TOPIC:

Hello All,
I have 2 KZ 550 LTDs (the dialy runner has an 81 motor with 6k miles, the parts bike has an 82 motor with 20k miles). I have had the motor with 20k miles for years and have done rings and hones through the time I've owned it. The 81 motor is somewhat new to me yet.

My question is in regards to compression. The OEM manual says 145 psi - 220 psi, with no less than 14 psi difference between cylinders. A third part manual says 130-160 is normal. I have always been in the 140-155 range on both of my motors, but I see other people confirm they have seen 180 psi. I know lots of factors can play into this (like carbon build up, hot/cold, valve shim clearance etc). If both of my motors are in reasonable good health and adjusted properly, what would you guys feel comfortable with for numbers? It seems like my motor runs fine, makes good power, is adjusted properly, doesn't start hard and really the only issues I have been having are carb issues that I dealt with last summer, but the fact my compression numbers are lower makes me wonder if something is wrong.

I did remember reading once on this site (and now I can't find it) that someone said you'll never get the higher end of compression unless the bike is either 1. brand new (which they aren't by a long shot) or 2. you just did a complete top end rebuild and everything is like new tolerance-wise.

Gauges can easily be off by 10 or more psi, though. You have to compare several, or calibrate the gauge against something reliable.

Compression zone on inline 4s is pretty basic. Head, valves, piston, cylinder or rings are the basic parts in the compression of air/gas mixture. So most lost compression will come from regular wear, lubrication faults or other damage (poor/excessively lean jetting). Your numbers are above the service limit and you have no operating problems. My advice is don't fix what ain't broke. R

i can tell you from experience that it takes 75lbs.(if you can spin it fast enough it may run,barely with less)of compression to create combustion in a gas engine.
if something only has 100lbs it needs rebuilt (or at least honed and re-ringed).

i don't care what size or how many cylinders 2 stroke or 4 stroke.

most stuff that hasn't had anything done to alter compression is going to be between 120lbs and 150lbs when freshly built.

the only time you are going to see numbers higher than 150lbs is when something has been done to alter compression.

hth,
leon

I have not seen any kz550 below 160, and range I see across multiple different gauges is around 165 to 190.
I have one gpz, suspected of being skimmed, that goes in the 195 psi range.

I have seen other kz's down into 120 to 140 range that seemed to run fine.

I guess I should have stated that my numbers are cold engine numbers... How much different would a hot vs cold motor make? 5, 10, more?

I usually do it very hot on a hot day. I also use the hold-in type gauges only. I've had a few faulty screw-in types and they always read lower.

Also, if the motor is cranking a lot slower by the last cylinder, you might want to try a car battery with jumpers.

Wanted to build oil pressure before I started my build .....so I checked comp cold un started and got 145 across all 4 with a push in gauge..... and 165 in a hot motor on all 4 after 500 miles so I was a little surprised that a cold motor didn't make that much of a diff (the increase was "probably" mostly due to break in) .........but then all motors will vary and hot is the right way.

OK, well, I know my battery is due to be replaced (its like 5 years old, cheap one from local hardware store) so that might be part of the problem...plus the motor hasn't been run much lately and it was cold.

Kinda wish I could use the wife's old sportster battery...its almost new, fiberglass one and 220 CCA instead of the 160 or so I got...but the posts are reversed and it just barely doesn't fit... I did hook it up though last year and she sure turns over much nicer.

Lou - are you kind of insinuating that screw in types are inferior to hold in type? I always thought screw in were supposed to be better. Maybe I'll add another gauge to my toolbox as a cross check.

No. Screw-in compression testers can be just as good, if not better. But if the check valve is not in the tip that goes into the motor, then it's not good. Some have a valve all the way at the end of the hose near the gauge. This adds a volume to the "effective" combustion chamber, which reduces the compression reading significantly. In my opinion, those gauges are simply made wrong, and don't do what they are meant to do. I think it may have been Harbor Freight selling those around 15 years ago.

Some have a relief valve placed away from the tip. That is fine. A relief valve, (as long as it's not leaking) makes no difference. But there needs to be a check valve to allow the pressure to build up in the gauge. Without it, the gauge loses its pressure on every stroke. The closer the check valve is to the combustion chamber, the more accurate the gauge will be.

OK, I think my gauge my be fine then. It looks identical to the one I have posted here. I do have to use the M12 adaptor to fit the plug hole though, which adds 4" of volume. If you look at the threaded end of the hose, there is a check valve there and the relief is on the gauge itself. So, that adaptor may influence my readings...but how much I don't know. If I remember correctly, the actual volume of the combustion chamber isn't all that much when the piston is at TDC, so the volume of the adaptor may be be 10-20% of the volume of the chamber (throwing numbers out for example), which would affect the numbers... It's been a while since I did the ol' P1V1=P2V2 (and assuming adiabatic), but if that volume of the adaptor is 10% the volume of the chamber and my reading is 140 psig, that means I could be cheating myself 14 psig, right?

Anyone got any idea what the combustion chamber volume is at TDC ?

The 550 I have with 3500 original miles that sat on the side of a house the last ten years had 180psi across all four when turned with the starter, all plugs out, carbs off, and cold. I used a cheapo HF compression gauge with the press in plug hole grommet. I first tried to measure by turning the crank with a wrench and got 120psi across the bank cold with the screw in adapter for the gauge. Either way, I wouldn't fuss or stress over a motor that has compression numbers above the service limit unless there are other symptoms. R