Low compression

[Makedon]

My GZ 250 didn’t want to start. I rebuild the carb , change the oil, new spark plug. I check the compression with my tumb ( finger) and it gave pressure on the spark plug hole. I ended baying a engine compression gage/ tester , and check the compression. It was showing 70psi. Very low compression. I opened the valves and did valve adjustment( they had zero clearance), I followed the service manual instructions, went ahead check the compression, and again it shows 70psi. And now I’m stuck. Is there anything else that I should do or check, or Should I start thinking about replacing the head gasket, or maybe even worse the piston ring. The bike is 2009 with 7700 miles on it.
Thanks for all ideas and suggestions.

[Vegas Street Rider]

I am not enough of a mechanic to help you, but someone on the forum will come along and give you the advice you need. I also have a 2009 and have over 20,000 miles on it without any issues. I wouldn't think you would need a new head gasket or ring just yet unless the bike was abused by the previous owner.

[wacio]

You probably solved this already - but for the next generation: If you have low compression there are two potential problems: 1 - piston leaks thru the rings, 2 - valve problems. How to determine which one. Do compression test like you tried - record the number. Remove tester - drip some engine oil into the combustion chamber (not a lot - teaspoon maybe) - thru the spark plug hole. Repeat compression test. If the number rose significantly - that means bed rings, cylinder seal. If number is the same - bad valve seal.
In your case - I would bet on the valve problem. Recheck your clearances. Recheck again. Retest compression. If no improvement - valves and/or seats might be damaged. Running engine with no valve lash clearance will burn the valve heads and seats. If damage is small - you could try to remove head and try to hone the valves in - but you may need new valves, rebuild head.

[Makedon]

Ok, so this is what is happening. I rechecked the valve clearance, it was good. Then I watched some YouTube videos, some mechanic was suggesting to put some Marvel Mistery oil thru the spark plug hole and let it sit for 3-4 days, I did that and didn’t help. So the next thing I did is, I put a tablespoon of oil thru the spark hole, did a compression test. The results war 150psi, on the first test, few minutes later second test was 120 psi. 45 minutes later, third test was back to 70 psi. This leads me to believe that I need new piston rings. Now I want to say that I got the GZ as a project bike to learn how to work on bikes. I think I got my money worth out of this one. I also have a parts bike too. Now the question is how difficult is to do piston rings replacement on the bike, and is it required to pull the engine out? I was searching through the forum to find “ how to” information for this job but I was unsuccessful, anybody has a link or advice where I can find more info. As always Thanks for all the advice and your time.

[wacio]

You can do the head, cylinder pull and piston, rings swap without pulling engine. Its little tight but can be done. I would think that with the mileage you have on the bike - something maintenance related caused the issue. Most likely the rings are stuck in piston groves due to lack of oil changes. First I would try to soak it few times with some penetrant (you already have Marvel). Use table spoon each time - let sit overnight - crank few times without plug, repeat. It may get them unstuck. If not take the cylinder off first before buying the parts - see which parts might be salvageable. Once you know - in addition to damaged parts replace all seals and gaskets you will be pulling - even if they still look good (head, cylinder, exhaust, intake) - cheap insurance.