5000 RPM at idle -- sometimes!

[spacesuit6]

Hey --

Just picked up a 1978 GSX400 and it's looking & running great, with one problem ...

When I pull in the clutch, or am coming down from speed (like to a stop light), the RPMs will stick up at 5000 RPMs. Sometimes, they'll stick there all the way through the red light. Sometimes, the'll settle back down to 1,100 RPMs and act normal.

When I'm at home, I can start the bike, and have it idle at 1,100 RPMs forever. But when I go riding, this sticking up at 5000 starts to happen.

I've been all over the idle adjustment screw, turning up, down, sideways, etc. The action at the throttle on the handlebars is crisp and fast to return from an open position. The throttle action from the cable between the carbs seems to be pretty ok -- it looks functional enough.

I do have a bit of Sea Foam in the gas tank -- the bike had been sitting and I'm sure the carbs could use a little dusting off with the Sea Foam.

Any idea what could be going on?

Appreciate all the help & insight.

Best,

Eric

--

2003 Kawi Concours, 30,000 miles
1978 Suzi GSX400, 15,000 miles

[pete]

Welcome to the forums!

Sounds like you have the notorious "hanging idle" issue, which is generally a leaky intake system.

You need to check that your airbox is sealed and your air filter in there is in place and oiled correctly, as well as checking the condition of the intake boots on the airbox side and the engine side.

Most likely the O rings in the intake boots on the cylinder head are brittle and cracking by now too.

Do some searching for "hanging idle" on here and no doubt you'll find lots to help you out

[terrylee]

Also could be your throttle cable, had similiar problem with 450, tried adjusting it and lubing, but still had problem. Bought new one, still had problem till I lube new one real well, no problem since. terrylee

[Tom R]

My first thought would be throttle cable. 5K "idle" seems pretty high from a vacuum leak, but it'd be a good idea to check out carb holders and such.

[BassCliff]

Greetings and Salutations!!

Hi Mr. spacesuit6,

I concur. It actually could be a combination of a leaky air intake system and a sticky throttle cable. For the cable; remove, lube, and re-install. For the air intake system, have a look at this article: Air Intake Repair

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Please click here for your mega-welcome, chock full of tips, suggestions, links to vendors, and other information. Then feel free to visit my little BikeCliff website where I've been collecting the wisdom of this generous community. Don't forget, we like pictures! Not you, your bike!

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[spyug]

Welcome.

What they all said. Air infiltration and gummy/sticking linkage somewhere. The first thing would be to pull, clean and rebuild the carbs along with a clean and lube of the linkage and throttle cable. Seafoam is good but its not likely to sort this issue.

Good luck with it and keep us informed.

Spyug