Vacuum issue?

[buzzlitebeer]

I have a 1990 Katana 600, and a while back I had idle issues, and Arsenic and some of you others thought I messed up the sync pretty bad. Anyway, after a lot of other stuff, I have finaly put this thing back together. I have synced and adjusted the carbs, and I have to say She was running great. No hesitation, great response, smooth idle, it just ran great. That was with the tank off, with the tank back on, I have the same idle and response problem that I had months ago. What could the tank being on or off have to do with it? Could the petcock be causing this problem, and has any one else ever heard of this happening?

[yngveny]

Not my area of expertise, but I suppose the petcock could be the problem (or the vacuum lines).
How did you set it up to run without the gas tank??

[Kreylyn]

If the only change is the gas source (the rest of the bike, air filters, so on were all together correctly) then your looking at a flow issue for the gas from the tank.

Petcock, kinked line, stopped up line, or something.

Krey

[Black_peter]

Good find, my ideas;

1. Plugged vent line on the tank. Try running it with the gas cap popped open.
2. Kinked lines. It might be right as the tank seats and out of sight.
3. Vapor lock (not really) a bubble or air preventing the gas flow.
4. Tree Hugging Syndrome, the carbs are being stifled by your oppressive attitudes about inside and outside spaces. It needs to see the sky man.. Stop oppressing it!!

[buzzlitebeer]

I fed the carbs from a "bladder", gravity feed, split at the carbs so I can feed both. Vacuum/fuel lines are clear and new. I double checked the the condition and routing of them and they seem to be fine. I did not, however, check the petcock to see if if had a flow problem because I don't know how that could be done.

[Kreylyn]

I fed the carbs from a "bladder", gravity feed, split at the carbs so I can feed both. Vacuum/fuel lines are clear and new. I double checked the the condition and routing of them and they seem to be fine. I did not, however, check the petcock to see if if had a flow problem because I don't know how that could be done.

If the tank if not full of gas, you could prop up the side the petcock is on, pull the 2 bolts out, and it will come right off.

Inspect the screen to ensure it is clean.
Make sure nothing is floating inside the screen.
Use a can of air or carefully use an air compressor/tank to blow it out. Check each of the petcock posistions.

Finally, take the small screws off the back of the petcock to inspect the diaphram and make sure it is in good condition with no holes.

Krey

[buzzlitebeer]

Can the petcock being clogged or messed up cause the idle to "hang at 3000 rpm? This will stumble real bad off idle, then rev high (around 4000) then it will come down a little, and then hang at around 3000 until i adjust the idle screw down then the cycle starts all over again.

[kevin2502000]

When you had the tank off, did you plug your vac lines with a bolt or something so it didn't suck air?

[buzzlitebeer]

Yes, I blocked off the line that feeds the back of the petcock.

[Kreylyn]

Can the petcock being clogged or messed up cause the idle to "hang at 3000 rpm? This will stumble real bad off idle, then rev high (around 4000) then it will come down a little, and then hang at around 3000 until i adjust the idle screw down then the cycle starts all over again.

Hang is caused because it's too lean.
Trickle of gas (not enough) to the carbs would give some to idle, but make it lean out. Giving it alot of gas would use up the little you have and it would die.

So, I'd say yes...

Krey

[buzzlitebeer]

Good Call!! I'm on it!! After I rebuild the petcock I'll let you know.

[redciv1]

I experienced the same situation. New bike purchased in Janaury and the previous owner thought he didn't seal one of the airbox connections tight. But when I got the bike home, a mechanic friend and I took off the tank and started the bike with the tank off and the fuel in the bowls. He was able to adjust the idle at this pointwith no problem. Even showed me how small increments make a big difference. So 5 minutes later, tank back on he starts it up for a test ride and the same thing.

[buzzlitebeer]

Are you saying it was the airbox? Or do you now have the same problem as I do? Runs fine with the tank off?

Although I must now admit that even with the tank off, It is starting to show the "lean" characteristics as before. So now I'm reeling from this new discovery... Suzuki Tech Needed!!

[Helmet_Hair]

If it does indeed seem to be better with the tank off, it would certainly seem to rule in a vacuum problem. Did you change out the vacuum line from the tank to the 4th carb? I'd also be sure that the vacuum line fits tightly both around the petcock and the nipple on the carb. That line seems to be the only wild card with regard to the tank and the carbs.