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Problems with compression test.

#1

C

cdestuck

I hope I don’t get thrown off for this, but I’m having problems with the compression test, but this is on a four-cylinder motorcycle. I didn’t know where else to ask, but I thought you guys could be a big help.

I bought a compression test kit at Harbor Freight and hooked it up as I should in a while. Turning the engine over with the throttle open. It only reads about 65 pounds for each of the four cylinders. I thought the gauge was faulty and I exchange it Harbor Freight for a new one does exactly the same. How can I only be getting 65 pounds. The engine would never run at that level. The engine does run fine, but I’m just trying to take a test to determine how the cylinders are. Thanks for any help you can give me.


#2

I

ILENGINE

Does the engine have compression release, which would account for the low compression


#3

C

cdestuck

ILENGINE said:
Does the engine have compression release, which would account for the low compression
Click to expand...
Good thought. So I found a video of someone doing a compression test on the same engine as I have. He went through it and made no mention of a compression release. He came out with readings over 100 and the only difference he did is to remove all four spark plugs to do the test. I just took out one at a time when I did mine so tomorrow morning I will replicate his procedure and takeoff four out. I will report back my findings


#4

M

Mark H_NO

I've never heard of motorcycles having a compression release. I've worked on a few, but haven't done compression tests on any.


#5

C

cdestuck

Mark H_NO said:
I've never heard of motorcycles having a compression release. I've worked on a few, but haven't done compression tests on any.
Click to expand...
Years back I did have a two cylinder 650 Yamaha had a compression release. It was a manual lever underneath the clutch lever that you would pull in to start the bike. Just more of a manual release.


#6

Tiger Small Engine

Tiger Small Engine

cdestuck said:
I hope I don’t get thrown off for this, but I’m having problems with the compression test, but this is on a four-cylinder motorcycle. I didn’t know where else to ask, but I thought you guys could be a big help.

I bought a compression test kit at Harbor Freight and hooked it up as I should in a while. Turning the engine over with the throttle open. It only reads about 65 pounds for each of the four cylinders. I thought the gauge was faulty and I exchange it Harbor Freight for a new one does exactly the same. How can I only be getting 65 pounds. The engine would never run at that level. The engine does run fine, but I’m just trying to take a test to determine how the cylinders are. Thanks for any help you can give me.
Click to expand...
If the engine is starting and running, you have considerably more than 65 psi compression. A healthy 4-stroke will have 110 to 160 psi. There has to be a compression release on camshaft. Try to find detailed engine specs online for compression readings on your bike. Better yet, if there are concerns, do a leak down test to pinpoint problems.


#7

C

CraigH

I've always been taught that you should have a fully charged battery, all plugs out of the engine and wide open throttle.
That method has never failed me.


#8

StarTech

StarTech

See this why you should always look up your equipment by the equipment model number. This way you know for sure if a compression release is involved or not. Just can't go at it unknowing what you have in front of you.


#9

Hammermechanicman

Hammermechanicman



#10

woodstover

woodstover

cdestuck said:
Good thought. So I found a video of someone doing a compression test on the same engine as I have. He went through it and made no mention of a compression release. He came out with readings over 100 and the only difference he did is to remove all four spark plugs to do the test. I just took out one at a time when I did mine so tomorrow morning I will replicate his procedure and takeoff four out. I will report back my findings
Click to expand...
Yes, remove all spark plugs when doing compression as well as full throttle. The starter is fighting the compression of the other 3 cylinders. You want it spinning as freely as possible for checking compression.


#11

D

digr

Let the piston compress at least three times


#12

Hammermechanicman

Hammermechanicman

The cheap HF compression tester doesn't have the Schrader valves at the end of the hose. You can't use it for small engines or motorcycle engines

It it ain't like this it ain't gonna work

Attachments





#13

C

cdestuck

woodstover said:
Yes, remove all spark plugs when doing compression as well as full throttle. The starter is fighting the compression of the other 3 cylinders. You want it spinning as freely as possible for checking compression.
Click to expand...
Well today, I did take all the spark plugs out and check the compression while holding open the throttle. Still got the same 65 may be a pound or so more. Then I also had the engine running and checked the compression in one of the cylinders and it was still the same. I’m really baffled. Could certain engines actually run with 65 pounds? I’m lost.


#14

Hammermechanicman

Hammermechanicman

I give up.


