Engine screaming on start-up

[steve-lloyd]

My son recently aquired a 82 model GSX400F.
On start-up the engine immediately screams straight to the redline and beyond!
I can find no reason for this, I have even disconnected the throttle cable and set the idling adjuster screw to minimum with choke off, and when turning the throttle mechanism by hand, it springs back closed strongly.
How can this be? Help!

[tkent02]

Got to have air coming in somewhere...

I had one bike that had the throttle slide in backwards so it could not shut. Could be something like that. More likely a large air leak into the intakes, it takes quite a bit of air to rev that high. Look for something missing that would allow air in.

[mark]

A botched carb synch could also have your slides way too high. I tried to synch my carbs once and forgot to plug the vacuum line. To even get it close the engine had to be racing pretty good. With the airbox pulled off you should be able to see how much distance is between the bottom of your throttle slide and the opening. Shouldn't be much.

[Dave8338]

Sounds like a suspect choke circuit and air leak combo to me. What happens if you use full choke on start up...do the rpms drop, stay the same, go up? If the slides are set correctly as Mark mentioned you should check, I'd take a closer look at the choke circuit...

[steve-lloyd]

The engine screams instantly at dangerous revs and it makes no difference if the choke is on or off.
Mark, can you clarify what you mean by throttle slides?
The carbs have throttle plates on the engine side and diaphram-controlled pistons on the airbox side, which are the only ones that can be seen with the airbox removed without taking the carbs off.
I really don't want to remove the carbs, as they are an absolute nightmare to refit on these bikes.

[Dave8338]

It sounds as if you're going to need to tear into things a little further. I'd suggest checking the slides by removing the four screws that hold down the covers on top of the carbs themselves. There will be a spring, about 2 inches long that will push the covers off, once the screws are removed.

Remove the spring and the diaphragm with the slide connected to it. Check to see that the slides are free from scratches and varnish. They should move freely up and down in the carb body. Look for pin holes and tears in the diaphragms and replace if needed.

Start here and let us know what you find.

[steve-lloyd]

Thanks guys for all your advice. The problem turned out to be multiple air leaks. The previous owner had made a complete hash of fitting the carbs and air box.
The carbs were not even seated properly in the intake stubs.
Until now, I would never have believed air leaks could make an engine rev like that!
I still don't understand how the engine received enough fuel to do that on a closed throttle.