Compression Test

Ok, im gona pull the plugs to check them out, so while i have the plugs out can anyone help me out with what i need and how to do a compression test??

thanks a bunch

 

you will need one of these unless you have an engine analyzer.

 

Just crank the engine over while the compression tester is in the spark plug hole, pretty easy.

 

To perform this test correctly, follow the bouncing ball.
1. Disconnect the connectors to the TCI. This saves you having to ground all of the spark plug wires, lots more work than needed.
2. Remove all of the spark plugs. This will reduce the amount of pressure from each revolution fighting against the turning of the engine, saving your battery and starter a lot of effort. Additionally, you spin the motor faster and get better numbers.
3. Crack the throttle WIDE open. This allows the engine to gulp as much air as possible for better (more accurate) readings.
4. Don't spin the starter motor for more than about 10 seconds without a few minutes rest. It isn't designed to be run very long and you will over-heat it if you crank continuously.
5. Do a dry test and write down your numbers.
6. Do a wet test and record your numbers.
7. Compare the two and see what you have.
8. Discuss your results with the forum.

 

Do you warm up the engine for a few minutes first the way you do with a car?

What are dry and wet tests? This one lost me.

 

I have the same questions floating around in my head. Any takers?

 

This even explains wet/dry.

Out of my 1980 XJ650G Service Manual:

1. Check valve clearance.
2. Remove headlight fuse.
3. Warm engine 2-3 minutes.
4. Remove all spark plugs.
5. Install compression check guage
6. Turn over engine with the electric starter with throttle wide open until the pressure on the guage does not increase further.

Standard: 156 psi, 11 bar
Minimum: 128 psi, 9 bar
Maximum: 171 psi, 12 bar

7. If pressure is too low, squirt a few drops of oil into the cylinder being measured. Measure compression again. If there is a higher reading than before (without oil), the piston rings may be worn or damaged. If pressure remains the same after measuring with the oil, either or both the rings and valves may be the cause.

8. Check each cylinder. Compression pressure should not vary more than specified value from one cylinder to any other cylinder.

Difference in guage reading: 14 psi, 1 bar

 

could you describe where this wire is and color. thanks grm

 

It's the ignition box behind the left cover on most models, it has lots of wires going into 2 plugs.
You could also unplug the 2 connectors to the coils, or just cross the solenoid--key off.
Also, it's a good idea to boost the battery on a charger while doing the testing.

 

I got a few questions if I could. You guys are such a great help, I wouldn't know where I would be without you (oh yeah, i'd be making payments on a harley that I couldn't afford and that i didn't know how to fix ;-P )

So, i'm taking my head apart anyways for some broken manifold cap screws, and I was going to lap my valves and replace the valve stem seal, I was told to check compression to see if there as any 'other' problems before I got started..

The thing is, my carbs are off the bike, how can I check compression? I can't really put my carbs back on, due to the broken manifold screws (however, my carbs are all nice and pretty now ;-p).

Probably a dumb question, but I appreciate all assistance i can get ;-)

thanks in advance!

 

You don't need to have the Carbs on the Bike to conduct a Compression Test.
The proper test is done with Wide Open Throttles; anyway.

You'll still get numbers that you can judge the condition of your engine by.
There's only so much Volume a Cylinder can Intake; any way.

Just be sure the environment is clean and nothing can sneak into the Manifolds during the testing.

 

Ok; I didn't understand why the throttles needed to be wide open (hence my question, becuase if the carbs aren't on, the throttle can't do anything).. looked it up (new to engines, forgive my ignorance) - its because you don't want a vacuum created that could screw up your readings, right?

josh

 

Exactly!
You want as much air as the Vacuum created by the Piston on its Intake Stroke can draw-in.

Be sure the Battery is Charged-up nice. That also accounts for a factor in determining what Compression readings will do.
A swift rotation will be better than a sluggish engine with a less than fully charged Battery.

 

Ok- beau-tiful!! Starting to get this then ;-P

Thanks for the assist! Much appreciated!

I have my battery fully charged, but I figured i'd throw the charger on it while I am running the tests too just to be sure ;-)

thanks again!

josh

 

ok - just ran some compression tests.. first time 'ever' doing them so I think i did them right ;-)

#1) 110 dry 140 wet
#2) 115 dry 150 wet (I think... i think I put to much oil in - initial wet reading I got was 210!, after1 minute it was 150.. then it dropped to 140... then i stopped testing this one ;-P )
#3) 100 dry 150 wet (same issue as above)
#4) 110 dry 140 wet

so... are these good numbers?? does this tell me that just the valve seals are bad?? (which i am going to replace anyways)...

