New rings, valves/seals - compression lower than it was!

I just finished new rings/hone new valves and seals on my 850. The valves lapped in and no leakage at all (several teaspons of WD 40 would not seep past any of the valves, even when left for several minutes). Put it together today, torqued everything down. Tomorrow I'll mount the tank and fire 'er up. Interestingly, I did a compression check. Before the rebuild, compression when cold, throttle wide open, was 134/138. After the rebuild it was 120/120 BUT even though the compression showed lower, it was noticeably harder to kick over. Wonder what's up with that? There was a lot of carbon in the combustion chambers/piston top that is not there now which might explain the lower compression even if it is sealing better. Though I guess it might also be related to the fact that I can't kick it through as quickly which might affect the static compression. Yes, no?

Only guessing, but wouldn't you get the true compression reading when your new rings are fully seated, rather than now when they are not?

compression should come up when the rings seat. At your altitude it's not going to be as high as if at sea level however. Don't expect 175 psi.

what was the reason for the teardown? Those pre-teardown readings sound fine to me.

Debby

Agree with Debby. Let those new rings seat, then check it again. All sounds pretty good to me. 120 PSI, same in both cylinder, cold, with brand new rings? Methinks you're gonna be a happy camper when those rings seat.

I'm sure you know this, but do change your oil after 500 miles if not before.

Best - BrianK

Thanks guys - we'll see how the comp looks after sealing.

"what was the reason for the teardown? Those pre-teardown readings sound fine to me."

Oil past the valve seals/guides. The old seals were essentially brittle hard plastic bits; also there was a slight bit of valve wobble. New valves/seals was the primary reason but after removing the head I just couldn't mentally handle leaving the existing rings in place! After pulling the pistons I also found a bit of horizontal scratching in the bores where it appeared to me that at some time in the past the rings may have seized up in the bores though I'm not sure about that. There was also a vertical scratch in one bore that went the entire length of the cyl and seems to me to have been a "factory addition" rather than dirt/grit that had gotten into the cyl - there was no mark on the piston itself that corresponded to that scratch. A particle that would have scratched the bore would definitely have scored the piston.

Here are some pics:

Pistons/CYl after head first removed


Head - the different color of the Ex valves is caused by the angle of the camera flash - the valves look the same on both sides.


Not a sharp pic but you can see the horizontal scratches - you could feel them with a fingernail. Cyl hone made a nice "new" surface - there was no measurable taper in the bores.


A bit cleaner now!


New Black Diamond valves. Absolutely zero wobble. I couldn't get every trace of carbon off the combustion chamber but what's left it is discoloration more than anything else and it cannot be felt with a fingernail.

Shouldn't worry about small amounts of carbon left on, the motor will be at it's baest when there is a lght coating of carbon all over. I lightly emery my combustion chambers then polish with Solvol (aluminium polish). You can leave the carbon on the dia. above the top rings, on the pistons, unless it is thick.

A question, how you're doing the compression check? I use a SUN screw in gauge. Kick through with no plugs, gauge in right or left cylinder and throttle wide open until the compression won't go higher - 5-6 kicks. With that procedure I got 185psi right side, 183psi left side two years ago with around 500 miles on new rings. '72 750 combat engine.

I'm using the same compression test procedure - screw in gauge, 6 kicks, WOT. I've done a few more checks today after starting/running for 20 minutes and got anywhere from 70 to 120 on the same cylinder! Seems to be totally related to how briskly I can kick it through. Bike started on the 1st kick. More like the first half kick so I was very pleased with that. I won't actually be able to ride the bike until this weekend to see how it feels in "real life."

Mex,
Maybe the gauge is stuffed? Try another one. Everything you have done sounds good and correct. With the rings seated and carbon around the piston edge it will increase a little.

Mick

I don't remember what the figure should be, but ring tension on a fresh motor can be checked by pulling a ringed piston through its bore with a spring scale. I think what you're feeling (harder to kick) is a combination of ring tension and friction between the rings and the freshly-honed cylinder. More important than a compression check is a leakdown test where compressed air is pumped into the cylinder and loss of pressure is measured with a gauge. Compression ratio is a measurement of cylinder volume at BDC compared to volume at TDC. You may be getting a lower reading because the piston is harder to move until break-in is complete.

