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Discussion Starter · #1 ·
The aforementioned BSA has all new piston, rings, bore. It has a new Pazon ignition, Podtronics reg/rect, new battery, etc. It was stored dry for about a year and a half, started on the first kick. Problem: Once it is nice and warm it will cheerfully idle for about 30 seconds to 2 minutes and suddenly lose idle rpms and stall. If you stand there and feather the throttle it will wake back up and idle once again...for about 30 seconds to 2 minutes.

The electrical system happily charges the brand new battery at 14 volts and there is no dimming of the fully functional lighting system during the stall. The Pazon ignition (which I REALLY like) is sold on the premise that it will run on crappy voltages normally associated with the crappy charging systems of old British motorcycles. I can find no other problems with the bike.

I removed the carb and cleaned it thoroughly. I made absolutely sure that the pilot circuitry was clean and clear and made sure that carb cleaner flowed through the outlet holes in carb body bore. The fuel screw responds to input insofar as screwing it in or out raises or lowers rpm corresponding to leanness or richness. The idle speed screw is secure in it's bore and is not vibrating out of place. Slide and needle shows normal wear and nothing that I would consider loose in it's bore (could be wrong though). It pulls strongly through the rpm range too.

So.....putt, putt, putt, putt, putt, for about 30 seconds to 2 minutes, then putt.....putt.....putt....putt.....then dies. Any great ideas? ThxIA.
272768
 

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well, if you're not going to ride it i wouldn't worry about it cutting out after two minutes.
Otherwise I would be looking at fuel starvation from air lock, collapsed fuel line, vibration affecting carb' float / needle.
 

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Very common issue with these old thumpers. Get an Amal premier carb.
 

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Have you got a heat insulation spacer between the head and carb? Overheating the carb causing fuel evaporation can be a problem.
 

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I doubt that from a cold-start the carburetor could become overheated in 2 min. the OP claimed it runs before developing issues. It sure sounds like a fuel delivery problem. I think if it was an electrical issue, it would die-suddenly, and not re-start until it cooled-down enough to allow the open circuit to become intact/conductive again. The way it starts to peter-out sounds like more of a fuel-delivery problem. The suggestion of an Amal Premier carb may help, or maybe a Mikuni of similar capacity/size and jetting.
 

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Discussion Starter · #11 ·
The bike has ran well in the past and did not have this problem. Therefore, throwing $300 at a new carburetor does not make sense. If it is indeed the carburetor then it should be able to be fixed. As far as it being a fuel problem: The insulator between the carb and the intake is present. If there was a problem with fuel restriction because the tank was not properly vented, it would be more likely to manifest itself during normal highway operation, not idle.

I ordered a new coil from the Bonneville shop. I believe that it is a reproduction part ($35), can’t be any worse than OEM. I’m not going to put a lot of faith in it as a fix but at least it will start the process of elimination.
 

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Could it be a blown head gasket? I had idle issues with my Bonneville, but only when it was hot. Some times it would idle low and stall and other times it would idle high. After I changed the head gasket it was fixed.
My Daytona on the other hand with its totally clapped out Amals and crap in the fuel tank (but with a gastight head) would idle low and smooth all day long.
 

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Discussion Starter · #14 ·
Could it be a blown head gasket? I had idle issues with my Bonneville, but only when it was hot. Some times it would idle low and stall and other times it would idle high. After I changed the head gasket it was fixed.
My Daytona on the other hand with its totally clapped out Amals and crap in the fuel tank (but with a gastight head) would idle low and smooth all day long.
I switched out the coil. Although I'm sure the old coil was inefficient and could have gone south at any time, it did not change anything. However I did get it nice and warm and sprayed brake clean on the area around the carb/manifold and joint where the head gasket is. It was hard to pinpoint but there was some evidence of a very small air leak. I used some 3 Bond on the manifold/insulator/carb area and let it cure but have not had the time due to work schedule to check it out yet.

Point is: there is a small oil leak emanating from the area of the head gasket.....another thing I simply haven't gotten around to. A head gasket leak does indeed make a lot of sense. If the carb body/manifold re-seal doesn't work, the head gasket is where I'm going next.
 

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Retorqe the head now that you have put it through some heat cycles. I'm assuming you annealed the head gasket when replacing the piston/rings ect. Also just for giggles open your gas tank fill cap and let the bike idle and see if that eliminates the idle issue. I had a plugged vent on my old Shovel and it would basically "run out" of fuel from the vacuum. It's always the simplest things
 

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I doubt that from a cold-start the carburetor could become overheated in 2 min. the OP claimed it runs before developing issues. It sure sounds like a fuel delivery problem. I think if it was an electrical issue, it would die-suddenly, and not re-start until it cooled-down enough to allow the open circuit to become intact/conductive again. The way it starts to peter-out sounds like more of a fuel-delivery problem. The suggestion of an Amal Premier carb may help, or maybe a Mikuni of similar capacity/size and jetting.
Did you not read this part "Once it is nice and warm it will cheerfully idle for about 30 seconds to 2 minutes and suddenly lose idle rpms and stall. If you stand there and feather the throttle it will wake back up and idle once again...for about 30 seconds to 2 minutes. "

However, i agree it could be a fuel delivery problem but i would be looking beyond the carb.
 

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Yeah l had one of these beasts back in the 80s
DEvil of a bike .. if its got tits or wheels ... you know the rest ...
 

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Sure sounds like some-sort of leak somewhere in the fuel/air delivery system, like an air leak. I'm not familiar with the gasketing or O-rings for the air delivery on the B50, but either that, or possibly something warped in the mating surfaces, or excessively-worn components allowing unwanted additional air sounds like what you're experiencing. On a twin-carb (or more carburetors) bike, I'd think a lack of adequate carb synchronizing as another possibility.
 
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