Bike was sitting Now won't start

[alantf]

Originally Posted by Water Warrior

I think JohnC is thinking a leaky vacuum line will suck air into the engine and therefore lean out the fuel mixture. I'm sure he will clarify that for us.

I hope he does clarify what he thinks. All a leaky vacuum pipe will do is to stop it sucking a vacuum, which means that the petcock valve won't open, so fuel won't flow to the carb.

[blaine]

Originally Posted by JohnC

Yes, if the vacuum line is missing or broken two things happen. First, the fuel won't flow in "on" or "reserve" and secondly, the engine will suck extra air in through the vacuum port, which is after the fuel metering section of the carb. This causes an extremely lean condition at idle and low speeds by reducing the flow through the metering section of the carb and hence reducing the amount of fuel metered. This mix is then combined with the unmetered air from the vacuum port resulting in a very lean mixture delivered to the cylinder. On a single cylinder engine the leak is sufficient to prevent the engine from idling, especially when the choke is off.

Yep....Any vacuum leak will cause a lean condition.

[JohnC]

Originally Posted by alantf

All a leaky vacuum pipe will do is to stop it sucking a vacuum, which means that the petcock valve won't open, so fuel won't flow to the carb.

It's not that simple, as I stated above. When the vacuum line is connected to the petcock, there is no air flowing through the vacuum port once the petcock opens. When the line is broken or disconnected there is a steady flow of unmetered air into the engine's intake, and no fuel delivered by the carb to go with it. The carb meters fuel based on the amount of air flowing through it. Allowing air into the engine that does not pass through the carburetor throat dilutes the mixture. So, even if the petcock is on "Prime" and fuel is being delivered to the carburetor, the mixture the engine sees will be very lean. As an aside, as engine load and speed increase, the pressure in the intake manifold increases and the air drawn in through the open vacuum port decreases both in absolute quantity and percentage of the whole, and the leaning effect eventually becomes insignificant.

[alantf]

So how is it, that if the vacuum pipe is cracked, and the engine won't start, switching to prime (which doesn't need vacuum) will let the engine start, and run o.k.? There's still the problem you mention, with extra air going to the carb through the cracked pipe, but the bike runs fine. :??:

[JohnC]

Pull the hose off and see if it will run fine then. Maybe there is a restriction in the vacuum port that limits the amount of air drawn in. A restriction wouldn't significantly affect the operation of the petcock since there's no flow involved.

Such a setup would only be suitable for a single cylinder bike as multi-cylinder bikes use the same ports for carb synchronization and they are necessarily unrestricted.

[QUICKSTRONG]

Originally Posted by JohnC

Pull the hose off and see if it will run fine then. Maybe there is a restriction in the vacuum port that limits the amount of air drawn in. A restriction wouldn't significantly affect the operation of the petcock since there's no flow involved.

Such a setup would only be suitable for a single cylinder bike as multi-cylinder bikes use the same ports for carb synchronization and they are necessarily unrestricted.

tonight I took the carb off again there is gas in the floatbowl but the rest of the carb seems bone dry???? does anyone know what is going on here?? no wonder it wont start

[jonathan180iq]

Good vacuum to draw fuel through the system and clean ports within the carb are necessary to get this thing running. If it sat for so many years, I would replace the vacuum hoses anyway. There are only 3 or 4, IIRC. Get some new hose from the auto parts store and go to town.

You're probably going to have to disassemble the carb to clean all the other inner ports.

There is no reason to rush carb work. Since this is the only method the bike has of delivering fuel and air into your engine, then you need to just go ahead and do it right the first time.

(Couple of mini-thoughts. You aren't trying to start the bike with the kickstand down are you? You're using choke? You will always have to use choke on the GZ. Did you ever test for spark?)

[QUICKSTRONG]

Originally Posted by jonathan180iq

Good vacuum to draw fuel through the system and clean ports within the carb are necessary to get this thing running. If it sat for so many years, I would replace the vacuum hoses anyway. There are only 3 or 4, IIRC. Get some new hose from the auto parts store and go to town.

You're probably going to have to disassemble the carb to clean all the other inner ports.

There is no reason to rush carb work. Since this is the only method the bike has of delivering fuel and air into your engine, then you need to just go ahead and do it right the first time.

(Couple of mini-thoughts. You aren't trying to start the bike with the kickstand down are you? You're using choke? You will always have to use choke on the GZ. Did you ever test for spark?)

Yes it has plenty of spark However, I am trying to start the bike with the kickstand down! I never thought about that, is the kickstand some kind of safety switch?

[blaine]

Originally Posted by QUICKSTRONG

When working properly the bike will stall if you try to put it in gear with the stand down.

[alantf]

There's no problem with starting the bike with the side stand down. So long as it's in neutral. The safety switch only kicks in (to turn the engine off) if you try to put it in gear with the stand down. This is a safety feature, so you can't ride off with the stand down.:2tup: