Checking Cylinder Compression

There have recently been a few posts regarding checking the compression of each cylinder. Just wondering a few things:

1st: What tools are needed to do this and are there any cheap ( and reliable ) alternatives if these tools are expensive?

2nd: What is the recommended compression per cylinder and how is that adjusted / improved, or do they all just have to be the same?

I understand that the compression is a factor of the valves, piston rings and head gasket. but I'm just trying to wrap my head around weather or not I should even care about such a thing.

 

1) Something like this.

2) 156 psi. All cylinders within 14psi. of each other.

Not much you can do about compression without major repairs.

 

The base Compression Test numbers will indicate if there is a problem or not.

Low compression ... or how the gauge behaves during the test can help determine the cause of the low reading.

Adding a couple of shots of oil to a cylinder with a low reading and retesting ... helps determine if the problems is Rings/Cylinder - or - Valves.

If the LOW reading IMPROVES after adding a few shots from the Oil Can ... the trouble is RINGS or Cylinder Wall related.

If there is NO change to a LOW reading after adding oil ... Valves or valve related problems are suspected along with the possibility of a Head Gasket Leaking.

 

I've run reliably on 120psi across the board. As long as they all balance, you should be ok.

 

This may be dumb, but do you test while running (obviously only on 3 cylinders ), or by turning it over by hand / with starter?

 

All Spark Plugs pulled.
Wide-open throttles.
Screw-in hose. Hook-up gauge.
Starter Motor used for turning-over engine.
2 - 3 Cycles tells the story.

 

Fully charged battery too. Get artificially low readings if it isn't cranking well.

 

Above is for dynamic compression check (cranking over with starter) Variance should not exceed 10%.
Also there is a static (leakdown) compression test which is done with each cylinder at TDC on compression stroke. This will more accurately determine the condition of valve seats and rings due to leakage (or not) past them. You should really do both for an accurate picture of the top end.

 

Why is it necessary to pull all the plugs?

 

Another tip is to check your compression on a warm engine. A cold engine (cold oil) can give artificially high #'s.

 

1) You need to pull them to check each cylinder's compression.

2) The starter will spin the engine faster when it's not fighting the other three cylinders' compression, so you'll get your reading quicker. It's also easier on the starting system.



As an aside, be sure to blow all the little bits of dead bugs, rocks and debris from the cylinder head fins near the plugs before removing the spark plugs. There's enough air being blown in and out the plug holes to inhale anything nearby that's loose, and it tends to stay in once it's down the hole.

 

also does the TCI unit need to be unplugged? where is this?

 

Wide open throttles mean the maximum amount of air is being allowed into the cylinders; if you didn't open the throttles it would take longer to get to the max reading and might attenuate it.

Yes you need to unplug your TCI unit; on your bike it's located under the left sidecover, directly to the rear of the rectifier/regulator (the thing with the fins on it.) It should have a blue or red "foil" label on it that says "TYPE TID-<numbers>" on it, and two large white connectors plugged into it. Unplug those two connectors.

 

thanks fitz. did a quick check (carbs off for rebuild) since I've replaced 7 out of 8 shims. only problem is i couldn't get a good reading from #1 hole (when I finally did it only read 35-40psi). the other three read in the range of 100-115psi. gonna put battery on trickle charger tonight and recheck tomorrow. maybe i drained the battery by time I got to #1 since I started at #4

only thing different at #1 is the intake boot does not have a plug since the tank & carbs are off. but this shouldn't matter, just bringing in more air to engine...

 

nope can't be the battery. just double checked all others and get same readings,, (roughly around 100-105 psi) but only 30-40 psi on plug #1. what's wrong?! engine seemed to run fine this past summer and only thing I've done past few weeks was valve adjustment manually turning the crank at starter

 

Ruh-roh.

First question: Just exactly how tight were the valves on #1?

Now, you need to do a "wet" test. Add a couple tablespoons of motor oil to #1, crank it over a few times, and re-test it. (Be sure to put a rag loosely over the hole when you first crank it over as it will spit some of the oil back out.

If the compression rises markedly with the oil, then you most likely have an issue with the rings. If there's virtually no change, then you probably have a burnt/stuck valve.

Which brings me back to my first question.

You can go ahead and do a "wet" test on all 4, but #1 is the one to be concerned about.

 

Be sure you are doing the Test with a Fully Charged Battery.

