Yamaha fuel pump- how does it work?

[sparkie]

Hi all, just stripped and cleaned the carburetor on my 8hp Yamaha for the first time, looked at the fuel pump, but didn't strip- (if it ain't broke etc etc), but couldn't figure out how it works, as there doesn't seem to be any external drive to it, it's just bolted to the side of the carb.. The manual tells me how to strip it but not how it's supposed to work. Anyone out there any idea???

Safe sailing


Sparkie

 

[boatbuilder]

They are usually just a diapam pump operated from the pressure of the piston going up and down.
I think /forums/images/graemlins/smile.gif

 

[VicS]

yes normally you will see a "fuel in" connected to the tank connector a "fuel out" going to the carb, but in this case it may be an integral connection and a connection to the crankcase. The later produces pressure flutuations on one side of the diaphragm. There will be small valves included in the petrol side of things and usually a filter

 

[sparkie]

Yes there is a "fuel in" conection but everything else happens within the carb itself. There are no connections between the carb and the crankcase except for the inlet tract so I guess the diaphragm must be operated somehow by the pressure pulses in the tract as the reed valves open and close. Dashed clever these Keihin people!

Many thanks both

Safe sailing

Sparkie

 

[jimboaw]

Get a spare pair of diaphragms. They stretch over time.

 

[sparkie]

Thanks for that advice Jimboaw, what symptoms would a stretched pair of diaphragms produce?? I'm having problems at the mo with slow running, engine keeps cutting out and won't restart without choke even when hot.

Sparkie

 

[Otter]

Just in case it isn't the fuel pump - we had exactly the same symptoms as you describe on our Yam 9.9 - and you really need reliable power when moving a cat in a marina with a cross wind! It turned out to be a combination of two things:

1. The throttle minimum was set too low
2. The throttle linkage under the engine cover was sticking.

After resetting the first (and there is a very narrow tolerance) and clearing the gunk that had caused the second, we haven't a had a single problem.

 

[sparkie]

Thanks Simon- haven't checked the linkage out yet- will do.

Safe sailing

Sparkie

 

[tross]

To set up you slow running. Set the throttle stop screw to give you a resonable tickover and the adjust the slow running mixture screw. Screw in the mixture screw - slowly and carefully - do not screw beyond finger tight- until the revs begine to drop and then back half a turn. rest the revs again on the stop screw and then check the mixture screw again. Oh , make sure the engine is wrmed up and the leg / waterline in the same as on the boat as theis wil affect the back pressure in the exhaust and therfore the mixture. It's sometimes a good idea to blip the throttle and let the revs settle down before finishing.

 

[sparkie]

Thanks Tross, not thought about the effect of the water pressure on the mixture, always set up ashore. Do you know which way the screw works, ie does screwing in richen or weaken the mixture?? The manual doesn't enlighten us.

Sparkie

 

[tross]

Hi Sparkie
Screwing in weakens the mixture. I set my Yam up this year - well I stripped it down to sort out a hole in the exhaust tube and as part of this, cleared the carb out completely. Come to think of it, last year I had to use the choke to start it even when hot but this year I don't ??? It may be I set it up better, maybe it was the exhaust tube weakening the mixture ? How old is your unit, mine is around 1992/4 vintage. You maybe able to drop of the lower part of the leg and shine a torch up to see if there is a hole in the tube. The cost of these tubes is around £20

 

[sparkie]

Thanks for that. Mine is a 1995 model, I intend to strip the whole thing next winter and do a thorough check on everything outside of the actual motor head, which seems fine, so I'll certainly have a close look at the exhaust tube. This could definitely affect the mixture as you suggest.