GS400S run problem

[atrent]

The bike starts and revs up to about 5k then slows to about 3k and then dies. I can keep it going by blipping the throttle and holding the rpm about 3K. It eventually dies this may be because I don't catch it in time. The carbs have been cleaned and the float level checked. Fuel is supplied from a 1 gallon can directly to the carbs so the petcock is not a problem. Although I felt the boots were ok the symptoms pointed to an air leak so I replaced the boots ($80) there was no change in operation. Tried a vacuum to check the slides for operation and they seem to be working just fine. I am getting very frustrated. ,

[tom203]

The bike starts and revs up to about 5k then slows to about 3k and then dies. I can keep it going by blipping the throttle and holding the rpm about 3K. It eventually dies this may be because I don't catch it in time. The carbs have been cleaned and the float level checked. Fuel is supplied from a 1 gallon can directly to the carbs so the petcock is not a problem. Although I felt the boots were ok the symptoms pointed to an air leak so I replaced the boots ($80) there was no change in operation. Tried a vacuum to check the slides for operation and they seem to be working just fine. I am getting very frustrated. ,

So you are satisfied it's not electrical ?- I reviewed your old posts.
Rigged up aux tank and even in this barely loaded state (parked), the bike needs you do blip the throttle to avoid dying? Have you checked the carb vacuum diaphrams for pinhole leaks? Carefully remove and hold them up to the light. No need to get frustrated- remember it got to the junkyard, cuz PO gave up on it.

[Nessism]

Did you do a full carb rebuild or just a quicky job? Soak the carb bodies in carb dip and replace the various O-rings? Airbox installed? Where are the pilot screws set?

[atrent]

Did you do a full carb rebuild or just a quicky job? Soak the carb bodies in carb dip and replace the various O-rings? Airbox installed? Where are the pilot screws set?

O rings on air screws replaced, o rings on boots replaced, carbs were cleaned rather thoroughly but not soaked. all the jets were cleared with a small wire. passages blown through with air and carb cleaner. air box installed. idle air screw tried at 1/2 turn increments from 1 1/2 to 3 with no noticeable change. A pin hole leak in the diaphragm was suggested but I can see no sign of any holes. Tried a vacuum to check the slides for operation and they seem to move freely and both carb slides react the same way to the application of vacuum.

[Steve]

How high is your aux. gas tank?

It should be about the same height as the stock tank.

If it is too much higher, the extra pressure can overwhelm the floats and flood the carbs.

.

[User]

How high is your aux. gas tank?

It should be about the same height as the stock tank.

If it is too much higher, the extra pressure can overwhelm the floats and flood the carbs.

.

Too much head. Also the first thing I thought of when you mentioned the aux. tank. Though I can't imagine that you could have raised it too much eh?
Has it ever ran right with this aux. tank? Did you modify the length of the fuel line?

[Nessism]

That aux. fuel tank needs to vent too. Sounds like the engine isn't getting enough fuel.

[atrent]

How high is your aux. gas tank?

It should be about the same height as the stock tank.

If it is too much higher, the extra pressure can overwhelm the floats and flood the carbs.

.

Aux tank is same height as stock tank, which has been removed. Cap is loose on can. I believe my problem is lack of fuel ? It appears that it is not getting fuel at low revs, however if I blip the throttle then it will pick up and I can keep it running but only above 3k RPM. I have the carbs off again, I am getting quite adept at this. I would like to make up a guage to check the bowl level while running but can't find a screw to fit (m6 x .75). My lathe is supposed to be able to cut this thread but it is a bit out. Chinese precision. McMaster Carr doesn't list this pitch either. What I need is a scrap carb bowl so I can salvage the drain screw.
How important is it to have no leaks in the air box? Are the air box boots prone to leak and would it matter? This bike was running on one cyl. when I got it, after cleaning the carbs and replacing the plugs it ran fine on 2. I rode it for about 100 miles and it stated to act up slightly so I pulled the carbs and at the time replaced the boot O rings. It hasn't run since so obviously I screwed up somewhere.

[atrent]

Too much head. Also the first thing I thought of when you mentioned the aux. tank. Though I can't imagine that you could have raised it too much eh?
Has it ever ran right with this aux. tank? Did you modify the length of the fuel line?

The fuel line is definately longer but I can't see this as a problem once the line is full of fuel. I have not run this bike with this aux tank but I have run my daughters 250 and my son ran his 450 with this same tank.

[tom203]

The fuel line is definately longer but I can't see this as a problem once the line is full of fuel. I have not run this bike with this aux tank but I have run my daughters 250 and my son ran his 450 with this same tank.

Well , it was just a thought! Can you take off airbox and observe carb action when you blip throttle?

[atrent]

Well , it was just a thought! Can you take off airbox and observe carb action when you blip throttle?

I am open to any suggestions at this point. I wonder if it is normal to have to fish out the spring, washer and o ring after removing the air screw? There is a wire mesh screen in the air box that would make it difficult to observe the slides and I think the air box has to be in place for the engine to run properly. Since I have checked the slide operation with vacuum I assume they would work fine with engine vacuum. There is good compression and the vacuum can be felt by hand over the snorkel.

[tom203]

I am open to any suggestions at this point. I wonder if it is normal to have to fish out the spring, washer and o ring after removing the air screw? There is a wire mesh screen in the air box that would make it difficult to observe the slides and I think the air box has to be in place for the engine to run properly. Since I have checked the slide operation with vacuum I assume they would work fine with engine vacuum. There is good compression and the vacuum can be felt by hand over the snorkel.

Ok, good vacuum, but is it pulling up needle out of main jet?

[atrent]

Ok, good vacuum, but is it pulling up needle out of main jet?

That I don't know for sure. But it runs ok at 3k with a little encouragement. Also when applying vacuum to the carb on the bench the slide lifts up quite nicely. However whether this happend when installed is unknown only assumed. My problems began after I removed the carbs the second time when it started to act up a bit. I also pulled the boots to examine them. I replaced the original boots with new o rings. Now I have a problem. The bike was running although not as well as I would have liked. Now it doesn't run good enough to even consider riding it. I have since replaced the boots with new ones with no improvement.