Vintage Dirt Bike Q & A

I picked up an MR175 that was mostly disassembled and missing a few parts earlier this week. The day after I picked it up I got it cleaned up, cleaned the carb well and mostly put back together. It started up great and ran and idled well. The next day I finished putting it back together by adding the air box/seat and getting the wires/cables ran etc. After this, it will only start by adding starting fluid and can't keep the idle going. It seems like it's running out of gas basically.

I removed the carb and cleaned and checked everything again to make sure nothing came loose and caused an issue. Everything looked good but no improvement. Took it apart again removed the air box and it still won't start or run right. It then started surging and accelerating uncontrollably.

Everything looks good with the gas and we have tons of spark and compression.

The carb is a Mikuni 28mm I'm pretty sure. The only identifying numbers are 28378 and Q1-Z1. Does anyone have any thoughts on what might be causing this or alternatively have a suggestion on a replacement carb and jets?

I had a similar problem when first getting my MR running. Long story short, it turned out to be varnish / crap / rust in the tank. I ended up going with a Clarke plastic tank. Just for grins, try filling up a clear 8 oz. glass almost full with fuel while sloshing the tank around a bit.

I know the 70's 2-stroke Keihins pretty well, the Mikunis not so much.

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

Thanks Ray. The tank is surprisingly clean but I'll definitely check it out. I think it was replaced fairly recently. I assumed something must have broken loose since it worked great to start with but the carb has looked pretty good each time.

Do you have a suggestion for a Keihin carb to replace the Mikuni if needed? I'm just looking for a good trail bike to play around with for this one so I'm not too concerned about lots of extra performance. I'd like it to be quick still but don't want to get into heavy modifications.

My tank looked OK from the fill opening, but down in the lower corners there was all sorts of stuff. I tried everything except coating the tank. With yours you may only have to keep the petcock strainer and 'snorkels' clear and add a fuel filter, but I wasn't so lucky.

Honestly, the stock 28mm Keihin carb setup works quite well if it's in good shape. The MR175 intake tract is tuned from the cylinder to the airbox base, and can have excellent throttle response and low/mid power. The needle and needle jet need to be in good shape (preferably new) and I found a 118 main / 48 pilot / 3.5 slide worked well. The airbox is quite restrictive - I cut three more holes in mine last week - but if you clean the filter often you should be OK.

Two things that can make a real difference are cleaning and/or drilling out the exhaust pipe ahead of the tailpipe insert, and more compression. If you can find .010" or .020" copper or aluminum sheet at a hobby shop, you could make yourself a high-compression head gasket (use the cylinder top as a cutting guide if possible).

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

The tank looks pretty clean on the inside but I added an inline fuel and will see how it goes. I'll look into getting a stock 28mm Keihin carb anyway. What would I enter in for the model? I was going to guess PWK28 but want to make sure I get the correct needle and jet etc. I have been checking at jetsrus.com

Another thing that might be causing my issue is that I don't have the compression I hoped I had. It's coming up around 125 and I read on here that is pretty low. Maybe it will just a ring job I hope.

Thanks again
Paul

Paul,

I believe the MR175 used a PE28 Keihin - the PWK would be the modern version of it. There's a decent OEM carb on eBay now:

http://www.ebay.com/itm/Honda-MR-175-19 ... dc&vxp=mtr

This seller has the motor this carb came off, which looks very low-hours to me.

125 psi compression is very low; these motors had around 150-160 psi when new. The MR175's power output is quite sensitive to compression and the OEM rings will break if run too long, so I would definitely recommend changing them. Search 'MR175 ring' at eBay, there's a ton of them there now. If your piston has a lot of miles on it, I'd change that too.

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

Thanks again Ray. I have my eye on a couple of carbs including that one. Also for the pistons I see several people selling ones for MR175 '76 & '77. Assuming my engine actually matches the frame, I have a '75. Do you know if there are any differences between the '75 and '76 & '77 parts. I've noticed this on several Ebay auctions and at first assumed it was a misprint for the date range. I wanted to double check. This piston doesn't look too bad but I might as well switch it out while I'm in there since they aren't terribly expensive.

Paul

All 3 years of MR175 use the same pistons; 13100-373-000 is standard, 13102-373-000 is 1st (.25mm) oversize, etc. In my experience most MR175's were still on the original bore when sold or parted out, but if you don't know the mileage it's obviously best to measure before buying. eBay sellers use various search engines for parts, and I've seen many listed for '76-'77 MR's that were used all 3 years.

I always replace the piston, rings, piston pin, and small-end bearing as a set, for what that's worth.

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

Thanks Ray, I am going to take the jug and piston in to the local shop and get them sized to make sure
what they are and get a new piston, rings, pin, bearing and gaskets...
Paul

A couple of years ago I looked for some pistons. This was what I found:

I contacted Wiseco for the pistons and the guy referred me to their custom order department. A few years back a guy had a special run of .5mm and 1.mm pistons made for MR175's. They still have the specs and can run them in batches of as little as 12 for about $145 per piston (including rings, clips and pin).

I contacted the guy who made the run and bought his last .5mm and one of the 1.mm. His website is www.themotoweb.com

Be sure to check the wrist pin bearing for the number of notches in the end. They are different diameter to fit the rod.

Absolutely! I test-fitted the 3 different small-end bearings on my first motor for what seemed like the best fit, used a larger bearing than the rod notches called for, and ended up destroying the rod bearing surface. This last rebuild used the correct bearing, and the rod still looks perfect after 4,000 mi.

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