Carb vacuum problem?Hey Everyone-

[dmoore]

Hey Everyone - I have a '93 GS500E with a tad over 5K that I bought a week ago. It ran a little rough when I went to see it but I bought it anyway figuring it was a minor problem since the bike sat for a while. Got it for $850 so there's room for the usual repairs you generally have to make. The electricals are all good, and the clutch and gear box are fine. No noticeable engine noises. No signs of oil or gas leaks. Cosmetically it's near perfect. No signs of damage anywhere. It doesn't look like it's been abused.

The problem is the bike won't run for more than a few minutes with the petcock in the ON position but it does in the Prime position although a little rough. I understand that the PRI setting allows continuous fuel flow where the ON position depends on vacuum for fuel flow. Before I go tearing into the carbs, I want to determine if everything is connected properly. It's about 30 degrees here in Columbus, Oh so I haven't done any disassembly to be able to get a look at the carbs. The throttle and choke function well, by the way.

I'm more accustomed to working on 70s CBs which have different carbs so these Mikunis are new to me. If I understand correctly, fuel flows from the tank via the main fuel petcock to the frame-mounted fuel tap then to the carbs. From the Haynes manual I see there is a tee connector for one of the hoses but it doesn't show where the other hose connects. Is the other hose a vacuum hose? I noticed that the clamps are different at the tap. One is a screw clamp and the other is a typical fuel hose clamp. If this is the case it may be that the vacuum hose is not connected properly or is damaged, crimped or obstructed in some way. Before going in I'd like to know if I have this straight. Any help is greatly appreciated.

Thanks.

[jestercinti]

Fellow Ohioan,

Look for kinks in the hoses.  Also look for poor vacuum hose quality.  The vacuum hose connects to a little barb on the carb.  My model (2009) this is connected to the RH carb, but some choose to connect it to the LH carb.  Overall vacuum petcocks work well...it's mainly something else.  The diaphram in the petcock may be old and hard too.  You may have to replace the fuel hoses.  They wear out after a while.  The size is 7mm...hard to find.  5/16 is the closest SAE size.

This is how a vacuum petcock works:  http://wiki.gstwins.com/index.php?n=Upgrades.Petcock

And this is proper hose routing:  http://wiki.gstwins.com/index.php?n=Main.HoseRouting

Just do one thing for me.  GO TO IRON PONY.  There is not a store like that in Cincinnati. 

EDIT:  The house routing shows where the vacuum connects to the carb.  Looks to be the LH side where the petcock is.  If I was in the 614, we'd have a beer and figure this one out together. 

Welcome!

[dmoore]

Terrific info, jestercinti. Thanks a lot. I'll check all the hoses and connections to see if there's an obvious problem. Eventually I'll  take the carbs off and have them cleaned at Venturi Moto here in the Columbus area. But I first want to know how it all fits up. Thanks again.

[badandy]

I had a dual sport that did the same thing.  The petcock had two fuel screens on it.  One for "on" and one for "prime/reserve ".  The "on"screen was totally full of crud and starved the carbs.  The "prime" screen was nice and clean.  Ran fine in that position.  Pulled the petcock and replaced both screens.

[dmoore]

Thanks, badandy. This seems very plausible. This bike has had carb cleaning and gasket and o-ring replacement done by a dealership a while back. But it sat around a lot because the last owner just didn't ride it as much as he thought he would. Not uncommon. We have three MAJOR bike re-sale shops here in Columbus filled with low mileage bikes from guys like this. Weather is partly to blame but it's mainly people who THINK they're going to ride but just don't have the time or the money. As you know, motorcycling is not cheap.

Thanks again.

[weedahoe]

I know a lot of people dont like doing it but I leave mine in PRI

[dmoore]

OK. I removed the air box and inspected the vacuum line from the frame petcock to the left carb. No cracks or crimps and the hose was clear of any obstruction.  The connections were good. I then replaced the frame petcock altogether. The bike still won't run in the ON position for any more than a few minutes but it runs fine on PRI all through the gears. I checked and double-checked the hose connections against the repair manual illustration so I know I have the hoses connected right. The engine is running smoother now after a few test runs than it did when I first turned it over. So since the petcock and the hoses and connections are no longer a factor I'm left to believe there is something in the vacuum port on the carb that's affecting the vacuum circuit.

How should I proceed from here?

Thanks.

[weedahoe]

I have the same issue. Addidas says it is from using an aftermarket air filter. I leave mine in PRI and ride. You can do the same. No need to over complicate things

[Snake2715]

I recently bought fuel line and its hit and miss if 5/16 or 1/4 is the better choice.. Just an FYI there as 1/4 is apparently able to flow enough fuel and fits the barbs much better.

[dmoore]

OK I noticed when I drained the tank that gas drained only out of one barb. I can't remember which one. But if it was the ON barb that wasn't draining that might explain why I can only run the bike on PRI.