At the moment, ... ALL your readings are Below the Minimum Requirement

Any reading BELOW 120 psi you need to re-check "Wet" (1 tbsp Oil).
See what happens when the Oil is added.

 

Where do you pour the oil? In the spark hole? (Yes I'm clueless and waiting for above freezing weather to check mine).

 

You pour (or Oil Can squirt) about a tablespoon of Oil right into the Spark Plug Hole.
After you add the Oil, you run a second Compression Test on the Hole and see what the results are: "Wet"

If there's NO Change, ... you suspect Valves or a Head Gasket issue.
(Particularly, if there's NO Change across the board.)

If the Compression Rises and approaches "Standard" pressure values, ... you are dealing with a problem related to Piston Rings or Cylinder Wall.

 

before valve adjustment, I measured .102 for exhaust and .051 for intake which were very tight clearances from my understanding. replaced shim Y265 to Y255 for exhaust and Y270 to Y260 for intake.

Just finished the wet compression test and all four increased. The 'troubled' one (#1) increased compression dramatically, like near 200 psi. This isn't even standard from what I believe right? How could it do such a thing???

I am fidgeting with the original fusebox too because i wanted to retry #1 but the darn starter wouldn't even turn and oil level light would just flash on. I will try one more time in the morning & also replace the fusebox. I just purchased an aftermarket one from chacal so its time for a change.

The other three increased appropriately to around 140 psi which I should be pleased I believe. So thinking maybe the rings need replaced in #1? What does not make sense to me at all is I've owned the bike for over 2 years. Feel ashamed this is the first time I've checked compression, after I've already done the valve adjustment. Could I have been driving like this for two years now or is this I may have done to the engine recently?

 

It;s possible that the other three were nursing the 4th (bad) one along.

 

It's more than possible, it's highly probable. These are 4-cylinder engines, and without a frame of reference (like two bikes you can ride back-to-back) you'll never notice that performance has slowly faded.

The slight increase in the other three is what we would expect to see. If #1 simply had a burnt valve, it wouldn't have increased like that. There's definitely something amiss in #1; it could be as simple as stuck rings (actually sounds like it from the dramatic increase.) That motor doesn't have enough miles on it for that cylinder to be that worn out when the others aren't.

Do you have a service manual? Next step is to pull the head and have a look...

Unless...

Just a hunch. Spin the motor over vigorously (surrounded by rags) and get as much oil pumped out as possible, and re-test. I wonder if maybe you HAD a stuck valve on #1, which has suddently freed up with all the oil in there.

But that's a real long shot.

I think you're gonna have to pull the head and have a peek.

 

If you're capable and have the right tools; you can recondition that Engine and bring it back to full performance by Lapping the Valve and giving the Lower-end a Ring Job.

Get a Manual.

You can take a Head off the Bike and "Level-it" up-side-down, fill the Combustion Chambers with Rubbing Alcohol and look for Bubbles or a Leak to detect a Burned Valve.

If you don't have a Burned Valve; take the time to LAP the Valves and Replace the Valve Seals while the Valves are Un-sprung.

After you've done the Head; you can tackle the Rings.
Having a Manual to guide you through the Honing and New Ringing process is Vital.
You'll need to use a Broken-in-half Ring as a Tool to Scrape-clean the Piston's Ring Grooves.
Honing a new "Cross-Hatch" on the Cylinder Walls can be accomplished with a Tool that they LOAN-Out at AutoZone.

Once you've done a Valve and Ring Job, ... you'll have taken-off those 18,000 Miles and need to Break-in the Engine like it's Brand New.

 

alright. let me get out there and recheck the compression one more time this morning. I wouldn't be too terrified at pulling the head apart and examining further because I do have the manual and such great advice from you guys as well... its just I thought I was about done tinkering around with the carbs (need to check float heights today or tomorrow & then reinstall do some balancing with colortune & manometer) but now its looking like a whole new process. I want this bike to perform at its best and wouldl like to get my moneys worth, plus i'm a student at college so I cant be spending big big $ if you know what I mean. its definitely worth tearing apart into at this point since it is only january & about 20 degrees right now lol. will get back to you guys shortly, thanks

 

definitely something different about #1. compression as followed this morning: 190, 135, 125, 125. I measured #1 three times and the average was 190, with little variance between each time.

