Cylinder compression test - failed ??? :(

So I got the bike running and did the first test ride around the neighborhood (just one small loop). I was really really happy. I then proceeded to check the compression using the compression gauge I got at harbor freight yesterday. I got readings of 80psi on 1 & 4 and 70psi on 2 & 3. I obviously got really worried but then after looking at the reviews, I realized that the tool may be the problem.

I repeated the results with my carbs off since I need to fix a float valve in the carbs. And I re-did the compression test. And here's the big bummer. For some reason, to see if the readings changed, I put my hand over the air intake of cylinder one while doing the compression test. I now don't read anything on this cylinder, and one reading I got was 30 psi. What's going on here?

Can someone please help me out here? Newbie is lost and confused.

The problem is that you put your hand over the air intake during the test. When you do a compression test you should have all spark plugs out and the throttle opened fully. Then crank the engine using the electric starter (if available) until you get the highest reading. If you have the carbs off obviously you don't need to worry about the throttle, you just leave the intake ports open to the air. Ed

Thanks Ed. But the readings I mentioned the first time were with the carbs on and I did not have my hand on anything, also had the choke open. Even with the carbs off the bike, the readings were similar. But when I put my hand over the air intake, the readings were off and ever since that time, the readings haven't returned to the previous numbers, that is 80 psi instead of the 30 or so I'm reading. Again, this gauge is supposed to the faulty according to all the reviews on harbor freight. But I'm concerned that my putting my hand over the intake while doing the test, I somehow messed up the engine. Is this even possible?

And this problem is only cylinder 1 which is the one I was testing and the one I put my hand over the intake.

Unlikely that you did any damage. But do the comp. test again and see what you get. If the bike runs then you should get pretty normal reading.

I found this in your other thread " I also opened up the carbs and found some missing choke flaps. Made some new flaps out of tin-can metal and installed them on the choke assembly." I´m worried that you might have had the same thing happen to your bike as happened to mine. The choke flap sucked into the nr2 cylinder and it mashed up the piston so bad the top two pistonrings was stuck. The bike hardly ran at all and I had to replace the piston. Usually and a bit lucky nothing happens with the flaps sucking into the cylinder since they are quite thin and will mash up and get spit out via the exhaust but sometimes it happens the way I described. Get that compr. test. And if you have a infrared thermoreader you can see the temp. on the headers which is a good tool to see that your cylinders are firing correctly. costs maybe 15£ on eBay

There is no way you can damage your engine by holding your hand over the intake.

Have you checked the valve clearances?

Ah the choke flaps, bummer.
I would try to make a Leak Down fitting from the HF compression Gauge, that is if you have a compressor. Or return the Compression gauge and get the Leak Down Tester instead. But only if you have a Air compressor that you can use.
Have you done a leak down test before? If not what you need is a compressed Air supply that can be put into the cylinder through the spark plug hole. With the cylinder to be checked at Top Dead Center (TDC) compression stroke, you supply 100psi into the cylinder and the gauge will show incoming air pressure then how much it holds of that, second gauge. You would Love it to be like only 10% leakage, but you may find 25% or so. If you have any more than that power is really going to suffer. The point is to listen for where the air is escaping, like at the oil cap- Rings, or at the Exhaust- EX valve leak etc.
www.harborfreight.com/automotive-motorcy...wn-tester-94190.html

Can perform a poor man's leak down test by introducing compressed air into a spark plug hole and listening to where it escapes from the combustion clamber. This may be done at TDC or any other cam positions where both valves are fully closed.

Hearing escaping air at the carb intake indicates that the intake valve isn't completely sealing when closed (for whatever reason).

Hearing escaping air at the exhaust indicates that the exhaust valve isn't completely sealing when closed (for whatever reason).

Hearing escaping air at the crank case vent indicates an issue (for whatever reason) with the piston, rings, and/or bore.

Hearing escaping air at some position along the edge of the head gasket indicates a leak in the head gasket.

A leaking valve may sometimes be resolved by adjusting the clearance to within specs.

A leaking head gasket may sometimes be resolved by torquing the head fasteners when the engine is stone cold (such as after sitting overnight).

Good Fortune!

Thanks Paroxyt & Motor Head. But I had the carbs out this morning after the first ride and the choke flaps that I custom made are totally intact and fine. So I don't think that's the problem.

So I'm guessing the compression testing is done after the engine has warmed up considerably. What should I expect to read on the compression tester on a completely cold engine?

faffi wrote: There is no way you can damage your engine by holding your hand over the intake.

Have you checked the valve clearances?

Try that with a 12 to one, and you'll have a pretty pink spot!

also, OP, my comp readings were low, lower than that on a basket case I had. Ran it and those numbers just kept coming up
If it wont start (haven't read the whole thread) poor some acetone and transfluid into the cylinders and let it soak- for a day or more, turn the engine slowly over with a wrench, spin the battery starter and that fluid will shoot across the room

OK, so I did the poor man's compression test with cylinder 1 that I managed to mess up with my hand trick and viola, I could clearly feel a lot of air coming out of the open carb intake (I took the carbs are out). I repeated the test on cylinder 4 which I did not mess with and I couldn't feel air escaping out of anything, especially not out of the carb intake for sure.

So it's certain that by putting my hand over the carb intake while doing the compression test, I managed to mess things up. But the question now is what is it that I did to that one cylinder and what can I do to fix it? Gurus, please help!!!!!!