ZL400 Possible leaking head gasket

[hicrg]

I have a ZL400 se which came to me as a non runner. I've put it back to standard airbox and tidied it up including rebuilding the carbs. I have a problem with it and want to pick some brains.
The bike now starts on the button (ish) with full choke. It'll kick onto all four cylinders after a few seconds and will run with the choke on. Once it's warmed up a bit, it is possible to back the choke off and let it idle but then it will stall. On trying to restart it, I get little puffs of smoke/vapour out of the small hole on the right hand side of the cylinder head and the bike won't restart until it's cooled down.
When it is running, it smokes white from the RHS silencer which gets progressively worse as the engine warms.
My theory, which I would welcome comment on, is that there is a leak between coolant and bore on one or both of cylinders 3 & 4 and the leak is opening up as the head gets warm.
No oil visible in the coolant and the oil isn't milky.
My questions are:
* does this sound plausible?
* can the head gasket(s) be changed with the engine in the bike?
* has anybody else experienced this?
* could there be another reason?
* what are the small holes on each side of the head for or connected with?

My intention is to try a compression test on the cylinders - the last time I tried, I couldn't get my tester into cylinders 2 & 3 so I may need to look for another tester.

The history of the bike is that it was obtained as a non-runner with another running bike. The bike had been converted to pods, (but only 3 were present). The carbs had a mixture of needles, (one standard and three for pods) and one metal slide and three plastic slides - so somebody had been into them. I also found that the inlet and exhaust cams had been swapped over so I suspect somebody's already been into the engine with two left thumbs. All four cylinders have some degree of compression (finger down plug hole). The rocker gasket has been replaced also.

I would welcome people's thoughts. At some point, I'm going to have to open it up, I think.

 

[hicrg]

Does anybody know what other models had the same head gasket as the ZL 400?
The ZL600 seems to have the same part number.
Any other models?

 

[87kawzl]

If the early ZX6 isn't the same pn, the difference would probably be valve size.

 

[R. J.]

Here in the US we don't have much info. on the ZL 400/500. Someone may chime in with some info that ownes a ZL 4.

 

[87kawzl]

There are a couple of tests to diagnose, pressure test the cooling system and leak down test should shed some light. You need to make sure the valve train is properly adjusted for leak down test. Sometimes the test kits are available as loaners from parts store with a deposit.

 

[Ed]

Looks like all the part #s are same for GPx/ZL/ZX(400/500/600) head gasket. In later models, the intake valve # changed. The later 500 engines are totally different.

 

[hicrg]

Little update if anybody is curious. Compression test was done cold so not the best of conditions. All cylinders were about the same but only about 80psi, which seems low to me but could be because the engine's cold, and they took a while to crank to top pressure. But they were consistent and comparable.
So I took the head off.
The exhaust crush washers looked fairly new and the exhaust nuts came off easily so I don't think the bikes done many, if any, miles since they were off before.
I discovered that the head gasket was in upside down - the manual says the gasket should be positioned with the marked "UP" side to the right. I wouldn't have thought that is likely to make a difference - it seems to be symmetrical.
Probably not as much as a difference as the head bolts, some of which came loose very easily and certainly didn't feel like the 23 N-m torque in the manual. So it could be that the head just hasn't been torqued down correctly. I seem to be missing a head bolt washer also. Intention is to obtain an new head gasket and to put it back together properly and see what that does.

 

[R. J.]

When doing a compression test the carbs need to be on the motor and have the throttle needs to be wide open. You might want to adjust the valves. Doing so will bring up the compression.

 

[87kawzl]

Look closely at the gasket for a spot where coolant may have leaked into your combustion chamber, causing white smoke. Also check the head with a good straight edge for warpage since it didn't seem torqued properly.

 

[hicrg]

When doing a compression test the carbs need to be on the motor and have the throttle needs to be wide open. You might want to adjust the valves. Doing so will bring up the compression.