I haven't done any homework/reading on my own yet about pulling heads so I'm not expecting a big "how to" from anyone. is it simple as removing valve cover, camshafts, cam chain, nuts & bolts, and lifting half the engine out (the head) ?

Also, it surprises me how much a change the compression had for just 2~2.5 tablespoons of oil I put into it. If the standard was 120 up to 156 psi, what mechanism has been changed to give readings higher than that for #1?

 

You still have alot of oil in there, taking up volume.

Personally - I'd run it rather than pull it all apart. Frequent oil changes with quality oil, get the tune-up right, ride!!

 

The oil is still affecting #1, that's the only explanation. Which would square with a stuck or "clogged up" ring; something's not right in that cylinder. Whatever's going on, the oil is causing a much better seal than it did in the other three. (See my theory, below.)

Yep, pulling the head is just about as simple as what you outlined, with some detail differences (cam chain stays in place, tied up.) It can be done with the engine in the frame, no sweat.

There are some o-rings that you have to watch don't come off with the head and then fall into the motor, otherwise you've got it.

You'll need a new head gasket and those o-rings; and that's pretty much it-- EXCEPT for what you find once you open 'er up.

My theory:

I'm betting that #1 (the lowest cylinder when the bike's on the sidestand) got rusty during whatever time the bike spent sitting, and the upper ring got all clogged up with the rust it scraped off the cylinder wall. Now the oil that you put in there won't drain down, so it's causing the high compression numbers while the other three are more "normal."

I may also be WAAYYY off base; there's only one way to find out...

 

TIME, he was only getting 30-40psi "dry" on #1. With one cylinder that low, it's going to be hard to "get the tune-up right" it's not gonna want to sync.

I'd pop the head and look; we've still got a lot of winter time to go up here. Riding it won't make it better, likely the problem will get worse. Better to figure it out and fix it.

IMHO.

 

not to hijack the thread or confuse anyone, but here is an interesting article on compression testing from over at the honda SOHC guys.

Compression check

 

makes sense. hoping my cylinder wall isn't scraped though. just finished putting new aftermarket fusebox in so I won't worry about that. looking good checked compression again & it was higher than other three; near 180psi again :x

 

would it be a good idea to drain all oil? gonna be doing an oil change soon anyways but didn't know if it would be good to keep oil in it or not. doesn't most of the oil sit in the crankcase anyways so I shouldn't have too big a mess when removing head?

already got camshafts off and chain is held in place right now by zip ties. making progress today! seems easier to get things going on parts big enough for your entire hand. messing with carb parts is tedious from what i've experienced replacing the throttle shaft seals. too bad I couldn't have just put the carbs back on, tune it up, and be done with it :x

gotta love working on 30 year old bikes... hopefully i'll be driving it again with a grin ear to ear

 

I know you're into it now, but any chance you can take photos to go along with the progress you're making?

 

No need to drain the oil now; wait until you're done and then change it, to flush out any cr*p you might inadvertently drop into the bowels.

Be careful "maneuvering" the head off; the whole idea is to NOT let the cam chain come off the crankshaft sprocket down below if at all possible.

 

Zip ties could break an old wire coat hanger won't. Just a thought.

 

guys' i'm afraid something bad broke. ugh................. 11 out 12 nuts came off the head but one is still on the rod and I can lift the rod out but the nut is not spinning. it happens to be #6 nut in order when following the manual (they give an order of which nut to loosen to keep things balanced)

gotcha covered maxim-x. pictures will be taken for my sake too.

fitz, how can the head be removed then if the cam chain shouldn't be moved off the crankcase sprocket? sooner or later the cam chain tied up will have to be loose so the head can slide out. right? suppose when that time comes I'll lift the head as high as I can, grab tension on the chain under neath and have somebody else lift the head off then. keep in mind i've got the engine still in the frame...

 

It's not the end of the world if the cam chain comes off the lower sprocket, but to be avoided if poss', when you have the cam psrocket removed hook the chain up to the frame (maybe a rubber bungie) then when you lift the head you can slide a kebab skewer through the links at the top of the clyinders, then you can release the bungie.

 

take a look below at what I did to keep slight tension on the chain. got the head broke loose from the engine and waiting for dad to come home from work to help me out so he can do more work.. just need another set of hands.

i like the idea of a kebab skewer. once the head is high enough I'll grab the chain from underneath.

now am i stuck with a broken rod too? number 6 nut refuses to break loose from the rod, which shouldn't be free to lift out of the head correct?

