Join Date: Feb 2015
Location: southern oregon
i guess that would depend on if you miced the jug and piston before you assembled everything. since you already have 2 used sets i dont think a third set the same vintage will help much. as a general rule you cant just mix and match those parts. when you assemble the top end everything need to be measured and fit for optimum performance. it is possible you could get a stock piston and hone the jug and get a reasonable fit but it is unlikely. if the jug has any wear the rings will not seat.
with compression that low and the valves properly adjusted my first guess would be an error in torquing the head and you have a head gasket leak.
the first thing i would do is check my compression tester to insure you data is correct.
did we rule out some sort of auto decompression?
second disassemble the top end slowly looking for issues as you go. you will need to check the mating surfaces of the jug and head for flatness both alone and mated, next is using an inside mic make sure the cylinder is both round and in spec. take the rings off of the piston and place them back in the jug and use a feeler gauge to make sure the ring gap is correct, inspect the cylinder, it should not be smooth you should see cross hatching ( if you dont know what that looks like google it it is important ). inspect the head and valves. look for leaks in the head gasket itself. this will tell you for certain if the top end is your compression issue. and it will keep you from spending unneeded money. this will take some special tools and the skill to use them. it is not rocket silence but if done incorrectly it is a waste of time and will get you nothing.
order a good name brand oversize piston and rings, take the whole mess to your local machine shop and have them bore the old jug to match the new piston, also have them set the ring gap (again special tools and an increased skill level but it is possible at home). have them replace the valve guides and measure the valves. have them machine the head to the jug. this would be a waste of time and money if that is not the problem but if you cant do the measurements your self it is the next cheapest way to rule out the top end as you problem
order a matched cylinder and piston off of ebay (if you ever had dreams of a big bore this would be the time to do it there is no cost differenc but it will open up some jetting/carb issues later). if you do this you will need to set the ring gap, have the head machined flat, replace the valve guides and inspect the valves to make this option a absolute rule out for the top end.
another guy pm me this weekend with the same bike and same issue. (ignoring the 90 psi) i cant help but wonder if it is a vacuum issue somewhere and the slide is not opening correctly. when i was doing some research for him i found several kits that convert that machine to a regular barrel slide carb. you might try googling your symptoms with barrel carb ans see if anyone else has been down this road. why would those kits exist if there was not a need.
i think plan b while likely the most expensive would be the best, as long as you have a good machine shop. factory jugs are generally better than aftermarket and all of the tec stuff would be professionally done. you are getting to the tough decision part of owning an atv. at some point you need to decide if it is worth it to YOU to fix. every machine can be repaired but some are not worth it. when you get there is determined by your skill, the value of the machine, your love for the machine, and the value of the education. i have a 5000 dollar jeep that i have 10 grand and hundreds of hours in. i enjoyed the jeep, the journey, and the education so it was worth it at the time. now that it is time for something different, eventually even your favorite toy grows old. i am faced with loosing thousands of dollars or finding a way to fall in love again. in other words is sucks to be so deep in a project you cant get out.
if there is no attachment to the machine you may be able to sell it as is to someone else and take the money and time you were going to spend on repairs and upgrade your machine and kick the hassle to the curb.