My 700 sportsman began bogging badly and when I pulled the carb off I found the spring on top of the main jet diaphragm sideways. I replaced it correctly and cleaned the entire carb out and now it is doing the same thing. It will run fine for about 1/4 throttle but as you accelerate it begins to bog and occasionally backfires. I have bypassed the fuel pump by running a fuel line from the tank to the carb. There seems to be a lot of blow by coming from the crankcase vent that goes to the air box. The atv has been very well cared for, not modified or run hard. The guy who owns it is older and never ran it over 25 mph if I had to guess. Always stored inside. I have run it with the air box off and tried to look inside the carb and it doesn't seem that the slide for the main jet is moving very much, not sure how much it should move. I have pulled the choke out when it bogs and it doesn't help. When it bogs down there is a strong smell of gasoline. My next step is to do compression test. I checked the fuel pump as per polaris manual and it holds vacuum and there was no gas in the pulse line that connects to the intake manifold. I did check the compression with a guage that I do not know if it is accurate and the compression was 70 psi on both cylinders. I will get a new guage tomorrow and recheck. Seems odd that both cylinders would be equally low. Both spark plugs looked fine, maybe a hair on the lean side.Please help me..............

 

Maybe he never revved it up that high? That style of carb doesn't like.being run with no air box lid on. Maybe pull the snorkel(4 Phillips screws) and see if it runs better. If it does, lean the main jet down. What jet is in it?