compression numbers

can anyone give me a idea as to whats is decent compression for a 1982 kz1000j?? bike has 28,000 miles engine has never been apart

Purely from memory (could be wrong) I think nominal compression is about 160 - 170 psi ballpark and minimum service limit is about 120 psi. That's with engine hot, crank on starter with wide open throttle.

thank you that makes sense i did the test engine warm with readings on 1,2,4 around 135 cyl3 was 120 t-spoon of oil in each cylinder was around 175..looks like a top end job..now my dilema is how for to go to get it fixed up again..i really love the bike but i dont want to sink a ton of cash in it..but im not gonna buy a new one either so..

bigkzfan wrote:

thank you that makes sense i did the test engine warm with readings on 1,2,4 around 135 cyl3 was 120 t-spoon of oil in each cylinder was around 175..looks like a top end job..now my dilema is how for to go to get it fixed up again..i really love the bike but i dont want to sink a ton of cash in it..but im not gonna buy a new one either so..

If you are getting a +40psi increase from adding a spoon of oil, it sounds like the rings are not sealing well at all.

time to freshen top end then right????????????

1978 KZ1000 standard cylinder compression is 128~156 psi with service limit of 100 psi. Best to have less than 14 psi difference between cylinders.

Many of these large fours perform surprisingly well with lower compression and wider variance between cylinders.

If not already done, would check and adjust valve clearances. Too tight valves are a common cause for reduced compression.

A leakdown test should reveal where compression is escaping each cylinder, whether exhaust valve, intake valve, piston/rings blow-by, or blown head gasket.

Meanwhile, I'd ride and enjoy the bike, and not get excited about doing a top end job.

The peanut gallery has spoken. :laugh:

Good Luck!

I agree with Patton about riding and enjoying the bike, rather than tearing into the top end, other than checking the valve clearances. What matters is: How does the bike run, and how do you plan on using it?

Also, has the bike been sitting up for a while, or has it been used regularly? If it has been sitting and you are just now getting it running, then the compression may improve as the rings and cylinder walls get well lubricated and free of carbon, rust, etc.

Or if you are engaging in serious competition, then sure, rings will help. But to do this right may require boring oversize with new pistons and rings. Unless the bike runs very poorly, is the cost worth the benefit?

Just my $$$ 0.02 worth.

well thats the funny thing is it was running perfectly fine. just happened all at once it seems...what about the higher compression it had when i put a t-spoon of oil in the cylinders??? that seems to me it would indicate worn rings right?? when i start it now it backfires real crazy and i am having a problem with each cylinder firing at start up to...sometimes 1 and 2 fire off and 3 and four pipes will still be cold???? it has new coils stage 3 in carbs pod filters dyna coils..and the carbs are in sync thanks to morgan carbtuner? it has me baffled..o and also i have a problem with number one carb filling with fuel and running out the back...it would be great to avoid a rebuild with money being so tight right now..i dearly love my motorcycle..o yeah and i have to repair number 3 spark plug hole that is stripped out..from what i read on here i am gonna use the napa thread repair kit..so that should be no problem there...thank you guys for all your help

it has been ridden regular all summer and ran very very well up untill now..it is not running good at all to even consider living with it..it has always been kept inside also

bigkzfan wrote:

well thats the funny thing is it was running perfectly fine. just happened all at once it seems...what about the higher compression it had when i put a t-spoon of oil in the cylinders??? that seems to me it would indicate worn rings right??

Correct. My 750 had about 75k miles on it before the rebuild and I was seeing about 30 psi shift when adding oil. My cylinder wall wear was about .006".

But as noted, suddenly running bad may not be the compression because it changes gradually.

bigkzfan wrote:

well thats the funny thing is it was running perfectly fine. just happened all at once it seems... ///when i start it now it backfires real crazy and i am having a problem with each cylinder firing at start up to...sometimes 1 and 2 fire off and 3 and four pipes will still be cold????

If you are having plugs that are dead when you try to start, check them for oil fouling. BTW: plugs stop firing in sets of 1-4 or 2-3 since they are wired in series on the coil driving them. If either plug in the set fouls, both quit sparking.

Worn valve seals will let oil seep down and foul the plugs. Mine was doing that a lot. It will backfire because the gas going by the fouled plug eventually "washes" it clean enough to fire and it ignites the accumulated fuel in the cylinder.... been there, done that many times.

bigkzfan wrote:

it has me baffled..o and also i have a problem with number one carb filling with fuel and running out the back...

That is a problem with the float (sticking or set too high) or the needle and seat are not sealing well. easy to fix. A flooding carb will knock out a cylinder because the plug can't fire when it's flooded and the thing run like a dog in general.