Is this a vacuum leak?

[dgxser]

I have been having issues with eratic idle. The bike sat for five years, so I knew the rings needed to reseat. I set all the mix screws at 2 turns out, and synched the carbs after I rebuilt them.

I get the bike good and warmed up, ride around ten miles or so. Then [ull the tank and synch the carbs, set the idle at 1500. Now put the tank back on and the idle drops to about 1000 or less. Hit the throttle and it ramps right up, drops back to about 2k then slowly drops off, may pick back up, but drops down to 1000 or less. I synched when I first got it running.

So, today I decided to resynch. Pulled the tank, hook up my aux fuel tank, gravity feed, no vacuum valve. Plugged the vac line with a bolt. Hook up my carbtune. It took some doing, but I kept two screwdrivers, on on the idle, one on the synch screw. Adjust the synch, the idle jumps, adjust the idle, synch some more. Eventually got all four nice and even.

Now I pull the synch tool off, recap the carb vacuums. Adjust the idle and it reacts as I would expect, set it at about 1500 RPM. Now I put the gas tank back on, vacuum line on the petcock, all the drain lines, install the tank. Now idle is 1000 and barely hanging there. So I adjust the idle screw, it now jumps to 2500 or more. Adjust it back down just a touch, no change. Adjust some more and it drops to 1000 and then eventually lower till the bike dies.

Since it only started this after the tank was installed, I am thinking the vacuum valve or its connection has a leak in it.

[92xjunker]

There isn't a vacuum valve, you mean petcock. Check the boots on the airbox.

[Kreylyn]

I would be more inclined to think it was carbs needing a very deep and good cleaning.

Krey

[dgxser]

Yes, I meant the vacuum valve on the back of the petcock.

As to the cleanliness of the carbs, while I thought I got them pretty dang clean, having cleaned more than a few of these Mikuni carbs before, I know they can never be to clean.

However, if the bike idles fine and the idle can be set and maintained with the gas tank off, a bolt in the vacuum line, and a tuning tank feeding the carbs through the same fuel lines. But with the gas tank installed be tough to get it to idle, and only with the tank on, turn the idle screw and get no response, then suddenly the idle jumps by over 1000 rpm.

It did not seem like it would be carbs since they are the same in both scenarios.

[92xjunker]

That's why I stated to check the carb boots. Thinking that the tank is contacting the carbs or airbox somewhere. maybe even a loose clamp

[CDNbakn]

So you're supposed to block the vacuum port on carb #4 when you sync? Seems a little odd since it won't be blocked under normal conditions when the tank needs it. How big of a difference does it make to sync the carbs while they're cold as opposed to hot?

[wing]

That's why I stated to check the carb boots. Thinking that the tank is contacting the carbs or airbox somewhere. maybe even a loose clamp

+1 if you don't position the fuel filter properly, the tank will sit on the carbs.

Also, how does it run on prime? Are you running it on Reserve or On position?

So you're supposed to block the vacuum port on carb #4 when you sync? Seems a little odd since it won't be blocked under normal conditions when the tank needs it. How big of a difference does it make to sync the carbs while they're cold as opposed to hot?

Carb #4 vacuum is blocked under normal condition (when plugged into the petcock). No air goes in, no airs comes out. The vacuum is just used to open the fuel valve.

Just another note with using a screw to block off the vacuum hose. If you're not careful, air can still get sucked through the screw's threads and into the intake. You could try using a c-clamp or something to pinch the hose.

[Stumpy]

Even easier - just bend the tube back on itself and tie-wrap it together.

[dgxser]

Wing, I do not have an inline filter on the system, just the one in the petcock. I also do not have the prime position on the petcock. The metal spring that actuates the PRIME function broke off. It is in the ON position with half a tank of fuel. It ran fine at highway speeds, as well as around town, so it seems to be fine on fuel flow.

I will try plugging off the vacuum more securely than the large bolt trick tomorrow. I will also pull the carbs back off and see if I can get them any cleaner.

I also do not have any clamps on the air box velocity stacks where they enter the carbs, none were on there when I got the bike. But the tank does not seem to hit on there.

[dgxser]

Parts sourcing

OK, so the first time I went through the carbs I got them good and clean and put all the same parts back in. However, I want to eliminate any question of the carbs being part of this problem, so I took them back out, everything still seems clean. Good spray through all the small orifices with Gumout carbcleaner.

In reassembling, those small black plugs over the pilot jet holes are not real tight. I squeezed them to get them tighter, but they are still loose now. So where is the best place to get those?

Also, I have a set of nitrile o-rings, had ones for the AF screws, the vac seal where the cap meets the body, and ones for the slide bodys where they meet the carbs. I do not seem to have any for the two on the float body. Anybody have the sizes of those? A source?

Finally the T at the top of the carb on the right side (between 3 and 4) looked fine, but apparently when I pushed the drain hose on it, it cracked apart. I can now look straight through it, the bottom is a hole. SO, a good source for that as well?

[Maverick.Marq]

Carb diagram

http://www.bikebandit.com/1993-suzuki-katana-600-gsx600f/o/m147847#sch279541

Carb fittings diagram

http://www.bikebandit.com/1993-suzuki-katana-600-gsx600f/o/m147847#sch258198