GY6 Compression Issue

[raj]

Hello, forum and scooter noobie, here.

So, TL:DR of this scooter is, my friend's parents who live near the beach bought
this 49cc "Kait" (Per the title) scooter and it sat since 2017 because they thought it had a carburetor issue.
They've just given me the scooter about a month ago.

So, here's what I've done so far and in the order I did them.

• Replaced battery
• Replaced carb and fuel lines and fuel filter.
  
• Replaced petcock valve and breather(?) line.
  
• Changed oil.
  
• Realized there's only 50lbs of compression
• Checked valves aren't leaking/damaged and adjusted lashing (.004 in and out) 

Today, I've just completely replaced the piston and cylinder (as well as all gaskets therein).
It was an idea that it may have been a problem, also a Christmas gift anyway, so, why not just start fresh. 

And the compression hasn't changed. 

It has spark, though, very dim and won't start on starting fluid. So, I've just now ordered the CDI box, hoping that resolves that issue.

However, I'm stuck wondering what's up with only 50lbs of compression?
Is there anyone who can give me some idea as to what's going on?

Any info is appreciated!

p.s.
I'm fairly sure I've gone over everything I've replaced or adjusted. If anything pops up that I remember, I'll add it.

[mikehailwood]

First, is the compression tester known to be good?

[FrankenMech]

Are you measuring the compression right?

[raj]

Appreciate the responses!

One of the testers is brand new.
I bought it because the other is +/- 10 years old. Both read the same.

The way I tested was by screwing the hose into where the spark plug goes and holding the throttle open while cranking.

Throttle closed is 25-30lbs.

[FrankenMech]

How long do you crank? It usually takes 5-6 compression hits to register. Is the valve core in the spark plug fitting working? Some compression tester hoses have a release valve in the hose near the gauge that can leak.

Gauges usually don't go bad unless dropped and damaged. The needle may not point to '0' if damaged.

A little oil in the cylinder will help determine if it is valves or rings. New rings will not seal well until they have worn in but 50# now and then sounds strange.

[mikehailwood]

How did you check the valves for leaking?  Have you used the compression tester on another machine to make sure it works correctly?

[Petro]

See if you can through the spark plug hole put pressurized air while the piston is at TDC. It must be something wrong with the head and You should be able to hear it leaking if so.

[raj]

How long do you crank? It usually takes 5-6 compression hits to register. Is the valve core in the spark plug fitting working? Some compression tester hoses have a release valve in the hose near the gauge that can leak.

Gauges usually don't go bad unless dropped and damaged. The needle may not point to '0' if damaged.

A little oil in the cylinder will help determine if it is valves or rings. New rings will not seal well until they have worn in but 50# now and then sounds strange.

I've cranked it for a good 30+ seconds most of the time. Or until the gauge levels out.

I've used two different compression gauges and get the same results, I don't think it's the gauge misreading or leaking. But I can get a third one in the next day or two and try that one, also.



I've put oil on the cylinder through the spark plug hole and the pressure rises by 10-15lbs.
That made me believe the valves weren't bad.

How did you check the valves for leaking? Have you used the compression tester on another machine to make sure it works correctly?

So, as I mentioned above, putting oil on the cylinder and gaining small amounts of pressure. But I've also done a leak test on the valves and nothing came through like this - youtu.be/yqMslJOf5W8. 

And I've used both on a lawn mower and get the same readings 110lbs. 

See if you can through the spark plug hole put pressurized air while the piston is at TDC. It must be something wrong with the head and You should be able to hear it leaking if so.

I will give this a shot as soon as I can and report back.


Thanks all, again!

[mikehailwood]

"And I've used both on a lawn mower and get the same readings 110lbs."

So you've established that the compression testers are good.


As for the video, if you have a leaking valve situation, the fluid you use will not necessarily leak as quickly as is shown in the video. It can take a minute before the fluid begins to seep through slowly.

[raj]

Jan 1, 2021 12:10:53 GMT -5 mikehailwood said:

"And I've used both on a lawn mower and get the same readings 110lbs."

So you've established that the compression testers are good.


As for the video, if you have a leaking valve situation, the fluid you use will not necessarily leak as quickly as is shown in the video. It can take a minute before the fluid begins to seep through slowly.

I gotcha, the valves were left overnight with a piece of paper underneath. If the paper was wet, or had any kind of stain, it would have been visible the next day. The video was just to show how I tested.

My Dad's idea. He was a full-time mechanic most of his younger days, and this even has him stumped. 

[mikehailwood]

Jan 1, 2021 15:44:33 GMT -5 raj said:

Jan 1, 2021 12:10:53 GMT -5 mikehailwood said:

"And I've used both on a lawn mower and get the same readings 110lbs."

So you've established that the compression testers are good.


As for the video, if you have a leaking valve situation, the fluid you use will not necessarily leak as quickly as is shown in the video. It can take a minute before the fluid begins to seep through slowly.

I gotcha, the valves were left overnight with a piece of paper underneath. If the paper was wet, or had any kind of stain, it would have been visible the next day. The video was just to show how I tested.

My Dad's idea. He was a full-time mechanic most of his younger days, and this even has him stumped. 

Okay.  I had to ask.

[mikehailwood]

What liquid did you use in checking for valve leaks?

Old head gasket showed no signs of leaking? Petro's idea of pressurizing through the spark plug hole seems worth considering.

[FrankenMech]

Jan 1, 2021 9:02:06 GMT -5 Petro said:

See if you can through the spark plug hole put pressurized air while the piston is at TDC. It must be something wrong with the head and You should be able to hear it leaking if so.

I usually use the compression tester hose (with mods) for this. Make sure you block the engine from rotating.

[raj]

Jan 1, 2021 17:00:28 GMT -5 mikehailwood said:

What liquid did you use in checking for valve leaks?

Old head gasket showed no signs of leaking? Petro's idea of pressurizing through the spark plug hole seems worth considering.

Purified water with food coloring. I'm going to try the pressurised thing when I can make it back over there. Hopefully by Sunday. I keep it at my friend's shop.

I usually use the compression tester hose (with mods) for this. Make sure you block the engine from rotating.

Got it. Thanks for the heads up!




Again, I appreciate everyone's responses! 😊

[mikehailwood]

I don't have a lot of experience with this but I did notice that water was incredibly slow to seep through valves on a low compression head. It seems as if I changed to something else to find leaks (kerosene, thinner, reducer). The viscosities of the water and of the solvents were actually noticeably different.

Keep us posted on the "pressurized air through the spark plug hole" test.