TOPIC:

Yes, an adapter can lower the result, but I can't guess how much. The 550's have nearly the smallest combustion chambers of all the KZ's (in the U.S., except the 250 twin), so they would take a larger drop due to any adapters. (An 1100 would have half the drop, all else being equal.)
I don't think I have any screw-in type right now to do a comparison. (And for a comparison, they would have to both be static tested to the same pressure, in the 150 to 190 range.)

Like R said, I wouldn't stress about it too much until you can get it running hot and do a proper test with a solid battery. When you do it, you have all of the other plugs out, and the coils disconnected or plug wires shorted, correct?

Lou - To answer your question, I agree, my compression numbers I took on this bike in particular, it was never hot (if you remember, this motor was donated to my daily rider from a parts bike, so I did the tests real quick before putting the effort in swapping the motors). So, I have maybe 10 miles on this motor since I had it, and it last ran probably 10 years ago, plus my battery is on the way out the door. But, in the past, on my other motor, the numbers were never the 160+ psig. Also, when I have done compression in the past, the throttle is open (or carbs are off) and the coils disconnected (this only protects from stressing the coils, right?).

Now, as for the adaptor - I did a little research because I was curious. I found I was using two adaptors (1 to fit the plug hole, the 2nd one to aid in the ease of connecting/disconnecting the hose and not burning ones hand on the motor). Now, the adaptors ID is very small (its like a piece of round stock with a small hole, like 1/4" bored through it), so I thought maybe it wouldn't make a difference. But I took a syrnige and filled up the adaptors with water. I got just over 1.25 mL...call it 1.3. The OEM manual does have a spec for combustion chamber volume in the head (its like 15.6-16something CC), lets call it 16 for ease of numbers. Im sure once aseembled its actully less because the domed piston should extended past the surface of the jugs into the chamber some, right? Anyway, if I take my 1.3 mL (same as a CC) and divide it by 16 I get 8.1%. So I'm adding 8.1% volume to my chamber. And if I'm getting, for example, 140psig reading (assuming the gauge isn't compensated for the adaptor, which I doubt it is because there are an assortment of different sized ones) the actual true reading may be higher by 8%, which would put me in the 151 psig range. Obviously these numbers are estimates and some bar napkin math, but I think the point is made that my adaptors are defintiely affecting the readings. I may borrow a friends or pick up a stick in one and have someone at work test both of them for accuracy and try them out side by side on the bike.

Since you've only ran it 10 miles in the last 10 years you could have the rings a little gummed up also ... that with warming it up could get you another 10 + lbs so I don't think things are looking that bad.

I bet if you used 10 adapter's and turned the motor over till the
gauge stopped climbing then took them off and retested you'd get the same number's.

Cam grind and timing play's a big part in the static comp. number's
test a stock KZ750 then put in some GPz cams with no other change's and you'll get a lower number
my 810 cc motor with Wisco 10,25 comp. piston's and 020 off the head only made 140 psi.
right before I started it for the first time.

zukdave wrote: I bet if you used 10 adapter's and turned the motor over till the
gauge stopped climbing then took them off and retested you'd get the same number's.

Cam grind and timing play's a big part in the static comp. number's
test a stock KZ750 then put in some GPz cams with no other change's and you'll get a lower number
my 810 cc motor with Wisco 10,25 comp. piston's and 020 off the head only made 140 psi.
right before I started it for the first time.

Yes, it's true that cams will lower the compression number if the valves have more duration etc. I presume that is why gpz550's have about the same test results as standard KZ550's.

But if one adapter is 10% of the combustion chamber volume, then 10 adapters would double the volume. That cuts the compression ratio in half. The compression test results will certainly not be the same.

The defective tester from Harbor Freight was only defective because it basically had a stack of adapters in the form of a hose. So we already know that adding volume to the tester before the checkvalve will give really low numbers.

I am no physicist, but I think their is way less effect due to adapter volume. Your first reading would be least reliable. But given the number of strokes it takes to build max psi, 5-10 crank rotations, I would think the mechnical forces at work expelling pressure are constant enough to overcome the volumetric inecfciency of any adapter. Again not a physicist, could be wrong. Wouldn't be the first time. R

R - If I understand you correctly, you would be correct if the adaptor or hose was on the gauge side of the check valve, but my adaptor is not - it's between the check valve and combustion chamber. So, yes, it's pressurized some on the compression stroke, but the minute a valve on the engine opens, the combustion chamber (and the adaptor) will be depressurized. Everything on the other side of the check valve will maintain its pressure though.

apbling wrote: R - If I understand you correctly, you would be correct if the adaptor or hose was on the gauge side of the check valve, but my adaptor is not - it's between the check valve and combustion chamber. So, yes, it's pressurized some on the compression stroke, but the minute a valve on the engine opens, the combustion chamber (and the adaptor) will be depressurized. Everything on the other side of the check valve will maintain its pressure though.

That is correct.

The reason for having to pump the cylinder multiple times is because the gauge has internal volume. This extra volume needs to have compressed air in it for the guage to hold a reading. That air comes from the combustion chamber. Extracting this air from the combustion chamber reduces the pressure in the combustion chamber. So it takes, and holds, a little bit on every compression cycle. When the pressure in the gauge matches what is in the combustion chamber, it no longer takes any more air from the combustion chamber. This is when the gauge finally stops moving, and you now know what the true peak pressure is in the combustion chamber.

If the gauge was not attached, and the spark plug was in place, that peak pressure would be achieved in the combustion chamber on one single compression stroke.

But if you let the gauge depressurize on every stroke, the combustion chamber never gets to it's normal peak value because the gauge will always take air away from the chamber on each compression stroke. ( It also gives back that air, which then exits through the exhaust valve.) This is what happens when the gauge is made incorrectly by having the check valve on the gauge end of the hose, instead of at the spark plug end. Using adapters has a similar effect, but not as great, since it is smaller than the hose, in internal volume. Using a bunch of adapters is like using the incorrectly made gauge.

Does my gauge suck then? see pic

No, not if the check valve is in the spark plug end, like shown here in green circle.
Then it's good.

Not sure if I'd say it sucks...I bet the check valve is in the brass part of the hose, which makes it similar to mine. If you put the rubber tiped ones on the gauge, I have no idea where the check valve is. My check valve looks indentical to a core in a car tire.

When you use the rubber tipped parts, does the gauge flutter with every stroke or does it keep increasing (never decrease) until it settles at a number and stays there?

Sorry me and Lou posted same time. My thought is if he uses the rubber tipped ones where is the valve?

BTW, when the gauges don't have a check valve there, they don't suck, they blow.
But either way, they read lower.


Here's what the tip of a hold-in type, or hold-in adapter should look like.