Kawasaki VN750 Forum banner

1 - 13 of 13 Posts

·
Registered
Joined
·
17 Posts
Discussion Starter #1
So I have noticed my bike revving with no throttle usually soon after starting the bike with choke. what's weird is the fluctuations in rpms. It can be at normal rpms(1100) and then jump up to 3k and sit there while idling and then after taking off it will drop. Is there anything anyone has encountered that might cause this? Recently installed new throttle cables. They work like a charm. Any help is appreciated.
 

·
Super Moderator
Joined
·
11,865 Posts
I'm not sure of what you are saying here. When you start the bike (cold) with the choke, the RPMs will go up as the bike warns up. That's why he choke is a progressive lever, so you can slowly back it down while it does.

You should not ride with the choke on. If you are saying that the bikes RPMs rise and fall after the bike has warmed up and ridden a few miles, you have a leak in your boots or a clog somewhere.
 

·
Registered
Joined
·
185 Posts
Revs

So I have noticed my bike revving with no throttle usually soon after starting the bike with choke. what's weird is the fluctuations in rpms. It can be at normal rpms(1100) and then jump up to 3k and sit there while idling and then after taking off it will drop. Is there anything anyone has encountered that might cause this? Recently installed new throttle cables. They work like a charm. Any help is appreciated.
My 2002 VN-750 revs higher during warm up and I just close the choke back back down a litle and it runs fine after that. BTW, the so called experts say to just warm it up about 15 seconds to get the oil distributed properly and then drive it gingerly without over revving it for the first couple of miles. Claiming thats the way to warm up any modern day engine without letting it idle wasting gas.
 

·
Registered
Joined
·
185 Posts
I'm not sure of what you are saying here. When you start the bike (cold) with the choke, the RPMs will go up as the bike warns up. That's why he choke is a progressive lever, so you can slowly back it down while it does.

You should not ride with the choke on. If you are saying that the bikes RPMs rise and fall after the bike has warmed up and ridden a few miles, you have a leak in your boots or a clog somewhere.
Oop's Knifemaker, didn't know you had made that post saying basically the same thing as I did! Ten lashes!
 

·
Premium Member
Joined
·
3,026 Posts
It is normal for the rpms to go really high with the choke all the way on. Mine will quickly go over 3000 rpm after the choke has been on all the way for a few seconds. You need to back off the choke to keep the rpms down. As KM said, do not ride it with the choke on. I warm mine up until the needle on the temp gauge reaches the second mark from the left before taking off. If it only does this with the choke on, I'd say it is normal. If you get fluctuations in idle speed with the choke off, then something is wrong.

I had an issue where the idle would remain around 3,000 rpm with the choke off, for 10-15 seconds after blipping the throttle, before returning to normal. It turned out to be a bad diaphragm on the coasting enrichener on the front carb. But many things can cause a fluctuation in idle speed. A vacuum leak, plugged jets or a passage in the carb, problems with the enrichener system including the cable, problems with the throttle and cable, problems with the vacuum diaphragms on the top of the carb.

Does it do this only when the choke is on?
 

·
Registered
Joined
·
17 Posts
Discussion Starter #7
I tried to be clear. The choke is completely off and the rpms fluctuate with no throttle with the bike just sitting there.
 

·
Super Moderator
Joined
·
11,865 Posts
I tried to be clear. The choke is completely off and the rpms fluctuate with no throttle with the bike just sitting there.
Well then, it can be a leaky carb boot, a clogged idle jet, torn diaphragm in the carb or other blockage, cylinder cutting out or even water in the gas.

I'd start with some SeaFoam and check the boots. Clean/replace the plugs and check for loose connections in the ignition system. Make sure the carb vents are clear.

Not holding a steady idle can encompass alot of things unfortunately.....

If it did not do this until you replaced the throttle cable I'd start there to make sure it's functioning properly.
 

·
Registered
Joined
·
17 Posts
Discussion Starter #11
It has had seafoam within last 200 miles, just replaced wires and boots, plugs only have 1500 miles on it. And it was occurring before the cable replacement.
 

·
Super Moderator
Joined
·
11,865 Posts
It has had seafoam within last 200 miles, just replaced wires and boots, plugs only have 1500 miles on it. And it was occurring before the cable replacement.
I think the reccomended proceedure for looking for air leaks is to spray WD40 around the boots and see if the RPMs change....might be difficult to judge given your problem. Check your airfilter and related ductwork too....along with all the carb vents and hoses.

You still have the stock airbox?
 

·
Premium Member
Joined
·
3,026 Posts
As a former private pilot (a long time ago, early '80s) I learned about properly warming up an engine before putting a load on it. Warm metal (as long as it is not too warm) is stronger and less resistant to wear than cold metal. I also used to be into air cooled VWs, and never knew one VW enthusiast that did not warm up their engine before taking off.

Anyway. As for the idle fluctuation with the choke off, it could be a lot of things. To me, there are two different types of idle fluctuations. One is where the idle is all over the place, with the tach needle dancing like crazy. That is usually caused by a misfire. That could be caused by dirty carbs, or an electrical issue. The other is the same, only much slower, where it will remain fairly steady for a few seconds, then goes up or down, stays there for a few seconds, then changes again. That type of fluctuation is usually caused by something that is moving. Could be caused by the vacuum diaphragms, (both the main ones, and the ones in the coasting enricheners), the coasting enricheners themselves, or sticky slides. Also possibly an erratic fuel delivery to the carbs, which can be caused by the vacuum petcock, or bad float needle valves. That type of fluctuation is rarely ever caused by a vacuum leak, clogged jets, or anything that remains constant. Also note that an air leak on the inlet side of the carb, like in the rubber ducts, airbox, or filter will not cause it. Those can lean out the air/fuel mixture, but will not cause an erratic idle speed. Mine will run just fine with no air filter at all. It is also almost never caused by an electrical issue.

I would start by removing and inspecting the coasting enrichener diaphragms. Hold them up to the light. You should not be able to see any light through them. One of mine was fine, the other had a lot of the rubber coating missing, and looked kind of like mesh underneath. I could clearly see light through it. Next, remove the rubber ducts that go from the carbs to the airbox from the carb inlets, push them aside, and look into the carbs while it is idling. If the slides are moving up and down, you likely have a hole in the main diaphragms (one or both) or they are not seated properly.
 
1 - 13 of 13 Posts
Top