What do these compression numbers mean?

On a lark I decided to check engine compression on my 700x.

Having started riding this spring I've put 6k miles on the bike without any of the regular maintenance- other than fluid/oil changes- that seems to be part of owning a 20 year old bike. Things like balancing the carbs or adjusting the mixtures.

So, having decided that a compression check was the first order of business, I ran tru once dry and 3x after adding a tsp of oil to each cyl (wet). The numbers that came up have me scratching my head:


Cyl__1_____2____3____4

Dry 140psi 90psi 65psi 100psi
Wet 160psi 190psi 180psi 240psi(!)
Wet 160psi 190psi 200psi 210psi
Wet 160psi 200psi 180psi 210psi

*Yamaha Spec is 154 psi on all cylinders



Total mileage is 20,500- 5,000 of which I have put on since April
The bike burns no oil
Oil has been clean the two times I changed it
Starts good
Idles smoothly at 1,000 rpm
Mileage 40 to 50 mpg US depending how I ride it
Spark plugs look good to me, but what do I know? Pictures below (I thnk)...


Does anyone have any idea what's going on with these numbers?
Is this a ring problem or a valve problem or both or neither?
How about those cyl #4 wet numbers- how can they be right?!

Should I mess with anything or should I, as my friend says, "It's 20 years old. Just ride the bike."?

 

Offhand it looks like you put WAY too much oil in. I'm very skeptical of your #3 dry reading if it's not burning oil.

Run bike for a few minutes.

Put battery on trickle charger.

Have a beer.

Sleep 12 hours.

Pull all 4 plugs.

Throttle wide open.

Put gauge on. Crank engine 3 revs or so. Record reading. Repeat each cylinder.

Report back.

 

Since I can't seem to figure out how to add pictures to the body a of a post, I put a photo of the spark plugs in my gallery...

 

When the wet numbers improve the Compression reading ... it Rules-out Valves as the Number One cause for the low numbers and indicates that there is some problem with the Rings.

The numbeer 2 & 3 Cylinders are what we call Siamese. Practically joined together they are so close together.

An overheat problem will "Spread" from 2 to 3 real easily.

I fear that the Low reading of No.-3 indicates you have a Cracked Compression Ring ... letting the Compression "Blow-by" the Ring that's cracked.

The fact that you are not Billowing Smoke out the exhaust probably means that your No.-3 Cylinder is running LEAN and the OIL is being burned as fuel.

Line-up the Plugs 1-2-3-4- and take a picture of the Ceramic ends surrounding the Electrode tip and let's have a look at what's happening inside the Combustion Chamber.

 

They're in my gallery 1 thru 4, left to right

 

I'd like a better look at just the electrodes ... But, either those are some Brand New Plugs ... or, you are running way too lean.

The pearly-white #-3 shows me that ... that Hole is burning CLEAN ... even with the possible Ring Problem.

Bring ALL your Pilot Mixture Screws OUT about the width of 2-Nickels.
You've been running Hot and Lean ...
That's NOT good.
I hope you haven't found-out the hard way!

 

Well, I pulled the battery and plugs. Battery's on the charger.

Rick, the plugs are kind of new- 1,400 miles or so. And the mixture screws were all set at 3 turns out (manual calls for 2 to 2.5).

Tomorrow I'll check compression again, try to take better photos of the plugs and post the results.

 

One more thing:

How do I know if it's burning oil per Rick's post. The level in the sight glass hasn't changed in the last 2,000 miles and when I drained it today it looks like I recovered 2+ quarts (manual calls for 2.6 quarts)?

 

We have a hang-up about where the Mixture Screws are "Supposed" to be set too.

Every Bike and every Cylinder on every bike could be different. It depends on the Condition of the Engine and the Condition of each individual Cylinder.

You think SYNC doing the Intake Air Supply and Matching the Air Volumes drawn-in.

You DON'T SYNC the Mixture Screws. You Dial-in for the BEST Idle and Just-off Idle need of the particular Cylinder.

The ABSOLUTE "Sweet Spot" is going to be where you have a Mixture capable of keeping the Bike running -- at Idle ... and, supplementing the the Opening of the Throttles with enough Fuel to allow the engine not to:
a) Hesitate ... waiting for enough Fuel from the Main Jets to Accelerate ...

b) Bogg-down the Engine with too much Fuel for an Instant until the Main Jet Supply provides the Intake with a Precisely Metered amount from the Main Jets.

The Pilot Mixture Screw is Ultra-Fine Threaded.
The correct Mixture is achieved within "Degrees" ... Not Fractions of a Whole.

The 2-1/2 Turns Out was Factory Lean to conform to EPA Rules.
Most Dealerships brought them out close to 3 Turns Out when the Bike got its Free Dealership 500 or 1,000 Mile check.
Some places NEVER took out the Plugs and those bikes need a tweak.

Your Mission is to find ... by Individually Tweaking each Mixture Screw ... the Ideal Mixture for Idle and Off-Idle (Out of the hole) performance.

A Colortune Plug will assist in bring you to the Ballpark.
Ballpark Colortuning gets you to "The area where the Sweet Spot is Found"

The BLUE you get from Colortuning is near the Window of Ideal Performance.

Adjusting the Mixture and getting a nice Cobalt Blue happening in the Combustion Chamber is an Air-Fuel Ratio that provides the assurance that ENOUGH Fuel is present inn the Pilot Air to support Combustion.

Once you arrive at that spot ... you Tweak for the way you ride!

Within a FEW DEGREES of a Turn -- (The width of a Dime or a Nickel) ... you can Dial-in:

High-Performance LEAN:
A Mixture where the Pilot Jets Supplemental flow will provide a small percentage LESS Fuel to the Chamber -- allowing for a POWERFUL and FAST Burn of the Mixture Present at Ignition ... for Rapid Accelerations, at the same time ... providing LESS Fuel too the Engine when the Throttles are closed ... providing a GREATER DEGREE of Engine Deceleration -- or, "Engine Braking."

Cruiser-Bike RICH:
A Mixture where the Pilot Jets Supplemental flow will provide a greater percentage MORE Fuel to the Chamber -- allowing for a SLOWER and Less Powerful, smooth application of Power because there is a Greater Percentage of fuel to burn during Ignition ... this provides a quieter and smoother application of Power ... providing additional measure of Fuel to the Chambers at Closed Throttles for Moderate Deceleration and "Coasting"

The SPOT is different for each hole.
The PLUG READ will show you where you are at.

The solution is not a "Quarter Turn" [90-Degrees]
The solution is nit an "Eight of a Turn" [45-Degrees]

It's less than 10-Degrees.

An "Honest-to-God" TWEAK.
Almost not moving it ... because you move it so little!

Chart your + & -- Know where you are and where you've been with those Mixture Screws.

They have ULTRA-fine Threads.
You can do some ULTRA Fine-tuning.

(It helps to have a gas line a bit longer to be able to Prop-up the tank and a Special Screwdriver to be able to get in there without bumping the top of the tank).

 

Reporting Back...

MICARL- Did all you suggested except for the beer which, as it turns, out is probably what I needed the most.

This morning I reran the compression test and got the following numbers both Dry and Wet, measured 3 times:

Cyl 1 130psi
Cyl 2 90psi
Cly 3 110psi
Cyl 4 130psi

I carefully metered the oil for the wet measurement- 1/2 ml in each cylinder.

What do I do now?!

 

Get in there and see what the Clearances are on your Valves.

You might be a one of this months Lottery Winners and find-out that you have some tight Valves that just need to be Re-shimmed.

Wouldn't that be a nice early Christmas Present!!!