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Discussion Starter #1
Hey guys, I'm having a problem with my 04 polaris predator 500. It will start up and idle great, but then it it gets to operating temp./ warmed up and idles up extremely high so I run over to it and shut it off. When I am riding it and pull in the clutch it does the same, idles up extremely high. The only way I can control this is having a load on the engine such as applying the brake when it is in gear to keep the rpms were they should be. And no, it is not the idle screw. I swapped carbs from a parts four wheeler and it does the same thing. I also saw the thread when a dude had the same problem and it was his throttle cable. I have yet to get the new one in to see if that is the problem. Leaning towards a vacuum leak but I've coated that thing with carb cleaner and couldn't find anything. Any help would be appreciated!
 

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Idle speed is controlled by spark timing and throttle opening. A vacuum leak is an uncontrolled source of air. The fuel mixture has to be rich enough that a vacuum leak only results in an increase in idle speed. If the leak is minor enough to raise the idle speed without affecting performance in other RPM's, it is easily found using soapy water to spray around the carb holder where the carb pushes into it and where it seats to the cyl head.

Other causes of uncontrolled idle; most common - not enough slack in or sticky throttle cable, not enough slack in or sticking enrichener cable, dirt, corrosion or debris preventing the throttle butterfly from closing against the throttle stop screw (idle speed screw) - less common - a perforated air cut valve diaphragm, high float level, throttle butterfly that has loosened and not closing completely due to binding in the venturi or damaged butterfly shaft, missing or incorrect size air jet in the bell of the carb - more obscure - a bent slide needle or damaged pilot screw orifice.

Good luck - it takes a good eye to see some of the less common to obscure causes - vacuum leaks are easy
 

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Discussion Starter #3
Yep that is the problem I am having, no problem when I get on it at high rpms when, for say, I max it out still runs fine, but I mean when it starts to idle up uncontrollably it flat gets it. It doesnt just go up a little bit it goes up to about 3/4 the throttle being pushed in. I will start with the intake connecting the carb to cylinder head. Is this what you would recommend?
 

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Discussion Starter #4
Well guys I replaced the rubber intake that connects the carb from the cylinder and it still have the problem. What should I try next or a direction I should head with it.
 

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Discussion Starter #6
Yes, I replaced intake flange, and tried using a different carburetor from a donor bike I have, the problem persisted so i tried to find a vacuum leak with soapy water and could not find a single one. I have also went through the carb many times, I am at a complete loss.
 

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Discussion Starter #8
Well late bird that wasnt it either unfortunately. It was not sticking. Also when I was doing this I looked down the barrel of the carb and the slide was moving up and down with use of the throttle. Have I missed something?
 

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Obviously!

You failed to identify the carb. The original carb was a constant velocity - there is no connection between the butterfly air control valve and the vacuum operated slide.

I now suspect you have a other than original carb. Are there identifying marks like a brand name - maybe you could attach some pics of the carb?
 

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Discussion Starter #10
Well I cleaned the carb that was on it. Everything looked slick and span and perfect condition even when taken apart. So I have a parts quad, cleaned its carb and slapped it on and it had the same problem, has anyone you have heard of had this problem and the reason were the valves somehow?
 

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I guess I misread your post or drew the wrong conclusion - when you were looking into the carb and watching the slide rise - I imagined this was happening with the engine NOT running - now I gather this was happening with the engine running? The slide will rise with an increase in the velocity of air passing through the venturi. Air is controlled by the butterfly valve. If the butterfly valve is completely closed, the engine has to be getting air from somewhere else and that air has to have a sufficient amount of fuel mixed with it to support combustion. I suggest you carefully inspect the enrichener and verify it is closing completely. Perhaps remove the enrichener plunger from the cable, inspect the rubber seal for damage and put it in it's port with just the spring and cap to hold it closed. Then see how the engine runs.

I've never seen valves cause this type of problem, cam timing might possibly do it and spark timing will increase engine speed, but that is provided the carb is functioning properly to begin with.

I'm at a loss without having it in front of me, but I'm 100% sure I could get it right if I could get my hands on it.
 

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Discussion Starter #13
So to answer your question,yes, it was when the engine was running. Slide was moving upwards as I used the actual throttle, but will not move when it is revving up on it's own. This is very confusing to me, when the engine is operating temperature it revs up on it's own. For example I'd be getting ready to go just drive around and go to grab something from my garage and it will idle up on it's own. while Just trying to warm up the bike. My assumption is I somehow missed a vacuum leak, because I've been in the carburetor so many times I could do it in my sleep, it looks mint. Also even if it is not operating temperature and I move the throttle in the carburetor with the throttle cable disconnected it will idle up until I shut it off then I can turn it back on then it will return to normal idle only when the engine is not operating temp. It will resume high idle if it is operating temperature. Maybe there is a missing peice of info that above that will make it an easy solution or at least an idea of what it is.
 

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I'm going to ramble here - it's part of deducing the problem - got to cover all the possibilities.

