GY6 Compression Issue

[raj]

Ok, so I managed to make it over there tonight for a few minutes. I took FrankenMech's idea and screwed the compression tester hose in with the gauge left off, and used an air gun with that big rubber tip for corking, whatever it's called, and pushed that into the end of the compression hose, where the gauge would plug in.

Anyway, if I placed the piston at TDC via the flywheel, it blows the piston back, and between three of us, we couldn't hear any leaks.

The only time air would noticeably escape is when we moved the flywheel to open a valve on either the exhaust or intake side. Air flows out through the muffler or the carb, depending.

However, before I went over there, just to rule it out entirely, I ordered a whole new head with the valves and a new rocker arm. It was only $26 for the head and $12 for the rockers, so what the hey.. And aside from the valve cover, it's the only original part on the "top end" of the motor.

This is mind boggling, for sure. But I'm determined to figure it out. I'll post back as soon as I get some of the new stuff installed. Parts should be here within the next few days.

[FrankenMech]

Jan 2, 2021 20:10:39 GMT -5 raj said:

Ok, so I managed to make it over there tonight for a few minutes. I took FrankenMech's idea and screwed the compression tester hose in with the gauge left off, and used an air gun with that big rubber tip for corking, whatever it's called, and pushed that into the end of the compression hose, where the gauge would plug in.

Anyway, if I placed the piston at TDC via the flywheel, it blows the piston back, and between three of us, we couldn't hear any leaks.

The only time air would noticeably escape is when we moved the flywheel to open a valve on either the exhaust or intake side. Air flows out through the muffler or the carb, depending.

However, before I went over there, just to rule it out entirely, I ordered a whole new head with the valves and a new rocker arm. It was only $26 for the head and $12 for the rockers, so what the hey.. And aside from the valve cover, it's the only original part on the "top end" of the motor.

This is mind boggling, for sure. But I'm determined to figure it out. I'll post back as soon as I get some of the new stuff installed. Parts should be here within the next few days.

You must block the engine solidly to keep it from turning, any motion is dangerous and can mash fingers etc.

Use a solid pipe connection to the gauge hose, not a rubber blowgun nozzle. That way you can concentrate on detecting leaks.

New parts can be a good idea but I prefer to know what caused an issue. That way we learn and sharpen our 'detective' or 'observational' skills.

The Asian aluminum these engines are made of is crap and barely adequate for the job. It deforms easily and the parts or engines are assembled by slave labor.

[raj]

Jan 3, 2021 0:30:32 GMT -5 FrankenMech said:

You must block the engine solidly to keep it from turning, any motion is dangerous and can mash fingers etc.

Use a solid pipe connection to the gauge hose, not a rubber blowgun nozzle. That way you can concentrate on detecting leaks.

New parts can be a good idea but I prefer to know what caused an issue. That way we learn and sharpen our 'detective' or 'observational' skills.

The Asian aluminum these engines are made of is crap and barely adequate for the job. It deforms easily and the parts or engines are assembled by slave labor.

The gun itself is solid, it just has that bigger rubber piece on the end.. Here's some pictures similar to what I'm talking about - 
This type of air gun with the larger tip here.

But I will try blocking the engine better when I have more time to go over there and tinker with it. We didn't really try to block it from moving too well.

The only reason I purchased the new parts is because I've exhausted my detective skills and the skills of those around me. That's why I've come to this forum to begin with. But if I'd have known that much about the production of these parts, I'd have probably reconsidered buying new. But that definitely explains why they're so inexpensive. 

Appreciate you! 

[FrankenMech]

I know the type of airgun, I have a couple. Plumb it solid like I said.

I usually attach a hose coupling fitting to the spark plug adapter or hose. The gauge fitting is probably 1/8" NPT.

If I had the parts in hand I could probably figure it out. You are educating yourself.

That engine moving under pressure could lose somebody a finger or two, or just mash them. Machinery under pneumatic power is dangerous.

[mikehailwood]

You may not be able hear the leak. I would try soapy water, like checking for natural gas leaks.

[raj]

Well, I went there tonight and put the CDI box on it.

I double-checked the old CDI box for spark at the plug and there was none this time around.
I put the new box on and it lit up like a Christmas tree, by comparison.

