Fuel tank vacuum problem?

[zulueffendi]

1990 kat 600 in canada. Last year had carbs rebuilt with new innards. When shop gave me the bike back i got 5 minutes down the road and it stalled out. I refired it. But idle was sub 1000 rpm and no throttle response. After 20 min it worked fine. Until i got to next stoplight. Same thing. Sent back to shop. They changed throttle cable and said it runs great. I go pick it up and same thing. 3 minutes down road. Stalled out when downshifting. After that. No start for months.

That brings us to this spring. I can get it started. It idles for 15 or so minutes. But when i shut it off. I can hear air sucking into the fuel cap

Question is. I shouldnt have negative pressure in tank or it would prevent fuel delivery right? If that is correct. What would be plugged preventing air getting into tank? 2 vent lines on bottom of tank or petcock problem?

Sorry im not much of a story teller or a mechanic. Thanks for help in advance. Enjoying digging around the forums

Read some more. Now i understand the 2 lines are drains. So petcock problem. Or is negative pressure ok?

[ratrider]

are you sure its sucking you can hear? its normal to hear a noise coming from the vented cap on the tank.
it may be worth investigating for vacuum leaks that may be preventing adequate fuel flow as a starting point. its possible that something was overlooked by your bike shop.
why were your carbs rebuilt in the first place mate?

[zulueffendi]

I took it in for a safety. To put it on the road after buying the bike for 800 bux. When i ran it at purchase it fired up and ran well. There are many signs its been dropped in its life. But mechanically it seemed to run great. That didnt last. And the shop showed me a float from the carb that was in really rough shape. They were going in to clean the carbs.

It seems to suck air for a minute or two. And seems to idle better if i leave cap open. So im not really sure where to even start

Its my first bike. And unfortunately hasnt been a good experience.

Could a cdi box cause the stalling issue after a few minutes of riding? Or am i driving badly (not revving high enough before shifting possibly flooding something?)

[ratrider]

its unlikely to be anything to do with the igniter unit. more likely by what you say to be a blocked vent in the cap.

[Kreylyn]

Disassemble and clean the gas cap. That is where the vent is.

There is even a detailed pictorial of someone else doing it for reference.

http://katriders.com/vb/showthread.php?t=126784

It's not hard, but it's definately 100% one of the issues your having.

Krey

[hardlydangerous]

fuel petcock or vac line to the petcock would be my guess.
or a very poor job putting the carbs back together.

make sure the enricher/choke is not sticking open.

A simple test would be to ride it down the street with the fuel cap open.
Also run it on Prime
chances are the noise is air pressure escaping from the vented cap from a build up of heat from idling for 15 minutes.

[92xjunker]

Does it stay running with the cap open during a ride? Your cap vent may not be functioning properly.

[zulueffendi]

Thanks for input everyone. I will have a look at your suggestions. Hopefully in the next few days i will have a chance to work on it. And ride around block. Last time i rode it i had to push it home. About a mile. Then the stupid thing started 5 minutes after i got home