 

You won't get the head off if a nut is still on there, maybe drill through the top of the acorn & get some kroil in there.

 

I have a length of 1/8" Nylon Cord I use to "Secure" the Cam Chain.
Pull the Chain straight-up.
Run a Plastic Tie through the Links to keep the Chain together and prevent it from coming-off the Crankshaft Sprocket.

Attach the Nylon Line to the Chain and knot it such that it WILL NOT come untied.
Lower the Chain down into the Engine, ... slowly.
Keep thje opposite end of the Nylon Line tied to the Center Tube. It's good to have PLENTY of Slack.

Pull the Head.
Once the Head is free; Untie the Line from the Frame and pass it through the HEAD.
Once the Line is passed through the Hehad, ... re-tie it to the Frame.

Use a couple of Old Tee Shirts and Stuff them into the Cam Chain Gallery to PREVENT ANYTHING from dropping-down into the Engine.
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Head Nut.

Use a DREMEL Tool.
Employ a "Cut-off Wafer Wheel"
Cut the Dome off the Nut.
Cut the NUT along its side.
Make the Side-cut "Bias" (On a slant, ... Not straight up & down.)

Once you have the Nut with a Bias Cut along its side, ... Use a Cold Chisel and SPLIT THE NUT Open.

(( The Bias Cut saves the Stud-end from being weakened if the Toll removes Threaded material. ))

 

I agree with Rick; "slice" the acorn and then split it open. Actually the cutting will probably free it.

And moellear that's a stud not a "rod." I just got home, checked your thread and saw all this "rod" talk and you had me worried.

Whew. (Rods are those wishbone-shaped things that connect the pistons to the crank.)

Don't worry about screwing up a stud, whatever one it is; as long as you don't snap it off at its base, it can also be easily replaced if you tear it up getting the nut off.

And although it's a little late now, invest in some Kroil: http://www.kanolabs.com/google/ Split the $12 with your dad, and give him a can for his very own. You'll both be glad you did.

Oh, and one more thing: Wiz is right on the money about kebab skewers, everybody needs a pack in their toolbox. They have a MYRIAD of uses in the shop.

 

my apologies. like I've mentioned, its a learning process for me and this is the biggest mechanical job I've attempted. normally just normal oil changes and lube for our family vehicles... anyhow, here's a picture of the stud.


looks like it didn't break after all. with it turning inside and the nut not able to come off, i was scared I snapped the stud. now its just a matter of getting the darn nut off :x

as a result, i got the top portion of the head off
the cam chain is tied by wire currently. tried using a skewer but it was too long. bending wire through the chain worked better while the head was slightly raised. then just a matter of lifting it off fully.

now correct me if I'm wrong but is the next step to lift the remaining portion of the head (cylinder block) out so I can examine the pistons and check out the rings? this looks like there's only one bolt holding the cylinder block, located on the front of the block where it also displays the size of engine "650". anything else holding it down? the manual suggests having another person available to support the pistons as it leaves the cylinder barrel. any other suggestions beyond this step?
feel like i've accomplished a lot in a days worth of work. now off to watch the buckeyes win the sugar bowl!!

seriously considering that kroil stuff fitz. slightly sore back from removing the 12 nuts, boy were they ever tough to turn 8O

 

Stop right there. You have the head off.

What's left is the cylinder block itself, A.K.A "the jugs."

You'll need to unbolt the cylinder base, it doesn't just get "sandwiched" in by the head; it has a gian-a** nut at the front holding it on and I believe the book says to remove the bolt holding the rear cam chain guide. I WOULD NOT COMPLETELY REMOVE this bolt, you might not be able to get the tensioner back in place if you're not splitting the cases; I would loosen it so the tensioner won't interfere with the cylinders lifting off.

But let's don't be too hasty---

Let's do some analysis of the situation first. We need to take a close look at that #1 hole before you pull the jugs off.

Get some nice close-up pics of the #1 cylinder with the piston at the top, center and bottom of its stroke, and let's see WTF before you yank the jugs off.

