so i bought a 86 yamaha moto4 80 and it was in peices! guy i bought it from said it had low compression it just needs piston and rings lol! i had 1 of these machines before and i know its usually in the valves w these four strokes! so i bought it for 100 bucks! i did put a piston n rings in it and one of the valves was not seating rite but we just pulled it out and cleaned it! it was built up w carbon so once we cleaned it it seats just fine! so i put it back together and it runs yeah! so i run it at work in an open parking lot n it runs great! then i took it thru a small field and it starts making a wierd noise at high rpms! my bro says its the valves hitting the piston? that is what it sounds like but im unsure of how to go about this! did i bend a valve or is it sticking? i was told to loosen the head and the valves and tighten the head down to spec. then set the correct gap on the valves? any body have any ideas? thanks in advance!

 

With only 2 valves and one cylinder if they where hitting I'm thinking the fun would all be over. One would bend in a heartbeat and the compression would be gone. End of story. Can't say I have been down this road with my ytm80, but I sucked a valve keeper through the retainer on my Mach 1 once, canted cylinder heads, (like the ytm). So the valve hit the piston at a angle and bent the stem into a S and wedged the valve up into the guide. only 7 cylinders after that, and the wild fluctuations on the vaccume guage told the story.

You can oil up the valve heads, put some modeling clay on top of the piston and roll it over, this will give you some idea of the valve to piston clearance and compare it with a manual. (I have one if you need it 6789438-Yamaha_Badger_80_Service_Manual_Repair_1985-1988_Yfm80.pdf )

I would also pull the valve, check the stems for straightness and look at the top of the piston. But I honestly think it is elsewhere. Did you dump the oil and check it for debris?
Ken

 

thank u mucho for the manual! that will help me out alot! i think im gonna pull it apart tomorrow or beginning of this week! ill def. keep u posted on my findings! and yes i did drain the oil and no debris! thanks again

 

Another thought that comes to mind is inspect your valve springs closely for anything bent, warped or perhaps just weak. Believe the manual has a section. If you really want it to run high rpm then some new one should be on your shopping list.
Might be a good time to pull the engine, split the case and look inside the clutch basket (worn clutch plates) and transmission (2nd gear end to wear it as it gets used the most)
Also never drive it around with the LH side cover off, the pin that holds the trannys shift forks can fall out, Busting gears in the tranny!
Ken

 

i will def. look into that! did some work on my bike today so ill get to the yamaha tomorrow or monday! thanks again for the help!

 

sorry it took me so long to get back to this problem but i got alot of projects going all at one time! but i did finally get to do some work on my yfm 80 and 1 problem was the cam chain tensioner spring was old and worn so we pulled it out and stretched it! did the job for now but will def. be replacing it! and also the intake valve needed adjusting! it ran great then the head bolts came lose so had to tighten them back down and she runs like a champ! super pumped i love this machine! thanks again for the manual and all ur suggestions!

 

No problemo, hope it helps. I ended up putting new valves in mine because the margins were worn down to to a sharp edge. So I did a new spring on the tensioner too.

Its running good now, even when the kids forget to shift it and wind it too the moon.

Now I'm wondering if I should be tapping a grease zerk into the seal rear diff and pumping some fresh grease into there.
Ever been in the rear end of one of these??
Ken