question about engine reving

[ff154]

New guy here, so please pardon my ignorance. I just got an 87 gs450l and am in the process of learning about it. The bike cranks and runs, but sometimes (more often when the engine is warm) the bike will rev up to 4500-5000 rpm on it's own. It will do this while I am riding (clutch engaged) and also while the bike is out of gear. When the bike is out of gear, I can just bump the throttle and it will jump to 4500 rpm. It eventually winds down, but I need to give it some attention. I have a Clymer manual on the way, but I was wondering if I can get some advise on where to start fixing the problem. Thank you in advance.

[semerm]

does it do this when you turn the handlebars?

[ff154]

Thank you for the quick response. It does not happen when I turn the handlebars. I forgot to add that the only thing I have messed with on the bike is the air cleaner. Cleaned out the air box and replaced the filter. All I have done is ridden a little to get a feel for what is going to need to be done to the bike. Other than crappy tires, this problem is tops on the list.

[semerm]

The reason why I asked about the turning when this occurs as it sounds like th ethrottle cable is binding on something. Will it do it when Ideling and not rolling?

[ff154]

It will do it while idling, but not until the motor is warm. When I first crank it, it idles just fine. But after riding for a while, it will rev up high. I have only noticed it a couple of times on its own, but almost every time I give it throttle. Also, if the bike is in gear, clutch engaged, it will rev but releasing the clutch to the friction point will bring it back down to normal until the throttle is rolled again.

[trepperuna]

Maybe I didn't exacly understood what happens, but are you sure that you don't have the choke cable stuck?
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[ff154]

I'm sure the choke cable is not stuck. I choke it when it's cold and it cranks and runs fine. Warms up, and the choke is turned off. It idles fine. Imagine driving down the road (at approx 3000 rpm) releasing the throttle and engaging the clutch; the engine revs up to 4500 rpm. It will do this from idle as well. Bike sitting on the stand, idling at about 1500, give it a little throttle, and it revs to 4500-5000.

[ff154]

I will be leaving for work soon, but I was thinking...could it be possible that I am getting extra air into the carbs from somewhere? Maybe from the rubber "boots" that connect the carbs to the intake? Just a thought. I will check back later from work. Thank you for the timely responses. This is a great internet site!

[Jethro]

I will be leaving for work soon, but I was thinking...could it be possible that I am getting extra air into the carbs from somewhere? Maybe from the rubber "boots" that connect the carbs to the intake? Just a thought. I will check back later from work. Thank you for the timely responses. This is a great internet site!

Yes, sounds like an air leak to me, check your carb boots. I find the best way to know for sure is to take them off, look right at the built in o-ring seal and squeeze the boot. If you see cracks, that thing has o go. I have tried testing with a spray bottle, even carb spray and had no change in engine speed, even though my carb boots were definitely cracked and leaning out my mixture.

Could also be a leaking choke plunger seal, leak from the petcock vaccum tube, or the petcock itself.

[KEITH KRAUSE]

Sounds like an intake leak to me.
If you like, you can FIRST try tightening any loose manifold clamps and re-test. If no joy, try removing the air box and spraying some contact cleaner (best) or carb cleaner towards the throttle plates because they could be sticking. If you can't reach them well enough, you'd have to remove the carbs and service them better.
With the carbs removed though, it would be smart to check the manifolds and any o-rings inside them for good condition.
Most of the time, an idle that rises with engine temp equals an intake leak somewhere at the manifolds. Once in a while you can have a sticking throttle plate.

[KEITH KRAUSE]

Ahhh. As I was typing, I see you're on the right track with the latest replies.

[semerm]

After the cable issue, that was my second thought (air intake leak). You could try spraying some carb cleaner near the air intakes (on idle) and see if the rpm's change. If they do it is leaking there.

[ff154]

Thank everyone for the replies. I will check the air intake avenues next chance I get. I will post the results when I find something out.