GY6 - Sudden Loss of Power, Bad smell, Running lean, Carb OK By kancur (Read 7,473 times)

[kancur]

Hi guys, thank you in advance for your help!

My scoot:
Keeway ARN 125
GY6 153qmi - 4 stroke

The problem:

I was driving WOT at about 90kph (55mph) for about 10 minutes and suddenly the scooter lost power and started decelerating, the engine was still working but I was only able to manage about 40-50kph (25-30mph) after that.

Since then, it can not exceed 50 kph (30mph), and having half throttle and WOT doesn't make much (any) difference.
The exhaust gasses smell horribly - I suspect lean condition. I don't have a thermometer but engine seems to be much hotter after a short ride than ever. The spark plug shows signs of lean conditions.
Engine was also firing into the exhaust on deceleration. 

It seems to rev just fine on stand. But doesn't have enough power on the street.


At first, I adjusted the valves. The intake valve had no valve lash at all.
Only effect I noticed was it is not popping or firing into the exhaust anymore, but this might just be a coincidence.

The engine idles, but it's pretty rough, but I have long term problems with setting this correctly.

The first 0-20 kph seems ok, but then the power diminishes and runs terribly. It does not boggle or anything, the sound is nice, but it just doesn't have the power.


Then I checked the carb.
Carburetor OK - all jets clean, I changed the diaphragm just to be sure. It definitely seems to be working OK. And anyway, it would be pretty strange if it suddenly "broke down". I can only imagine three ways carburetor can make a problem like mine: 
1. Damaged diaphragm (It's not)
2. Clogged main jet (It's not)
3. Problems with fuel level (This might be it, but I don't see it happening "suddenly". The float level is not adjustable on my carb, and it definitely seems to be OK to me. I would say this is NOT the problem)

Air leaks: I checked for air leaks, and found none. I've had problems with air leaks before, but they were mostly present when idling. When the butterfly in the carb opened under WOT, under these low vacuum conditions the vacuum leaks never were a problem. So I'm pretty sure air leaks are not the cause.

I recently changed bad CDI for a new one, which worked flawlessly, so I don't really see how this could cause a lean condition or loss of power all of a sudden.

My exhaust is in a pretty bad shape and I wouldn't be surprised if there are problems there, but how could it make engine run lean? I don't know.

Can engine running lean be caused by something wrong with the engine itself? Like piston rings or something? 

I'm pretty lost. I'm thinking about opening the engine and checking the piston etc. (I've changed my piston and piston rings before) but it's quite a lot of work and if it's useless, why do it. 

Feel free to challenge my conclusions. I'm definitely not a pro and might be interpreting the symptoms all bad - maybe it runs rich in the end..

Thank you for your help! 

[cyborg]

I'm sure diver will chime in here but it sounds like a soft seize to me,,,did you check the oil level?

[kancur]

Thanks for the answer.
Yes, the oil level is fine. The engine is pretty easy to turn by hand, but it has compression - Seem just about normal. And the problem doesn't seem to get worse when engine heats up - it just works bad all the time now.

Another thing came to my mind. There's a big difference on how the scooter idles on the stand vs. on the ground. Don't know if it means something.

EDIT:
Maybe the engine running lean isn't the original problem. Because I tried moving the needle in the carb after that incident but it did not help. Actually, I don't think it was running lean when the problem occured - the needle was in it's upmost position. I actually think it might have been running rich when that "thing" (loss of power) happened when I was doing 90kph. 

Anyway, the carburetor is in it's original setting, with original jets, which always worked just fine, and I don't see a reason for it to run lean due to carburetor suddenly. :/ 

But are there any reasons except carburetor and air leak why engine can run lean or hot?

[czmdiver]

Ok slow down there partner and lets confirm some things before chasing tail in the wrong direction.
You may have a multiple problems here.
How many miles on her? or her current top end?
We Really Really need to confirm either the lean or rich condition, please take some pictures of your spark plug and post them.
You mentioned you adjusted the Valves and there was near or at "0: lash. Not Good! Go back and Triple check your work.
You may consider checking the local parts store for a loaner Compression Tester to Confirm Compression...
Also post some pics of your exhaust, I had my OE exh come all apart internally and got so hot it burned the mounting straps. I pulled it down and found a couple problems.
If your exhaust was or is my like my OE it will jack up the tune.
You mention it runs different on the side and center stand, When was the last time you serviced the Vari? is it and the clutch releasing at idle? pull the cover and put it on the stand and have a looksee.. take some pics and post them is the best way we can help you...

[czmdiver]

The can or Muffler portion completely fell apart internally and with the flange you see the big goober of weld and the flashing blocking off a good percentage of exhaust port had the engine running like crap...

[czmdiver]

I neglected to adress the Piston question, No the Piston assy, won't make the engine go lean so Do Not pull the cylinder off until you positively confirm with a Compression tester..

