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Hiya, have a GPZ900R - it has a vacuum petcock fuel tap. Been having a few fuelling issues. After cleaning the carbs, it's currently running well (maybe a touch rich), idling very smoothly, pulling through the rev range really well, maybe a slight pause when rolling power on from e.g. 3k revs but pretty good.
However, after 10miles, it'll suddenly act as though starved of fuel. If I very briefly turn the fuel tap from 'on' to either prime or reserve, briefly and then instantly turn back to 'on', it recovers immediately and all is well for 10 miles. If I solely run on reserve, all is well.
Have ordered a diaphragm kit for the tap, but so far have:
- dismantled the tap and checked for blocks, screen ultra clean, not sign of blockage, diaphragm looks intact and clean
- stripped the filler cap - there's a complex diaphragm in there which I couldn't remove, but looks clean
- stripped and cleaned carbs
- manually sucking on the vacuum hose to tap creates a flow on 'on' which stops when sucking stops, and no leaks
- submerging vacuum hose to tap in water, blocking one end and blowing hard on the other, no bubbles, so presumably not leaking
I'm not clear on how the fuel tap works i.e.
- if it's OK on reserve, then the tank must be venting OK and the vacuum feed to the tap is OK???
- if OK on reserve, prime then carbs OK???
Am I right in focusing o the tap?
Just confused! Thanks
How much fuel in the tank?
It occurs with fullish tank and nearly empty, and 8n between
Have you checked the air breather into the tank? IE is it clear?
Water in the fuel?
I don't know if it's the same but on the bikes I've had the reserve intake is separate and a lot shorter than the main intake. Could the main intake have a moveable blockage or just a piece of plastic that moves a bit over time so restricting the flow. On mine it looked like a long straw and could be detached.
if you can detach your may be worth running it like that to see if the problem still happens. Just don't forget to put it back or there will be no reserve
Most taps have two little filters on the inlet (inside the tank). It may be that the 'On' filter is partially blocked as uggski says. They can get gummed up with sludge over time, not necessarily solids.
Some can be replaced, or you can blast it with compressed air and agitate it with a toothbrush in some solvent (petrol would be ideal).
Hi all, and for those who may be interested ... I think(!) I finally understand the vacuum petcock. I replaced the diaphragm (altho the original looked excellent under close scrutiny) and did a 30 mile run no problems. So cautiously optimistic.
Once I got back ... I started on doing the rear wheel conversion - switching the original 18" wheel out for a slightly wider 17" so I can get some modern tyres. Machined up spacers and looks promising so just waiting for final bits to arrive and then have a go at fitting it all.. and then a decent ride to check out the fuelling (carbs currently off too, checked float heights all good, cleaned and waiting for new gaskets ready to refit).
Thanks for the pointers, really appreciated.
