Low Compression

[Mally]

Think I am avoiding the obvious but seek opinions from you folks.

Zed been standing idle for a long time, got around to refurb , just checked compression and getting around 100 PSI per cylinder max.

Cold engine and just had oil change after a couple of weeks drained down , no carbs or exhaust on and just cranking it over by hand.
Was initially showing 80 PSI on each cylinder but came up to 100 PSI on them all after a lot of cranking by hand. Engine turns free with plugs out by hand via kick starter ( checked via crank very slowly with a spanner first, nothing nipping up)

Clymer manual says 100 PSI minimum but question that ??

Thoughts please , guessing its a top end strip down. Ta.

[gray]

I'd be inclined to charge the battery, refit carbs and exhaust, fresh fuel and get it running. Rev it a few times, (not too hard), and turn off, then let it cool then restart. Get a few heat cycles thru it . Then recheck compressions when its hot , turned over on the starter. The rings might just be gummed into their grooves.
Don't commit to a top end rebuild unless you have to.
Was it lying a long time with the carbs and exhaust off, and the ports just open to the air? ( please say no)

[Mally]

No, carbs/exhaust's were on when its been stored, oil down the bores and turned over a few cycles , plugs back in . Carbs drained and tank drained down ....but tank liner fell apart but I have re done that now.

Its on the bench just now but ports have rags in them to keep crap out, tried some oil down the bores to see if that brings them back up compression wise , to point away from the valves being the issue. I don't think its the valves as shims are still spot on and I had re lapped them not so long before it was lain up. What i can see through the plug hole etc is clean valve lips but cant see them all.

[Barnett]

100 is lowish, are all cylinders the same or similar? As others have said had similar on a z650b1, after a good long ride compressions were up 20psi across the board. A leak down test will give you much more information.

[Mally]

Thanks for the info, yeah all around 100 PSI which is encouraging they are similar , think maybe add the carbs and pipes and run it a little to see if there is any improvement,

Any other comments from anyone greatly appreciated.

[gray]

Im not far away, pm me if you want a hand over the next few weeks

[Mally]

Cheers mate , appreciate the offer , will give you a shout if I need a hand.

Put the battery in yesterday to turn over a few times on starter .......ignition light/oil light on .....nothing else, all fuses ok.

Need to sort that out first

[Mally]

Finally sorted out wiring , earth fault , new harness and reg/rect from ZPower on order as its totally fkd .......will by 50 years old next year!!

Hooked up starter motor direct to battery , turns over freely, more importantly getting 130PSI on all cylinders , guess the kickstart wasn't enough to raise the compression sufficiently.

So top end strip down not required ! Happy days.

[gray]

top news, and compression might get even better after its run a bit

[Mally]

Yeah , would hope to get closer to spec once its running ....a wee bit away as yet but working through the resto.

New wiring harness and reg/rect unit ( ZPower) going on at the moment, 50 year old wiring is brittle and connectors loose , been hacked about a bit by me over the years , surprised it lasted this long.

[kev edwards]

Compression tests need to be done on a hot engine, no plugs in throttle wide open, when you do this disconnect power to coils, at present you are not achieving much at all.

[Mally]

Apart from getting 130 PSI.... which gives me an idea that the bores and valves are probably gonna be ok.