Kzrider Member Needs Help ASAP with 1000 Rebuild

Hey Gang - I have been helping a friend (and fellow Kzr member) with the rebuild on his 1977Kz1000 LTD. Nice bike.

He just did a top end rebuild after finding the power was way down. A compression test showed all cylinders in the range of about 75lbs. He even checked the compression with another guage and got the same 75lbs in each cylinder.

Here is what he has had done and where he stands.

Valve job by machine shop. new valve seals.

Newly honed cylinders and new rings. Confirmed cylinder bore correctness.

Checked valve clearances.

New gaskets.

We put the jugs on and head on a couple of nights ago He (Barry) was going to put the cams, carbs etc. on tonight when he called me saying that the compression was still only around 75 lbs.

I suggested 1. checking that the timing marks were dead on. 2. Recheck valve clearances. 3. A cylinder head leakdown test with carb cleaner might show if there is a problem with the seats, but it is very unlikely that all of the seats/valves would be off together. Could it be that the rings need to seat themselves properly with a short run-in period?

On the two rebuilds I did on my bike, I put it together, lit 'er up and was good to go without putting the compression guage on the cylinders. BTW finally got around to doing a compression test last week when the motor was good and hot and everything came in around 155-165. This is good:)

So what is your guess?

Barry is rather frustrated and wants to get at least one ride in with the new motor before the snow and freezing temperatures envelop us.

Cheers-Colin Firth-Ontario Canada
1977KZ650 Custom, 14,000 miles on bike bought new by brother, Latest rebirth includes 810 big-bore, WG Mikuni 28's and lottsa go-fast goodies tucked away inside.

Uuuhhhh...I hadn't been following the sparring on the "Low Compression" post just above this one. Similar topic but different bike and owne,r so let's start fresh. Any ideas gang based on Barry's circumstances?

Cheers-Colin Firth-Ontario Canada

Maybe cylinder walls and new rings need breaking in. I would get a good 500 miles at least on rebuild and try to break in somewhat gently then see what comp is.

Definitely an oddball.

Go for the leakdown first. Yeah, the odds are long on the seats being bad, but stranger things have happened.

Another longshot possibilty - who installed those rings? Are the gaps staggered? If they're lined up in a neat row, that'd definitely cause a compression leak - though I'm not sure exactly how much since I've never done that one myself.

There's a limited number of components that determine compression; valves, head gasket, pistons and rings - and one or more of 'em ain't right.

Another oddball notion is if the correct torque was applied to the head bolts in the correct pattern.

With a good quality hone and new rings with a liberal film of oil, I'd expect to see figures in the 130-140psi range, rising to 160-165 after seating.

This is gonna be one of those *head slappers* when you find it.

Start with the leakdown and work backward - you'll find it eventually.

Post edited by: Pterosaur, at: 2006/10/27 22:35

PT the rings are properly installed, offset and the oil scraper ring is not overlapping onto itself.

Keep 'em coming gang.

Cheers-Colin Firth

Some shops do valve jobs with interference angles [angles that don't match]. It's an old school technique.

I've seen heads that came from machine shops with valve jobs like that. They need time to be run in for the surfaces to mate. This is a possibility.

Post edited by: larrycavan, at: 2006/10/27 22:34

larrycavan wrote:

Some shops do valve jobs with interference angles [angles that don't match]. It's an old school technique.

I've seen heads that came from machine shops with valve jobs like that. They need time to be run in for the surfaces to mate. This is a possibility.

That's in order to improve flow dynamics around the valve head - the best seat is actually a narrow ring between the valve lip and the seat, rather than a wide band. If the band is wide, one side or the other of the valve will release pressure first, setting up turbulent flow that can mess with combustion.

Lapping the valves before installation should preclude any necessity to "run them in", though hey, whatever works...

Post edited by: Pterosaur, at: 2006/10/27 22:46

I'd recheck the cam timing.

steell wrote:

I'd recheck the cam timing.

Sounds like a cam timing issue to me too. I'd double check that first. One tooth makes a big difference...


Also, degreeing the cams can make a big difference too. Was the head milled? Is the cam chain stretched? Maybe the cams are out of time but installed on the correct teeth?? Just a thought.

Post edited by: 79MKII, at: 2006/10/28 01:33

Try to eliminate the obvious first. Do a leakdown on all the cylinders to pin point where your pressure is running off to. At TDC on each cylinder, air coming out of either of the ports OR out the breathers (or oil cap) will give it away.
Cam timing could still be off but the leakdown will get you looking in the right direction.

The head was not milled.The machine shop only did the valves and put a triple angle cut on the seats. I had suggested doing a leakdown test on the head's intake and exhaust runners to make sure the valves were seating properly but it is unlikely they would all be out so much.

I am leaning towards cam timing myself and triple checking the clearances but I wanted to hear other opinions. Keep 'em coming gang.

Cheers and thanks -Colin Firth

Not necessicarily.....in fact, over lapping of the valves is detremental to flow.

If you're using a lapping compound that is coarse enough to cut that much material, you're spoiling the benefits that a proper valve job could have provided to begin with.

I've never seen where mismtached seat/valve angles improve flow.

Turbulence generation is more of a function of port shape, surface irregularities and velocity mismatches than seat angle.

I thought you already checked the cam timing....

Why do guys put engines together without absolutely being certain the cams are in properly and then head back after experiencing problems to verify the cam timing?....:dry:

Engine building 101 - rule #1

Take the time to do it right....or make the time to do it over....

Post edited by: larrycavan, at: 2006/10/28 07:34