Did a compression test....not great | GTAMotorcycle.com

Did a compression test....not great

K

Knight

Well-known member
So my 1985 nighthawk 650 seems to run great even though I know it's on the lean side. I just did a compression test after it was all warmed up after a ride and my results were disappointing.
Cyl1 80psi spark plug was normal
Cyl2 80psi plug real lean
Cyl3 70psi plug real rich
Cyl4 85psi plug a bit lean

And yes it does burn some oil I can see a faint blue at idle at a stop light when it's hot
Now in the Clymer manual it says the pressure range should be 156-184 psi. Is it time for a rebuild.
Gh8HqYL


I tried to attach a picture of the plugs but this site is not letting me attach a picture from Imgur.
Gh8HqYL

umk2v







umk2v
umk2v
 
Knight said:
So my 1985 nighthawk 650 seems to run great even though I know it's on the lean side. I just did a compression test after it was all warmed up after a ride and my results were disappointing.
Cyl1 80psi spark plug was normal
Cyl2 80psi plug real lean
Cyl3 70psi plug real rich
Cyl4 85psi plug a bit lean

And yes it does burn some oil I can see a faint blue at idle at a stop light when it's hot
Now in the Clymer manual it says the pressure range should be 156-184 psi. Is it time for a rebuild.
Gh8HqYL


I tried to attach a picture of the plugs but this site is not letting me attach a picture from Imgur.
Gh8HqYL

umk2v







umk2v
umk2v
Click to expand...

If they were all 100psi or over I would have said still ok ish to go. Are you sure its BLUE smoke? You may have multiple problems with your old girl. 70 psi cyl spark plug shows rich because it doesn't have enough compression to fire. check your head for tight valves?
 
fastar1 said:
Did you check/adjust valve lash before running the test?
Click to expand...

Honda 550 and 650 Nighthawks of that vintage have hydraulic valves.
IF IT WERE MINE -I'd shoot some combustion chamber cleaner down the spark plug holes, fire it up, take for a ride then recheck.
 
Were these readings done at wot with several rotations of the engine?

Sent from my SM-A500W using Tapatalk
 
pretty low numbers, but consistent

any difference between wet/dry?

if a few squirts oil into the cylinder give you a bump in compression
across all cylinders, you are likely looking at rings and walls being worn out


anyone do these engines? any chance of a hone and re-ring working out?
or do you need to bore and go oversize?

sounds like a good winter tear-down project
 
See if you can hook up some low-pressure, regulated compressed air to a plug adapter, bring each cylinder to TDC on compression and see where the air is leaking out.
 
Since you already have a compression tester... maybe you could spend some time this winter checking out parts bikes for a decent engine - private deal will be cheapest. 130+ psi on all cylinders would be good. A rebuild will mean doing both the rings and or pistons/rings, along with a rebuild of the head- $$$$ more than the bike is worth. you cannot do one without the other on this old girls engine. I think you could find a motor in the $200-300 range if your patient or lucky- and persistent in your search.
 
Last edited:
A common mistake when doing a compression test is to not hold the throttle wide open. If this test was done without the throttle held wide open, please re-do it that way and report back with new numbers. If it was done correctly ... she needs work!

Has it been consuming oil? I'm in the midst of a re-ring on an oil-burner. Mine had good compression and ran well, but drank oil (about 1 litre every 1000 km) and it was only letting out the occasional puff. In my opinion, the cylinder walls were too smooth to allow proper ring sealing (this is the second time I'm in there) and the shop manual incorrectly says not to touch the cylinder walls. I gave them all a touch with the deglazing tool this time to give it some better cross-hatching. (this is on an engine with plated aluminum cylinders - yours are regular cast iron liners)

On an elderly Honda, I wouldn't exclude the possibility that the valve stem seals are shot, too.
 
Re engine swap versus fix this one ... One of the headaches with vintage stuff is that anything you buy could very well have the same problems as what you've already got, sometimes more. Either way the engine needs to come out of the chassis. With that done, it's not all that much work to take off the valve cover, tensioner, camshafts, and cylinder head, and have a look and make some measurements. If the bore is within diameter specs about 1/3 of the stroke length down from the top so that you aren't into re-bore for oversize pistons, then it's not a difficult or expensive fix as long as you can do this sort of work yourself or between friends. (and you may have already deduced that I have the tool that you will need for deglazing the cylinders!)

If it's into needing a re-bore with new oversize pistons and some serious machining work to make that happen then it's time for the question "what's this going to cost versus what's the bike worth" ...
 
boyoboy said:
Since you already have a compression tester... maybe you could spend some time this winter checking out parts bikes for a decent engine - private deal will be cheapest. 130+ psi on all cylinders would be good. A rebuild will mean doing both the rings and or pistons/rings, along with a rebuild of the head- $$$$ more than the bike is worth. you cannot do one without the other on this old girls engine. I think you could find a motor in the $200-300 range if your patient or lucky- and persistent in your search.
Click to expand...

some good points have been made about used engines. Even though an engine tests good for compression doesn't mean nothing else is wrong with it.
it is a bingo-could go either way. but by checking the compression of a used engine beforehand it does rule out one major pit fall.

edit- an engine out of frame can still be compression tested with power to the starter. be in neutral.
 
Last edited:
Something wrong here.
A motor with those compression numbers would hardly run. All the plugs would look rich because of incomplete burn.
I suggest you have done the test incorrectly.
 
^ That's my suspicion, too, hence the earlier suggestion that perhaps the throttle wasn't held wide open during the test ...
 
Wow thanks for all the info guys I will re try the test again in a couple days at wide open throttle and also try dropping some oil into it. she does burn some oil I'm not quite sure how much. it does also have 34k+ miles on it.

And when I did the test I did go through compression cycle 6-8 times till the needle stopped going up.
 
Do the test with WOT without any oil before adding oil. Adding oil will skew the numbers upwards but is only really a diagnostic tool once you've got the initial numbers from a proper WOT test without any extra lubrication.

From what I could dig up online stock compression should be around 170 in a perfect world. I have to agree with others that 70PSI would probably be too little to even get the engine to run, so assuming it's still running and now blowing blue smoke constantly, I too suspect your compression test wasn't done right.
 
I don't think the compression will rise more than 20psi a cylinder at best with the throttle held wide open ...shall we start a guess the new psi pool lmao.
 
boyoboy said:
I don't think the compression will rise more than 20psi a cylinder at best with the throttle held wide open ...shall we start a guess the new psi pool lmao.
Click to expand...
Your compression is consistent -- that's a good sign. My guess is you're readings are wrong. You need to have the carbs off the motor to get the Clymer reading, try holding the carb sliders wide open (I don't think just holding her WOT will do).
 
Mad Mike said:
Your compression is consistent -- that's a good sign. My guess is you're readings are wrong. You need to have the carbs off the motor to get the Clymer reading, try holding the carb sliders wide open (I don't think just holding her WOT will do).
Click to expand...

take the carbs off? cheaters ! not worth the effort. For this older engine, I would consider anything above 130psi (wot) a good reading.
 
Last edited:
You must log in or register to reply here.

Back
Top Bottom