help diagnosing a rough idle?

[Ruffus]

@Iahawk,
good to see you're tracking it. Pls compare old/new belt by lenght. In my case the old one (same brand Bando, same numbers on, same width, etc.) was 15 mm longer than identic new one. Was whipping and caused a rough ride-off.
Best bet IMHO is to change the whole part strain (rollers, belt, spring-coil, clutch) at once and from one brand, for me Malossi. Running, accelerating and decelerating smooth since then.
Only with the belt I did not stick to Malossis Kevlar item, for I read somewhere about rare cases of disintegrating (could have been an instaling issue too) after a while.

[Iahawk]

all right, thought I'd provide an update on my attempts to cure my rough vibration.

Scooter has been idling with much more vibration than it used to. It also seems quite a bit louder, both exhaust and motor.

after some suggestions from the members, here, I did the following:

-pulled the carb, fully disassembled and cleaned everything, twice
-replaced about half of my vacuum lines with new silicone ones
-replaced my fuel line and added an inline fuel filter
-replaced my intake manifold O ring (old one was completely flat but still appeared to seal)
-replaced belt with new OEM..appeared same length as old belt but was difficult to measure exactly
-replaced OEM rollers with Dr Pulley sliders, 13 gram
-disassembled, cleaned and regreased rear driven puley
-replaced contra spring in rear pulley
-cleaned and scuffed inside of clutch bell and sanded clutch pads
-cleaned and scuffed front drive pulley faces
-new NGK plug
-new air filter
-checked valves earlier, all in spec

reassembled everything, set idle, checked for any intake or vacuum leaks multiple times using both propane and starter fluid, spraying on all connections...no change to idle with any of these

adjusted air/fuel mixture screw to try to get the smoothest idle. Even reset the throttle position sensor (TPS) on the scoot using info from an old post.

After all this? no change. Idle is just as rough and vibrating. It kind of feels like my scoot is turning into an old Harley.

The upside is that with the changes it accelerates faster than ever from a stop and my top speed seems to be back where it was.

So I've checked the main suspects - carb, intake leaks, vacuum leaks and cvt. Nothing. Still vibrates a lot. Oh, well, at least I had a lot of fun tearing into things.

added a few pics of the carb and the new vacuum lines (red)

[Ruffus]

@Iahawk, seeing you did all possible, only one more question.
Did you check the axial play on your front boss-pulley?
(pls see pics).
If there is any up-down play, might cause this vibrations. Had this in my Mallossi drive.
Wait a couple of hundred miles with Dr.P's. They improve gradually.

[Iahawk]

Ruffus, thanks for the suggestion but there was no play in the front boss and pulley. I made sure to clean them up and used a little metal polish on the boss to make it as smooth as possible. It slides in and out but does not have any other movement.

It's vibrating enough that yesterday the front plastics were resonating...something I haven't noticed before..and that was after the idle had settled down after it was warm.  I guess the mystery continues...

[Kansas kymco]

Sadly this is a area where Kymco USA is lacking in help troubleshooting.  I've had this comment from my Kymco dealer.  I will ask him if he has any ideas next time I'm down to his shop.

[Ruffus]

@Iahawk, one of my last straws:
-could you check the rubber motor mountings?
This two or four rubber bullets on that teeterboard.

[Iahawk]

Ruffus, yes, that was one area I checked. Both rubber bumpers are still in place on the engine hanger.

One last thing..my exhaust is a lot louder than it used to be. I can't imagine that an exhaust (possibly rusted or broken internally?) could contribute to a strong vibration at idle?

[Ruffus]

@Iahawk, I see at this below

https://www.kymcoforum.com/index.php?action=dlattach;topic=30858.0;attach=19776

that you have an kick-start option. Is this assambly properly fixed within its CVT housing?

For the muffler I would simply take a piece of let's say 12mm / 1/2 inch reinforced plastic airhose, put it from behind into the COLD silencer and start shortly the scoot, to see whether there is any significant change in vibration or idle. But pull it out quickly, might give an strange odor when melting.

But reading all over, I still think the crux is somewhere in an airhose or carb.(pls see pics)
I read and know you did all this checking on your carb and airsystem, but watching following pics might give you an additional idea where to look at.
Pics are from a125 ccm LIKE, but your carb should look similar.

