Topic: Engine compression test help

[redrider736]

Going to look at possibly my 1st SOHC bike, a 1978 CB 550k. I want preform a engine compression test to the bike.
I am just not sure of the proper steps I need to take to preform this test ? or the Honda specs for this bike ?

Any help to this newbie would be greatly appreciated !!

[MoMo]

Throttle wide open, choke open while kicking or cranking.   Reading should be at least 120(but that depends a bit on the gauge you're using) and all cylinders should be within 10 % of each other.  Good luck with the bike if you get it...Larry

[MCRider]

Remove the spark plugs. Squirt a tiny amt of engine oil down hole. Be frugal, or not at all as this will come shooting back out of the hole. Stuff lightly a rag in each plug orifice, but not into the threads, to catch the spray.

Then use MoMo tips.

IF you're leg's like mine, you'll need a strong battery and elec start. Turn kill switch to off.

[redrider736]

MoMo- using OTC compression kit. http://www.amazon.com/gp/product/B002W53J6S/ref=ox_ya_os_product

MCRIDER- what dose adding the oil do ??

[MCRider]

MoMo- using OTC compression kit. http://www.amazon.com/gp/product/B002W53J6S/ref=ox_ya_os_product

MCRIDER- what dose adding the oil do ??

I should have prefaced with: Is it a runner? If its been running then you don't need to add any oil, just start it first, then pull the plugs. Pulling the plugs will allow it to spin faster and give a higher reading.

If it doesn't run, a little spritz will wet the rings and give a truer reading.

[MoMo]

The length of those hoses may give a reading a bit lower than actual but as long as the readings are within 10% you should be good to go.  I assume the bike does not run and you want to get an idea as to compression?...Larry

[redrider736]

The bike runs, I just thought performing the compression test would be a good idea too do ??

[MCRider]

The bike runs, I just thought performing the compression test would be a good idea too do ??

Sure couldn't hurt, if the seller allows. Don't cross thread a spark plug.   

[Dino]

If it sounds smooth and the cylinders are all firing and it idles smooth. You're good to go. If it's lumpy at idle and at sustained RPMs, you might want to do a compression test.
Follow the instructions below.

Dino's compression test method.
Remove spark plugs., leave ignition OFF.
Screw the compression gauge into one cylinder's spark plug hole.
Hold throttle wide open, and give the engine three solid kicks on the kick starter. Try to be consistent with the kicks and do the same number on each cylinder! If the bike has a starter, crank it over five revolutions on each cylinder. Consistency is key.
Read gauge, and write the cylinder number down with the compression next to it.
Repeat for remaining cylinders.

IF one cylinder is 10%-20% LESS put a few drops of motor oil into the spark plug hole of that cylinder and repeat the test as described above.
If the compression comes back up a significant amount, the rings are worn. What the oil does is plug the gap on worn piston rings.

 

[TwoTired]

IF one cylinder is 10%-20% LESS pour an ounce of motor oil into the spark plug hole of that cylinder and repeat the test as described above.
If the compression comes back up a significant amount, the rings are worn. What the oil does is plug the gap on worn piston rings.

Holy crap! 1 Fluid ounce is 30 CC!  A 550s combustion chamber is only 15CC. That amount of fluid is guaranteed to raise the compression of a brand new perfect engine! (any perhaps blow the head gasket).

 I wouldn't let you compression test any of my engines with that procedure.

[Dino]

IF one cylinder is 10%-20% LESS pour an ounce of motor oil into the spark plug hole of that cylinder and repeat the test as described above.
If the compression comes back up a significant amount, the rings are worn. What the oil does is plug the gap on worn piston rings.

Holy crap! 1 Fluid ounce is 30 CC!  A 550s combustion chamber is only 15CC. That amount of fluid is guaranteed to raise the compression of a brand new perfect engine! (any perhaps blow the head gasket).

 I wouldn't let you compression test any of my engines with that procedure.

Whoa! Right you are!! That might work in a V8 but not a bike! Not sure what the hell I was thinkin there. I corrected that to a few drops. lol

[redrider736]

Current owner says it runs "rough" untill the bike is warm. Is this a Carb , or Compression issue ??

[MCRider]

Current owner says it runs "rough" untill the bike is warm. Is this a Carb , or Compression issue ??

Within reason, this is the nature of the beast. IMO.  Assuming its warm outside, it should cold start on full choke, move to half choke within 20 seconds, and be fully warmed up within 2 minutes. You can ride it sooner than that, but it may still be rough.

[redrider736]

ThX All, I appreciate the time and help !! I am sure I will have a few more questions to fire at you all..

[redrider736]

Have to say I am a little frustrated right now....... I don't consider myself a great mechanic by any means, and that is what I love about Owner groups like this one and "Goggle" to answer allot of questions.

The current owner by my request sent me the compression #'s from his 78' CB550 . This is what I received !! PLEASE help me all.

(Email from the current owner) "Here is the info you wanted. The compression was actually pretty good I thought. From Left to right starting at number one cylinder.  #1@135 psi / #2 @130 psi / #3@125 psi #4@135 psi. The test was done twice on a cold engine with a compression tester designed for higher CC engines (cars not bikes). If you do the test with a tool designed for smaller engines I assume the results may be a little different."

Everything I have read tells he should have done this test on a "warm" engine ?
Are his #'s F***'d or useless to me at this point

[750K]

Warmed up is preferable but as long as they're consistent across each cylinder I wouldn't be to worried, you should see the numbers come up even more when the bikes warmed up.

[redrider736]

Should I be doing a 2nd test while I am there going over the bike before purchase ?? I have a Comp tester, is it rude of me to question or be concerned with his #'s ??

[dave500]

pressure is pressure!if its not blowing smoke and rides ok itll be ok,itll need a tune up no doubt?,like points set/tappets cam chain tension and carb sync.

[750K]

If I was really suspect of what to owner was telling me about it I might do my own prior to buying but like Dave said if its running half decent and not leaking oil anywhere I wouldn't worry that much. 125-130 cold isn't that terrible, I bet it'll go up a fair bit when it warms up. I'd be more worried about motor making any weird noises than I would be about those compression numbers.

[redrider736]

ThX BoyZ

Keep you all posted !