The laws of physics govern how or if an engine runs - it takes air and fuel - together and in the right amounts - control the air fuel mixture and you control the speed the engine runs at. For the engine to accelerate, it has to get more air mixed with the correct amount of fuel - where is this source of air? A vacuum leak may increase the engine speed a little, but without mixing fuel at the point of the leak, the engine will also surge and usually spark knock because the leak causes the fuel mixture to become lean.

When looking in the carb, you saw the slide rise as the engine speed increased. This is normal - technically, the operation of the carb is this: the butterfly valve is the primary control of the air/fuel mixture allowed into the combustion chamber. The slide and slide needle is a secondary control of the air/fuel mixture. When the engine is running at a constant speed, the vacuum controlled slide rises to a position dependent on the velocity of air through the carb. As an example, let's say the butterfly valve is half open and the slide is also half open - if the throttle is opened gradually, the slide also rises gradually as the engine speed increases. As the slide rises, more air is allowed into the engine and the needle in the slide is withdrawn from the needle jet which meters more fuel being mixed with the increased volume of air which is now traveling at a higher velocity. However, if the throttle butterfly is opened suddenly from the 1/2 position to the wide open position, the vacuum is reduced and the velocity of the air passing through the venturi slows - the slide drops, partially blocking the air flow and reducing the amount of fuel mixed with the air. The engine does not slow down because the primary air control is open and the slide has a stop set at about the 1/4 open position. With no load on the engine and the primary air control wide open, the engine will rev to near maximum RPM. If you want to test this - remove the spring from the slide and replace it with a piece of material that will prevent the slide from rising. Start the engine and open the throttle slowly to find out how high the engine will rev. When the engine fails to increase RPM, from that point and on changes in throttle position will not affect engine speed. NOTE: when shutting the throttle from wide open to the normal idle setting in this exercise the engine may stumble or die due to the fuel air mixture being overly rich upon the sudden closure if the primary air control. If you close the throttle slowly, it should return to normal idle.

Now, the engine will not accelerate if it is only given more air or more fuel - it has to get a proper ratio of air to fuel. If it is only given more of one or the other the performance will suffer and the engine will either increase it's speed only slightly or die.

Therefore, where is the engine getting the increased air/fuel and why? Answer these questions and the engine will perform as designed. Cam timing, spark timing, compression, grade and quality of fuel, jetting, air density and engine temperature only affect idle speed a few thousand RPM at most. The properly performing spark plug has a less than 1% effect on engine idle speed, acceleration or maximum RPM. 98% of idle speed is controlled by the throttle.

Definition of throttle; Definition of throttle - transitive verb -Definition of throttle - transitive verb

1 a (1) : to compress the throat of : CHOKE
(2) : to kill by such action
b : to prevent or check expression or activity of : SUPPRESS

2 a : to decrease the flow of (something, such as steam or fuel to an engine) by a valve
b : to regulate and especially to reduce the speed of (something, such as an engine) by such means
c : to vary the thrust of (a rocket engine) during flight

I did experience 'run-away' acceleration on an old Kawasaki 2 stroke motorcycle. It was a heat related issue similar to what you have described. The engine was idling normally - already hot from 1/2 an hour of riding on a 90 degree day - sitting at a stoplight, suddenly the engine started to accelerate. I immediately hit the kill switch shutting off spark to the plug - it continued to run (dieseling) - I turned the throttle wide open - no change. I locked both brakes and let the clutch out slowly to kill the engine. After the bike cooled for about 20 minutes, it started and ran normally again. 2 stroke gasoline engines were derived from diesel engine design, so dieseling is not so uncommon.

In a 4 stroke engine, runaway acceleration is always attributed to throttle position failure - I could find no documented cases of runaway acceleration in gasoline engines where the carburetor or throttle body was not the culprit.

Now I suppose if you had a fuel that was supplying oxygen as well as hydrogen (water) (it's the hydrogen in gasoline that burns - the rest of whatever comprises gasoline ends up being pollution). Alcohol absorbs water - have you drained the fuel tank and tried a fresh fill of non-ethanol 91 octane or higher gasoline?

Perhaps under ideal conditions a blend of water contaminated alcohol and gasoline in a hot engine........

If you were willing - I would like to examine one of your carbs. If you are conducive to this idea, let me know. I'll PM you with an address and instructions. I'll even pay the shipping.
 

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Discussion Starter #15
Okay I'll do as you said tonight and check out the carb and remove the spring and test it,and double check for air leaks. I'll check back in, in a few hours.
 

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Don't forget to install something to prevent the slide from rising - might just be a paper tube made from a toilet paper roll cut to length and split so it can be reduced in diameter.
 

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Discussion Starter #17
So I did the test and it still had the issue, I'm going to buy a brand new OEM intake flange,clamp, and bolts and see if that is the issue.
 

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Discussion Starter #18
I took your advice and got a full carburetor rebuild kit with the new slide and all. It fixed the problem, thanks.
 
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