As for sparkplugs, I had sometimes, if nothing else helped anymore (particularely on early BMW 1100 oilhead motors) good success with BRISK 4-pronged plugs.(pls see pic)

[Ruffus]

.just two more pics to this subject

[Iahawk]

Wanted to update with some thoughts on my rough idle issue after a season of riding (edit - the idle is very smooth, it's the scoot that vibrates like an old Harley). After eliminating all of the usual suspects (carb, air leaks, cvt) I'm leaning toward Kansas Kymco's suggestion of an out of balance crank. The scoot still idles like a Harley but runs great.

After looking at more youtube videos of other scoots with bad vibrations...and many commenting about out of balance cranks causing these issues...and hearing from (who I thought was) Kansas Kymco's friend/bike shop owner..this may be the culprit.

It seems my (our) single cylinder Kymco motor has a 1 piece connecting rod on a pressed together crank (as opposed to a forged crank with a 2 piece connecting rod). It's possible for these to come unbalanced. When they do they will shake like a..well..shake a lot.

Not sure about the long term implications of riding it like this...may just be an irritating vibration...or it may eventually shake the scoot apart. I see that it is possible to remove and rebalance a crank but if I went to the trouble of removing the crank i'd just replace it with a new one ($145 USD).

I currently have no desire to pull the motor, crack it open and replace the crank. I'm not even sure I have the skills to properly do this.  I sure don't want to pay anyone to do this as the cost would likely far exceed the value of the scoot.

I've had the scoot for 6 years and have put under 2000 miles on it..as I have 2 other bikes to ride.

I can continue to ride it as is...I only feel the vibration at idle, not when riding. Wondering what you guys think about this or what you might do in my situation?

[Stig / Major Tom]

Only 2000 miles??
Vibrations shaking the scooter.....?

some pasta on the kitchen walls then.....:
wheel bearings
engine mount bushings
rear axle bearings
broken frame weld
screwy roller weights
clutch spring broken
stretched belt / wrong belt

Problem here is that measly 2,000 miles - can't see any of the above parts failing on a Kymco in only 2000 miles - including an internal engine part.
As an old medic - we liked to work sometimes by "ruling out" things.....can we rule out O.C.D.?

Stig

[Ruffus]

@hawk, I do have a crank here in my storage, will check it tomorrow and so some pics, how and where and if, such a rod exists...

[JJJoseph]

This is possibly not what you want to hear, but these are not really smooth-idling scooters, especially when cold.  My suggestion would be to simply ride it until you can't stand it any more, and then have another look at the various tunable items.

[Iahawk]

scoot has about 3800 miles..I meant that I'd only put about 2000 miles on it since I've owned it. 2010 model.

Earlier this spring I'd isolated it to the motor...I removed the variator, ran the scoot.. and the vibration was still there.

It seemed to occur with the first start up of the year this past Spring. Much stronger vibration at idle than ever before. that's when I went through it and cleaned, inspected and replaced about everything I could.

I'd never even heard of an out of balance crank until KK mentioned it as a possibility.

The scoot looks brand new, rides well, is nice and quick (with the Dr Pulley sliders) and runs great...except for the 'Harley like' vibration.

So I may do as JJ suggests..and just live with it! (unless Ruffus comes up with a secret solution!)

[scooterfan]

You mentioned that the carb has been cleaned but depending how the carb has been cleaned, I still think the problem has something to do with the carb. Or maybe an air leak in the intake system.

Maybe you should have a look whether the carb has been mounted properly, and make sure the bolts are tight. Again, make sure there is no air leaks in the intake system.
Depending on how the carb has been cleaned, I would remove the carb, remove the main jet, and submerge everything except the needle and seat spring in white vinegar for about 24 hous. Just make sure the seat spring does not get submerged in vinegar - because the vinegar could destroy the seat spring and  then you will be sitting wit a bigger problem. I’ve had such an experience on another application  before, and due to that mistake the coplete carb had to be replaced.

After about 24 hours in vinegar you can just blow out the the main jet and carb channels with an air compressor if you have one. Just make 100% sure the main jet channels are clean.

Side note - I cleaned my son’s electric generator’s carb this way two weeks ago. My son was at the brink of buying a new carb, but since the carb has been cleaned properly the engine now  runs like new.

Good luck, maybe this helps.