600 Sportsman carb adjust questions...

[brassmagnet]

Yes yet another carburetor tuning question... Ok for starters this my first time tuning a carb, and this is my very first atv so please be gentle. I have a 2003 Sportsman 600 and I recently rebuilt the carburetor. The float needle was fairly worn and the pilot screw/needle looked like someone tried tightening it too hard and it had a nice step in it – replaced those. I also replaced the pilot jet since it was cheap. After giving the carb a good cleaning I reassembled everything. Using a caliper I measured the float height (unloaded) and it was within the specified range shown in my service manual. The jet needle looked good – no shiny or worn spots and its associated e-clip is in the middle position. I also purchased a new main jet (both the stock 155 and the next size higher). I currently have the stock main jet in the carburetor (i.e the 155).

Being that the pilot screw is in such an easy spot to get at I have the atv somewhat torn apart so I can partially pull the carb off and adjust the pilot screw. That being the case I am only trying to adjust the pilot screw while the atv is idling in neutral for now – not actually riding it or spinning the tires. The stock setting for the pilot screw is 1.5 turns. After some trial and error I have it currently set just a little under that (a little richer) which seems to produce a good idle once everything is warmed up.

With the atv good and warmed up (choke completely off) I was testing the responsiveness of the carb by very quickly snapping the throttle open about 1/4 of the way. About 50% of the time the engine responds and quickly revs up, the other 50% it will momentarily bog. If I ever so slightly ease the throttle open (i.e. I don’t snap the throttle open quite as hard) the momentary bog disappears.

This momentary bog that I am sometimes seeing… is it due to the pilot screw needing to be adjusted to make the idle mixture slightly more lean/rich? Or is this just a function the physical lag associated with a carburetor? Could it be a symptom that I should adjust the jet needle up/down a slot? Or is it just because the motor is unloaded and not actually driving anything. Thanks in advance for any feedback!

Rob

Fyi… I completely drained the tank and refilled it (treated with seafoam) so old fuel wouldn’t be an issue. I have also replaced all the filters/breathers on the atv, and replaced all the fuel lines with brand new rubber. I also added a fuel filter directly between the tank and the fuel pump.

[Russ C]

adjust pilot screw to 2.5 turns out and put the larger jet in it, should start and run better

[brassmagnet]

I will try that tonight when I get home. I just wanted to try and get everything going using the stock set-up since that was what was originally in the carb. Based on what I had read, my understanding was that the pilot screw/jet controls the idle and throttle up to about 1/4 throttle, followed by the main jet needle from 1/4 to about 3/4 throttle, and the main jet from 3/4 throttle to wide open. Based on that line of thought I figured in theory that if I had the pilot screw set-up or correctly for the 155 jet, I should pretty much be able to swap out to the 157.5 since its main influence is on top end.

I'm also going to pull off and double-check the rubber manifold between the carb and engine and make sure its not cracked at all. I had already replaced the molded tube between the airbox and carb since that was getting dry rotted and begining to crack.

[Russ C]

to check boot, take side covers off, start engine and spray around boot with carb cleaner, engine will speed up or slow down if boot is cracked

[brassmagnet]

Well I put the 157.5 jet in and backed the pilot screw out 2.5 turns and let the engine warm up. Quickly snapping the throttle open 1/4 of the way while idling didn't produce any noticeable delay or bog. I also sprayed carb cleaner around the intake manifold like you suggested and didn't see any noticeable rpm increase. I'm guessing on the idle rpm setting and playing things by ear since I don't have a tach.

Thank you very much for the help Russ!

[Russ C]

let it idle as slow as it will without die ing when you blip throttle or shift gears, this will make it easier to shift, run 1 oz per gallon of seafoam in your gas occasionally and turn gas off when not in use, should not have any more problems with it