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Discussion Starter · #1 · (Edited)
Hoping you guys can help me figure this out.

71 CB 350 Twin

The bike runs excellent, starts with one kick (needs choke for a few seconds on cold mornings, but no choke on warmer days). Idles at around 800 rpm and super smooth.

BUT, after riding it for a while, if I come to a stoplight and put in neutral and take my hands off the bars, it will idle fine for maybe 15-30 seconds, and then slowly the RPMs drop, even to the point where it will die unless I throttle it up. When I do throttle it up, response is immediate (no hesitation or lag). Once I get driving again, performance is fine, and maybe the next stoplight it won't do it. But then the next light it might do it again.

It seems to be worse (more often) when the gas tank is full. I rebuilt the carbs a few months ago, and they're fine, and this didn't really start until I replaced the inside of the gas cap (the spring-loaded plunger and seal) with one I took from a spare tank I bought. The old one definitely had a deteriorated rubber seal (so it would have been constantly venting). Now that I have a better seal on the tank cap, I definitely hear an "eeeeep" sound coming from the cap, and I can hear/feel slight pressure release when I open the cap.

The "new" cap plunger is from the same year/model, and does have a vent hole that appears to be clear (see photo).

So, I'm trying to work the science here. Would a pressurized tank cause a strangled idle? Should the tank be vented?

EDIT: Things I've already thought of:
If the pressure in the tank was causing any kind of fuel starvation (empty float bowls), I don't think throttle response would be so immediate (if the float bowls are empty, attempting to throttle it up would cause it to die, not recover)

It does not appear to be electrical-related. If I turn off the headlight there is absolutely no change in idle (I thought maybe the old rectifier and notoriously poor charging might be weakening the spark). But if that were true, I would expect that turning off the headlight would produce some slight improvement in idle (even when it's not beng strangled)




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Yes the tank is vented, sounds like the replacement cap vent is clogged.
 

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^^^Yeah, I think the "eeep" sound you're hearing is suction released and not pressure - I agree with the vent being at least partially clogged, which would affect flow and possibly worse when there is less air space in the tank
 

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A pressure buildup in the tank would cause a higher float level( and a richer mixture), it wouldn't empty the float bowls.
 

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I agree with the VENT is CLOGGED and NOT Venting and what you hear is a Vacuum Release NOT Pressure.

The Needle & Seats on these bikes are known to leak just from gravity pressure when the motor is not running.
(Why we ALWAYS recommend you turn the Petcock - OFF)

Pressure would cause the carbs to FLOOD and run rich by dumping fuel past the Float Needle & Seat.

Vacuum will Starve the system of fuel.
 
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