Rebuilt Engine Cylinder Smoking - w/ compression results

[jensey]

Hi Gullepumpe,

I see that you are asking the same questions on the Hondatwins forum, started a thread at 10-10, after signed in.
Why are you asking the same questions on both forums, and are there more forums you ask the same questions, the same time ?
How many people do you want to spend time to solve your problem ?

No offence, but you're new on the hondatwins, new here (and maybe on other forums for the same reason), and the only thing you do is asking questions, and asking them in the wrong sections, at least here on this forum.

On both forums you did not introduce yourself, you show no occupation, and place were you're living. Are you running a garage / shop or something ?, are you earning money with working on bikes for others ? If so, it would be nice to donate a percentage to the forums you use.


Jensen

[Gullepumpe]

My background is as an aircraft engine mechanic. I have rebuilt several of these engines, bored oversized and honed. Each time I got no smoke at all, and compression was around 125-130 using the pump up compression test. (Denver, Co altitude) Each time I got a bit of air out the breather and oil cap using the leakdown test... more on that later, but in each case I personally knew the cylinder dimensions and bore job quality, as well as the hone pattern and roughness.

If I were you I would put it back together because it seems you found the source of the oil getting into the cylinder and causing it to smoke. Was there another indication of poor compression like being sluggish or running poorly? Chances are the smoke caused a great deal of concern, and that's why you took it apart. Also I would apply a little (a little goes a long way here) sealant around the o-rings and use a new head gasket, torque it very gently, wipe up anything that squeezes out (hopefully you didn't use too much and nothing comes out) and let it sit overnight. This way the sealant can firm up a bit, then when you final spec torque, you will have a great seal here and wont squeeze sealant anywhere you don't want it.

I'm not the best typist, and I don't want to drone on forever, but some things to think about on compression tests on an engine this small:

1) When using the pump up gage, does it have a check valve? (it should if you want any results you can trust) Is it a sensitive check valve that opens at very low differential pressures? Is the check valve in the spark plug boss, or is it halfway up the tube to the gage? Is the throttle open? Now imagine how small the volume in the combustion chamber of this engine is when the piston is at TDC. If the check valve is 1 foot up a tube between the spark plug boss, and the gage, (as many cheap ones are) then you have effectively doubled the volume of the combustion chamber, and the reading will never be very high. This is so because each time the cylinder exhausts, and then comes to compression again it must compress the air in the combustion chamber AND the air in the hose (prior to the check valve) from scratch. This effect is considerably lower on engines with LARGE combustion chambers, because the hose volume is proportionally low compared to the combustion chamber. The CA95 combustion chamber is tiny.

2) Now imagine that it takes 10psi differential pressure to open the check valve. The reading on the gage might be 10 psi lower than actual because near the end of the test, the higher pressure in the combustion chamber may not be enough to open the check valve, and read on the gage. I personally only use this type of gage if the check valve is very good condition and located IN the spark plug boss. Its harder to find these but they are out there. Don't use a tire Schrader because the spring is too stiff.

3) **The following is opinion** Leakdown tests are great for locating faulty valves, (because there should be NO leakage here during this test, which you can hear) but be careful on taking too much away from how much leakage there is or how much air sounds like is escaping. This test should be performed while at TDC, with the engine hot ( that's when the piston is biggest, and roundest...Aluminum expands a lot more than the cylinder casting) and sometimes tiny movements make a great deal of difference on the amount of leakage. This is easy to do with an airplane, because you have a 7 foot lever on the crankshaft called a propeller that you can move a bit to get the best test. This is really hard to do on a motorcycle engine. The leakdown tester I use is designed for aviation engines and is calibrated for a piston approximately 6-7 inches in diameter. A lot of leakage is pretty normal over a circumference that large. The CA95 piston is tiny! Not sure how much leakage would be normal or what your gage is calibrated for, chances are its a big engine.

4) I would torque the head and leave the carb off... with the engine not even installed in the bike yet, you can use the electric starter to turn it and get a pretty good idea of the compression. Especially now that you know you did the test wrong before.

Hope this helps.

Thanks for posting this, it's a big help. Some great information in there! The compression tester I have does in fact have the valve right at the plug hole and the valve feels pretty easy to depress. Good point about the amount of air volume being tested on these small cylinder bikes. We've been using Permatex High Tack gasket sealant on all the gaskets and it's been working great. The mating surfaces on this old engine aren't pristine anymore so many of the dry gaskets just weren't sealing 100% before the Permatex.

What should I expect for comp. numbers if I run a bench test with the engine cold like you said?

Thanks again for the help.

[Gullepumpe]

My question is how much would not having the throttle open hinder the compression results? !

Quite a bit.

Can't remember exactly, but memory says maybe 30 psi or more.

Yea, looks like that's accurate. I found this article showing a compression test with and without throttle opened:





Full article: http://cx500info.com/honda-cx500-how-to ... mpression/

[Gullepumpe]

Hi Gullepumpe,

I see that you are asking the same questions on the Hondatwins forum, started a thread at 10-10, after signed in.
Why are you asking the same questions on both forums, and are there more forums you ask the same questions, the same time ?
How many people do you want to spend time to solve your problem ?

No offence, but you're new on the hondatwins, new here (and maybe on other forums for the same reason), and the only thing you do is asking questions, and asking them in the wrong sections, at least here on this forum.

On both forums you did not introduce yourself, you show no occupation, and place were you're living. Are you running a garage / shop or something ?, are you earning money with working on bikes for others ? If so, it would be nice to donate a percentage to the forums you use.


Jensen

Feel free to shoot me a PM if you think I'm using the forums incorrectly. I'm new to rebuilt engines and this bike so just trying to get some insight.

[Geeky160]

Good job Gullepumpe, thanks for the compression link. It was a big help. ;)