GS400XB Bogs on Stops

[rymac11]

Hey Guys,

So I was having some carb issues and I had the carbs overhauled, cleaned, and one of the floats replaced (because it had a small hole in it). Since then it has been running better but it bogs down to about 100 rpms upon braking to a stop and also the engine cut a couple times while sitting in traffic. It idles between 1200 and 1500 also normally but it waivers. Any ideas? I heard people talking about petcock problems, maybe this could be the issue?

[tom203]

Sounds like your low speed carb circuits didn't get properly cleaned, so bike has trouble at low rpms. But it could also be an intake leak at carb boots to cylinder head interface giving you a false/random idle (so you set idle stop screw too low) - did you or someone replace the gaskets at cylinder head ?

[rymac11]

Sounds like your low speed carb circuits didn't get properly cleaned, so bike has trouble at low rpms.

Is there an easy way to check for this? Maybe like inspecting the spark plugs or something? I just put a fresh set on after the carbs were overhauled because the old ones, which I replaced when I got the bike initially, were all fouled up with carbon.

But it could also be an intake leak at carb boots to cylinder head interface...did you or someone replace the gaskets at cylinder head?

He said he replaced all the needles and gaskets. I'm not sure if these gaskets are included in most overhaul kits. If they are, he replaced them.

...giving you a false/random idle (so you set idle stop screw too low)...

Is the idle stop screw the air/fuel screw that adjusts the idle? If it is, it could be set too low however it seems to idle at the correct rpm (~1200) most of the time while sitting still permitted the engine is fully warmed up.

[tkent02]

If you want something like this done right, you usually have to do it yourself.

Sounds like leaky intake boots and messed up carburetor adjustments.

[rymac11]

If you want something like this done right, you usually have to do it yourself.

I can understand that, however, the guy I had do the overhaul has a lot of experience with motorcycles and owns his own shop. I also don't have much experience in the area of carbs.

Sounds like leaky intake boots and messed up carburetor adjustments.

What is the best way to check these and/or seal them up? Or are they leaking because they are old and deteriorated? Initial inspection doesn't show any serious signs or wear.

[tkent02]

Take the boots out and look at them. If they are cracked, hardened or misshaped, replace them. Should be soft pliable rubber. Replace the O rings between the boots and the head. They are supposed to be round, soft and fatter than the slot, so they are above the flat surface of the bot a little bit. Heck, just replace them, they are cheap.

Then do the entire carb cleaning routine on Cliff's website, might as well do all of the other neglected maintenance as well before something expensive burns up..

[tom203]

Looks like you got a gasket at head/ carb boot interface- unlikely to be doing its job after 35 years! Inspect boots carefully for cracks. You need to ensure a tight seal in this area.

http://www.alpha-sports.com/spst/1977%20GS400X/01.htm

[rymac11]

Ok, thanks for that information. Yes, I have been trying to have things done/do what I can myself to make up for all the potentially neglected maintenance due to previous owners. The thing is, this bike only had 3000 miles on it when I bought it.

I do have an added detail. I let the bike warm and idle. When I put pressure on the front forks and depressed them, thats when the idle dipped from about 200 rpms. I assume that when I stop, the amount of force applied to front forks is much greater than that I can apply with my body. So if we assume that the carbs were cleaned and overhauled correctly and that the boots are in fine shape and not leaking, does this hint to an issue with how the floats are set?

[rymac11]

Looks like you got a gasket at head/ carb boot interface- unlikely to be doing its job after 35 years! Inspect boots carefully for cracks. You need to ensure a tight seal in this area.

http://www.alpha-sports.com/spst/1977%20GS400X/01.htm

Ok, in the meantime I will remove the boots and inspect the insides for cracks since the outsides look fine. Age and sitting can definitely ruin the integrity rubber.

[tom203]

"I do have an added detail. I let the bike warm and idle. When I put pressure on the front forks and depressed them, thats when the idle dipped from about 200 rpms. "

This happens instantly?

Only 3000 miles on this? wow, definitely worth springing for the few parts that likely need replacing. As tkent02, it's really best to do the work yourself.

[rymac11]

Yes, as soon as I depress the front forks the idle drops and then raises back up to normal upon the forks returning to normal position.

Yep, only 3k on it but it was sitting for a while so I know that can cause some things to dry rot.

[tom203]

Yes, as soon as I depress the front forks the idle drops and then raises back up to normal upon the forks returning to normal position.

Assuming you weigh less than 1000 lbs, your carb float levels are off!

[rymac11]

Haha I think I can safely say I weigh much less than 1000 lbs.

[rymac11]

Ok, so just to end this saga with a bit of a mystery, this behavior by the bike has stopped and it seems to run normally. Maybe it just needed to be ridden a bit after everything was cleaned/replaced?

[tom203]

Ok, so just to end this saga with a bit of a mystery, this behavior by the bike has stopped and it seems to run normally. Maybe it just needed to be ridden a bit after everything was cleaned/replaced?

I love happy endings! but if your random idle sneaks back, suspect an intake leak. Enjoy