engine revs by itself

nhibberd · October 02, 2011, 12:19:42 AM

Hi, I'm new to this site and I'm looking for some advice. I just got a 01gs500 for close to nothing. It needed some engine work and being an automotive mechanic I have the tools and was able to repair it. Anyway my problem is now that I've repaired the engine, it wants to rev up to 5k with just bumping the throttle either in neutral or in gear with the clutch in. It will sit and idle just fine around 1300 rpm hot or cold until the throttle gets snapped. The same thing happens when the bike is driven, in any gear the engine will accelerate to 5k unless I put it into a higher gear where the engine can't pick up.
Now I've checked everything I can possibly think of. The throttle cables aren't pinched or binding and the throttle valves are working normally. I've completely taken the carbs apart, cleaned everything and replaced every O-ring. I have a half tank of fresh premium gas. I've sprayed carb cleaner over every inch of the intake, carbs, and all the mating surfaces without finding a vacuum leak.
I need any suggestions to help fix my bike. Is there any reason to think that the rubber diaphrams on the top of the carbs that hold the needle is the culprit?

gtscott · October 02, 2011, 02:05:26 AM

any freeplay in the throttle cable, there should be a little bit at the start, so maybee its holding the throttle open just a tiny bit

burning1 · October 02, 2011, 02:17:20 AM

Stupid question, but have you checked the idle speed adjustment? I'd also suggest ensuring that there aren't any issues with the fuel enrichment circuit.

I had an issue with my race bike where it wouldn't want to idle cleanly after a fresh carb rebuild. Ended up cranking the idle up a lot to keep it running, but once the revs came up, they would stay up just fine and wouldn't want to fall.

BaltimoreGS · October 02, 2011, 06:40:34 AM

Did you adjust the idle mixture screws on the carbs? From the factory they are covered with 2 brass plugs that have to be drilled out.

-Jessie

nhibberd · October 02, 2011, 09:15:45 AM

I've wanted a motorcycle for over 15 years and now that I have one it's killing me that I can't even ride it! Anyway there is a slight bit of free play in the throttle cable. The throttle opens and closes completely and smoothly. After i got the bike running and it's good and warmed up I adjusted the idle speed and set it to around 1300rpm. The bike will sit there and idle at the same speed all day long but as soon as the throttle gets touched the engine jumps to 5K and doesn't ever drop back. I also have the jet neeedle in the 2nd groove from the top and have the pilot screw set to 2 3/4 turns out. I did see the 2 brass plugs that cover the mixture screws but i did not touch them. Are the 2 mixture screws covered by the plugs the enrichment circuit?

burning1 · October 02, 2011, 11:58:31 AM

If you start the bike, and blip the throttle so that the engine revs rise to 5000 RPM, and then reduce the idle, what happens? Are you sure that your choke cable is routed properly, and that the choke is completely disengaged (on a motorcycle, there isn't a choke per se, there's a fuel enrichment circuit that will make the engine rev up to 3-5K RPM when fully open.) Is it possible that the enrichment circuit isn't closing fully, or that for some reason it's stuck open?

angelocardoc · October 02, 2011, 12:46:33 PM

My bike was doing that too.

I turned down the idle to 1,000rpm. Problem went away after that.
I don't know why? Motorcycle carbs are different than car carbs.

Dr.McNinja · October 02, 2011, 01:49:07 PM

Quote from: nhibberd on October 02, 2011, 09:15:45 AM
I've wanted a motorcycle for over 15 years and now that I have one it's killing me that I can't even ride it! Anyway there is a slight bit of free play in the throttle cable. The throttle opens and closes completely and smoothly. After i got the bike running and it's good and warmed up I adjusted the idle speed and set it to around 1300rpm. The bike will sit there and idle at the same speed all day long but as soon as the throttle gets touched the engine jumps to 5K and doesn't ever drop back. I also have the jet neeedle in the 2nd groove from the top and have the pilot screw set to 2 3/4 turns out. I did see the 2 brass plugs that cover the mixture screws but i did not touch them. Are the 2 mixture screws covered by the plugs the enrichment circuit?

Do you have any experience playing with carbs? You could've inadvertently brought this upon yourself playing with all the settings. The two mixture screws covered by the screws ARE the mixture screws. They set the air/fuel mixture. What is the "pilot screw" you are turning out?