#15

C

cdestuck

Hammermechanicman said:
The cheap HF compression tester doesn't have the Schrader valves at the end of the hose. You can't use it for small engines or motorcycle engines

It it ain't like this it ain't gonna work
Click to expand...
I am not at the bike now until Sunday. I will check to see what the bottom of the connector looks like. It advertises that is good for motorcycles and small engines. Of course that’s their advertisement


#16

woodstover

woodstover

cdestuck said:
Well today, I did take all the spark plugs out and check the compression while holding open the throttle. Still got the same 65 may be a pound or so more. Then I also had the engine running and checked the compression in one of the cylinders and it was still the same. I’m really baffled. Could certain engines actually run with 65 pounds? I’m lost.
Click to expand...
Did you remove all the plugs when you checked the compression after the engine was running??


#17

C

cdestuck

woodstover said:
Did you remove all the plugs when you checked the compression after the engine was running??
Click to expand...
Yes I did. I’m lost over these findings.


#18

B

bertsmobile1

Mark H_NO said:
I've never heard of motorcycles having a compression release. I've worked on a few, but haven't done compression tests on any.
Click to expand...
All single cylinder motorcycles over 250 have a compression release
Most kick start twins had a compression release
IT changed when our friends at Nippon Denso managed to make a reduction gear starter that was small enough & cheap enough to fit to a motorcycle
Not sure about old HD's but there is no way any one under 300 lbs will kick my G 14 BSA over without a compression release


#19

B

bertsmobile1

cdestuck said:
Yes I did. I’m lost over these findings.
Click to expand...
First of all what motorcycle are you talking about ?
Second what model compression tester are you using ?
Screw in or rubber bung ?
Next try spraying all around the plug hole and the tester tubing with soapy water I would expect you will be blowing bubbles every where
Does the tester have solid tube or flexible hose ?
I am tempted to say what did you expect from a $ 2 Chinese compression tester from the biggest cause of over consumption on the planet ?
HF = Junk ( 99 % of the time )
FWIW the compression tester I use cost $ 120 ( Aus ) back in the days when $ 200 was a weeks wages and it still works perfectly
When I started to do small engines it was fine for mowers but hit & miss on hand held gear so I bought another from Goodwoods because their stuff always works .


#20

Tiger Small Engine

Tiger Small Engine

cdestuck said:
Yes I did. I’m lost over these findings.
Click to expand...
Around 75 psi (give or take) is about the minimum for a 4-stroke small engine to start and run. Obviously, you have a compression release, or your gauge is inaccurate. Put a little oil into the cylinder and test it again. If the reading increases substantially, you have worn piston rings. Don’t get too focused on compression numbers if engine is starting and running strong. Perform a leak down test if it is bothering you that much.


#21

Hammermechanicman

Hammermechanicman

"All single cylinder motorcycles over 250 have a compression release"

My kawasaki 350 Bighorn didn't have a compression release.


#22

B

bertsmobile1

Hammermechanicman said:
"All single cylinder motorcycles over 250 have a compression release"

My kawasaki 350 Bighorn didn't have a compression release.
Click to expand...
That is because you Yanks are so tough . or perhaps just heavy .
To be honest the last new bike I bought was an SR 500 Yamaha ( Street version of the XT ) and it had a valve lifter on it as did the XT .
Smaller engines can be walked past TDC but big ones tend to kick back .


#23

C

cdestuck

bertsmobile1 said:
All single cylinder motorcycles over 250 have a compression release
Most kick start twins had a compression release
IT changed when our friends at Nippon Denso managed to make a reduction gear starter that was small enough & cheap enough to fit to a motorcycle
Not sure about old HD's but there is no way any one under 300 lbs will kick my G 14 BSA over without a compression release
Click to expand...
This is a 4 cylinder 650 1980 Yamaha with electric start. I really don’t believe there is a acr on this bike.


#24

Hammermechanicman

Hammermechanicman

bertsmobile1 said:
That is because you Yanks are so tough . or perhaps just heavy .
To be honest the last new bike I bought was an SR 500 Yamaha ( Street version of the XT ) and it had a valve lifter on it as did the XT .
Smaller engines can be walked past TDC but big ones tend to kick back .
Click to expand...
Anybody who had a big 2 stroke single knew how to "straight leg" start a bike. Work the engine around to the compression stroke. Shove yourself up high enough to extend the leg on the kick start out straight and drop your body weight down on the straight leg. Trying to start it sitting on the seat and just kicking it usually resulted in a painful kickback.
Screenshot_20240719_092902_Chrome.jpg

Pic off the internet. There is a reason the Kickstart lever on this bike was over a foot long.


#25

C

cherami

Mark H_NO said:
I've never heard of motorcycles having a compression release. I've worked on a few, but haven't done compression tests on any.
Click to expand...
Hello Mark yes in fact many older single cylinder motors had a valve lifter to ease starting, but not on modern engines that I know of.


