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RSRBOB, Technician
Category: Motorcycle
Satisfied Customers: 1078
Experience:  Former Factory Service Rep, Dlr Line Tech, Service Manager, General Manager, Store Owner
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I have a 1997 Polaris sportsman 500 atv, It has a vacum cotrolled

Resolved Question:

I have a 1997 Polaris sportsman 500 atv, It has a vacum cotrolled slide in the carb that is not responding to throttle. the butterfly opens all the way with the throttle cable but the slide is only coming up half way. Its not tight when its not running i can use my finger to raise the slide with no problem the atv will start up and idle perfect but when you throttle it down it goes to around, what seems to be half throttle and will not rev any higher. You can ride it as long as you dont need alot of power. I've cleaned the carb, checked cam, new piston, HELP!!!!!
Submitted: 9 years ago.
Category: Motorcycle
Expert:  RSRBOB replied 9 years ago.
There are a couple of issues that could create the scenario you are experiencing. The first would be if there is any kind of a hole or perforation in the actual diaphragm for the carb slide.
The second would be if the outer edge of the diaphragm is not properly seated into the groove cut around the carb body for the lip on the diaphragm.
You can always test the carb to see if the slide is going to lift all the way, which is a good idea to do anytime you had the slide out. Take compressed air and blow in the passage above the venturi. The slide should lift all the way to the top, leaving the venturi unobstructed by the slide, leaving only the needle visible in the venturi.
Take the top off the carb and first look to see if the diaphragm was properly seated all the way around forming a seal, the last time it was installed.
Next, remove the slide and hold the diaphragm up to a light and try to look through the diaphragm as you stretch it to reveal any miniscule holes or perforations. Any perforation of the diaphragm will result in improper slide operation.
Customer: replied 9 years ago.
I did pull apart the top of the carb and stretch the diaphragm and could not see any holes or tears and the seat looks fine. I have cleaned the carb about 4 times now but I guess that does not mean that there could not be a slightly plugged line in carb. Is this the only thing that would cause this problem.
Expert:  RSRBOB replied 9 years ago.
Actually, unless you have ingested sand past the air filter, a dirty carb would not affect the slide movement. If that were the case, then it would be from the grit effectively jamming the slide by reducing the working clearance between the slide and carb body.
Since the slide is vacuum operated, anything that has to do with creating or maintaining that vacuum is suspect. The two suggestions I gave you resolve 99% of slide lift issues.
Did you try using compressed air to blow in the back of the carb to see if the slide lifts completely like that? Until it will pass that test, it will never work when installed on the motorcycle.
It is good that you didnt see any holes in the slide diaphragm. The next question becomes the installation process. When re-installing the top over the slide, before you put the screws in, have you tried sliding the top around on the top of the diaphragm to make sure it feels like it sliding over it and not rolling an edge or feeling it rub the top of the carb body?
Exactly how are you determining that the slide is not lifting all the way?
Have you tried putting a new spark plug in, taking the atv out to ride and then doing a plug reading to see what color it is when the engine is acting up?

Another thought came to mind, but it is leaving the realm of normal repairs and going into heavy speculation. Is it possible someone has installed a spring for the slide that is too strong?
The other issue with the spring would be making sure that it is correctly seated in the slide and also in the carb top. If it isn't fully seated, it could increase the spring pressure on the slide and cause it to resist opening.
Just remember, no matter what you do while trying to resolve this, you can acurately simulate operating conditions by blowing into the passage above the venturi on the air filter side. When everything is right, the slide will lift all the way very easily.
Customer: replied 9 years ago.
Ok, The atv was running good and then this happened, so there was no modifications done anywhere before this happened. It started just not revving past what seems like half throttle and will perfectly stay there no backfiring, the plug has been changed by it looks a little rich. What kind of air pressure do think it might take to simulate the vacuum to check that operation I did not know you could do that.
Expert:  RSRBOB replied 9 years ago.
It doesn't take much pressure. I would say 5 or 10 psi should do it.
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