1975 st90 smoking issues

[vanrodd]

hi, i recently purchased a 1975 Honda st90 mini trail. it starts runs an sounds good only problem is it smokes worst than mt. saint helen an has oil dripping out of the exhaust port while its running. i took the head and cylinder off an replace the seals hopping that was the problem, no luck. this thing smokes so bad you can only start it for a couple of minites without smoking up the neighbor hood. any suggestions, thanks rod

 

[robby12]

Well i don't have two much back ground info on mini trail bikes but if its not the valve seals or doesn't have badly worn valve guides id have to say its blow by witch is caused by worn out piston rings or a worn out cylinder probably time for a top end rebuild

 

[vanrodd]

yea the guy i got it from said he put a set of rings in it, probably from his lawn mower, i guess its time to investigate, thanks

 

[Fat Boy]

sounds like the rings are not seating . mine had top overhaul when I bought it and I had to put new rings and put a cross hatch in the cylinder on the ST 90 that I have then they seated after riding it for a few days and then riding it hard for a few days and it doesn't smoke or use oil now .

 

[minibikedude]

Honda horizontal engine heads have a rubber seal that sits inside a cone that goes over the exhaust valve stem and sets against the valve guide before you put the valve springs back on. That rubber seal could be deteriorated. There is not one on the intake valve.

 

[vanrodd]

well this third time ive had the head and cylinder off , i took the rings off inspected them cheked the ring gap the cyliner ,piston put it back together new seals took the valves out inspected guids, seals put it all back together started it without the exaust because i know its full of oil, started right up secound kick still smokes like a house on fire, oil still dripping out of the exaust port im about ready to buy a china moter. BENT

 

[jesse8931]

While some of the aftermarket motors are real nice you will be sorry in the long run. It could be a cracked jug or head Do a leake down test

 

[zeeman]

I agree with Jesse. The Lifans and cheaper china engines are not near the quality of a Honda. I would fix the Honda engine, probably for a lot less than you can get a knock off for. Then, just ride it for another 40 years.

 

[jesse8931]

Also even if it was eating massive amounts of oil it should not be smoking that bad.

Is the gas in the tank fresh ?

 

[minibikedude]

You said you had the valves off the head there should be a dome with a seal inside it on the exhaust valve stem under the valve spring. Those engine pump oil up one side of the head bolt and and drain back down the other bolt on the bottom. If that lower valve seal is bad, it lets oil run into the exhaust through the stem of the valve cause the blue smoke. There is a green O ring seal the goes under the valve guides. but that guide has to be pressed out and pressed back in if its bad. But i bet its the valve guide seal, check out the links i posted.

VALVE GUIDE SEAL 50/70/90'S ATC/TRX70'S ATC110's_TO_85 (B22H)

Valve parts break down. seal is number 6 on diagram.
http://ep.yimg.com/ca/I/dratv_2159_70396670

 

[minibikedude]

Here is the break down of the Honda 90/110 head, check your valve guides when you have the head apart, If the valve guides are worn you will have to get them pressed in and out and change the 2 green O-rings that are part 11 on this diagram.

 

[vanrodd]

i looked at the seal that your talking about it was soft and had a tight fit on the valve stem. should i try a leak down test if so how do i do it

 

[minibikedude]

You know for a 1.99 I would try the valve seal first and go from there. Your local Honda dealer should be able to do a leak down test.

 

[wayne]

one ? please, must it run a l;ittle befor it smokes OR will it smoke as soon as
you start it ?