Fourtrax 300 sputter when giving gas - Honda TRX Forum
 
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post #1 of 5 Old 01-14-2019, 10:29 PM Thread Starter
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Fourtrax 300 sputter when giving gas

I have a 1997 fourtrax 300 I'm working on building it has a brand new carb on it it will start up fine with no problems idles great and runs good when ur in the throttle but if it is sitting and idling and u rev it up it sputters when you first give it gas then it's fine same way if your driving it I've looked on a thousand different forums and everything says clean the carb but I know that's not my problem really hoping someone on here can point me in the right direction, thanks.
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post #2 of 5 Old 01-15-2019, 02:00 PM
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When you open the throttle, you ARE giving it gas, but it's the wrong kind of gas. Air is a gas, but it won't burn. What you desire is for the carb to mix the correct amount of COMBUSTIBLE gas the air that you allow into the engine to make it perform. If you understand the operation then it makes it easier to solve the problem. As you open the throttle and allow more air into the engine, several things happen at the same time.

With that being said; there were 3 TRX300's produced 97 - the TRX300 and the TRX300FW used a VE91C A carb and the TRX300EX used a QB02A A carb

I'm going to presume you do not have a 300EX as the EX carb had an accelerator pump to get you past the stumble inherent to the VE91 carb - the VE91 is a CV carb without an accelerator pump, so you need to figure out if the sputter is caused by being too rich or too lean then you can start addressing the cause.

With the CV carb - as you open the throttle the velocity of the air passing through the venturi slows and there is an accompanying drop in vacuum - it is the design of the CV carb to attempt to keep the velocity of air 'constant' - this is achieved by using the loss of vacuum to lower the slide to increase the velocity of the air passing through the venturi - consequently, as the slide lowers, the slide needle reduces the amount of fuel allowed into the air stream. If the slide falls too quickly, the fuel decreases rapidly and too little fuel is mixed with the air and the engine misfires due to too little fuel - this is usually accompanied by a backfire through intake. If the slide does not fall fast enough, the velocity of the air in the venturi slows and too much fuel is mixed with the air. The engine loses power and 'blubbers' until the mixture corrects itself and starts running properly again.

If you have a 300EX and it's sputtering just off idle, it is usually caused by a maladjusted accelerator pump linkage or a faulty check valve in the float bowl accelerator pump fuel passage.

There have been entire books written addressing the problems of each style of carb. Please consider what I have put forth here and reveal what kind of carb you are working with, what model TRX you have and what type of fuel you are using (brand and grade).
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post #3 of 5 Old 01-15-2019, 02:23 PM Thread Starter
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It's a trx300fw not sure exactly what carb it is I'm pretty new to this but it's a caltric carb that it supposed to be for my fourwheeler and I'm just running regular 87 pump gas
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post #4 of 5 Old 01-15-2019, 08:48 PM
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OK - the recommended fuel is 86 or higher octane - if using ethanol blended fuel use at minimum 2 octane points higher - you should use at least 88 to 89 octane if blended with ethanol. Higher octane is OK , but offers no performance advantage. I recommend using non-ethanol fuel to avoid the problems associated with the corrosiveness of alcohol. And the fuel needs to be fresh - as fuel ages it's volatility decreases. Ethanol fuel should be discarded after 60 to 90 days - non oxygenated fuel may last up to a year in a fuel tank before degrading into 'kerosene'.

All adjustments must be made with fresh fuel, clean air filter properly oiled and properly installed (if a K&N filter is used or 'performance' modifications are made to the air box, complete carburetor re-calibration is required). The valves should be correctly adjusted and the spark plug should be new. The air box should have the lid on it with the intake tube in place. The engine should be at normal operating temp - run the engine (not just sitting and idling) for at least 15 minutes. Adjust the idle mixture screw for the smoothest idle, if the engine still stumbles coming off idle while riding (not just standing stationary), adjust the fuel screw counter clockwise in 1/4 turn increments to attempt to eliminate the bog. If the fuel screw is turned more than 3/4 turn from it's smoothest idle setting, the pilot jet needs to be increased one size or the slide needle need to be raised one notch or both.

This should get you started
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post #5 of 5 Old 01-15-2019, 10:10 PM Thread Starter
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Thank you for the help I will try what u suggested.
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