 

in other words, you're saying i can manually turn the crank? if i recall the book says to keep it at TDC (top dead center) line until the camshafts are back on securely. also, if the crank is turned, the chain will turn too if still on the sprocket in the bottom of crankcase. keep tension on it so it doesn't fall off the sprocket as I turn the motor?

sorry for silly questions but just want to make sure so nothing else gets screwed up. pictures to follow late this evening or tomorrow

 

Don't sweat the questions. A lot of us learned all of this the hard way, I wish I'd had a "me" to ask questions of, maybe I wouldn't have learned so many hard lessons.

The thing about not turning the motor over without the cams being TIMED is to keep the pistons from coming up and bending valves that are in the wrong place at the wrong time.

Now that the head's off, that's no longer a concern. You just don't want to have the camshft chain get all ensnarled in the guides or come off the crank, so go slow and help the camchain along.

You can turn the motor over, slowly, CCW ("forward") as viewed from the left. Use a 19mm open-end wrench on the camshaft timing plate (under the left crank end cover, look in my valve adjustment how-to.) Be careful not to crash the wrench into the timing pickups. (Ask me how I learned that one.)

 

you need 80 pounds of compression for a cylinder to fire. anything less is considered dead weight. and usually results in a gas fouled wet plug from lack of compression to fire accordingly.

run your fingernail along from bottom to top of cylinder wall and feel the lip at the top of the wall. the cylinder wall wears the most in the area where the piston rings touch. they don't touch the top of the wall so there's no wear at the top. resulting in a lip. your weakest cylinder might have a bigger lip then the other 3 cylinders.

the bigger culprit might be broken piston rings though.

like bigfitz mentioned. post some pics of the dead cylinder. clean up the valve chamber and post a pic of that also. so everyone has a very thorough look of both the engine AND the head for that one cylinder.

 

am I in between a rock and a hard place? sure feels like it. how can i tell if the rings are broke besides the following pictures i took? seems like only way to determine that is to remove entire cylinder block

here's what I've seen and took pictures of anyhow. we're dealing with cylinder #1 & the compression was VERY low until I completed the wet test adding about 2.5 tablespoons of oil. then the compression increased dramatically higher than normal ~around 180 up to 205 psi~ not normal for these engines. something is wrong with the piston rings I believe. but pictures don't really show wear on the cylinder wall so I'm confused.


piston bottomed out.


cylinder wall in detail. take a look closely. are those rings from the rings on the piston? I would imagine so... look normal?


piston up the chamber halfway.

I snapped a picture of cylinder wall #2 to compare. all four lips seem to be almost exactly the same. I felt the lips which is the dark ring at the top of the cylinder walls and they all seemed to be fairly even.

this is cylinder wall #2

lastly I took a picture of the underside view of head for cylinder #1


hope the pictures help see you fellows see what I'm seeing. if the pictures are too small, take a look at my gallery. they will increase when you click on them individually in my gallery. not sure why they wouldn't load on this page the size I specified... i gotta get a photobucket page to load bigger pictures sorry. any conclusions though before I tear it apart even farther?

 

here's where the test gets tricky. but you might be able to buy the tool at harborfreight if you have one out there.

need to measure cylinder taper. it's a micrometer tool that you slide up and down the cylinder wall. you don't measure the lipped portion though. the rings don't touch that area so there's no wear. the top portion will be wider then the bottom. how much is teh question.

engine shops do this test to determine if the cylinders can simply be honed or if they need to be bored. i think the spec is .003 inches. over that and honing needs to be done.

another test would be to see if you can slip a feeler gauge in between the piston rings and cylinder wall. with the postion at top dead center. using the most thinnest gauge.

only way to see if the rings are broken is to pull the pistons or jugs.

i wouldn't think the compression would hit 200. the engine had a top compression spec of 172. and that would be at sea level altitude.

 

sorta confused at what i need to measure with the micrometer tool. you are saying though that the cylinder gets wider as the piston travels up the cylinder. correct? the micrometer to measure this goes side to side then to cover the diameter of the cylinder from what i gather and can think of.

i'll check dad's toolboxes. and no, the closest harbor freight from my home,,, well idk, the closest mall is 30 mins away lol. when i go back to school there's a harbor freight of course in the big city

did the other test with feeler guage and at one location, the very forward part of piston i could get .004 inches in between the piston and cylinder. good/bad?

 

Check your local auto parts store for loaner tools to do the measuring. You want an ID micrometer. you measure the top and bottom diameters for taper and you can also measure twice 90 degress apart to check out of round.