help diagnosing a rough idle?

[Iahawk]

Hey guys, I was hoping this issue would go away on its own but it hasn't. Starting with my first start this season (2 weeks ago) my scooter is idling much rougher than it did previously. I'm used to some normal vibration at idle but my current vibration is about double what it used to be. It runs fine and I don't notice anything different when riding, just when idling. I've done about 4 rides and 75 miles this month. Scooter sat in garage for past 2 1/2 months from last ride last season to start of this season.

Fresh oil change last December, level full. I checked valve clearances but they were all in spec so i changed nothing. Carb seems to be working fine, although I do have a new carb bowl gasket on order and am planning on taking out carb and giving it its first thorough clean in its life. Still on same tank of ethanol free fuel with stabilizer that I filled it up with in December.

Scooter is 2010 People S200, 2 valve, air cooled 163cc with carb. 3100 miles. Iridium plug was installed around 1100 miles ago. Scooter fires right up but idles a lot rougher than it used to.

In diagnosing this I can only think of a few things that could cause a rough idle...

ignition coil and plug (potential ignition issue causing misfire?)
carb
engine mounts (will look closer but don't see anything that appears off/broken/loose)
something in the CVT (haven't pulled cover, yet)

Any other suggestions or places to look to diagnose the rough idle? It seems like this should be an easy one to fix..

Thanks for any help.
IaHawk

[Ruffus]

@lahawk,
for me the first suspect would be my carburator. (for it sat and didn't have any symptoms before)
Is it surging or just limping on idle?
Surging would mean, IMHO, lack of fuel and stuttering
I would put to an air leak somewhere or just a speck of dirt in an bypass.
Or, if this roughness comes from your CVT section, rollers and plate could be a little bit corroded too.

[stuo]

If all its doing is vibrating but running steady, not missing, look for an air leak. What you describe is a too lean situation.

[Stig / Major Tom]

Only 3000 miles in 11 yrs. I would suspect carb needs cleaning....
Are you using a Denso or NGK,  Iridium plug?
I found my 163 engine preferred the NGK...giving a stronger idle RPM.
Look for air leak, if OK... then run a shot of that cleaner.... Chevron Techron.. Stuff got my old Jetta through its last E-Check.
Good luck.
Stig

[Iahawk]

good ideas for me to investigate, thanks!  Stig - it's an NGK iridium plug ..

so air/vacuum leaks or dirty carb...2 things I can definitely do. My carb bowl gasket should be in this week so I can pull it apart and do a full clean and I'll also look for any air leaks. I'm off work this week to coincide with my kids' Spring Break so I will have plenty of time.

Doesn't seem to be missing...just vibrates a whole lot more while idling. Thanks, again, for the quick responses.

[Ex-KLR]

Has your idle speed decreased? That might cause vibration.
That said, I prefer the carb as the culprit.

[Stig / Major Tom]

Has your idle speed decreased? That might cause vibration.
That said, I prefer the carb as the culprit.

+1

Stig

[john grinsel]

First 11 year old scooter with no miles----bound to have some fuel system problems-   Me, I would change spark plug to proper NGK heat range standard plug, making sure plug outer is clean (so doesn't fire around), fresh tank of corn free fuel, some fuel system cleaner in tank-----go out, run tank of gas thru in one day---refuel, set idle correctly.  Might be ok.   Bike that sits as much as this one does, should be parked only with full tank of fuel, something like Staybil added.

[Iahawk]

Great advice from all. I'll look for any intake or vacuum leaks and still plan on giving the carb a full clean. I don't think the idle has changed significantly but I'll get a tach on it and confirm.

It's funny..I think I have a pretty good handle on most things mechanical and then something as simple as a much rougher idle initially stumps me. Thanks for all the input and I'll report back later in the week with what I come up with.

[Iahawk]

to follow up...removed carb and fully disassembled it and gave it a good clean. Also replaced carb bowl gasket, as old one was squashed very flat. Replaced fuel line while i was in there. No change to the idle roughness. I initially looked for an intake leak using one of the methods suggested by many online...hitting it with a small propane cylinder...didn't find any leaks. The other method suggested is to hit the various connections with carb cleaner, looking for a change in idle...will probably give that a try, too.

Put a tach on the scoot and got the idle adjusted to roughly 1700. It's been years since I've had a tach on it..interesting to see the idle speed fluctuate by up to 150 rpm.

Drained fuel tank, replaced with fresh, non ethanol fuel, added marine stabil and some techron.

 Will try to get in a ride this week. Last week the weather was beyond awful around here and then I had to travel out of town for the funeral of the husband of a young lady who works for me. She is 33, he was 38. Very sad week.

[Kansas kymco]

I would look for a vacuum hose leak. There is a couple of house going to the fuel petcock and the emissions smog pump. If one of those is cracked/split it will cause a rough idle .

[Iahawk]

thanks, Kansas Kymco, I don't see any clear cracks or breaks but the vacuum lines are all original and kind of stiff. I went ahead and ordered new vacuum line (in red, should look good!), a package of T, Y and straight connectors and package of assorted spring clamps, since the ones on there are a pain to take off and put back on...and I even broke one pulling the carb and doing the fuel line.

Shouldn't be difficult replacing the vacuum lines, just kind of a pain. Must be an age thing because man my back gets sore leaning over to work on the motor from the top!

So the journey continues and I'll update later in the week. Amazon shipment will arrive on Wednesday.

[Kansas kymco]

thanks, Kansas Kymco, I don't see any clear cracks or breaks but the vacuum lines are all original and kind of stiff. I went ahead and ordered new vacuum line (in red, should look good!), a package of T, Y and straight connectors and package of assorted spring clamps, since the ones on there are a pain to take off and put back on...and I even broke one pulling the carb and doing the fuel line.

Shouldn't be difficult replacing the vacuum lines, just kind of a pain. Must be an age thing because man my back gets sore leaning over to work on the motor from the top!

So the journey continues and I'll update later in the week. Amazon shipment will arrive on Wednesday.

As Dad use to say it sucks to get old but sure beats the alternative.  Probably would be good to have a motorcycle lift to work on scooters. Good luck getting the issue resolved.  I will ask my local dealer owner next time I visit with him. He has 40 years mechanic experience on motorcycles.

[Ruffus]

@lahawk, just as a shot....
if you take your CVT cover off, please check your drivebelt whether its knocking at its housing on idle.
Then spray kind of WD40 or similar on your clutch-shoes, trough the openings. Revv up a little
(on mainstand and rear wheel in air !!!) and then, what does it do on idle again?

[Iahawk]

Ruffus - your suggestion may be the winner. I have clearly visible black belt marks inside my cvt case on top and bottom. The belt also shows a clear mark on the back side where it's been rubbing.

I'm still waiting for a new belt and also ordered a new contra/torque spring for rear driven pulley. i disassembled both my existing rear clutch/pulley and my spare..(practiced on the spare, first)...cleaned out old grease, re greased and also measured length of spring. Both were quite a bit  below minimum length, per manual.

So to attempt to fix vibration at idle I went through the following things:

cleaned carb
checked for intake or vacuum line leaks (replaced about half the vacuum lines, wasn't necessary but looks better)
checked for belt hitting inside of cvt case.

more to come..