HI All,

I have been messing around with this old polaris 2 stroke for a while and couldn't seem to figure out what is the problem till coming across this site.

Basically I have a hard time getting the engine started without starting fluid. It is hard to keep running... I haven't quite figured out the carb adjustment screws(are there 2?).

So tomorrow im gonna pick up a compression gauge and test the compression. I know im looking for 100 min so if I dont hit this number I guess I need to do the crank seals.

I was wondering if anyone knew where to buy/and if there are any write up on the process.

Also how would i Remove the driver pulley? I dont want to have to buy the puller if there is another way.

I think it is a 1990 something polaris Trailboss 250. The bike i have has all identification removed...

Thanks for any and all helpful advise.

 

if you don`t hit 100psi compression you need to do the top end, piston rings and cylinder. once you have good compression and it still is giving you a hard time then you should be looking at the crank seals. How it works is with good compression it draws fuel and oil from the crank case to the combustion chamber and at the same time sucks fuel from the carb. With bad seals it will suck clean air from the bad seal area and not very well from the carb making it hard starting and almost impossible to adjust the carb.
If you need to rebuild the top end do that first. then if carb adjustments are not working and its still hard starting you know where to focus.

 

Update:

I checked the compression, on the first stroke it was 60 psi, second about 90 psi, then it built up to a max of about 120 after about 5 strokes. Is this normal? If so I shouldn't worry about the rings, then what about the crank seals?

Also I completely cleaned every port on the carb now I need help adjusting the Adjuster screws. Does anyone have a process that usually works and gets it right.

Thanks

 

not sure about the compression but i just did my crank seals, not that bad really with the help of randyneff. check out my thread its basically a complete troubleshooting guide stemming from a hard start. i didnt mess with it yesterday but i will today

 

Hey football4life:

Here is a quick set of step to adjust the Mikuni carb for the low end, copy/pasted from a post I did elsewhere on this forum:

low-end carb adjustment on a Mikuni VM is done via a screw (philips) at the carb (on the right side). That is the "air screw".

Turn it clockwise to close (careful, do NOT overtighten, when it snugs up, call that "closed"), then back off 1 and a half turns counter clockwise. (another caution: *3* turns back is the max, after that the screw could come out)

That is the theoretical "correct" position for starting with the carb adjustment procedure.

From there, with the ATV idling, you can do slight adjustments either way to bump the idle speed up. Once it is warmed up, you can test the low range of the throttle, adjusting the screw further as necessary (again, *slight* adjustments)

A good test is if you have slight slope to run the ATV up; if it handles that without bogging down and you seem to have good power output with the throttle in the low range (i.e. ATV is moving slow), that is a rough way to determine it has been adjusted "good enough"


Note that the high end adjustment is basically for performance, and then you are getting into jetting, etc. I don't have any experience with this aspect, but you could probably search this forum for tips.

 

One other thing, separate from the carb adjustment:

Try to check that the crank bearings seem fine.

Last year I had a piston skirt break, and in the course of addressing that (top end job, wiseco kit), I checked the bearings. Rather than taking everything apart to check, the way I did this is to get at the flywheel, and checked for play when I wiggled and rotated the flywheel. I didn't like how much play I could feel, and sure enough, after opening the crankcase, all 3 bearings were in bad shape and all had to be replaced. My guess is that had something to do with the broken piston skirt.

Point is, if your old Trail Boss had been sitting for years like mine had been when I got it, you might check into this and prevent a problem further on.

 

Thanks for all the replies, I will Get the carb back on it next weekend and see how it does. I'm gonna see if there is any play in the flywheel also,

I'll update how it went then,

thanks to all

 

If you have 120psi compression I wouldn`t worry about doing the rings or cylinder thats plenty good enough, if your finding it hard to fine tune the carb once its running then I`m willing to bet its the crank seals. Like paraord said check out his thread I explained how to change them its an easy job. 1 thing you might want to have a look before ripping into the seals is the carb boot (thats the rubber piece between the carb and head) make sure its not cracked if it is replace it dont try to repair it the repair will not last and you`ll end up leaning out the motor and seizing it.