Why is my motorcycle engine RPM rising when I apply the clutch?
January 13, 2011 11:20 AM Subscribe
I have a 1982 kz750 LTD (inline 4) with stock parts. For about the last three months, the engine RPM has increased 1-2k when i apply the clutch. Additionally, over the course of about an hour, the idle RPM will increase to around 3.5k.
I have the idea that this is either a valve problem or a vacuum leak, but google hasn't been much assistance. I've done most of the work on this bike since i've owned it (including carb & starter rebuilds, electric work, etc) so i'm fairly confident that I could fix this if i knew where to start.
I have the idea that this is either a valve problem or a vacuum leak, but google hasn't been much assistance. I've done most of the work on this bike since i've owned it (including carb & starter rebuilds, electric work, etc) so i'm fairly confident that I could fix this if i knew where to start.
Does the idle RPM change with clutch in/out when the transmission is in neutral? I can't think of any connection between clutch position and idle on a bike that old, other than more/less mechanical drag/friction through the transmission.
My first guess for the change over time would be temperature-related. If you run it for an hour (so the idle has crept up), then shut it down for a few minutes, then start it up again, does it go to the high idle quickly/immediately?
At any rate, I'd start by looking at hoses, carb boots and so on, trying to find something that's cracked and admitting too much air. Maybe the choke system?
posted by spacewrench at 11:42 AM on January 13, 2011
My first guess for the change over time would be temperature-related. If you run it for an hour (so the idle has crept up), then shut it down for a few minutes, then start it up again, does it go to the high idle quickly/immediately?
At any rate, I'd start by looking at hoses, carb boots and so on, trying to find something that's cracked and admitting too much air. Maybe the choke system?
posted by spacewrench at 11:42 AM on January 13, 2011
Response by poster: No change to idle RPM with the engine in neutral - only when in gear and rolling.
Running for an hour, shutting down, and starting it up a few minutes later results in the RPM going back to the point it was at shutdown.
posted by casconed at 11:51 AM on January 13, 2011
Running for an hour, shutting down, and starting it up a few minutes later results in the RPM going back to the point it was at shutdown.
posted by casconed at 11:51 AM on January 13, 2011
Weird idle problems often are related to vacuum leaks. My first thought is there's a small split on one of the rubber boots that mounts your carb to the cylinder head. One way to check is to see if the idle changes when you spray WD40 onto each boot (if there's a leak, the engine will suck in WD40 rather than air)
posted by zombiedance at 12:00 PM on January 13, 2011 [1 favorite]
posted by zombiedance at 12:00 PM on January 13, 2011 [1 favorite]
Isn't this just a case of the clutch slipping?
posted by Cool Papa Bell at 12:10 PM on January 13, 2011
posted by Cool Papa Bell at 12:10 PM on January 13, 2011
Best answer: I had that exact same bike with a very, very similar problem. Those carbs suck a LOT of air, but they're very finicky about just exactly what they're getting. I think it's one of the following:
-heat related vacuum leak, either in a carb boot, vacuum line, or carb-to-engine-gasket.
-Something to do with your airbox---those bikes are soooo finicky, mine tended to run better when I stuffed a rag in the airbox with the filter. I don't think this is your problem though
-A sticky carb needle that's not springing back closed
-Those bikes also have very sensitive electrical systems. It's entirely likely you've got something messing with your spark, although I don't think this is it either.
The way the motor increases rpm's is to open the carb flutter, which lets it suck more air, which raises the floats which opens the gas line which dumps more air/gas into the cylinder...etc. So basically, your problem is air, because more gas w/o more air and it would choke.
posted by TomMelee at 12:14 PM on January 13, 2011
-heat related vacuum leak, either in a carb boot, vacuum line, or carb-to-engine-gasket.
-Something to do with your airbox---those bikes are soooo finicky, mine tended to run better when I stuffed a rag in the airbox with the filter. I don't think this is your problem though
-A sticky carb needle that's not springing back closed
-Those bikes also have very sensitive electrical systems. It's entirely likely you've got something messing with your spark, although I don't think this is it either.
The way the motor increases rpm's is to open the carb flutter, which lets it suck more air, which raises the floats which opens the gas line which dumps more air/gas into the cylinder...etc. So basically, your problem is air, because more gas w/o more air and it would choke.
posted by TomMelee at 12:14 PM on January 13, 2011
Clutch could definitely be slipping too, although hopefully the rider would notice the weird shifting and/or odor. That makes a lot of sense really with it only happening in gear...
Clutches on that bike are easy-peasy, worth a look.
posted by TomMelee at 12:16 PM on January 13, 2011
Clutches on that bike are easy-peasy, worth a look.
posted by TomMelee at 12:16 PM on January 13, 2011
Best answer: If your revs go up as the bike gets warm, if sounds to me like the clutch pushrod is out of adjustment. As the engine heats up, the case will expand and the rod will go further out of adjustment, and the clutch will slip (as if you were pulling on it part way) and the revs will go up.
The fact the revs go up when you pull in the clutch indicates further that your adjustment (rod or cable) is not in and you've raised the idle to compensate. In other words, you're clutch is always in part way and you've raised the idle to compensate.
So do this: First, back the adjustment off at the lever so you have slack on the cable. Next, find the adjustment where the cable connects to the rod. Back it out until you're sure the connection is broken then slowly push it back until it just contacts the rod. Now set the idle on the carbs (it should be high). Now set the adjustment at the lever to what you are comfortable with in terms of pull to the friction point.
It's possible your plates are worn down because of this, and replacement is a bit more of a job, put you'll notice a swampy smell if they're worn down.
If it's a vacuum leak, you can test by spraying wd-40 or carb cleaner around the carb and listening of a change of RPM when you spray close to the leak.
Also check your timing, and make sure it's not going into "advanced" timing (the timing for full chat as opposed to idle) too soon.
posted by 1f2frfbf at 5:21 PM on January 13, 2011
The fact the revs go up when you pull in the clutch indicates further that your adjustment (rod or cable) is not in and you've raised the idle to compensate. In other words, you're clutch is always in part way and you've raised the idle to compensate.
So do this: First, back the adjustment off at the lever so you have slack on the cable. Next, find the adjustment where the cable connects to the rod. Back it out until you're sure the connection is broken then slowly push it back until it just contacts the rod. Now set the idle on the carbs (it should be high). Now set the adjustment at the lever to what you are comfortable with in terms of pull to the friction point.
It's possible your plates are worn down because of this, and replacement is a bit more of a job, put you'll notice a swampy smell if they're worn down.
If it's a vacuum leak, you can test by spraying wd-40 or carb cleaner around the carb and listening of a change of RPM when you spray close to the leak.
Also check your timing, and make sure it's not going into "advanced" timing (the timing for full chat as opposed to idle) too soon.
posted by 1f2frfbf at 5:21 PM on January 13, 2011
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posted by cmoj at 11:28 AM on January 13, 2011