Vintage Dirt Bike Q & A

Hey guys,

First off I just want to let you know what has brought me here. Last fall I came across and old dirtbike i could not pass up. When I got it home I found out I am the pround owner of a 1976 MR175cc dirtbike.

So anyways When I got the bike it ran pritty well and was able to move under its own power. When I got it home I tried to start it but couldnt so i changed the plug which was pritty black looking. The bike fired right up... It would rev and idle all day in nutral but when I put it in gear and tried to get it going it seemed to bog out and not have much more than enought power to get me rolling without stalling. So I figured the carb was dirty so I cleaned that all up but same problem occured. So then I tried to ajust the carb float but didnt get to install it untill after the winter and now the bike will not run at all. I playe with the float some more so i think I set it back to noramal but I cant get this thing going.

I have tested the spark and got a nice strong blue spark.

I have just tested the compression which tested at 90-100psi! I think this means my rings and/or my piston is in need of repair. my repair manual is comming in the mail is 100psi to low?

Thanks

100psi cold compression is very low, and will go even lower if/when the motor's running. I really don't know what my MR's compression was stock, but now it's around 155psi.

If you need rings and are on the original (standard) bore, Wiseco 2605KD rings work great once the end gap is set (http://wiseco.com/ProductDetail.aspx?ItemID=2605KD).

Welcome to the MR175 experience. Hopefully you'll be spared the ignition, carb, and exhaust problems a lot of these bikes have.

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

Hey thanks for the reply ray... seems as tho your the mr175 guru around here.

So an updated.

I got a new repair manual for the bike in the mail that took about 3 weeks to come but it did arrive. So today I grabbed some new plugs and started to work on the bike. I did a compression test again and got 100psi which the book says should still be enought to get it running. I checked the spark and it was nice and strong. So I worked my way to the carb, as I was pulling it apart I tugged on the needle and it slipped a bit. upon further inspection I noticed the clip that holds it in place got bent and was not holding the needle at all. After setting the needle to the middle clip spot the bike started on 1/4 kick.

Now the problem I noticed was after I kicked it over and it started, the kick start slammed up againsed the bike with some strong force.... I started it a few times and it seemes to only stick sometimes, can anyone tell me what is causing this? Another thing I noticed was it was smoking quite a bit does that mean its running rich?

Thanks for the help guys! this Forumn rocks!

When I first got my MR, the compression seemed a little low, but I thought 'what the heck' and continued trying to get it running rather than go to the trouble of pulling the cylinder. What a mistake that was.

It turned out the rings were so worn that the end gap was .2" (ten times the allowable maximum), and within a week the top one snapped in half and snagged in the exhaust port. I was extremely lucky to be able to salvage the cylinder.

At one point the kickstarter engagement pawl popped out of its circular retaining spring and the kickstarter dropped down against the footpeg. I don't know what would have happened if the motor had started when this happened, but I'm certain it wouldn't have been good. You may want to try kicking the bike over a number of times with the kill switch off, and if the kickstarter does anything strange at all, it's time to pull the clutch cover and check it out.

I don't mean to make you paranoid about your bike, but so many of these motorcycles are parted out with less than 2,000 miles on them, and there's got to be a reason. Mine has over 3,800 miles on it now, which is the most mileage I've ever seen on one of these (does anyone know of one with more?). If you plan on keeping the bike, I would assume that every maintenance item excepting the crankshaft seals needs to be done, and done soon.

I found the stock jetting (122 main jet, 52 pilot jet) to be very rich at low throttle openings, and ended up using a 48 pilot and 120 main jet along with a 3.5 throttle valve. I would at least change out the pilot jet. I use a 36:1 mix of Bel-Ray H1R, and get practically no visible smoke at all after warm-up.

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

back from vacation..

ray im not to worried about doing the work on the bike as it gets me out of the house I will take things as they go this season and tear it right down this winter.

