Hello to all, I'm new to this forum & thought I would introduce myself. Been riding & wrenching on motorcycles for the last 27 years. I recently traded for a 1985 Honda ATC125M for my 15 year old to ride. The thing appears to run pretty good, but it smokes like a freight train. I have ordered a service manual for it, but it will not be here for another week or so. I do have a manual for 70s & 90s, & the motor seems to be pretty similar. My first thoughts were that the rings were shot. I have not performed a compression test on it yet, but it seems to have plenty of power, so maybe it is not the rings. I have read that the exhaust valve has an oil seal, possibly this is the smoking problem. I figure when I tear the motor down, I will probably do a top end w/ new piston/ rings, & the oil seal on the exhaust valve. Any thoughts as to if I am on the right track would be appreciated.

 

You hit it on the head!
Valve seal or rings. Compression test should tell the tale. The ATC 125M is a sweet ride!
Nice deal!

 

Thanks for the reply Richie. Is there any problems with this model that I should be aware of ? It seems to be a pretty "bullet-proof" design, but I am not very familiar with these Hondas. Can you tell me how I access the rear drive chain to check tension ? It is an enclosed unit and I do not see how to open it up. Also, how do you adjust the tension ? I realize this is a question that will be covered in the service manual I have on order, but it won't be here for a week or so.

 

I believe that there is an inspection hole with a rubber stopper in it. Pull out the rubber stopper and check slack with your finger. Adjusting is taken care of in the back of the bike. It's been a while but adjusting should just be to loosen a few bolts and tighten the adjuster.
Have fun and ride with saftey gear![img]i/expressions/face-icon-small-cool.gif[/img]

 

I have one, the one inherent problem is a broken stopper paw on the shift mechanism. This will cause the atc to suddenly come out of gear. It is a $30 fix (and 2 hours). Also the brakes go fast, let's not even talk about the front brake.

As for the chain, most of the riders back in the day, including me now, remove the chain case for easier access to the chain.

But generally they are very reliable and lot's of fun, wheelies galore.

 

Thanks for the info Rob. Ya, the brakes on the rear of this one are shot. I ordered a new set for both front & rear from Dennis Kirk last night. That boy will probably break it if it is possible to do so ![img]i/expressions/face-icon-small-happy.gif[/img] He likes to do them wheelies & such, like most 15 year olds would ! Good thing it's not an expensive fix . I'll check on the chain tonite when I get home. I saw the rubber plug & figured that was the purpose.

Oh...another question til I get the service manual...does the gearbox have it's own oil supply ? If so, where do I check it at ? That is, if I can pry the boy off it long enough to check it ?

 

The gearbox uses the engine oil for lube, there is only one place on the right sidecover to add and check. Be sure not to screw the stick in when checking, just push it in. That engine is a virtual knock off on the age old CT90 trailbike engine with a few twists like electronic ignition. It is really easy to work on. Trx250rob is right, the stopper does like to break and sometimes the boss on the crankcase it's bolted to breaks also (a part of the right crankcase). If it smokes, it's very nearly always the rings, and this isn't always obvious via a compression test. It could still have decent comp if the problem is with the oil ring. Also, with all that excess oil on the cylinder\piston, you could get higher readings because of it. If the rings are just plain shot, which they probably are, your best result would be to go with a new piston and bore to specs. On rare occasions, the oil galley to the top, which runs along one of the cylinder studs thru the cylinder and head, could leak internally, more likely to see it leak pretty good on the outside too if this is the case.

 

Thanks for all the information guys. I got the rear brakes changed out over the weekend. Pretty easy to see why they were not working. They were full of a slimy muddy goop all over everything. The seal on the drum was shot, so I replaced it as well as the shoes. All is fine now. I am still waiting on the service manual, so I have another question to ask. I read where this ATC should use 20w50 oil, but not certain what type to use so I don't screw up the clutch plates. Will Castrol 20w50 be okay to use in it ? If not, what would you recommend ?

 

10w40 ATV oil - only use ATV oil