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BSA B25/Triumph T25 #780641 08/05/19 9:07 pm
Joined: Feb 2007
Posts: 785
E
edunham Offline OP
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I was having a problem with my little hot rod unit single which Ed V has laid his hands on. It would start right up cold, but not when it was warm and it wouldn't idle. Runs great otherwise. It would go down to 2000 rpm or so, hang up, then drop to 1000 and die. If I adjusted it, it would either idle at 2500 or not at all.
I cleaned out the carb and used my .078 drill bit to no avail. My buddy ran it through his ultrasonic cleaner. No difference. Went through my ignition. Ignition advance was not sticking. Reset points gap and timing. Barely noticeable difference. I use grease on the o-ring, so pretty sure there is no air leak at the manifold.
Took carb off again. Noticed that the bowl was wet. Screws were tight. Took bowl off. Gasket was dry on the bowl side, but wet on the top side on one of the carb. Eureka moment! The bowl is warped and the carb is sucking just enough air to cause the problem at idle, but not enough to cause a problem the rest of the time. Starting is an issue because I have screwed up the adjustments trying to compensate.
I have heard of this happening before, but never personally experienced it before. Carb has 6000 miles on it and the rest of the engine is all rebuilt, so I have ordered a new Premier. I'll fix the warped bowl as a spare.

Ed from NJ

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Re: BSA B25/Triumph T25 [Re: edunham] #780695 08/06/19 6:54 am
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Allan Gill Offline
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Good find, I’ve found this myself on one of my carbs, comes from being over tightened but to be fair to you and myself i never had the impression that I had over tightened it


beerchug
Re: BSA B25/Triumph T25 [Re: edunham] #781303 08/12/19 7:24 pm
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edunham Offline OP
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The new carb had not arrived yet, so I thought I would take a shot at fixing the old one. I took the bowl and gave it a couple of figure 8s across some sand paper on a flat piece of granite. The low spot was immediately apparent. there was an area about an inch long, where better than 3/4s of the gasket surface was untouched. Holding the bowl sideways, it almost look like someone had carved a little slice of the gasket surface off with a knife. I had an old float bowl that was pretty straight. Cleaned it up, set the float level, and the bike started right up. Took it for a 5-6 mile jaunt and it ran fine.
The mystery is why there wasn't a problem with the carb before. The bike wasn't running for some time while Ed V rebuilt the lower end. I may have freshened up the carb at that point. I think there may have been a thicker bowl gasket in there.
Anyway, the new carb will still go on the bike when it gets here. This one will go for a trial run on the Parilla. That SS-A is a pain in the rear!

Ed from NJ


Moderated by  Allan Gill, Jon W. Whitley 

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