wondering if it helps to do a simple mill of head to raise compression for better overall performance?
looking at the mating surface of the head looks to have a significant step to the dome area
plus I'm wondering if this causes this model to have lowerish compression
just did the top end and its still less than I was hoping for 100-110
Thanks

 

Yes this is a low compression engine at 6.1/1 but if you've done a new top end compression should closer to 130 psi or more if cylinder was bored and honed correctly to the next over size piston. If you just honed the cylinder and stuck in another piston,that could be the problem.Plus you can take 0.10 off the head to bump compression up a little.

 

I bought this atv broken
the top end had been done but not correctly
new piston, rings, etc...
they had too hot of a spark plug in it
they did not service the carb, fuel system, or bleed /check the oil injection
the sides of the piston were scored looked like lean condition
probably both fuel and oil
took carb apart found lower end jet too small
appears used carb was put on it but from another model:
looking up specs 300 express had smaller lower end jet
put in the correct jet cleaned service carb
fresh gas, new good quality 2 stroke oil, bleed air out of oil injection system
verified its dripping/ injection oil into intake(are these pumps pretty lasting?)

bought a used piston and cylinder that was in good shape, installed,
got it running ran good for a while(few days)
went to restart it, doesn't want to start, tried priming it
in the process noticed no vacuum or suction pulling through intake side
do you think this is because of bad crank seals?
I pulled the mag side apart and notice some oily debree / residue on components.
need to order a puller to get clutch side apart
Thoughts?
thanks

 

Yes crank seals first as they always need to be changed when you do a top end. If that doesn't help,take the best cylinder,have it measured for the next over size piston.You're just taking a chance slapping on a used cylinder and piston. Have a cylinder bored out for a new piston and rings then hopefully your problem will be over. Plus always premix the first tank of fuel 50/1 to allow the oil pump to catch up and help cylinder break in.

 

still have concerns about the oil pump
do you think I should pull it off, spin it up and do a volume output test?
don't want to sink a bunch of money in this have more engine damage
also it looks like the oil pump is pretty hard to get to for R&R
thanks again for your time

 

Main thing to check on the oil pump is that whether it has a plastic drive gear or a metal one like used in the 250. First couple years of the 300 Fugi did slip some of the plastic gears in.If you have one,just get a metal one. Some used ones on ebay or a new one. 70 bucks at Partzilla.Other than that these 300 pumps were as dependable as the ones on the 250 engines. Just open the 10mm plug on top of the pump until all air bubbles are removed and close the plug. When you first crank it up on a new top end with the tank premixed,just lift the pump arm all the way up at an idle. This causes the pump to flow at maximum rate and look to see oil go from the pump to the inlet tube on the carb. Let the arm down and go for a ride.

 

got it
thanks again

one other question
being this model doesn't have a reed valve
do you think its less in performance, usability, value, etc... to own?
you know kind of like weak, a lemon or inferior design

 

Simple piston port engine like the 250 and hundreds and hundreds are still running around.They got people where they were going and even the all wheel drive models surprised a lot of people as to what they could do. If set up right they could run on the lowest octane fuel that would gag other machines.If you want to get from point a to b and not in a great hurry,like things simple,and if you get it in good running shape,the rest of the machine is decent,no reason to sell it unless you want to.It's certainly not a lemon or inferior engine. Just as simple as an engine can get. Yamaha,Suzuki and Kawasaki had variation of this type technology for years also.

 

Hey old Polaris tech
got the crank seals replaced in this
started to clean up the mag & ignitor coils from oil and debree
notice the paint on the coils surface facing the flywheel magnets is starting to lift and separate
what do you suggest I do with it?
clean off the old lifting paint and leave it bare metal or repaint with a light coat
don't want to disrupt any of the components ability for spark and or charging

thanks for the advice

 

I'd shoot a little Krylon on the coils in case of shorts,leave the stator ends bare.