750 LTD Good Compression + low vacuum = exhaust valve leak?

New to the forums but I've owned a '80 KZ750 LTD and '78 KZ400 for 3 years now, got them both running when bought but both need work after sitting 2yrs.

question today is about good compression and low vacuum pull on cyl 2 of the KZ750. She's almost road worthy but cyl 2 wasn't firing, I found no fueling, then good spark and compression (120 psi on fresh battery across all cylinders) but the vacuum is about a quarter that of the others . I noticed cyl 2 exhaust valve has a good leak so i'm assuming after sitting 2 years and not rotated shes a bit sticky, possibly gumming the valve guide, yada yada.

Does my theory sound about right? Here's a viddy link
Youtube Video Demo

Leads me too the dreaded "while you're in there" top end rebuild. What else should I be on the look out while I'm in there?
Parts to replace
All Gaskets base and up, Piston rings, Valve seals, Shims, Possible CYL 2 valve train, seat guide
Do I replace?
Timing chain, auto timing chain tension. honing probably unnecessary

I'm gonna start build threads for both bikes soon, i update my Instagram often
Thanks - BJ

Before pulling the head and tearing your hair out.........................

1. Buy a Kawasaki Service Manual for your specific model.
2. Check the valve clearances using the instructions in the manual.
3. If needed, replace the current shim with one that is the correct size.

Ed

If your compression is good on all cylinders, that should rule out valve leaks. Did you do the compression test with the carbs off? If not, make sure the throttle is held wide open when cranking.

What makes you say the exhaust valve is leaking? A compressed-air leak-down test? If it was leaking badly, it wouldn't have a good compression.

Why aren't you suspicious of the intake rubber causing a vacuum leak? That is the usual suspect.

650ed wrote: Before pulling the head and tearing your hair out.........................

1. Buy a Kawasaki Service Manual for your specific model.
2. Check the valve clearances using the instructions in the manual.
3. If needed, replace the current shim with one that is the correct size.

Ed

I have manuals for all my bikes. Valve clearances are wack so I was planning doing those first before head removal but the bike ran perfect when parked besides some top end jetting.

loudhvx wrote: If your compression is good on all cylinders, that should rule out valve leaks. Did you do the compression test with the carbs off? If not, make sure the throttle is held wide open when cranking.

What makes you say the exhaust valve is leaking? A compressed-air leak-down test? If it was leaking badly, it wouldn't have a good compression.

Why aren't you suspicious of the intake rubber causing a vacuum leak? That is the usual suspect.

I haven't a leak down test as I don't have access yet to tools. I've done compression testing without carbs and the vacuum hand test i did in the video is the same results with/without the boot, rubber is good. Vacuum hand test on exhaust results are similar symptoms to the intake, a weak push, I wanna say I can feel some vacuum but may just be valve overlap.

I'm think the compression is pushing the valve the exhaust valve closed fully but remains cracked open after valve overlap. I've looked into the cylinders via spark plugs and all valves seem to be working properly.

BJ

A slightly bent or burnt valve can be stuck in the valve guide or did not seal anymore on the valve seats and will "stay open" all the time.
You will never get a good compression on a low vacuum cylinder = low vaccum is a sign of a low compression by a valve leak or hole in the Piston dom.

I don't understand why you are messing around with checking compression and/or vacuum before adjusting the valves. You have the process backwards.

Nessism wrote: I don't understand why you are messing around with checking compression and/or vacuum before adjusting the valves. You have the process backwards.

Ik ik cause it ran great when parked and I don't have parts yet

Without raining on your parade, compression testing should be done on an up to temperature engine, hand vac test is not a reliable method, best bet is put it back together, warm up, test compression, whilst waiting for warm up observe the running apply WD40 to one inlet boot at a time to test for Vac leaks, if all OK balance carbs and ride.

Well, I've wondered about this ... good compression, but low vacuum.

I suppose that could happen if there is a vacuum leak somewhere other than the mouth of the intake port.
What if there is a crack letting air into the intake port from the valve cover area? Yes, I realize this is unlikely.
Or what if a valve guide seal is disintegrated and the guide is worn and letting air into the intake port through the valve guide? In that case, I would expect that cylinder to blow some blue smoke when running.

What other possibilities are there?

I agree that all of the easy stuff should be done first before tearing the motor apart.

Faulty vacuum gauge?
Leak around the vacuum port?
Crack between the bolt hole for one of the carb holder bolts and the intake port?
Carb not fully seated in the carb holder.
Midget zombie apocalypse?

BJkraabel wrote:

Nessism wrote: I don't understand why you are messing around with checking compression and/or vacuum before adjusting the valves. You have the process backwards.

Ik ik cause it ran great when parked and I don't have parts yet

There isn't a whole lot to add other than to emphasize what has already been mentioned. Until you do a valve adjustment check you are chasing the proverbial tail. If as you say its a weak valve spring you should be able to make that determination with the cam cover off. All else hinges on correct valve adjustment. You may not even have to wait on tools to do this as you should be able to easily locate a set of feeler gauges locally. Nessism is 100% correct, do valve adjustment check first! We discussed doing a leak down test even without a leak down tool and it can be done, but you won't know what percentage of air you are losing. You WILL know if you are losing air pressure and it's up to you to figure out where it is going, crankcase, exhaust port or intake port., but it is imperative to do valve check and correction first.
Rick H.

The valve clearance goes away with mileage as the valves sink into the head. Eventually the valves don't seat and start to carbon up and eventually burn even. Good news is that KZ engines are pretty tough so even if there is no clearance there is a fair chance that after installing thinner shims and putting some miles on the engine that the valves will be okay. Immediately after installing the shims the compression may still be low though (until the carbon on the valve faces is pounded off with some good Italian tuneup riding!)