Hi Mike,

I tested my Mk3 this past summer after 150 miles on the rebuild and got 148 psi in both cylinders. I'm essentially at sea level with the jugs sealed at the bottom with Yamabond, i.e. no gasket. These readings are completely reproducable by the grace of 5-6 rotations from my electric starter; I didn't have to depend on a consistent kick from my antique legs. If we adjust our results by the atmospheric pressure at 7000 feet, 11.3 psia, then I'm getting roughly a 10:1 compression ratio indication and you appear to be doing quite a bit better with your old sealed rings. When you check the cylinders again after 100 miles or so, you'll be reading 130 psi or so.

I'm still a bit confused as to how to compute the true CR from the initial and final cylinder pressure. Simply taking the quotient of the two numbers overestimates the CR by 10% or so. From illf8ed's data, a simple ratio predicts a CR of 12.7, when he likely has a true CR of 10-11, or 10% less. Adjusting my numbers by 10% would produce an estimate of CR=9, about right for a fresh MK3 engine with a 0.012" bottom cylinder gasket missing.

I found a formula for engine compression pressure adjusted for altitude. The 120 PSI reading that I got at 7400 ft (Mexico City alt) would be a bit over 150 if the test was done at sea level.

Mike,

Your formula is likely a simple ratio between the seal level ambient pressure (14.7 psia) and the value at 7400 ft (11.2 psia), which scales your 120 psi to 157 psi. I really believe that you're in good shape regarding your compression. I'd still like to understand the relationship between compression ratio and the initial and final cylinder pressure.

In order for the pressure ratio to equal the volume ratio the temperature has to remain constant. Compressing the gas in a cylinder is not a constant temperature process and the final pressure is higher than what would be predicted at a constant temperature.

littlefield said:

In order for the pressure ratio to equal the volume ratio the temperature has to remain constant. Compressing the gas in a cylinder is not a constant temperature process and the final pressure is higher than what would be predicted at a constant temperature.

Click to expand...

Plus you have to have a cam with no overlap-not highly likely.

MexicoMike said:

I found a formula for engine compression pressure adjusted for altitude. The 120 PSI reading that I got at 7400 ft (Mexico City alt) would be a bit over 150 if the test was done at sea level.

Click to expand...

For a cold engine with brand new rings, that sounds pretty respectable...

Okay, you inspired me to get off the couch and do my rings. Have had a set sitting on the shelf for months (but more on that in a moment).

Pulled my head - looks pretty much like your "before" picture, Mike. But my pistons looks pretty darned close to your "after" picture - what's up with that? I do run some SeaFoam through the tank regularly, and did spray water through the carbs while running the bike last fall, but wow - the danged pistons look like they haven't seen much combustion (I've put 5K on the bike, dunno it's prior history).

I do know this now, though: DPO installed .020 over pistons. Which means those nice standard rings on my shelf can stay there.... arrgh!

Interesting - can't think how you could have deposits on the head/valves and none on the pistons but hey, it means less work cleaning! I discovered .020 pistons when I pulled mine apart as well; luckily for me I had not purchased anything before disassembly. Even more interesting, the NEW .020 rings had a wider gap when fitted in the cyl than did the rings that were on the pistons...

Don't know if you saw this but FWIW check my post re rings/gaps - last message on this page: post52590.html?hilit=ring%20gap#p52590

Thanks Mike. Yeah, really weird - you'd swear the head and pistons came from different engines.

Old rings measured .012-.013 the whole length of the cylinder - virtually no taper at all. Seems a bit tight of a gap, but it worked, and I'm happy I'm not in need of a(nother) bore. Crosshatch even looks okay so I'm not even gonna hone.

Course the bad news in all this is it's starting to look like my oil burning problem is not ring-related, but perhaps "all in my head."

BrianK said:

... starting to look like my oil burning problem is not ring-related, but perhaps "all in my head."

Click to expand...

...as in Valve guides?