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Discussion Starter · #1 ·
I have a 96 fourtrax that I just purchased. I am trying to adjust the mixture but have found that whether i run the mixture screw in all the way (lean) or all the way out(rich), it has no effect on how the motor runs. It always appear be running rich.
Spark plug is black and there is black smoke coming out of the exhaust.

Any ideas?
 

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The pilot jet is too large, the float level too high or the air passage from the front of the carb to the fuel/air mix chamber is clogged. I would recommend a complete inspection of the carb and parts for damage and/or modification and a through cleaning of the carb.

I just worked on one that the complaint was it would not idle below 3000 RPM - found that a shade-tree mechanic had removed the throttle butterfly from the shaft and did not install it correctly. It was binding against the throat of the venturi preventing full closure and holding the idle up. The idle screw was not even touching the linkage.
 

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Discussion Starter · #3 ·
The pilot jet is too large, the float level too high or the air passage from the front of the carb to the fuel/air mix chamber is clogged. I would recommend a complete inspection of the carb and parts for damage and/or modification and a through cleaning of the carb.

I just worked on one that the complaint was it would not idle below 3000 RPM - found that a shade-tree mechanic had removed the throttle butterfly from the shaft and did not install it correctly. It was binding against the throat of the venturi preventing full closure and holding the idle up. The idle screw was not even touching the linkage.
Thanks. This is a brand new carb according to the guy I bought it from. I took it apart yesterday and from the looks of it I would agree that it is new. Didn't see any debris in there but I will inspect it. Is it possible the guy ordered the wrong carburetor for it. Everything fits but maybe he got one for a larger engine and it's jetted different?
 

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It's kinda unusual for someone to buy a new carb. They are expensive. If your TRX is an EX model, the carb will be marked QB02B B - if it's a plain TRX300 or 4x4 it will be marked VE91D A - a 4x4 FOREMAN model, the carb is marked VE92C C and the prices are $360, $337 and $409 respectively

Carbs are jetted for the amount of air they flow, not the size of the engine they go on, but the jetting could be all wrong for the intended use. Perhaps it is jetted for alcohol or other fuel.

Tell me more about the carb - why did the guy buy it, where did he buy it, what brand name is it and what size is the venturi?
 

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Discussion Starter · #5 ·
I don't know why he bought the carb although the question was in my mind. At the time I bought it, it was running fine so I didn't think too much about it.
As for the other questions, I have to carb off the bike at the moment so when I get home, I will post up that info.

How can I determine the size of the venturi?
 

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OK - it appears to be a Chinese universal carb (about a 30 to 32mm). It can work, but it's up to you to get the jetting right. Just changing the jets to the same size as Honda recommends is not the cure, because the flow characteristics are unique to each design. It's possible (due to poor quality control) the carb has failed internally in some way and needs replaced. I recommend trying to find a proper OEM carb - new or used.

BTW - what is the VIN of the vehicle?
 

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Discussion Starter · #12 ·
OK - it appears to be a Chinese universal carb (about a 30 to 32mm). It can work, but it's up to you to get the jetting right. Just changing the jets to the same size as Honda recommends is not the cure, because the flow characteristics are unique to each design. It's possible (due to poor quality control) the carb has failed internally in some way and needs replaced. I recommend trying to find a proper OEM carb - new or used.

BTW - what is the VIN of the vehicle?
Not sure of the VIN. I'll have to check when I get home.

FYI, I contacted the PO and he said he still had the original carb. Not sure he's willing to part with it yet though and not sure why he changed it in the first place.
 

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Carb kits don't usually fix them. It needs to be taken apart, inspected, everything chemically cleaned, then inspected and any defective parts replaced. You only need to replace the bad parts, not just throw new parts at it. Sometimes putting in new parts won't fix the problem if it's not a replaceable part that is causing a problem.

You can buy carb kits from Dennis Kirk, Rocky Mountain ATV or EBAY
 

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Discussion Starter · #16 ·
Carb kits don't usually fix them. It needs to be taken apart, inspected, everything chemically cleaned, then inspected and any defective parts replaced. You only need to replace the bad parts, not just throw new parts at it. Sometimes putting in new parts won't fix the problem if it's not a replaceable part that is causing a problem.

You can buy carb kits from Dennis Kirk, Rocky Mountain ATV or EBAY
Ok, thanks. BTW, can I run Mobil1 synthetic in this thing?
 

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Discussion Starter · #18 ·
I picked up the original carb from the po last night. I completely disassembled it and with a few parts from the new knock up carb (bowl gasket and primer pump) got it reassembled after a thourough cleaning. It started right up and ran much better. I am now able to adjust the mixture down to where it dies (lean). There's a couple of things that still isnt right though. My understanding is that when adjusting the idle, turning clockwise runs it lean and CC richens it. I started with it 2 1/2 turns cc from seated and it run pretty well there. I expected it to die or run poorly when I lean it out from there which it did. I also understand that there should be a point where richening it will make it run poorly as well so the idea is to find the sweet spot in the middle. However, what i observed was that as I richened the mixture (CC) the idle got continually faster. It never began to run poorly. Any advice on this issue?
 

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You might have a vacuum leak at the intake manifold in which case more fuel is better, however, I would not exceed 4 full turns from lightly seated. You could have a partially clogged pilot jet which may clean itself over time and if it runs well at all other speeds, I wouldn't worry about finding the "sweetest spot".

Congratulations on solving a problem
 

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Discussion Starter · #20 ·
You might have a vacuum leak at the intake manifold in which case more fuel is better, however, I would not exceed 4 full turns from lightly seated. You could have a partially clogged pilot jet which may clean itself over time and if it runs well at all other speeds, I wouldn't worry about finding the "sweetest spot".

Congratulations on solving a problem
Thanks for all your help.
 
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