Starts and Runs, Won't Start, Starts and Runs

Has anybody had this before?

I have a 1979 Z 650, been stood for a long time and then rebuilt. Bike eventually started and runs great, carbs rebuilt, timing spot on, leave it for a day or more and won't start, it just won't suck fuel through even though the carbs are full the plugs stay dry. Put a hand over the back of the carbs it nearly pulls the skin off, sucks through fuel like a porn star and runs great, starts off the button all day, leave it for 24 hours and the same thing again. It does have a nearly almost knackered starter clutch that I'm about to repair so it may be the motor just isn't spinning as much as it should but..........any other ideas?
Bike has a Harris 4/1 pipe and K&N filters, all jetted correctly.

Any ideas most welcome before I kick the living daylights out of it!

You are using choke to start cold right?
Have you checked to make sure the choke circuit is clean/clear?
Pilots are clean/clear.
Mixture screws are adjusted correctly...

Yep, all done, spotlessly clean, even had a second set of carbs on it and the fault is identical.

Whats the resting voltage of the battery?

You say the plugs stay dry?...
Richen pilot circuit @ the mixture screws slightly and see...

Could be starter clutch too...

Checked the input voltage to the ignition coils? It should be the same as the battery. If a lot lower, start cleaning the many and various electrical connections.

Silly thought, but if you have a vac. petcock, you might try switching to PRI, for a minute before cranking after extended sets. My thought....everything in the VM carb is related to vacumn/fuel height...Ma kaws' vac petcocks have become "iffy" w/age. & cranking vac could be insufficient to refill 'evaporated/heat related fuel height"in float bowls. useless if manual cock, but a thought. G

Thanks for all the replies;

Battery fully charged and charging, bike has had a full rewire 2 weeks ago to finish off the rebuild as the original loom was scrap. Voltage at coils etc is fine.
Not going to mess with the jetting as the fault is still there using another set of carbs and I know they are spot on.
Taken the vacuum off the tap for now and only using prime.

We shall see, the starter clutch repair kit arrived today so it should be in next week, i'll let you know. Dave

I had an issue similar to this on my last bike. It ended up being the cheap fuel filter I was using. It must have had a small leak that was somehow pressure locking the fuel line as it sat. When this happened, if I pulled the gas cap off and pressurized the tank, the bike would fire right up. I replaced the cheapo filter with a good quality one, and the problem never came back.

You also may want to check that the gas tank vent is clear.

Next time it won't start, try pressurizing the tank for a few seconds and see if it will start then.

Keep us posted.

Well it's been a while since my last post and the result is........starter clutch repair has cured my starting issue in relation to fuel not getting through. It would seem that the engine just wasn't spinning over fast enough to suck the fuel through. Fitted the starter clutch repair kit and engine spins over fine now, loads of fuel getting through even when the bike is left for days at a time it starts straight away. Great result!

Nice to hear of a succesfull outcome.