It works! Low compression?

[Noobi06]

Hello, I am back!
I finally got the gasgas together and it works! Im super happy and special thanks to everyone that helped, especially Anders, Zman, F5 and 5valve!
As you may have read from one of my threads, I screwed my old cylinder and piston which I had insane compression on! It was kind of tough to even kick it with my foot, and its a 125cc!
After screwing the cylinder, I took a cylinder and a piston from my other gasgas 125cc, which the cylinder and piston is basically brand new.
Now the bike works but im not happy with the compression.
Its really easy to turn over with your foot, and tough with the hand, but considering that there was a lot of resistance with foot before, im not really happy.
Is it normal, or is it because the cylinder is almost new and maybe needs to get ridden a bit?
Thanks in advance and thanks for the help from everyone! You made a 15 year old go from no knowledge to now being able to fix a whole ass bike!

[Zman]

Very glad to hear you got it back together!

Did you put the old head on the new cylinder or vice versa? I believe some of the early GG125 years had a domed piston and maybe the newer later years had a flat top piston. Putting a flat top piston under a head intended for a domed piston will result in poor squish and compression similar to what you are describing. Also putting the flat top head over a domed piston will yield high compression and make it hard to start and detonate.

There was a MX version of the 125 engine and a EC enduro version.

Which year and model head did you use and which cylinder did you use?

Can you get a compression tester and run a test to see where you are now.

[(F5)]

Hadn't ridden a 125 for ages but did last month. Was quite easy to kick over. But yeah as above. Compression testers are a very blunt tool, many people put way too much trust in them
-but if you have circa 170psi or so you should eliminate a massive mismatch of parts.
Borrow one with right thread for your plug. Not push in type. Throttle wide open. 10 quick kicks.


We tested a mates 125 Aprilia Road bike on Sunday. About 30psi. Had stopped running suddenly. Yep. That'll do it. Stuck ring for sure

[Noobi06]

Thanks guys. Ill get a tester.
Zman, the cylinder also came from a gasgas ec 125 2005. I own 2 identical ones so I don't think the cylinder, head or piston should be a problem.
Ill see if I get my hands on a tester though, maybe it's not even bad, but having the compression I had before, awhh man that was really nice.

[Zman]

Great points on the compression test.

Did you use the piston that came with the cylinder or did you buy a new one? Does it have a dome or is it a flat top?

[Noobi06]

Is dome when it has like 2 weird holes on the top?
Either way, I took both the cylinder, top and piston. The top of the piston is flat, I don't know what a domed pistol is but it is flat.
Piston head is the same type as my other cylinder which had awesome compression. Therefore I assume that it is correct.

[Zman]

A domed piston has a slightly rounded top. A flat top piston is flat.

If you have two cylinder heads, they can be different and change your compression even if they look very similar. It is hard to see less than 1mm difference between two heads but it can make a big change in compression.

The new cylinder can also be slightly taller which will also change your compression ratio.

If you used the thicker base gasket under the cylinder it can change your compression ratio.

How does it run?

[Noobi06]

Oh wow okey, I didnt know this. I haven't tested how it runs, but first 30 seconds Ive started it seems good. Its snow and ice so I cant ride it and im honestly so afraid its gonna go boom so I dont even have the balls to start it in this cold. Im waiting till like march. Ill start it and see how it runs and see if compression goes up, maybe its just because it had a full rebuild? If not, I can try see if Its a domed or flat etc.
Now if My head is domed and my piston is flat, or vice versa, it affects compression but does it matter?

[Zman]

If the bike was hard to kick in the past and is easy to kick now, something has changed.

Do you have a dial caliper? If not they are pretty inexpensive and you should get one if you want to get the engine optimized so it runs best for the fuel you plan to run.

If you have a flat top piston and a head for a domed piston, it will have really low compression.

If the new cylinder is slightly taller than the previous cylinder, it will lower your compression ratio. Some people cut the base of the cylinder to increase low end torque and move the power band to a lower rpm.

If you used a thicker base gasket under the cylinder it will raise the cylinder and move the power band to a higher rpm and it will also lower your compression ratio.

I would run a compression test after you break it in. It should be somewhere between 160 and 180psi. A little higher is ok but you will need to run higher octane gas. Lower than 160 I would probably make some changes.

Did you install a new piston and rings? If so what part number?

If you really want to know how well it is set up I would also try to run a squish clearance test with a piece of solder.

https://youtu.be/1dfBB-_5rQ8

You can drain the coolant and pull the head off as shown in the video which will be the most safe and accurate.

You can also try to bend a piece of solder 90 degrees and stick it into the spark plug hole and pull it up high so the end of the solder is in the top corner between the head and cylinder. When you cycle the engine with the kick starter it will deform "squish" the solder. Once you deform/squish the solder you can take it back out to measure it to know your squish clearance and angle.

If you insert the L shaped solder through the spark plug hole you need to be careful you do not insert it too far so as to get the solder stuck into a port where it will cut off by the piston and it will drop into your bottom end where it will do "bad" things.

I would run it for a little bit before you do the compression test. You should be able to run it around the block now but be careful not to rev it to high as the cold weather often requires richer jetting. If you can get some AV fuel or race fuel without ethanol in it, it will run richer and compensate a bit for the cold weather.

[Noobi06]

Thanks a lot Zman! Im not gonna give it attention now, as it is too cold for me to even go to the garage. We live in an apartment and we share a cold garage with others so its not warm at all. As soon as the weather is warmer and the jetting as you said is in its season, I will follow all your steps to get the compression back! Having high compression to me wasn't only performence, it was almost an achievement as we ride 50cc dirt bike mopeds in Sweden which basically have no compression.
Ill update you in 2 months