Well I was checking my float height with the U-tube method (nailed it, for the record ), and I noticed that the float bowls were filling freely no matter the position of the petcock. From my understanding, this should only happen in the prime position. I fear this could be preventing my bike from running. So here's my 3-part question:

1. Is my understanding of the petcock accurate (i.e. should it only flow freely in the prime position)?
2. Is there anything else that could cause this problem?
3. What should my course of actions be from here?

My bike is so close to running! I got it to fire....once.

#5118

First be sure all hoses are connected correctly. Maybe a hose is connected from the tank to the carbs by accident. It is unusual for them to always let gas flow.

Then, you might need to replace the petcock. It is inexpensive and easy to do (I know the "anti petcock" lobby will tell you they are always crap but they aren't - then tell you to cobble up something different. Millions of bikes have vacuum petcocks and they can go bad like any other part).

It could be a simple as the o-ring being bad. You can remove it. Use a clean tube to blow air through it and check it for function. Or the diaphragm or spring in it is broke. Easy to replace for $25 or so or post a want-to-buy as people here probably have spares laying around.

Unless the bike is running, it should only flow in the prime position.  You need to replace the petcock.  I would replace it with a new one or you run the risk of getting another warn out one.  It will cost you about $50 to maintain the convenience of a vacuum petcock.

However, I don't think this would be preventing your bike from starting if the gas level in the bowls is correct as you said.  You might want to check the integrity of the rubber diaphragm in the petcock to make sure it's not sucking gas through the small vacuum line.  You could just cap off the vacuum line at the carb and see if you can get it running temporarily that way on prime.  If not, then you are not done sorting your carb tuning.

That wouldn't stop the bike from running so long as the float heights are right.

As far as fuel still flowing when not on prime... I'm not sure how long you let it run out, but the gas line between the petcock and carburetor can still hold and flow a reasonable amount of fuel before it runs out from the petcock being off. Make sure that's not the case before you go tearing into the bike.

There's also a fairly simple method of converting the petcock to a manual (non-vacuum) petcock that can be found on the wiki, but I'm not sure if it requires diaphragm/spring integrity to function or not.

#5119

True - a free flowing petcock will not be a problem if the carb float levels are correct and the float needs are working and can stop the flow of fuel - which is what they do with the bike running.

People go to too much work when they want to eliminate the vacuum petcock.

IF VACUUM NOT WORKING - instead of fixing it they replace it with something cobbled up. They forget that PRIME bypasses the vacuum so PRIME = ON and ON/RES would be off. Simple. Nothing to change if you want the inconvenience of manually turning off fuel. Zero time and zero cost.

IF ALWAYS FLOWS: That means fix your carbs. If you don't want to, add an in-line fuel valve from the petcock. 1 minute fix.

Remember that when ON or RES and vacuum working, fuel freely flows. The carb needles keep them from overflowing. So if that's what's happening - first fix the carbs.

Thanks for the help, guys! Went out and did some stuff, and I have some updates. The petcock is working fine. The float heights are set spot on, according to the u-tube method. No fuel seems to be coming through the diaphragm or vacuum line.

The one (well, two) thing I didn't check is the mixture screws. I noticed there were caps blocking access to the mixture screws and just assumed they were set right (3 full turns from snug, correct?). I may have assumed wrong.

Have you cleaned out the carb thoroughly?  If fuel flow is not the issue, then you might have a clogged air/fuel passage.  Clean out all the passages physically with guitar strings, then spray through them with carb cleaner.  Don't forget the choke circuit.  Clean out that small fuel jet on the bottom of the float that leads to the choke circuit.  You might just need to choke the bike to get it started.

Pulled the carbs back off, did some more cleaning. Found both of the choke jets were badly clogged, and cleaned those out. Thanks bombsquad, I never would have noticed them!

Regarding the mixture screws, here's a concern I have. The carbs I took off the bike and had to replace had open access to the mixture screws. The replacement carbs I bought have the mixture screws thoroughly blocked off.

The original:


The replacement:


Is this normal, or should it raise some concern?

That's completely normal.  That's how it comes from the factory because it's preset and they don't want you to change it for emissions regulation reasons.  You need to carefully drill out the brass plug to adjust the mixture screws.  If you take the mixture screw out completely to clean, make sure you don't lose the tiny washer oring and spring, and remember the order they get replaced.

Alright, well in that case I guess its safe to assume the mixture screws haven't been messed with. Tomorrow I'll recheck the float height, put it back together, and try again.

Thanks again for the help! I want nothing more than to finally get this bike running right now.

Good luck man.  Let us know how it goes.

idk if this is what your talking about i had gas leaking out of my carbs alot what i did is replace the gaskets and its perfect now rember gas leak is also a air leak and since gs500s are vacuum fuel injection the vacuum is not wokring so gravity is forcing the fuel to the carbs. Best guess for me

Not quite what I was getting at, but I do appreciate the input. The float bowl gaskets are brand new, and the petcock is apparently working fine; not sure what was going on earlier, most likely user error.

Well I've got some good news and some bad news. I got the choke jets cleaned out last night, adjusted the float height, and put the carbs and tank back on the bike. Did the u-tube check just to make sure....and I was WAY off. Fuel was leaking out of the intake trumpets. Didn't feel like pulling the carbs back off, so I managed to get the bowls off with the carbs on the bike, adjusted the floats, and got them at a more appropriate level.

The good news: IT WORKED! Started up with no hesitation at all, warmed up great (the idle could use some adjustment, but thats easy enough), and ran for a little while in the garage.

The bad news: I might not have time to ride it today!

Thanks again for your help! After two weeks of work, it finally runs, and I couldn't have done it without you guys! It needs a couple tweaks (idle is a bit low, it doesn't respond well to sudden jumps in throttle, and the brakes need to be bled), but getting it started today was a HUGE step. I'm still shaking from adrenaline!

Awesome!  Glad we could help.