#26

I

ILENGINE

cdestuck said:
This is a 4 cylinder 650 1980 Yamaha with electric start. I really don’t believe there is a acr on this bike.
Click to expand...
From the parts diagrams appears to be DOHC with no decompressor.


#27

Hammermechanicman

Hammermechanicman

Never seen a mechanical ACR on a multi cylinder 4 stroke bike. Honda did have one on the XL500S. It was a 500cc 4 stroke thumper that pretty much killed off the 2 strokes used in flat track racing. Back in the heyday of big single racing 2 strokes it was common to have a compression release activated by a lever on the handlebars. Wasn't used for starting but rather modulation of power while holding WOT.


#28

Etbrown44

Etbrown44

It's an interesting question, but it's mostly academic. Your best compression test is to run the motorcycle. If it runs smoothly and accelerates well, you have good compression, and that's all that matters.

It's often the test gague or your procedure. I've seen 80 psi on a chain saw, yet it performed perfectly. Always believe performance over a gague, unless you are a pilot flying on instruments in an aircraft, and in the clouds!
.


#29

R

RevB

cdestuck said:
I hope I don’t get thrown off for this, but I’m having problems with the compression test, but this is on a four-cylinder motorcycle. I didn’t know where else to ask, but I thought you guys could be a big help.

I bought a compression test kit at Harbor Freight and hooked it up as I should in a while. Turning the engine over with the throttle open. It only reads about 65 pounds for each of the four cylinders. I thought the gauge was faulty and I exchange it Harbor Freight for a new one does exactly the same. How can I only be getting 65 pounds. The engine would never run at that level. The engine does run fine, but I’m just trying to take a test to determine how the cylinders are. Thanks for any help you can give me.
Click to expand...
Continental aircraft engines are good down to 40# per the mfg. 65 is not spectacular but is probably much higher when running. Should run.


#30

R

RevB

cdestuck said:
Good thought. So I found a video of someone doing a compression test on the same engine as I have. He went through it and made no mention of a compression release. He came out with readings over 100 and the only difference he did is to remove all four spark plugs to do the test. I just took out one at a time when I did mine so tomorrow morning I will replicate his procedure and takeoff four out. I will report back my findings
Click to expand...
Stick the plugs back in the caps and ground them.....give the spark somewhere t go.....


#31

J

Joed756

Why did you do this compression test in the first place?
I wouldn't worry too much, if each of your four cylinders is reading about the same you most likely have a bad gage even though you replaced it. You go to any well known auto supply store and borrow a compression gage or better yet, your engine is running on all four cylinders, don't go looking for problems you don't have.


#32

C

cdestuck

Joed756 said:
Why did you do this compression test in the first place?
I wouldn't worry too much, if each of your four cylinders is reading about the same you most likely have a bad gage even though you replaced it. You go to any well known auto supply store and borrow a compression gage or better yet, your engine is running on all four cylinders, don't go looking for problems you don't have.
Click to expand...
I have come to that conclusion. I just have to figure out why my number two cylinder is not firing. I have spark, and I had all four carbs off to rebuild them and was relatively sure they were all OK. But this week I’m gonna pull them back off and open up that one just to go over it. I’m sort of baffled over this whole deal.


#33

J

Joed756

When did number 2 stop firing? It has spark, so the issue has to be fuel. Why would you take all the carbs off if only one cylinder isn't firing? I think you should check valve gap on number 2 before anything else.


#34

R

RevB

Joed756 said:
When did number 2 stop firing? It has spark, so the issue has to be fuel. Why would you take all the carbs off if only one cylinder isn't firing? I think you should check valve gap on number 2 before anything else.
Click to expand...
Why if they're all 65psi? 🤫


#35

VRR.DYNDNS>BIZ

VRR.DYNDNS>BIZ

CraigH said:
I've always been taught that you should have a fully charged battery, all plugs out of the engine and wide open throttle.
That method has never failed me.
Click to expand...
Lets get to the basic reason of plugs out and battery charged. rotational rpm increase wil increase the reading significantly. A quick test we perform on recoil start engines is a slow pull and at tdc if little resistance, the rings and cylinder or valves are leaking a bit to maybe to much. Adding a few drops of oil then will confirm rings as the source if an increase of at least 10%. What I suggest is plugs out and you will see a rise in the reading. This is all due to the dynamics of leakage does exist and at what level over time which is the rate of revolution.