So I have run a few tests on this kick starter problem and it seems the spring is gone or broken because it does not want to return at all after the kick.

anyways I pulled the cover off and removed the kickstarter mech. and I do see a spring but its deep in there and it is to large to get out of the hole its in. Is this the return spring? and how would I reset it? I will take a few pictures tonight and get them up to better describe it.

thanks

If your kickstarter return spring is broken, I'm afraid that's bad news. With the kickstart gear/pawl/etc. removed, can you can easily turn the kickstart shaft in either direction by hand? If so, the spring is broken.

This spring (item 6 in the drawing below) is inside the main engine cases, with the transmission. To get to it, you basically have to disassemble the motor. I've been looking for a new one of these springs for almost two years - they're extremely rare.

If you want to get your bike running for a reasonable amount of money in a fairly short time, what I would suggest is to pull the clutch off (I would take the opportunity to replace the clutch plates with new OEM ones and use Barnett 501-29-06072 springs), remove the kickstart idler gear (item 2 in the drawing) and then reinstall the clutch and side cover, leaving the kickstart lever off.

The kickstarter will no longer work, but the kickstart shaft (item 3) should retain one half of the broken spring, with the other half retained by the engine case - so the broken spring should not be flopping around inside the transmission. Since the idler gear is gone, it really doesn't matter whether the kickstarter pawl engages or not, since it will only be engaging the stationary gear on the shaft (item 1). The shaft will also serve to seal the hole in the engine cover.

I know it's not an ideal fix, but for what it's worth I ran my CR with no kickstart lever (as a weightsaving measure) for at least 12 years, and the only time I use my MR's kickstarter these days is if I stall in tight woods.

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

well I am bringing this thread back from the dead...

I had lost all interest in this bike for a while now and after trying to sell it without any luck I decided to tear it down to see what it needs done... The kick start return spring is pretty much impossible to find a replacement which sucks. So push starting is the way to go, to bad a fin broke before I came to this conclusion. Anyways.... I pulled the top off the motor and got the piston out. When I took the top off it had 2 head gaskets which I haven't figured out why yet. The bore does not have any stamping that it had been oversized so im assuming that it is original. The piston looks to be in good shape but the rings do look warn. so my plan is to measure the cylinder and bore to determine what I should do next. Compression was 90 PSI before I put another 5 or so hours on it so that seems to be the problem. Im going to grab a set of those rings ray recommended and I think I may be in business.

I have a vernier would that work to get the bore size?

I will get some photos up of the top end rebuild... looks easy... maybe to easy lol but we will see.

So only one fin broke taking the cylinder off? That's actually not too bad, considering the eBay cylinders I've seen. I've got a procedure I use to get the cylinder off undamaged, I thought i posted it (maybe not, and it wouldn't help now anyway).

Two head gaskets will KILL the compression on this motor; they're hard to find so keep them both if they're usable, but only use one. I still think it's a great idea to replace the rings, and hone or sand the cylinder. How many miles are on the bike? It's quite likely the cylinder hasn't been bored. Five of the six cylinders I own are standard bore, along with the vast majority of the eBay ones I've seen. The Wiseco rings will work with the standard and possibly the 1st oversize (.25mm) piston as well, and I have a spare set of .25mm OS rings I could part with if you need 'em. For bigger overbores, there's a guy on eBay selling Wiseco piston/ring sets in .020" (.5mm) and .040" (1mm) oversizes, I believe.

Try measuring the piston at several different places on the lower skirt, front-to-back, side-to-side, etc. If you get 2.599"/66.0mm or less, it's almost certainly standard bore.

