Running Issues

[Jdan]

First off, the bike is a 2000 m750.

My bike has been running fine, just with a few problems starting, the past few weeks
and now after it sitting for about two weeks, I can't get the damn thing to start.

It wouldn't fire at all in the beginning until i sprayed a little gas in the carbs.
After that it fired right up and ran for maybe 30 secs before the revs dropping really low
then completely cutting out.
It wouldn't start again without spraying a little more gas into the carbs again.

Battery is fully charged.
Fuses are good.
Plugs are recently changed.

I'm thinking that the problem is that the bike isn't getting fuel at all. Maybe there is a
blockage of some sort because there is no crimp in any of the fuel lines.

Any help would be appreciated.

[Howie]

Fuel contaminated?  Stale (smells like paint thinner)? 

[Jdan]

It smelled fine too me but I'll check it out again.

I've also been reading about these vacuum powered petcocks.
I have a manual one that the shop put in, but it looks like they left the old ones in.
When/if these do fail, will they block all fuel coming in, or just let everything go?

[koko64]

They can stay shut when they fail. Or they can restrict fuel flow more and more over time as they degrade. 

Many pull it off and stick with the manual tap. Just remember to turn it  on and off as needed, to avoid flooding your motor with fuel overnight if the carbs flood. Or to avoid stalling down the road if you forget to turn it on.

Bypassing the tap or fuel pump is a good test to find the problem. Clogged fuel filters cause similar problems.

[Howie]

They can stay shut when they fail. Or they can restrict fuel flow more and more over time as they degrade. 

Many pull it off and stick with the manual tap. Just remember to turn it  on and off as needed, to avoid flooding your motor with fuel overnight if the carbs flood. Or to avoid stalling down the road if you forget to turn it on.

Bypassing the tap or fuel pump is a good test to find the problem. Clogged fuel filters cause similar problems.

I am one of the many.  Since you have a manual petcock get rid of the vacuum shut off.  Drain some fuel out of the carburetor float bowls into a clear container to see if (a) there is fuel and (b) if it is contaminated.  On the right side of the bike towards the rear of the engine you will see two plastic tubes.  Follow them up to the carbs.  Where they connect to the carbs you will see small screws (8mm. heads if I remember correctly)  These are the drains.  Somewhere on the board is a good photo.  Water will sink to the bottom.  No fuel comes out?  Suspect the fuel pump or fuel shut off.  Make sure vacuum is getting to the pump before condemning it.  As koko said, the pump can be bypassed.  Just make sure the level of fuel in the tank is higher than the carb inlet.

[Jdan]

I'll try and get into it this weekend, I planned on rejetting the carbs anyways because of some other issues but this seems like a good idea anyways.

[Jdan]

So I've rejetted the carbs, cleaned it fully, and replaced the needles.

Still no fuel.

With my manual petcock turned on I still got no fuel coming out.

Is this the fuel filter? This image is from underneath the tank.
The main line exiting it is soft and pliable. I can compress it very easily.


Also, Is this the vacuum powered petcock? This is on the right side of the bike, bolted to the frame.

[He Man]

first pic is the vacuum petcock, the 2nd pic is the vacuum fuel pump, the fuel filter is the one before the petcock.

[Jdan]

There is nothing before the petcock on my bike. The lines go straight from the fuel tank into the petcock.

[koko64]

Looking at thr pictures confirms the advice given so far.

Fit a filter, (look in the manual and see how it fits), I run an after market filter between tank and tap. You need a fuel filter. A hell of a lot of crap has entered the pump and carbs if you have no filter.

Fit a manual tap, there is every chance that vacuum tap has failed.  Bypass that tap and see if fuel flows, it should.

[Howie]

You need a filter somewhere before the pump.  
Stock set up:



If you have a manual petcock time to remove that vacuum shut off.  That should be your first step.  The bottom hose is fairly soft, but with age can collapse in use.  If you replace it with clear Tygon hose diagnostics will be easier.

Bypass the vacuum shut off, remove hose to carbs, place hose in a clear container and crank engine.  If no or little fuel comes out the problem is from the pump back.  

[Jdan]

When I do remove the vacuum petcock, there is three hoses. One going to the carbs, one coming from the tank, and a third going to the engine. What do I do with the third hose?

[koko64]

You need to block it off. It runs to an inlet manifold. Some people remove the hose and use a short bolt or screw that fits the manifold, some shove a small screw/bolt into the hose and clamp it which is easier. It must be blocked off. Otherwise there is an air leak into a combustion chamber.

[Jdan]

So it's not necessary then.
What exactly is the point for it?

[koko64]

It provides a vacuum from the inlet manifold to the tap for the tap to open. Its only needed for the vacuum tap. Your fuel pump also has a line from a manifold and this is needed.