#36

Hammermechanicman

Hammermechanicman

cdestuck said:
I have come to that conclusion. I just have to figure out why my number two cylinder is not firing. I have spark, and I had all four carbs off to rebuild them and was relatively sure they were all OK. But this week I’m gonna pull them back off and open up that one just to go over it. I’m sort of baffled over this whole deal.
Click to expand...
If you had the carbs off and apart have you synced the carbs?


#37

R

Red Good

Motorcycle mechanic here and you must have all the plugs out and stick them in the plug caps and lay them on the head to not fry the ignition then with the throttle wide open spin it over till the needle stops bouncing . record and go to the next hole . Best of luck


#38

T

TobyU

Put the comp tester back into the toolbox.
It's just not needed.
I have fixed 1000s of mower engines and have never screwed one into one.
Well, not at the first time when I realized they are not like cars.
The only tester you need is your finger
If it blows it out nicely, it has enough compression to run


#39

R

Red Good

is the compression tester a rubber tipped one or a screw in one ? Rubber tipped ones are hard to get accurate readings off of with all the other things you are doing . Make sure you have spark on #2 by testing it by itself . The fact that it runs makes me believe it does have sprk and it is a carb related problem . I would use the screw on the bottom of the bowl and drain that carb with the petcock shut off , when done draining turn the petcock on and let some fuel drain again and then shut the screw off and see what it runs like then . Those 650 4's were little rippers when new .Good luck .


#40

J

jviews12

I also doubt you have 65 lbs on all 4. Sounds suspecious.


#41

G

glyn1718

Full throttle. Plugs out. Check all cylinders and note results. Then insert a teaspoon of oil in each cylinder and retest. If the reading are significantly different you are loosing compression usually via rings. But I say don't worry, if it runs, get out on the road and enjoy it. Only consider compression readings when the engine is playing up as part of diagnostics


#42

G

Gord Baker

cdestuck said:
I hope I don’t get thrown off for this, but I’m having problems with the compression test, but this is on a four-cylinder motorcycle. I didn’t know where else to ask, but I thought you guys could be a big help.

I bought a compression test kit at Harbor Freight and hooked it up as I should in a while. Turning the engine over with the throttle open. It only reads about 65 pounds for each of the four cylinders. I thought the gauge was faulty and I exchange it Harbor Freight for a new one does exactly the same. How can I only be getting 65 pounds. The engine would never run at that level. The engine does run fine, but I’m just trying to take a test to determine how the cylinders are. Thanks for any help you can give me.
Click to expand...
Check the Spec's for that engine. Your engine seems OK but low across the 4. It would run at that level I believe.


#43

G

Gord Baker

glyn1718 said:
Full throttle. Plugs out. Check all cylinders and note results. Then insert a teaspoon of oil in each cylinder and retest. If the reading are significantly different you are loosing compression usually via rings. But I say don't worry, if it runs, get out on the road and enjoy it. Only consider compression readings when the engine is playing up as part of diagnostics
Click to expand...
:) Yup.


#44

F

fastbroshi

Etbrown44 said:
It's an interesting question, but it's mostly academic. Your best compression test is to run the motorcycle. If it runs smoothly and accelerates well, you have good compression, and that's all that matters.

It's often the test gague or your procedure. I've seen 80 psi on a chain saw, yet it performed perfectly. Always believe performance over a gague, unless you are a pilot flying on instruments in an aircraft, and in the clouds!
.
Click to expand...
I agree with this. The compression may be reading low per spec, but you have to consider you're not using the same tools used by Yamaha when they created their spec. Not to mention if this bike hasn't ran in a while, it's possible it's got some gummed up rings, valves, etc that may affect your reading. Did you ever try adding some oil to the cylinders before a test, as others have mentioned?


#45

I

IronTom

A few years ago, 1957, I had a early Matchless 500 single. ( Big 'jump' up from my Whizzer ).
To start it, I coasted down the Vernon street hill, and released the clutch in 2nd gear.
There were many failures,,, push it back up the hill,, try again.
Uncle (machinist) installed a 'flip-valve' by drilling a tiny hole through the head.
It cut the compression down to where little-skinny me could use the kick-starter !
It also would often, shoot fire from under the gas tank !
which resulted in Dad trading it in on a brand-new Cushman Eagle !

Where were we ? Your 65 lbs compression is just enough pressure to 'fire' the engine up with that shrimpy-small battery starter. Once the RPMs increase, the valves return to normal high-compression operation, the electronic ignition advances the spark timing,,, and the beat goes on!
A simple 'what-me-worry' test would require a few drops of motor oil onto each piston, then re-test the compression. If the oil changes the 65 value, that indicates normal piston-ring wear.
Thats not a problem. Write the numbers in your manual,,, 'till next year.
B.T.W.
Do not try to roll-down-the-Vernon-street-hill, that modern engine may throw you over the bars !