Any (ALL) help appreciated. I only ask this many questions when i am new to a topic - so bear with me!

thanks!

josh

 

Nope.
Those results indicate that you need rings.
Right across the board.

If there was anything wrong with the Valves the readings wouldn't change between dry and wet.
There's nothing wrong with the Cylinder Head. Good News for you!

You have low compression on all four Cylinders.
You need a Ring Job.

 

ring job = something I can do - or I should pony up some $$ and get someone to do it?

I've been reading up on this topic (and valve job) through posts in the forums here.. Some people say a valve job is more indepth then a ring job, and others - vise versa..

Still others say, just get the pig running and maybe that will loosen things, lube things, de-crapify things and increase your compression...

This bike was last registered in 2000, its got 17k on it, and from what i can tell (and if the carbs were any indication) its been sitting for at 'least' the last 5-7 years. So is the whole 'get her started and see what the compression is after' better then trying to tackle all of this??

I have to take my head off anyways to get the manifold screws drilled out - so if i am 'there' already and its not to involved, i can do something... I mean I can always just get a new gasket - put it on right, get everything started and run her a bit, then see how she is, then I can strip down the engine over the winter....

Basically what I am asking is, is it better to just leave the valve / ring job when I take the head off for drilling, and just get her back together and get her running to see if that helps with anything? Then work on it all over the winter? Or....... buy a new engine (which I don't want to do, bought the bike for 200, got about 400 invested in parts already.... she's rough and needs a lot of cosmetic work, see gallery, etc... if I am going to drop another 300-500 on an engine, i might as well just buy another bike....)

inputs appreciated, you guys are a lot better at this stuff then i am!!

josh

 

Since the Head is coming-off anyway, it's another couple of hours to remove the Block.
If your Compression numbers were marginally higher you might have had a chance ar resurrecting some Compression, but probably not too much.

The wise thing to do is pull the Block and have the Holes HONED and Cross-hatched.
Then, determine what size Rings you need for the repair job.
I have a feeling you might be close to stock rings, but you might nened the 1st Oversize if the Cylinders got rusted within the travel of the Compression and Oil Control Rings.

After the Ring Job, you'll likely be restored to near optimum Cylinder Compression.

Since the "Wet Test" raised your numbers right up to the very-good to excellent range; the Head isn't likely to need having the Valves and Seats re-cut.
But, when a Head comes off and you have access to the Valves; it's always a good measure to have the Valves cleaned of any Carbon deposits on their stems and Lapped-in to restore the Gas Tight seals between the Valve Faces and the Seats.

The Shim Bucket and Valve Springs need to be removed anyway before you can replace the Valve Stem Seals.
While the old Valve Stem Seal is removed it is a very easy step to push-out the Valve, high-speed wire brush off any carbon deposit from the stem and near the valve face and clean-up the exhaust port of any build-up when the valve is out.

Lapping the Valve and seat takes but a few minutes.
Compound is added to the Valve Face and the Valve is closed upon its Seat.
A suction cup tool is used to rotate the Valve around upon its Seat while the Compound renews the narrow gas tight band where the Valve and Seat mate.

After a Ring Job and Valve Lapping-in ... you usually treat the Engine as brand new. Taking it easy and letting the new rings and valve surfaces break-in before running the engine at high revs.

Full performance is restores and with proper maintenance the engine will behave like new for thousands and thousands of miles.

 

ok... sigh... guess thats what i'll do then ;-P

do I have to take the engine off the bike or can I keep it on there?

 

Question for Rick, when lapping the valves, I have always been careful to replace each valve from whence it came, but how important do you consider that to be?

 

Vitally important!
You can't mix-up any of the parts to the Cylinder Head.

When I get ready to do a valve seals and a lapping of the valves ... I wipe-off every shim and bucket and mark everything with a Magic Marker.

The Valves get pulled and put into a Valve Rack (or a cardboard sheet)
with the Intakes and Exhausts poked through the cardboard just the way they come out.

 

Hey jgb1503, did you ever get an answer on the removal of the engine? I think that I'm going to have to do a ring job in the near future, and I really have limited space, leaving the engine in the bike would be best for me, but then again what do I know, I'm just another noob trying to turn a wrench with the ethics of a doctor, "first, do no harm"