Hello!
Wondered if anyone could help me here.
I've got a, "nice in every other way", 94 Polaris 400l that I bought with this problem. I knew it was an issue when I bought it but I am very used to tinkering on this sort of thing so I figured I could fix it. Wrong so far!
Anyhow, it starts fine and seems to idle ok, but for the most part you can't get it to go past 1/4 throttle without a bad bog issue (doesn't die). Occasionally it will clear up and go like crazy but as soon as it drops down to an idle you are back where you started, dead throttle (steady bog @ mid rpms). I assumed it was a main jet/jet tube issue, but I have gone through the whole carb a couple times, replaced parts, and can't seem to change it one bit.
I have confirmed the reeds are fine. Also when I first got it, it would sometimes fail to shut off with the key or kill switch so I got suspicious of the CDI and replaced it with another used one which seemed to cure that problem but didn't change the bog.
I have sprayed all around the carb/carb boots/ intake, etc with carb cleaner, but the engine doesn't fluctuate at all.
I removed the black wire from the rev limiter without any results.

The only clue I have at this point is when I try to rev it up just sitting there in neutral, I can make the bog go away by putting suction on the carb bowl vent tube.

This is a nice atv that I'd really like to make work, but it's stumping me. Please help and Thanks in advance!

 

Needle and seat replaced?,float arm adjusted level with the carb body? Is float arm and pin worn to where it's wobbly side to slide? If so this could be the source of the problem as it doesn't come in carb kits and most people over look it. Plus does it still have the rubber carb dampener mounted to the transmission? This was to help stop bogging and stop the carb from dumping fuel from vibration. 94 models had an update kit that was supposed to be installed by dealers to help this. If it doesn't have one you can order one from a 95 model. Item #8. Dirt Cheap Yamaha, Honda, Arctic Cat & Polaris OEM Parts & Accessories – Cycle Parts Warehouse

 

I did replace needle and seat. I will take a closer look at the float level and check for that carb dampener. I wanna say there isn't anything mounted to the tranny. What does the dampener look like? Rubber? Foam? Seems like it's actually the airbox that touches the tranny.... I'm not looking at it right now obviously so I'll shut up!
Just so I know, when I put suction on that vent tube am I causing it to lean out or get richer?

Thanks for the help so far!

 

Just click on the link I posted! Item #8. Sucking on the vent tube is removing air from the carb,essentially making it richer.

 

Took the carb out again, verified float level was even with the carb when "closed". Also confirmed that the two plastic "floaties" rise up and down normally.
Float arm and the ears on the carb don't appear to be worn and the float arm seems to move up and down smoothly.
It doesn't have, or look like it's ever had the damper like it should and I will sure get one on there, but I must have another problem too, cause there is no way somebody has been driving this thing while running this poorly for 3500 miles!
Sometimes like I say it clears right up and runs super smooth if you drive SUPER aggressively, but if you start riding like normal or like you would down a atv trail, (slower speeds, on and off the throttle) it's all boggy and overly lean.
In neutral holding the throttle steady at higher rpm's, it has a mild, steady, surge also.
To top it all off, sometimes (but not all the time) when riding it hard, it will run strong and then just cut out as if you turned the key off, then as soon as you come to a stop (and sometimes let it sit a few seconds), or coast for a while, it will come back to life and go again. I have a clear fuel line at the carb and you can see fuel moving through it before and when this happens even though it acts like the bowl went dry.
Do I have crank seals leaking or what? I sprayed carb cleaner around the oiler too with no results, as I have heard tell of air leaks around there.....

This thing is kicking my booty!

 

Crank seals were what I was going to mention next if the carb wasn't the problem. Running better at high speed can compensate a little for leaking seals,but at low speeds leaking seals can show up and cause problems.

 

Is there any surefire way of confirming it is crank seals?

If it is the seals, I am stuck with splitting the case, right? No way to replace them in the chassis I'm guessing....

 

Takes a leak down tester to do this and most people don't need this special tool for a one time use. Engine doesn't have to be split,but the front clutch,flywheel and stator have to be removed to replace them,plus you have to have clutch puller along with a harmonic balancer puller or good steering wheel puller for the flywheel. Here's a leak down tester like I have,plus here's a link on a pdf service manual that you can look over that will explain this engine better than than me typing.Index of /ORV Manuals/Polaris/1985-1995 - All Models http://www.ebay.com/itm/Motion-Pro-2-Stroke-Leak-Down-Tester-W-Box-/111167533264?pt=Motorcycles_Parts_Accessories&hash=item19e219f0d0&vxp=mtr

 

Did you ever figure this out? I know its a long shot years later, but I'm currently getting whooped by the same issue.

 

Yep I did figure it out!

The intake boot between the carb and the engine had a tear in it that couldn't be seen until the engine was under rpm's or a load!!
It would flex a bit in those conditions and open up a crack that would suck air causing it to lean out and bog.
Look for a crack right side of one of the clamps. You may have to bend the boot around to actually get the crack to open up enough to be seen.