#46

Smithsonite

Smithsonite

Some machines have a compression release on the intake cam, deactivated once over 300 RPM or so by flyweights.


#47

A

awkerper

cdestuck said:
I hope I don’t get thrown off for this, but I’m having problems with the compression test, but this is on a four-cylinder motorcycle. I didn’t know where else to ask, but I thought you guys could be a big help.

I bought a compression test kit at Harbor Freight and hooked it up as I should in a while. Turning the engine over with the throttle open. It only reads about 65 pounds for each of the four cylinders. I thought the gauge was faulty and I exchange it Harbor Freight for a new one does exactly the same. How can I only be getting 65 pounds. The engine would never run at that level. The engine does run fine, but I’m just trying to take a test to determine how the cylinders are. Thanks for any help you can give me.
Click to expand...
This conversation still provided me with some good info. Thanks all!


#48

B

bud&dusty

cdestuck said:
I hope I don’t get thrown off for this, but I’m having problems with the compression test, but this is on a four-cylinder motorcycle. I didn’t know where else to ask, but I thought you guys could be a big help.

I bought a compression test kit at Harbor Freight and hooked it up as I should in a while. Turning the engine over with the throttle open. It only reads about 65 pounds for each of the four cylinders. I thought the gauge was faulty and I exchange it Harbor Freight for a new one does exactly the same. How can I only be getting 65 pounds. The engine would never run at that level. The engine does run fine, but I’m just trying to take a test to determine how the cylinders are. Thanks for any help you can give me.
Click to expand...


#49

S

sheenist

cdestuck: Run a compression test on all four cylinders and record the numbers. Then squirt a couple of shots of oil in one cylinder and try it. If the compression is noteably higher, then yon may be leaking some compression past the rings. If the reading isn't much higher, you are probably leaking past the valves. Good luck.
Sheenist


#50

G

Gord Baker

This is getting like the 'winter storage' episode! Most important is that the Compression is near equal in all Cyl's.
It appears to be so just 'run it'.


#51

Hammermechanicman

Hammermechanicman

This thread took a left turn a long time ago.


#52

B

BVT

What motorcycle are you working on. Compression releases are still on some current production motorcycles. I would recommend getting a better-quality compression tester also. Depending on the motorcycle you're working on you may be able to find a service manual online or got to one of the online motorcycle parts stores and look at the parts diagrams and find the camshaft, look and see if it has any type of a weight on it that would be the release. the other thing to check is (again depending on what motorcycle you are working on) the valve clearance.


#53

L

lbrac

Hammermechanicman said:
Never seen a mechanical ACR on a multi cylinder 4 stroke bike. Honda did have one on the XL500S. It was a 500cc 4 stroke thumper that pretty much killed off the 2 strokes used in flat track racing. Back in the heyday of big single racing 2 strokes it was common to have a compression release activated by a lever on the handlebars. Wasn't used for starting but rather modulation of power while holding WOT.
Click to expand...
Manual 2-stroke compression releases were used for braking, when the drum brakes back then would wear out in one enduro, sometimes before it was over. Disc brakes solved that problem.


#54

Hammermechanicman

Hammermechanicman

Buddy of mine raced flat track back in the day with a big 2 stroke single. Once up to speed he held the throttle wide open all the time and used the compression release to modulate power in an out of the turns. Keeps the engine revs in the powerband all the time. Brakes? What are brakes? I helped him in the pits sometimes. Those guys had more balls than brains.


#55

L

lbrac

Does #2 cylinder miss at all rpm ranges? Idling or gradually revving through the rpm range while sitting still; riding while coasting or under different loads/power levels? If it misses only at a certain rpm range or loads/power levels, it might help identify which fuel circuit in the carb is at fault, assuming that is the cause for the miss. I worked at a Yamaha/Suzuki dealer in 1983-84 and never heard the mechanics or parts people mention anything about a compression release on your model, or any 4 cylinder models of that time frame. Nearly all had electric starters, and most had done away with kick-starters.

A parts guy there had a friend w/ a new HD who would dare different people to try to kick-start it, and we got a lot of entertainment out of that. If you weren't serious about follow through, it would throw you over the handle bars when it kicked back. They tried it with me and, to their surprise, I cranked it on the first kick, despite being a relative lightweight. But unknown to them, a friend of mine had a 450 Husky that I was used to kick-starting.


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