did st90's have any major issues or common problems?

[hrc200x]

Checking to see if st 90s had any common problems or mechanical issues? Like ct 70's, seats pans crack, stripped foot peg and intake bolt holes, bent or broken headlight ears, shift forks bent.

I'm working on one that has 80 psi compression. I've only had time to set the valves and it didn't help. Any problems with the valves not seating? Going to try some oil down cylinder and check timing when I get time and see if that bumps it up. Does the cover/plate thing thats behind the points cover need to come off to check the timing?

Is there any tranny issues such as bent shift forks? Crankshafts hold up to quite a few miles? This one has around 5500 I think.

Both front and rear suspension on this one seem a pogo stick. Very springy with not a very slow rebound.

 

[Enginedoctor]

i'm really not old enough to know this for sure, but i am a gossip of all things honda mini... so i'll relay what i've heard as well as some things i do know.

i've head the 1st gear in the transmission has issues.... those issues, i'm not sure what they are. the seat pan is similar to the ct70, but NOT THE SAME (don't be fooled), so if left out in the rain, sure they can have issues.

It's a 90 base motor.... usually bulletproof for the most part. sure, bolt holes strip out, and have other issues, but normally they're good engines.

on an 80 psi motor, i'd suspect the rings and bore. sorry. toss a few squirts of 30w oil or ATF in the cylinder (sin spark plug) then re-test the compression see if it jumps. if it does, needs em (rings/bore) if a valve was bent and thus not seating, compression would likely be 0 or close to it. I've seen schmoes put a 110 spark plug in a 90, (it's longer) and then mar the piston, as well as bend both valves i had to rebuild that motor

to check the timing, yes, remove the cover plate with the advancer and it's the same as the 70's more or less.

the cranks are good. at 5500, probably due for the top end.

on the front forks, a good rebuild will help. on the rear.........?

 

[b52bombardier1]

I own two of them and nothing much needing excessive attention comes to mind. The bike engine shares much with the CT90 motor minus the dual range transmission.

Your engine will either not run or at least not very well at 80 psi. One hundred psi is about my minimum and she'll likely smoke even then. A good top end rebore is indicated here along with two new valves and a three angle cut. Your valves will likely be tuliped at this point like every other Honda engine that we deal with around here.

Crankshafts hold up but you will need to study your's when you get it out. The ends of the con rod would be my biggest concern and not much unless ran with poor oil service a lot.

Your fork tubes probably need a rebuild. Easy to do, hard to put back together without rigging a way to get the top bolt back in. I can explain it better with pictures but not in the McDonald's Play Land where I'm currently sitting with my grandson.

Rick

 

[b52bombardier1]

Hello,

I was wrong about the ST90 front fork tubes - they are easy to reinstall. I had to go home and look at things again. It is the CT90 fork tubes that need a roughly foot long bolt in the same thread pitch to "catch" the tube by the top bolt hole so that the tube can be pulled high enough for the headlight bucket ear pinch bolts to tighten around the tube. Yea, this is an odd design for fork tubes but the extra long bolt works to make this job easier.

If your chrome-faced ST90 fork tubes leak due to rust pitting and torn seals, they can often be renewed with a thinly layered and sanded smooth application of JB Weld. Wire wheel the pits clean and then degrease the pits with Brake Kleen before applying the JBW. The rust pits will tear a new fork seal so you need to at least try this method before wasting a brand new seal.

First gear failures??? Yes, perhaps. My Topaz Orange K2 had a gear problem when I got it - chipped gear tooth. However, this problem has not re-occurred and I could blame previous owner abuse from a high rpm shift into first from neutral secondary to a poorly maintained bike in general. I really have no idea about the previous history on this bike before my ownership but I have boatloads of fun with it now.

Rick