These bikes really are a lot of fun when they're running right. It seems you may have lucked out and gotten a good ignition, which means you'll just have to set the timing, sort out the carb ($30 max for an 80% improvement) and clean/drill out the exhaust when you get it back together - at which point it may run well for a long time, or at least be much easier to sell. A 13T front sprocket will really blow your mind 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

The fin broke off when the kick starter kicked back because the return spring is broken... when I removed the head i can see how someone might brake a fin because i could feel them flexing a bit but I was careful and used a 2x4 to pry slowly on it. I was quite surprised when I saw two head gaskets on there and that must be the reason for the very low compression. So I checked the rings and the gap was about .035 " but I cant find a reference on whats the maximum limit. The rings are not square on the ends they have a 45 degree angle on them? Im guessing this is bad so rings will be needed. When I went to measure the cylnder at the top it was just under 2.6" but when I measured the bottom it was 2.610 or so but I may think I was measuring it wrong ... what do you think I should do next ray? this is my first rebuild so im a little lost.

Thanks

Wow, that must have been some kickback - you're lucky the kick arm didn't punch through the clutch cover!


Excellent! The locator dowels tend to rust into the cases, and it can be a real nightmare to break them free sometimes.


The minimum is .008" and the max is .020" (Honda shop manual), so it's definitely time for new rings. I set the end gap on the Wiseco rings to .010".


There may be an easier way... on the top ring surface to the right of the gap, look for '.25', '.50', etc. stamped into the ring (oversize). If there's nothing stamped on the right, they're standard-bore rings (all sizes will have an 'N' stamped on the left). These marks can be hard to see on new rings, let alone used/worn ones - you may need a good magnifying glass, bright lighting, fresh coffee, etc.

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

Ray

So I took a look at the rings and they have ( N 50 ) on them... should I assume that it is .50 over size and grab a set of those rings off ebay? I have a gasket set coming from elsinorestuff on ebay and want to get all the parts here, just don't want to order something I don't need.

Thanks again for your help

OK, so much for my guess about it being the original bore... The 50 is most likely .50, or .5mm oversize. The Honda part numbers are 13013-373-003/-005/-013/-015.

There's a set on eBay now for $100, which is a really high price. You may want to use Google with all four of the above numbers and see what you come up with. I did a quick search and found them at http://tiaromotorcyclewreckers.com and http://www.motoretro.com.au (both in Australia), and there may be others as well.

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

Well After looking into more items on this dirt bike the engine needs to be bored out and I would need to get new rings and a piston along with that. These bikes are not cheap to repair or find parts .... so I have come to the conclusion that I am going to put it all back together. I'm going to get it running and just sell it. I'm hopeing with just the one head gasket it will raise the compression enough to keep it from bogging.

Thanks for all the help ray!

I would definitely give it a good test ride before you sell it, especially since it looks like you've found one with a decent ignition. At least now you know you know what you've got (worn rings, good piston) which is a lot better than what I was riding around with originally (dead rings, worn/scored piston).

If you hone or sand the bore and piston a little, everything may seal a bit better and last quite a while. My original rings were sealing OK when they failed at .200" end gap/2000 miles, which means they were sealing better-than-OK at .035" end gap and well beyond. If you're patient, oversize rings will appear on eBay for a reasonable price - in the meantime, the bike is completely rideable - and I doubt the piston will need to be replaced for at least a couple of ring changes, if ever.

Trust me, you'll notice a difference using just one .050" head gasket. Milling my head .020" made a real difference in compression, and .050" would have been too much for sure.

And re the bogging - be sure to drill out the exhaust and clean out the spark arrestor openings! A clogged exhaust is a total power killer, and the stock pipe acts semi-clogged even when new.

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

Ray you keep me interested in what seems an impossible project considering it is my first bike...

So I went ahead and put it all back together with just one head gasket this time... I do have a copper one which seems a bit thinner than the black one i put on there so if it does lower the compression but not enough I will try out the copper one. It seems like the compression went up but I didn't have the compression tester on me so... anyways I tried to kick it over a few dozen times with only one small spudder. I figured will clean the points and see what condition they are really in. I got to the flywheel and could not get it off with anything I had on hand... Is there a cheap flywheel puller I can make or could I just grab a universal one from the auto store near by? I saw on you tube you can get it off a moped with a hammer but I don't really want to risk bending it out of shape. what have you found to be the best method of getting this thing off.

Thanks!