CB350 dying mid ride and can't idle.

figliolio

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May 9, 2023
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Location
Philadelphia, PA, US
Hello.
In need of some help diagnosing my bikes issues maintaining idle during rides. Initial startup is normal (relative to my bike) but after riding it'll die if I'm not giving a little bit of throttle constantly. Then, when I tried to limp it home, after starting it I can't accelerate or it'll die again after some coughing.
Am I loosing compression somewhere? Is this a sparkplug or coil issue related to the 4-stroke sequence? A fuel issue related to the carbs?
I don't know where to start testing to narrow down the problem.
Any help appreciated!
Thank you.
 
Try loosening the gas cap when it bogs down. A plugged vent could be preventing adequate fuel flow to the carbs.
 
The gas cap has a vent hole in it so air can enter the tank as fuel flows out of the tank to the carbs, if it didn't have a vent the fuel flow would stop eventually because of a vacuum built up in the tank.
 
As J-T suggested and ancientdad explained, opening the gas cap will allow the pressure in the tank to equalize with the atmospheric pressure and that will allow gravity to pull fuel down to the carbs. If this is your issue, opening and closing the cap should allow you ride again without issues until the vacuum builds up in the tank again. That won't be very far, but you shouldn't get too far from home with this issue unresolved.

If the gas cap is not the culprit, it could also be poor fuel flow due to clogged inline fuel filters or a buildup of gunk in the petcock itself. I've experienced it myself with a clogged fuel filter. Another possibility, less likely based on your description, would be improper float height in the carburetors. If the float height is too high, the supply of fuel in the carburetor might not keep up with demand, especially when giving a lot of throttle. In any of these cases, trying to accelerate as slowly as possible is your best bet for limping it home.

You've had the bike about a month or so based on your intro. This issue developed only recently, though? May be worth checking the condition of the tank inside. If it's showing lots of rust, it could explain blockages in the petcock, inline filters, or in the carb jets.
 
It seems that the issue was indeed an aftermarket gas cap that I am using. The tank is clean and has a coating inside that I can't recall the name of but it's white and bumpy. I did a water rinse, then used marvel mystery oil and rinsed with gas before installing the gas tank and running fuel. I've been having some issues with my throttle cable which I think is playing into the inconsistent idle... on top of the vacuum. I'll be making a separate post about this shortly. Thank you!
 
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