Tried starting it and I could tell the motor was having a hard time turning over.
I thought, "Man, this made it WORSE!" So I stopped. Then I went to try it again and VROOM it starts
and runs like a dream.

I threw the seat back on and drove it around, I still can't believe it. It's been
a month of fighting with this thing and scratching my head. But I've learned a lot, at least.

I didn't have time to go back and check compression afterwards, but it's got to be higher, right?
It cranks up first time every time, so far.

I don't know how the CDI box had anything to do with the compression, but for whatever reason, the thing runs like a champ now.

youtu.be/625p_Y-VuIg

Anyway, I appreciate all the input I was given here! Hope to be a part of this forum for a while. Next step is cleaning it up, and hydro-dipping the panels. If it turns out like I'm imagining in my head, I'll post some pics here.

[mikehailwood]

Sounds like something was terribly wrong with that compression test.

[FrankenMech]

Compression does not vary with a CDI box. Your compression test must have been way way off.

[raj]

I'm not sure how with two different compression testers I got the same readings on two different engines.
What are the odds of them both failing at the same reading?

[FrankenMech]

You were performing the test incorrectly.

-Operator error-

[raj]

I don't think so.

[GrumpyUnk]

I think it has to do with the volume swept by the piston stroke. A high quality gauge will need minimal volume to deflect the bourdon tube, and make the gauge indicate actual pressure. A lower quality gauge, with coarser mechanism, apparently will not respond and indicate true psi unless it has a good volume of air. It will report psi much lower than the actual pressure. If either or both gauges used are not known brands, I would call them suspect. It seems they will function adequately in a car or truck engine as there is enough volume being compressed that the amount used to operate the gauge is insignificant. It does take some volume to move things around, and the lower the quality, the more volume of air gets bled into the gauge. If you cannot make the piston pump quickly enough, it will not be able to handle that percent lost to the gauge, and will give false readings.
IOW, get a good gauge, and I bet you will have more psi shown when testing.


FWIW, after reading, until it worked, I was suspecting the cam/valve timing was off.

tom

[FrankenMech]

It takes 3-4 strokes to make a car compression test work but 6 or more for a scoot. The gauge internal volume will make a small difference. Leveling out is the endpoint for any gauge regardless of sensitivity or internal volume. Still 50PSI is real low for leveling out after 30 seconds of cranking like you mentioned. You also got good readings on another engine and with different gauges. You are doing something drastically wrong on the scoot.

I am not going to argue with you, I have done hundreds of compression tests on engines.

[mikehailwood]

I've not done a lot of compression testing. But...when I have done them it was usually done on motors where I could see the spark plug hole and get a good grip on the compression tester hose. Crank it in there securely with both hands. That has not been the case with an installed GY6 motor - especially with that plastic shroud installed. I'm surprised when I finally
get it set up and get an acceptable reading. I'm with FrankenMech on this one

[GrumpyUnk]

Frank, I can attest to what I described, and could post pictures of the different readings using an ebay gauge set obtained to get a scooter plug sized adapter. I have an old compression tester from JC Whitney, likely bought in the 1980's.
The new gauge produced readings in the 30psi range. The old gauge(made in USA) produced readings in the 70psi range. I took a picture of the gauges connected to the scooter, sitting on the battery tray, with the gauges showing the different readings. The ebay gauge did not produce readings that were anywhere near accurate.
I understand about the volume essentially filling the gauge volume after multiple strokes, it should. But it did not and the only explanation was that the bump in pressure produced at each TDC was not enough to move the cheap gauge. It leveled out, as you say, but not anywhere near the proper reading.
Could have been the rough cut of the gear teeth in the mechanism that moved the needle? Just not enough 'pop' to get it to move would be my guess.
Anyway, getting and using a good gauge is a lot more likely to produce useful results. This particular gauge set was only good for the adapters as the old set only had two sizes.
I did not try the gauge in a larger engine. If you get the adapter screwed into the spark plug hole, and the O-ring seats tightly, I don't know how you can screw up a compression reading. Open/closed throttle will likely make a difference, but I don't think as low as reported. Could be I am in outer space on this one.
tom