inlet tube is cracking so I'll make a new one to take advantage of a 34mm Mikuni, but since my throttle cable keeps breaking I'll have to get a new one which of course doesn't have the oil injection cable take-off, so I guess I am going with pre-mix.

I got a rear sprocket for my MT at PBIsprockets.com
The price was reasonable and it is very nice.

You're certainly not going to melt the piston on this motor by going down one jet size on the main. I went from 112 to 132 on my MR's main jet (stock is 122) with no issues at all, in WOT dirt and street riding. The plug varied from light to medium chocolate brown the whole time (plug readings can help with a number of things, but fine-tuning carb jetting that's already close to ideal is not one of them).

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

Ok, Buchanan has the specs on these and they are about $113 for each end. It broke another spoke today, so I'll probably order them tonight or tomorrow.