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Vapor Lock ???


Veteran Member
Jun 24, 2020
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Hey folks-.
SO my 72 CB350 runs great for about 20-30 minutes when all of a sudden it starts sputtering and then cutting completely out. Fortunately, I'm able to get it started up quickly but then 5 to 10 minutes later it starts doing it again. The tank is clean, it has a new fuel petcock, new fuel line and filters. I run ethanol free gas in it along with some seafoam additive. It seems to me that it does this once the engine gets nice and hot and am wondering if it could possibly be having vapor locking issues as it runs great for the first 20-30 minutes. I don't ride on cold days so I have no idea if the outside air temp has anything to do with it, I'm only riding on warm days.
Also it has new Mikuni VM30 carbs on it with less than 300 miles on them and the carbs were set up by a long time Honda mechanic who is quite familiar with the Mikunis .
Any thoughts or help any one could offer are greatly appreciated.
Did you try opening the gas cap right after it happens to see if it fires right back up? I'm wondering if the vent in the cap is clogged a bit.
Hey Tom, thanks for the reply.
It also has a brand new gas cap .
Could still be the gas cap... have a look here for an example where a brand new gas cap includes a piece of wadding by design that effectively causes this problem.

If the tank is clean and the petcock is new, I wouldn't think you should even need inline fuel filters. They're just one more potential problem in the fuel delivery system. Or did you mean new air filters?
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How do you get behind the vent? My cap has a screw through the middle of the plate ( for lack of a better explanation) and seems to have spring tension behind it and the screw won't budge even if you relieve the tension behind it.
Oops, can't seem to load the pics of the cap from my phone.
I bought a similar cap from 4-into-1 and removed the wadding. The screw was fairly hard to get started, so I would just make sure your screwdriver is grabbing in the head well and give it some oomph. A hand impact could be used if necessary, but put something under the cap to protect the finish.


You may need to remove the rubber gasket to get at the wadding — it's been long enough that I don't recall exactly where it was.
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Oops, can't seem to load the pics of the cap from my phone.
Are you still using Tapatalk? If so, you might stop using it and choose "Desktop site" in the 3 dot Chrome menu, others here are using the new (and apparently way better) mobile version of the new software.
It appears to be the same style that we've seen from 4-into-1, which have included the wadding. You can see my picture above with the plates removed.
I really hope so.
Haven't tested anything yet but will in the next few days .
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