Odd Z50 no-start problem

[Eggbert]

Being new to this forum, I'm not quite sure the best place to post this, but I'll try here.

Back in 1988 I acquired a 1972 Z50 that came in a number of boxes of parts. The engine and transmission were all torn apart. I rebuilt the engine/tranny and it's worked quite well ever since. However, it was time for a make-over which included new rings as it was smoking quite a bit.

After that, it did start and run for a few minutes, then kept dying getting harder and harder to start until now it just won't start. Also, it was misfiring the last few times it ran.

Here's what I've done so far:

1/ installed a new plug
2/ cleaned carb
3/ checked spark (nice and bright blue)
4/ checked flywheel to make woodruff key hadn't sheared and thrown the timing off
5/ checked transmission breather vent wasn't clogged (driving oil down the intake valve guide)
6/ checked cam timing and lash (intake only so far... will check exhaust too)
7/ checked compression and got 90 psi, however that's typical with my compression gauge when using it on a small displacement engine as it has a long hose between the gauge and spark plug hole adapter which absorbs pressure. So before it reads right, the piston has run out of air. Last time I did a comparison, that 90 would be more like 125 to 130)
8/ with carb off, I tried gas, then a little propane and then a small squirt of ether down the intake port. Didn't fire.

I'm out of ideas, at least temporarily. Any thoughts on the matter? Thank-you.

 

[Rapidrob]

What happens with these engines is that the point gap will wander. I would double check the cam alignment first and the the coil gap
.012-.016 max.
Make sure the timing chain is tight and has not jumped a tooth.
You want to be on the middle mark for the points.
Valves clearance. should be .002
My data for my engine says the compression should be 150 PSI with new rings.
a simple test of the engine together is to have someone push you with the bike in 3RD gear and you pop the clutch when at a runners speed.
The bike should start instantly or at least back fire a lot telling you the timing or carb is a fault.
A "weak" coil/condenser will also do the same thing. This happens all the time and is hard to diagnose.

 

[Eggbert]

Thank-you for your reply. I double checked my compression gauge as I was going by memory on it reading low for small displacement engines. Unfortunately it read about 180 psi on my 40cc chain saw (2-stroke though). So maybe my compression is down...

Now, when I rotate the flywheel by hand, there's come a point where I can hear air leaking past "something". Next step is to remove the cyl head, inspect the valves, and with the piston at the bottom of the bore, inspect the cylinder walls.

Still, it should, start the way it is.... I will test the condenser as I know a bad one can keep an engine form starting, but with the very fat huge blue spark, I would suspect it's OK.

 

[Davis]

I’d check that exhaust valve.

 

[Eggbert]

Thank-you for your replies. I think I have found the problem, and I'll get to it in a moment.

First, I double-checked my compression gauge and it does have a Schrader valve in the sparkplug end of the hose, so that 90 psi reading was more accurate than I thought.

Second, as Davis suggested, I started to suspect the valves, so I removed the head.

Lo and behold, it appears the head gasket was leaking air and oil into the cylinder (see pic below). Sucking air caused the low compression, and the ingress of oil was what was causing the engine to run worse and worse until it died (fouled plug). A new plug did help for a few moments, but since then it's a no-start situation.

I'm not unfamiliar with fixing these engines, so I can't quite understand why the gasket leaked. It must have been me, although I was very careful to tighten the head nuts in a criss-cross pattern, and I tightened the little side bolt last. This may have cocked the head a bit as I see that bolt side didn't appear to leak oil. So maybe the gaskets were soft and I should have re-torqued them? Or maybe the new gaskets were thinner and the those acorn nuts that hole everything together bottomed out on the rods?

I have checked the cylinder head with a straight-edge, and it's not warped.

Any thoughts on why this happened (well, could be I'm just a doofus...)

 

[Rapidrob]

For what ever reason it is obvious by the oil on the gasket that it was not seating correctly.
Is all of the old gasket removed from the head/block?
I would reinstall the gasket and re-torque the head bolts.
A trick I learned in the 60's was to seat the gasket and torque the bolts, loosen the bolts again,and then re-torque.This will seat the gasket a little better.
My manual for my Honda says to run the engine up to operating temp,let it cool and re-torque the head bolts again.
Make sure your reading Inch pounds/foot pounds correctly.

 

[Eggbert]

I was just looking at it again, and it seems the head is hanging up on the top, right hollow dowel pin. They did all fall out when I took it apart, so although they should be identical, they may be mixed up, installed backwards (compared to how they were) or something (bevel may be more pronounced on one end vs. the other). The dowel pin was pushed back in the cylinder quite a bit, but came out easily. I can see a small gouge in the head where it's supposed to go.

So, putting the recommended torque wasn't enough to fully seat the head and cylinder due to some sort of hollow dowel pin alignment.

Not 100% sure what's going on there yet, but putting the dowel in the head first might help.

 

[Rapidrob]

Today, when I take something apart for the first time, I pull out my cell phone and take several photos. This way I can't screw up the reinstalling of the parts.

 

[Eggbert]

I finally got the engine running, but things didn't quite work out as I expected.

First of all, I took a little exception to Rob's post implying I didn't put things back together correctly. However it turns out I did make a mistake, so I can't put Rob in my "book". LOL. Turns out, I had the thick o-ring on the lower right cyl. stud instead of on the lower left, and nothing on the lower left. Hence the oil leak, the fouling plug, and the excess exhaust smoke.

I put everything back together with the top right dowel pin in the head instead of in the cylinder and the head seated without trouble. Yet, still it wouldn't fire. The compression still wouldn't climb past 90 and I was starting to become more concerned about that.

Even though I had a nice blue spark, I looked at the ignition first. I opened the gap up a bit to spec. Tired it again, and it fired. Wouldn't run more than about 2 seconds, but at least it was firing. I doubt the gap had much to do with the quality of the spark, but adjusting the gap is how you adjust the timing on these bikes.

In desperation, I checked the valves. The intake lash was a bit loose, so I adjusted it. The exhaust lash was non-existent even though I thought there was some lash, I apparently felt side-to-side movement when wiggling it, not up and down lash. So I adjusted the exhaust valve. I did have one clue that the valves might not be sealing. I added a squirt of oil through the sparkplug hole and the compression didn't change, so that made me think I could rule out the new rings as a problem.

Bike then started right up! Sounded good too. Idles a bit high, but that's an easy carburetor adjustment.

So... looks like Davis was correct, and though I had many issues, I think the exhaust valve not quite seating was the main culprit.

Thank-you all for your responses.


ps: By the way, and just for interest, my feeler gauges are the long straight type for car engines, and couldn't be effectively used in this case, so I used my dial indicator. The magnetic base wouldn't work on the aluminum engine, but it did clamp to a front fork (for the exhaust valve) and the crossover plate where the fender mounts for the intake valve. Works pretty slick. You do keep the forks all the way over to one side so things won't move, and then adjust the dial indicator so it's in its range of travel when touching an adjuster screw. You simply move the rocker shaft up and down by touching the adjuster nut and the difference in readings in your lash. I like it better than trying to determine the lash by feeling how much resistance is felt when sliding a feeler gauge around. Just a bit of a pain setting up the indicator, plus the front fender and wheel need to be removed.

 

[Rapidrob]

It is easy to make a mistake. That is why there are erasers on pencils....Glad you got it running.

 

[Gearfinger]

Your hand is the only thing need to tell if a valve is loose or tight. Many times,modifying tools you have is necessary to complete the work you started.