Fuel Petcock PRI

Hey everyone. I'm working on fixing the last few problems with my 81 KZ550. Anytime the bike has been sitting for a while(more than 2 hours)I need to set the petcock to the PRI setting to send fuel to the carbs. It wont start if I set it to on or reserve. I'm familiar with newer bikes like the ninja and know that they are supposed to start without needing to prime the carbs first. Is the KZ550 supposed to use the PRI setting when first starting up or is there a problem somewhere?
Thanks

Hi, I don't know the exact answer to your problem but my KZ650 is the same if the fuel level is getting low, like about to go on to reserve but still running on main when stopped.
It doesn't like starting then and I have to use PRI rather than main or reserve.

Hope this doesn't mean I've got problems as well!

I dont believe you should have to prime it every time you want to start it. First off it sounds like your carbs could be leaking somewhere. Maybe out the overflow or into the cylinder. Even after you shut your bike off it should still have and keep fuel in the bowls and lines unless theres a leak allowing the fuel to drain down, causing you to have to prime it every time. Also the petcock is ran by vaccum. So you shouldnt have to go to prime to often. Maybe you have a cracked vaccum line or something allowing it to not pull a full vaccum. Have you messed with the clean air system any? That should give you a idea on a couple things to look into

uncle nick wrote: Hi, I don't know the exact answer to your problem but my KZ650 is the same if the fuel level is getting low, like about to go on to reserve but still running on main when stopped.
It doesn't like starting then and I have to use PRI rather than main or reserve.

Hope this doesn't mean I've got problems as well!

If iam not mistaken its not like reserve. Prime allows the petcock to be turned on and allow gas to flow with gravity. The other posisitions like on and res require a vaccum to open the petcock.

As Powerstroke_fan suggested, is there evidence of fuel leakage after a run when the bike is parked, or a failed vacuum supply (for whatever reason) to the petcock?

Less likely, but conceivable, that there might be a cracked or loose overflow tube which is allowing fuel inside the bowl to escape via the overflow circuit.

Good Fortune!

There is no evidence of fuel leaking from the carbs. It was the first thing i checked when I noticed the problem. Underneath the carbs are completely dry. I took the petcock off to make sure nothing was broken and all the o rings are still good

Whichever carb has the vac hose attached between it and the petcock is supposed to be furnishing adequate vacuum to the petcock.

Perhaps the cylinder for that particular carb isn't performing well enough to generate the necessary vacuum.

Could attach the vac hose to a vac nipple on a different carb, and test run to see whether it makes any difference. Be sure to cap/plug the formerly used vac nipple.

Would fit a new vac hose, or at least carefully inspect the existing vacuum hose which may be ill-fitting at the end(s), or cracked or torn.

If part of the vacuum is escaping before reaching the petcock, there might be enough vacuum being supplied to the petcock at normal running rpm, but not enough vacuum at crank-over.

Similarly, if there's an issue with the petcock diaphragm, the diaphragm may be functioning at normal running rpm, but not receiving enough vacuum to function at crank-over.

Good Fortune!

Another possibility is that one or more of the carbs is leaking so slightly that the escaping gasoline evaporates before dripping from the bike.
Such leak could be through a drain screw that isn't fully sealing, or past a float bowl gasket.

On some carbs, it's possible to over-tighten the float bowl screws, whereby an ear on the float is slightly bent just enough to disallow a sufficiently close fit between the mating surfaces on each side of the gasket. Seapage past the gasket may then occur regardless of tightness of the screws, when the bike is leaned over resting on the side stand.

The bent ears can usually be restored to their original positions.

Is there any discoloration on exterior of the float bowl(s) to indicate minor fuel leakage from the gasket area?

Good Fortune!

On tk22 carbs i believe there is only one hole where the vacuum hose can attach to. I'll check the hose for any rips when i get home. The carbs were cleaned by the previous owner and i recently polished them so any residue from the gas would be wiped away
There is no new gas stains on or under the carbs however

I think patton pretty much has you covered. Its about gotta be one of them. Theres way to test the stuff. Just because someone has went thru something or its new dosent always mean it cant be the problem. Ive seen new stuff have problems, stuff incorrectly installed and on and on. So i would go back and check everything over again. It sounds like to me your not getting proper vaccum or the diaphram in the petcock is bad.

^^^^^
What he said! I've purchased Yamaha petcocks 3 and 4 at a time finding that they were all crap and leaked right out of the box! :blush:

If the bike sits for a couple of days it is normal to use the prime. As the bike sits the gas in the carbs will evaporate from the bowls through the bowl vent. It is normal. If you ride the bike everyday then you won't have to use the prime function.