[kancur]

Hi czmdiver!

It has more than 40 000 km (25,000 miles) on it. 

If by current top end you mean speed (sorry I'm not a native speaker), it is about 50 kph (30mph).

I'm gonna dive into it right away and will post some pictures.

Yep, the day after that thing with losing power happened, I checked the valves and intake valve had zero lash. I tried to check if valve spring is not broken but I haven't found a problem. I probably made one pretty bad mistake when adjusting the valve when I think about it, I rotated it while I was pushing the spring with my thumbs. So when I adjusted the lash, the valve might not have been seated properly.   --- DEFINITELY have to recheck that ASAP.

My exhaust is like 4 times welded. But never caused any visible problem before. Will check that too. 

"If your ex did or is my like my OE it will jack up the tune.
"

Sorry I don't understand what that means. Can you explain it in some other words please? 


About it running different on center stand and without stand at all (in riding position):

It has higher rpm when on idle and on center stand - The wheel doesn't rotate and clutch works well. When I put it on the ground, the rpm drops. This is probably due to the fuel level changing a bit in carburetor.

I serviced the vario last year. And yes clutch is releasing. 

Thank you, Will post pictures and updates. 

[kancur]

I need to slow down for sure. I really mixed a lot of stuff together. Back to the facts:

I was on a short ride - and the sound definitely is not the same as before. The sound from the muffler is not the same, it is more popping instead of humming (Sorry really don't know how to describe it in english ). But I noticed the different sound mostly in the first few minutes only.

There's no misfiring or anything, the engine runs nicely, but has no power.

I took some photos and will upload them probably tomorrow.

Anyway, I detached the whole engine from the scooter as I decided I should change some things (head gasket and seals). And put a C clip instead of a G clip on the piston. So I guess I will start from the bottom anyway.

[czmdiver]

Mar 7, 2018 11:38:06 GMT -6 kancur said:

I need to slow down for sure. I really mixed a lot of stuff together. Back to the facts:

I was on a short ride - and the sound definitely is not the same as before. The sound from the muffler is not the same, it is more popping instead of humming (Sorry really don't know how to describe it in english ). But I noticed the different sound mostly in the first few minutes only.

There's no misfiring or anything, the engine runs nicely, but has no power.

I took some photos and will upload them probably tomorrow.

Anyway, I detached the whole engine from the scooter as I decided I should change some things (head gasket and seals). And put a C clip instead of a G clip on the piston. So I guess I will start from the bottom anyway.

Hey bro unless there is some immediate need to take the piston off the rod Don't Touch that Clip Please until you report with a picture. Just leave it alone for the now. 

[kancur]

Thanks for advice. Why is it so? I changed my piston and piston rings last year, I mostly wanted to gain some experience and it was fun to do.

After the job I got an advice about not using G clip - (They said the extra part on G-clip in comparison to C-clip can break off and damage my piston/cylinder). (The original piston came with a C clip but the new one had a G-clip in the package)

I definitely don't think it's necessary to open the head and do this stuff, but I must admit I kind of enjoy it

Will post pictures

Thanks

[speeddog]

Mar 7, 2018 20:50:00 GMT -6 czmdiver said:

Mar 7, 2018 11:38:06 GMT -6 kancur said:

I need to slow down for sure. I really mixed a lot of stuff together. Back to the facts:

I was on a short ride - and the sound definitely is not the same as before. The sound from the muffler is not the same, it is more popping instead of humming (Sorry really don't know how to describe it in english ). But I noticed the different sound mostly in the first few minutes only.

There's no misfiring or anything, the engine runs nicely, but has no power.

I took some photos and will upload them probably tomorrow.

Anyway, I detached the whole engine from the scooter as I decided I should change some things (head gasket and seals). And put a C clip instead of a G clip on the piston. So I guess I will start from the bottom anyway.

Hey bro unless there is some immediate need to take the piston off the rod Don't Touch that Clip Please until you report with a picture. Just leave it alone for the now. 

[kancur]

Some images:



Looking at the spark plug now, it seems quite ok to me. Isn't it?









There are some holes there from which air escapes under that. (can be felt when engine is running, and also black soot is visible in there)





Yep, the muffler was welded several times, Looks horrible but worked OK (There's a smaller diameter tube inside that weld, it's not blocking the air flow)



I want to change head gaskets as I have some small amounts of oil leaking there. I also want to change those G circlips for C circlips.

The muffler is in really bad condition... But I don't know how I can test if a problem lies there.

[cyborg]

Rich condition ,,,and it's time for a new exhaust

[cyborg]

Is it just me or does it look like a piece of the piston crown came off by the edge of the valve pocket ?

[cyborg]

That may be the cause of the sudden power loss,,,some of the piston material is stuck in between a valve and the seat