srx fuel cock - how's it supposed to work

[Julian217]

Hi,
My recently aquired SRX 400 ( twinshock) has what I think is a fuel tap problem.
It will start with the fuel cock  in 'prime' position. (Which seemed to be off on my previous bike). It ticks over nicely at standstill, but move the tap to on and the engine races. 

Then, running it down the road it, in prime position it sounds like the engine is fuel starved - have to keep the revs up to keep moving. Move the tap to on when on the move, if you see what I mean, and it backfires and stalls.

So its like its getting too much fuel in on position and not enough ( to ride) in prime. 

Reserve has the same symptoms as on.

any thoughts? I know they have a diaphragm, though I'm not really sure what it's meant to do.

cheers
Julian

[guest18]

Ok I'd read that as no fuel, or not enough, when it's set to on. A rise in tickover often appears when the fuel is leaned off and most will have noticed that just before they run onto reserve the bike will tend to backfire and the exhaust note will "harden" slightly.

A vacuum tap should give no fuel until the running engine creates a vacuum (strictly a depression and not a vacuum as such... but lets not go there!  ) and opens the tap. The prime position will provide fuel regardless of whether there is a vacuum or not.

Sounds to me like the tap is providing no fuel when set to "on". Have you run it on prime and checked the plug? Often the most certain way to find out if you're getting too much fuel or too little.

Good luck anyway!
Smudge

[andy230]

Check the vacuum hose from tap to inlet rubber.  Must seal!!  Vacuum lifts diaphragm off seat to let fuel flow when engine is running (ie negetive pressure).  "Prime" jsut by passes this and lets grafity supply fuel.  Means when engine stops, float needle is still holding back a head of 15 litres of fuel!

Then maybe rebuild tap??  Rubbers available from NRP in manchester (or they were)

www.nrp-carbs.co.uk

sorry 4 typos!  big rush

a

[guest27]

And having checked the vacume thingy is working - make sure that the filter etc in the tank are clean, mind some fuel is getting through so maybe not

R

[beeman]

As Andy230 said when in prime position the tap is straight through on the other two positions it is off when the engine is not causing a vacum.
It sounds more like problems with the carb.
Does the fuel line to the carb run with petrol when on prime. If so at least you know fuel is getting there.
I would take the carbs off and clean with a carb cleaner in an airosol (did I spell that right) also the tank often gets crap inside it from rust. an inline filter sometimes works.

beeman

[Julian217]

Thanks for all the info. The bike's not been used for a while so carb stripdown was next on my list.
cheers
Julian.

[andy230]

if the vacuum pipe is leaking, not only would it not open the diaphragm, the leak would also allow air into the inlet manifold weakening the mixture.

A 2-fold disaster.    Mr Yamaha was not content by making thing complex with 2 taps, he also needed the lower tap to have 3 postitions, and have the potential to alter the mixture once the rubber pipe get too old!  Genius.

Plus he made the pipe out of special rubber which welds itself to the (expensive) inlet manifolds....!

a

[bullet350]

am i the only one who missed taps with off/on/reserve and no vacuum?

i went through all this with a cbr tap that never really sealed, with the added bonus of near down draught carbs allowing the engine to fill up if i didn't sort the problem.

Got a £10 tap from the breakers with no vacuum, no more leaks.

bullet350

[Julian217]

a quick inspection revealed the following;  pipe to inlet boot not held tight, tap to carb tube perished, fuel leak from angled outlet at bottom of tap - so loose it came out. Two new tubes from tank to tap, both massively long and pinched by the tank.

So I've cut the tubes to a more suitable length, used the cut off bit to replace the tap to carb tube and more firmly attached the vacuum pipe.  Have yet to start it as neighbours kids were going to bed by this time....

any thoughts on what to seal the angled outlet pipe joint with?

[Steve H]

any thoughts on what to seal the angled outlet pipe joint with?

Mines weeping at the moment. I had thought about soldering it in place, but that may be a bit excessive. Superglues another option but I dont know how it reacts to petrol.

[guest27]

A bit of rtv?

R

[andy230]

any thoughts on what to seal the angled outlet pipe joint with?

I wound the stub with PTFE tape, forced it in, and lockwired all removable "stubs" in place to the tap.  Cable ties would do....

I have had the lower (ie. to the carbs) stub come off.  Both on the road and in the pits.  Both messy AND terrifying.

Not a pretty solution, but it works

a

[Julian217]

Delving a bit further..... leaves in the airbox,  filthy filter. Rusty mess in the float bowl.  Took the tap apart, can't see any faults there.