polaris 250 compression test

Tyler14 · #1 (**permalink**) 12-28-2010, 10:27 AM

I picked this quad up off of a very reliable buddy of mine knowing it needed some work to get it running. The entire topend is new at .030 over, otherwise its bone stock. Got the whole thing assembled and ready to test run/break in and now I'm having some problems...

The bike will start and run on full choke and then continue to run on half choke. There is absolutely no idle capability when off choke. Also, the engine seems to lag quite a bit when the throttle is opened. I also get some exhaust detonation that pops the headpipe apart at the joint between the 2 sections, usually after a strong rev when the bike is idling back down. This includes a proper amount of warm up time. The carb has been completely dis-assembled, cleaned and checked. All intake boots are crack free and no air leaks are detected. As far as I know, the jets are stock. Compression is good and I have installed an oil injection block off kit with single pull throttle cable and I am running 40:1 premix. I have adjusted the ETC gap to spec and even tested the bike with it bypassed. The carb slide has complete closure and full range of motion in the carb.

I've tried just about everything I know to try on this thing and I'm coming up empty. I have many years of experience tuning 2-strokes, but all of my time has been with high performance rides with Keihin carbs. This Mikuni is becoming a pain in my d**k.

The only things I have yet to investigate but intend to are proper float level and maybe needle position or maybe even slightly bigger jetting.

I think I have provided as many details as I can remember so far...any ideas?

Thanx in advance...

**Bruce!** · #2 (**permalink**) 12-28-2010, 02:05 PM

Welcome to the board!

3inthedirt · #3 (**permalink**) 12-29-2010, 05:34 AM

I'm sorry but you still have an air leak on or at the carb somewhere. I would soak the entire carb, minus the orings and rubber parts, in Berryman's for a few days and blow it out with an air compressor if you have one.

Manaen · #4 (**permalink**) 12-29-2010, 08:42 AM

if you are sure your carb is working properly (and I mean absolutely) it could be a poor case seal. If the case is leaking air it will not idle properly.

My bet is on the idle circuit of the carb (most common issue) . However since it is a fresh rebuild you could have some issues with an air leak somewhere in the case. The only way to tell is to pressure test the case. You may also want to perform a compression test. Keep in mind that on a two stroke the compression is only part of the story you still need a good case seal to create proper flow through the intake ports.

Before doing all that double check everything on the carb, reset the float level, clean/clear idle circuit, make sure your jets are the stock numbers and clean, adjust the needle and idle mixture back to stock. Then go from there.