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Diagnosing fuel pump problem

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I’ve been battling with reliability issue on my pre-97 Honda BF100 4-stroke. And I think it might be the fuel pump.

 

If I prime the bulb and then crank it over it sputters about 2 seconds then stops. Afterwards if I open the drain valve on the carb very little fuel comes out.

 

I’ve disconnected the fuel line into the pump and primed it, that’s certainly working.

 

I’ve also disconnected the line from the pump to carb and that’s priming fine too. However if I crank the engine I just get a small puddle of fuel come out, certainly nothing like when priming.

 

Is that expected? I want to be sure it’s the pump before tracking one down from overseas.

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when working the primer bulb i'd expect to see 50 to 100 mils (cc's) of fuel per squeeze , but while cranking it over  the little diaphram pump would only put out around 5 mils every two crankshaft revolutions. if its not steadily doing a 5mil squirt on the starter then a likely cause would be a bit of dirt or sleaze on the 2 valves in the pump - a careful clean of the pump could avoid a visit to ebay...

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I’ve had BF100 and Yamaha 15hp 4T engines . Both were awesome when running but really prone to carb issues .

 

Try disassembling the carb and cleaning with soaking carb cleaner not spray can cleaner . Poke super thin wire into all the little galleries and crevices and blow out with compressed air . If I didn’t do this once a year I would have starting and idling issues

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the biggest clue is very little fuel in the bowl when you undo the drain - i would expect roughly 40 or 50 mils to be in there after its turned off (at least thats about what i get out of my 9.9 yamaha). perhaps try taking the drain out and squeeze the primer a few times to see if you get plenty of fuel getting into the carb -that will tell us where the problem lies

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Have a look at the fuel line connector - the o rings can get munted resulting in irregular fuel flow - easy fix too - just buy another fitting and put on the hose . You need to have a close look at that fitting as it can get damaged just disconnecting and reconnecting over time .

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