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2002 Vinson 500
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Discussion Starter · #1 ·
Hi, every one! I'm a new member from California. I owned a used 93 kingquad 300, old but unbeatable. I maintained and fixed all the issues myself and it's going strong! I recently purchased a used 2002 vinson 500. Low mileages but the previous owner used to ride it in mud and swamps. So the first thing I did was cleaning all the wire connections and that fixed the 4x4 engage issue so as the poor ground causing poor spark.
Last week, I did a carb clean and rebuild, followed all the steps from the service manual included readjust the fuel mixture. But I encountered an issue after the rebuild, the atv will start right up and idles fine, but after I to push the throttle the engine started to bog in the midrange, and after the midrange the engine runs normal again. So the engine only bogs when the throttle is at midrange. I rebuild my 93 kingquad carb before and never have this issue. I took the carb off and cleaned it 5 more times with carb cleaner and compress air. Made sure all the passages are clear but the problem still there. Sorry to post a problem as my new member opening thread. Many thanks!!
Wheel Tire Vehicle Plant Automotive tire
 

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2002 Vinson 500
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6 Posts
Discussion Starter · #3 ·
I hope you don't go in mud with those tires.
This atv came with ITP mud lite tires, but our property road are mostly paved, so I changed to street tires for smoother ride also save the mud tires for future fun.
 

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2020 KingQuad 750
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116 Posts
If its running good at idle and up to 1/4 throttle, then the pilot jet is probably good. If it runs well after 3/4 throttle, then the main jet is probably good. If it's bogging in the middle, it could be the needle jet .... thinking back to my dirt bike carb. Either the needle jet or the needle itself isn't in the same position that it was. Just guessing here.
 

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2002 Vinson 500
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Discussion Starter · #5 · (Edited)
If its running good at idle and up to 1/4 throttle, then the pilot jet is probably good. If it runs well after 3/4 throttle, then the main jet is probably good. If it's bogging in the middle, it could be the needle jet .... thinking back to my dirt bike carb. Either the needle jet or the needle itself isn't in the same position that it was. Just guessing here.
You mean the float that controls the needle jet right? I adjusted the float height according to the service manual (17mm) with Venier caliper. Yea, the float height will affect the air/fuel mixture and if the needle jet not close properly will cause engine flood.
 

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2020 KingQuad 750
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116 Posts
So when I rebuilt the carb on my Yami WR400 (Keihin FCR39), the kit I used came with 3 new individual jets that screw in at different places and different levels in the body. Last time I had that carb off was like 10 years ago so I'm going from memory. My understanding is that each one of these actual jets control different areas of the throttle.... idle to mid-range to WOT. Your problem with your throttle sounds to me like you still have a clogged jet. I do know that even if they look clean and you can push a thin wire thru them, they still may be clogged. I dont know if yours were actually replaced with new or were just cleaned. Sorry I cant be more helpful. Its been a while since I was inside a carb.
 

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2002 Vinson 500
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Discussion Starter · #8 ·
So when I rebuilt the carb on my Yami WR400 (Keihin FCR39), the kit I used came with 3 new individual jets that screw in at different places and different levels in the body. Last time I had that carb off was like 10 years ago so I'm going from memory. My understanding is that each one of these actual jets control different areas of the throttle.... idle to mid-range to WOT. Your problem with your throttle sounds to me like you still have a clogged jet. I do know that even if they look clean and you can push a thin wire thru them, they still may be clogged. I dont know if yours were actually replaced with new or were just cleaned. Sorry I cant be more helpful. Its been a while since I was inside a carb.
Thanks for the help!! Yes, I replaced all the jets with a brand new kit. and the problem happened right after. Now I kind of suspect the problem might be from the pilot crew, because the previous owner set it to about 1 turn out, but the factory preset is 1 7/8 turn out (and I set it back to factory). There might a possibility that the pilot crew hole is somehow "enlarged" when the old owner rebuild the carb. So if I set it back to factory will be actually too "rich" for the mixture. The pilot screw washer was missing and the o-ring was torn, that's why I got a new rebuild kit. This ATV's engine was running ok when I first bought it, the only issue was the choke needs be on a long time for every cold start. Let me pull the spark plug out and check. .
 

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2002 Vinson 500
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Discussion Starter · #9 ·
I took the spark plug out today and it's black, so the carb is running rich. Tried to tune the pilot screw and found that it doesn't kill the engine when the screw is fully close. I was right, the previous owner did "enlarge" the pilot screw hole when he rebuild it, probably over tighten it. Now I need to find a "leaner" pilot for my carb. The carb is Keihin CVK36. Does any one know what website to find a "leaner" pilot?
 
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