Sounds like it could also be an intake leak. Get your engine to idle and *carefully* spray WD40 on the boots between airbox/carb and the boots between engine intake/carb. Listen for changes in noise. If you hear changes either your clamps aren't tight enough (unlikely) or your carb boots are shot and need to be replaced.

nhibberd · October 02, 2011, 06:23:05 PM

I didn't drill out those brass caps and mess with the screws underneath. The only screws I adjusted were the pilot screws under the carbs that are accessible without taking the float bowls off. I have a repair manual and it says to screw them all the way in and back them out 2 3/4 turns and thats what I did. I have sprayed the intake boots before and after the carbs and the entire carb itself with carb cleaner looking for a vacuum leak and found nothing. If I was dealing with a vacuum leak the engine should rev a bit by itself when I sprayed over the leak and that would mean I have a "full time" vacuum leak. My engine is acting like a vacuum leak because it won't drop below 5K, but the fact that it idles just fine is telling me something else is going on. Also when I took the carbs apart and cleaned them, I took pics to make sure everything went back in the way it came out. Not sure what else I can do.

BaltimoreGS · October 02, 2011, 06:42:48 PM

Not sure if we are all talking about the same thing. Also not sure what year these carbs below are off of. The mixture screw I'm talking about is under the brass cap, what screws are you referring to?

-Jessie

tboyer · October 02, 2011, 08:10:39 PM

@NHibbard,

We must be in parallel universes. I was recently given a 2001 GS500 that over-revs on start-up.

Take a minute and see if you note any fuel in the oil. I found that my frame mounted petcock/fuel valve was leaking.

...todd

burning1 · October 02, 2011, 08:12:05 PM

I've only rebuilt older, 1993 carbs... But I did take a peek at the Ron Ayers Fiche. You said you didn't remove the brass caps, and only adjusted the pilot screws? AFAIK, the only external adjustments are the screws under the brass caps, and the idle speed screw. Hopfully someone with experience with your carbs can confirm.

nhibberd · October 02, 2011, 09:30:05 PM

From everything I've read the gs500 stayed the same from 89 to 00. In 01 and 02 slight changes were made including an updated carb. For whatever reason they didn't produce a model in 03. From 04 until now everything should be the same. My bike is a 01 with the revised/updated carbs. Tomorrow I will post a pic of the pilot screw I'm referring too on my carb. With a light and a mirror I can see the pilot screw and the brass caps. Also I checked my oil and there's no smell of fuel. I don't know if there were 2 brass caps on each carb and the previous owner removed them or what but going by my repair manual one of the steps is to set the pilot screw. Thanks for everybodys help and I'll post a pic of the screw I'm talking about tomorrow.

nhibberd · October 02, 2011, 11:30:11 PM

Ok decided to post the pic of the pilot screw I've been talking about. I've labeled the picture. The pilot screw is the only thing I adjusted other than the idle speed. From what I've read this carb was only used on the 01 and 02 models. Sorry the picture is so small I tried to resize it but can't seem to get it right. If you need to see a bigger picture of it you might be able to copy and paste it in an editor program and expand it. I will try to make it larger in the morning.

burning1 · October 03, 2011, 01:47:22 AM

The little red circle looks like the carb drain screw, and the yellow circle is the brass cap that covers the pilot screw. I can't tell what the blue is.

BaltimoreGS · October 03, 2011, 03:28:13 AM

Yea, the red circle is the float bowl drain. Tighten that screw ASAP!

-Jessie

nhibberd · October 04, 2011, 12:30:58 AM

OK so I did a little bit of tinkering today and believe to have found something. In the past I have sprayed carb cleaner over the carbs and boots searching for a vacuum leak but never found anything. So today I thought to spray around the mating of the cylinder head and the block....and bingo! There's a sweet spot on both banks that kills the engine. I am going to recheck the cylinder head torque, if its out I'll re-torque it and see what happens. If it's in I'll pull the head and check it with a straight edge, repair it if necessary, and get another head gasket. It may be not be straight for all I know because I got the bike for free and the engine and all parts came completely taken apart in a box. Once I know if I got it fixed or not I will let everyone know. Thanks for everybody's ideas for my problem child bike.

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engine revs by itself