1977 GS400 High RPMs

[Zach]

Hey Everyone.. I just bought a gs400 to start as a winter project. I'm new to this and will probably be utilizing this forum fairly often. The bike starts and idles decent for a minute or two but then the rpm's slowly start to climb on its own and won't stop until the engine gets rev'd. It'll calm back down for a short while then start the process again. My initial thoughts are that the carbs are probably completely gunked up... Any suggestions?

Zach

 

[Guest]

Air leak,
Either the intake boots are cracked or the seal's behind them have rotted and cracked.

Quite common on something that old.

 

[Zach]

It doesn't have the stock airbox... It has two cone air filters attached directly to the carbs if I remember correctly

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[koolaid_kid]

The intake boots connect the carbs to the head. Either they or the o-rings (if your bike has them) are leaking.

 

[Zach]

Oh alright... I have a lot to learn... thanks!

 

[koolaid_kid]

You are welcome. And remember, everyone on this forum started out on square one. Don't be afraid to ask questions, many of us are happy to assist.

 

[Guest]

Welcome Zach! And I'll give a +1 to the intake leak comments.

It's also possible that the carbs have been jetted too rich on the pilot circuit but I'd say the intake leaks are more likely.

 

[850 Combat]

I see the cheap pod filters, and the mufflers appear to have been sawed off. Has anything been done to correct the fuel mixture for those changes?

 

[Zach]

I got a 2 to 1 exhaust that I'll be putting on after I get some adapters for it. Threw the stock one on without the mufflers (that were hacked off) just to see if it started and just haven't taken them off yet. No adjustments to my knowledge on the bike after the pod filters were installed though.

 

[Nessism]

Check the newbie mistakes thread in my signature. Several issues going on all at once...

 

[850 Combat]

I got a 2 to 1 exhaust that I'll be putting on after I get some adapters for it. Threw the stock one on without the mufflers (that were hacked off) just to see if it started and just haven't taken them off yet. No adjustments to my knowledge on the bike after the pod filters were installed though.

It won't run right until corrections are made. Either a jet kit, or stock parts. In addition, air leaks between the head and carbs need fixing, carbs need rebuilding, and valves need adjusting.

These bikes are sensitive to changes in the balance of vacuum leaks upstream and downstream of the carbs, and also decreases in back pressure in the exhaust. Particularly on the inlet side.

 

[koolaid_kid]

Check the newbie mistakes thread in my signature. Several issues going on all at once...

Absolutely. The rising idle is clearly an intake leak. This must be fixed first before you can work on any of the other jetting issues.

It won't run right until corrections are made. Either a jet kit, or stock parts. In addition, air leaks between the head and carbs need fixing, carbs need rebuilding, and valves need adjusting.

These bikes are sensitive to changes in the balance of vacuum leaks upstream and downstream of the carbs, and also decreases in back pressure in the exhaust. Particularly on the inlet side.

I agree. Once the rising idle is fixed, do not expect to jump on it and ride into the sunset. The next thing you will have to do is install the exhaust. Do not expect to be able to do any jetting changes of value until the intake is set and the exhaust is installed.
After that, the fun begins, which is to start taking stabs at jetting, which is an adventure in itself.
As 850 Combat stated, you will need to adjust the valves before tackling the jetting. You can do this at any time, even while you are waiting for the intake parts to arrive. But it must be done prior to the start of your jetting adventures.