150cc GY6 scooter lost top speed

[sc00ter]

Are you sure you have the throttle cable ran correctly, and the slide is opening fully when you twist the throttle wide open? I've seen throttle cables ran wrong before, usually not seated correctly. There is also a barrel adjuster to take out the slack under the throttle. The throttle should only have a tiny bit of slack at idle.

So make sure the slide is completely opening and there is no excess slack in the throttle. I'll keep thinking......

[Battercake]

Originally Posted by sc00ter

Are you sure you have the throttle cable ran correctly, and the slide is opening fully when you twist the throttle wide open? I've seen throttle cables ran wrong before, usually not seated correctly. There is also a barrel adjuster to take out the slack under the throttle. The throttle should only have a tiny bit of slack at idle.

So make sure the slide is completely opening and there is no excess slack in the throttle. I'll keep thinking......

Yep, it's opening all the way to the top stop, and it closes all the way to the idle stop screw. The stator idea is plausible. I've read about them a bit today, and although I'm not having any of the other symptoms of a stator going bad, it's still possible because the windings are independent.

As I approach 46, it just starts to feel like I'm starving for something. It feels like I'm hitting gentle waves on a lake in a boat. Up to that point, though, everything is smooth and runs like it should, as far as I can tell. But there's some constriction in something that won't let it go any faster.

[Battercake]

Decided to ride to work today. The first few miles it let me go as fast as 51 for a while, but eventually it reined me back in to about 48 for the rest of the ride. This afternoon (when it was up in the 90s) it started out with its usual maximum of 46, but within a few miles decided that 41 was gonna be my new best friend the whole rest of the way home. I checked the spark plug cap, but it was secure. I can definitely feel like it's maybe not completely misfiring, but definitely underfiring a lot up near wherever that ceiling is at the time, but still seems to run perfectly at lower speeds.

[sc00ter]

So you said you swapped carbs and the same thing is happening. I'm gonna rule out a carb issue.

You checked/replaced the fuel supply lines. I'm ruling out fuel issue.

You said the spark plug looks normal.

You said oil level is correct.

You went thru the variator and said all looks good. Belt looks good and I assume you checked the rear clutch assembly. I gonna rule out a transmission issue.

Verify the earth ground is attached before proceeding.

You need to test the stator and see if its supplying the needed juice. YouTube has some good how-to videos.

Also, check the CDI. Again, YouTube. They will sometimes fail with little or no warning.

Finally, check/test the coil. Same symptoms as above. YouTube is your friend.

Don't get discouraged at a electrical issue. I hate trouble shooting electrical but have learned a lot with YouTube videos. We have a Subaru Baja that my wife refuses to get rid of. It has 270,000ish miles. I've learned so much fixing it myself, both mechanical and the dreaded electrical stuff. Still hate messing with it but its not a daily driver so it can sit while I'm learning.

[Battercake]

I haven't had a chance to check the CDI or coil again, but the stator seems to be okay. Whatever the problem is, it's consistent. This morning it was 66 degrees and I was able to go 50 for a while, but this afternoon it was 93 degrees and 40 was the best it would do, and even then it wanted to miss. Opening the throttle more just makes it louder but doesn't make it go any faster.

[kz1000st]

Check your vacuum lines. it sounds like it's running lean. I don't know if it's been said but i had a problem with a plugged exhaust pipe that made it do the things you have said it's doing.

[Battercake]

I'm checking vacuum lines. I pulled off the intake manifold. The gasket looks partly melted. (See attached image.) Is that normal?

[Battercake]

A bad vacuum line seems to have indeed been the cause of my idling issues, but this was a relatively recent problem and now most assuredly not related to my original problem of hitting a ceiling between 42 and 46 MPH. Should I replace the exhaust? I just keep throwing money at this thing, so why not?

[sc00ter]

Just to play it safe before tossing anymore money at it, did you check the compression? Watch a few You Tube videos on compression checks and do 3 checks, for a overall average number. Completely install and remove the compression tester between each test, this will rule out sealing issues.

If the compression number looks within spec then I still suspect something electrical related. Something either building up heat or back charging?

How did you find the vacuum leak? Did you spray it down while at idle with a flammable cleaner or was is just a visual check?

[Battercake]

Originally Posted by sc00ter

Just to play it safe before tossing anymore money at it, did you check the compression? Watch a few You Tube videos on compression checks and do 3 checks, for a overall average number. Completely install and remove the compression tester between each test, this will rule out sealing issues.

If the compression number looks within spec then I still suspect something electrical related. Something either building up heat or back charging?

How did you find the vacuum leak? Did you spray it down while at idle with a flammable cleaner or was is just a visual check?

Flammable spray testing didn't seem to be getting me anywhere, so I started blocking off and swapping out pieces of the vacuum lines. Nothing changed until I swapped out the hose that goes from the manifold to the tee that splits between the petcock and the cutoff valve. Cold starts are still difficult, but I have a much more consistent idle once it does start and it doesn't surge after I've been riding a few miles like it was before.

Now, as for compression testing, which was a lot more fun than it should have been thanks to a misalignment between the plastic engine cover and the spark plug port, I got 145 psi after a slow climb on a gauge from HFT. I know those cheap gauges can be a bit wonky sometimes, so I'm gonna check it again with a parts store loaner this afternoon.

[Battercake]

The other gauge built up much faster but confirms my previous reading: 145 psi.

[Battercake]

Could my intake manifold gasket be damaged?

[sc00ter]

I always just replace those intake gaskets, no matter if they look good or kinda smooshed like yours. Certain gaskets you just replace. Order 3 so you have some on standby. I also use a bit of assembly lube on rubber gaskets as well.

If that smooshed gasket was sucking air the flammable spray would have revealed it.

Nothing else is popping in my head. Ever since I got into ebikes I quit working on gas powered scooters, unless its my personal transportation or a good friend with a Yamaha Xmax. I've gotten rusty with my troubleshooting skills. It's a case of use it or loose it. Did any YouTube videos point you in any good directions concerning your loss of power? YouTube had a good channel (I forgot the name) with how-to videos on trouble shooting various issues.

[Battercake]

Carburetor. 100%. Full send, Regis. Final answer.

[sc00ter]

Wait, I thought you had a known good carb, and swapped it out with the same results? I'm pretty okay(ish) at working on carbs and can still do a tune with acceptable results. I still hate those stupid diaphragms though.

IF you suspect the carb recheck the diaphragm for proper seating again. Also make sure a jet(s) didn't work loose. I had a main jet that was slightly stripped and it would work loose. Very annoying. I've also seen people mangle the float when taking the bowl off, knocking the float height out of whack. Measure the float height and set accordingly if suspect. I've already mentioned my hatred for electric chokes BUT my Genuine Buddy 125 still has its electric choke installed, so they (electric chokes) are not all doom and gloom.

Keep us updated if you find out what caused the issue!