Yep, carbs were on the engine and throttle was wide open. Valves had been adjusted.

 

[hicrg]

Look closely at the gasket for a spot where coolant may have leaked into your combustion chamber, causing white smoke. Also check the head with a good straight edge for warpage since it didn't seem torqued properly.

I couldn't see any obvious signs on the gasket.
A good straight edging is next on the list whilst waiting for the gasket to arrive, also I'll put the sparkplugs back in the head, flip it over and test with a little petrol in each compression chamber for leakage on valves and sparkplug.

 

[Ed]

Sounds like somebody's been in there before. Hard to say if you should also pull the cylinders to check pistons and rings since you're already there or put it back together and hope the only problems are what you've found so far. I'd at least, measure the bore to see if it's been bored out.

 

[hicrg]

Sounds like somebody's been in there before. Hard to say if you should also pull the cylinders to check pistons and rings since you're already there or put it back together and hope the only problems are what you've found so far. I'd at least, measure the bore to see if it's been bored out.

It makes sense to measure the bores. It probably makes sense to pull the cylinders but I'm strapped for free time.
As the bike had some degree of compression, and it didn't change significantly when I popped some oil down the cylinders, I'm crossing my fingers for rings and bore. There aren't any visible scores on the bores.

When I had the bike running, it started reasonably easily and only started smoking with some suspected coolant on one side after it had started to warm up, so I don't think it's unreasonable to stop at the head for now. I'm going to lap the head on glass and fine wet & dry to take out any warpage.

 

[87kawzl]

oversized pistons that I have used and seen are marked on top with +.030 or such. Granted, I've only been exposed to usually American engines that have been bored. I would think there would be a X.Xmm overbore marked if someone slapped new pistons in.

Who knows, at some point a 600 top end may have been used.

 

[Ed]

I wouldn't try to flatten the head until it's checked with a straight edge. Also, glass isn't always going to be perfectly flat from one end to the other unless it's very thick or supported so it doesn't flex. A granite surface plate is what I'd use. I have a 12"x8"'x3"one for smaller stuff. Here's a couple that should work for the ZL:

https://www.amazon.com/dp/B01LW4167T

https://www.amazon.com/gp/aw/d/B006JYKIVC

 

[hicrg]

I wouldn't try to flatten the head until it's checked with a straight edge. Also, glass isn't always going to be perfectly flat from one end to the other unless it's very thick or supported so it doesn't flex. A granite surface plate is what I'd use. I have a 12"x8"'x3"one for smaller stuff. Here's a couple that should work for the ZL:

https://www.amazon.com/dp/B01LW4167T

https://www.amazon.com/gp/aw/d/B006JYKIVC

Cheers Ed,

I have a sheet of 6 or 9mm toughened that I've used for years for adjusting shim rings for a BM. Intention was to use that on a piece of 25mm ply. I've run a straight edge over the head and there are a couple of low spots but I need to clear the workbench for a proper look. I'll check the glass with a straight edge also.

 

[Ed]

The glass might be flat but when you push the part down on it, it'll try to flex to the shape of the part and the edges will probably get more material removed than you want. It might work if it's supported in the right places but there's no guarantee. The granite will be flatter than anything you could rig up and stay that way under pressure.

 

[87kawzl]

https://www.motortrend.com/how-to/impp-1007-cylinder-head-distortion-check/

I've always used a straight edge and this method. It's what was taught to me back in high school when I worked at a parts store and machine shop. There were only a few aluminum heads coming thru back in those days but the procedure is sound.

 

[Ed]

Having a large surface plate is nice because you could sand almost anything as flat as possible. I originally bought mine to blueprint a r/c OS/NSU wankel. Beautiful motor on the outside but the iron combustion chamber wall was too wide and badly machined. I put it in a small pylon racer called the Yellow Fever. It was way too fast for me so I sold it. Don't know if I have pics because it was long before cell phones.

 

[Ed]

Found one taken after I sold it.