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Discussion Starter · #1 ·
New to the site and need help on the Rancher 350 TRX350 FM, 2003. Just bought the bike, will start quickly but will only idle. Any throttle will get the carb diaphram/piston bouncing and very little response on speed. Should I expect so much up and down on the diaphram? Cleaned the inside, all jets etc. Moved the main needle up and down with no change.

Thank you Marten
 

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Discussion Starter · #3 ·
Thank you. Please review these additional comments. I can see the fuel spraying from the needle/main jet. The obvious issue is the bouncing of the needle body as I open the throttle. It wants to speed up but the fact that the valve body is jumping up and down with the speed of the motor, makes it impossible to dispense an even amount of fuel and that seems to be the problem. I would assume that the valve body should be more stable as you speed up or slow down. The carb, jets etc are clean. I spray cleaning fluid through each jet to make sure that the liquid it is getting into the main throat. When I hold the valve body to stop the up & down, the motor does respond much better, but not perfect.
 

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The slide is lifted by vacuum - on a single cylinder engine the vacuum comes as pulses and the "bouncing" is normal, but at a steady throttle setting at elevated RPM it should lift to a more steady position. It's called a CONSTANT VELOCITY carburetor. The bouncing of the slide is the carburetor's attempt to keep the flow of air through the venturi at a constant speed.

The diaphragm that controls the slide is rubber. Spray carb cleaners can melt the rubber (as can ethanol blended fuel). You might want to take the top off the carb and inspect the diaphragm. A quick test: with the engine is not running - raise the slide with your finger and then release it - if the diaphragm is good, there will be a slight resistance (more than just the spring pushing down) when you lift it and you will hear the escape of air within the carb, Then when you let go, the fall will be slowed by vacuum - not just an instant drop to the bottom of the venturi.

Check it out - if the diaphragm is bad - a new slide is about $68 from Honda
 

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Discussion Starter · #5 ·
Great feedback. The diaphram is supposidly newish, looked good, black with no obvious cracks. I will carry out your test tomorrow. I might try to extend the spring a bit to give it more resistance in an attempt to reduce the bounce. The previous owner had installed the spring on the bottom of the plunger into the throat, no BS. That may have compressed the spring.
 

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stretching the spring will not help - the spring installed as you describe would have held the slide nearly all the way up at all times - if it did not hold it up then it's either not the right spring or damaged and needs replaced. It has a calibrated tension - it would be a challenge to stretch it and get it to work right. Just buy a new one - part number 16050-HM5-L00 - about $8.50.

Also, I would suspect other things not right in the carb. Look at a parts breakdown (here's a link to one) to make sure all the parts are there and in the right order.
 
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