1974 Honda Z50 - Smoking and Head Problem

[allenp42]

Well I thought I was all set to take a break for the holidays, but I've got one nagging problem where I need some advice. This little 50 "appeared" to be the pick of my litter and a good match for my shade tree mechanic skills. I thought that I tested it out well enough prior to the purchase and before I began the process of fixing it up. About the only thing that "stood out" was the clutch would slip a little when kick starting. No problem - clutch adjustment or rebuild. When I pulled the motor to make it easier to clean and paint the covers, I noticed the rear mounting bolt had broke out the entire section of the mount in the right side case...a long time ago (left side was fine). Opted to replace the right case -and- freshen up the top end even though compression was good (130lbs), no smoke and it ran great.

I used OEM Honda gaskets, rings, and replaced the exhaust valve seal, lapped the valves, etc. Now I have a problem. It smokes really bad when warm. No smoke on start but after 10-15 mins at ideal or 5 minutes of riding, it looks as if I'm spraying for mosquitos. Can let it cool for a few hours and again ....no smoke on start until the engine warms up a bit.

Removed the muffler to rule out oil in the residual oil causing similar symptoms (no change)
Removed the head and made a few checks. Intake valve is clean as a pin. I see some oil around the exhaust valve. Top of the combustion chamber is covered in oil, as well as the exhaust port. Installed another new exhaust seal and tried another new set of o-rings in the head.

Made several measurements around the circumference of the exhaust valve. No issues here (measures same as a new valve I have). All measure above the service limit of 5.38.

Tried a 79 head with a valve seal on just the exhaust so I could compare apples to apples. No smoke; problem cured.

Will do some more reading later tonight, but by "feel", the slop in the valve guide seems no worse that the 79 head that works just fine.

Is it possible that the valve guide is worn out?
The o-ring in the valve guide has "checked out"
Defective head gasket (I used an AFT when I tried the head from a 79)
Warped or crack in the head?

 

[Enginedoctor]

hold the valve with your fingers, most of the way through the guide and see if it shakes back and forth in the guide. if it's loose, the guide is worn out. guides are replaceable, assuming one is bad. a local machine shop

did you install a new piston and rings? or just rings? that could be the cause. sometimes rings get gummed up and stick in their land. there could even be scoring of the cylinder that doesn't affect the compression too much, but allows oil to weep by.

less than likely, you can check for a plugged crankcase vent. that would lead to pressure in the case, forcing oil past the rings and guides… but again, i doubt it.

Another cause could be running way too rich. this leads to 'washdown' of the cylinder wall. then the oil thins out, and gets into the chamber. it would have to be pretty rich for that to happen, but again, it's possible.

crack or warped head is less likely, but possible. make sure the oil is able to return to the crankcase, as well. you can check the head with a straight edge. did you install the o-rings and dowels in the head and cylinder?

thats a lot of oil in there. something is leaking by pretty bad. those are just a few guesses.

 

[kirrbby]

Did you stagger the gaps in the rings so they don't line up. Must have since the other head worked good. The early heads only have a seal on the exhaust valve, none on the intake. Pay attention to the head gasket when you first remove it to see if you can see a oil trail between the combustion chamber and one of the oil passages. If there is oil getting to the back side of the intake valve via hole, crack or leak, the motor will suck that oil in with the fuel and then pump it straight out thru the exhaust valve. There is lots of oil in the cam/rocker area of the head but no oil to speak of between the valve guide and the combustion chamber. I would expect a problem there, intake valve guide, oil leaking past the gasket, crack or hole thru to cylinder stud oil passage.

 

[allenp42]

Initially, I just installed new rings. The ring gap was ~0.2mm. Yes, I staggered the rings. Went one step further - I install a brand new OEM honda piston, rings, wrist pin, retainers and a NOS Honda Cylinder. Even after this I had the same problem.

Checked the crankcase vent while on the bench - Not plugged. Even has a new vent hose.

Not running rich - No odor or even a hint of running rich. The smoke is blue in color and smells like burning oil.

New exhaust seal - Actually tried a 2nd seal - No difference

I have had the head off 2-3 times. Each time there appears to be oil coming from the oil supply cavity across the head gasket. Another visual - More oil on the top of the combustion chamber near the exhaust valve than on the intake. I did try a 2nd set of new o-rings on one of the many tests. No help.

The 79 head works perfectly

I have a new exhaust valve the diameter measures the same on my cheap micrometer (~5.43mm)

I have not checked the movement of the valve in the guide yet. By feel, it does not appear to be close to the service limit (0.1mm) specified in the service manual (will try to rule out the "feel factor" get a definitive measurement). I have not checked (yet) to see if the head is warped, or has a hairline crack that opens up when warm. However, I feel certain it is not the piston, rings or cylinder -

I am not an expect by any stretch, but I am leaning toward: warped head (will confirm); small hairline crack somewhere, bad head gasket, or valve guide.

 

[69ST]

There's a seal between the valve guide body and the head casting. It's rarely ever been mentioned however, any seal can fail. Your original head may be warped, allowing oil to be drawn into the combustion chamber...once it's fully up-to-temp and flowing freely. Thermal expansion might also be widening an unwanted gap between two sealing surfaces. If a head swap corrects the problem then, obviously, it's not a ring sealing issue.

 

[allenp42]

Thanks racerx. I think I have found the problem. I checked the head on the flattest and straightest surface surface in the house (granite counter top) and there is a noticeable "wobble". As compared to another head head I have, and for sure against the service limit in the manual (0.05mm), this head is warped. Will take to a local machine shop in a day or 2 to confirm.

 

[allenp42]

Machine Shop checked the head. Is not out of tolerance. Doesn't say a lot about my granite counter tops. Ordered new valve guides, o-rings and will then go back to the shop for installation. I have a new set of Honda valves just in case....

 

[Enginedoctor]

haha you need a straight edge to check that kind of stuff... and in some cases, on such a small surface area, it's tough to catch anyways. a warped 4 banger head will show daylight across the deck mating surface with a straightedge on it. on a 1 cylinder, much tougher. I'm not saying the machinist is wrong... it's just using a countertop to measure the warpage in a head is probably more likely to show you warpage in your granite.. like you said. it's polished, sure, but machined... not so much.

This ones getting odd. thats a LOT of oil in there. can the machinist vacuum the head to check for leaks??

something fairly major is up here, in my guess.

 

[allenp42]

The machinist used a straight edge to check it diagonally across the head at 2 points. No light peeping through. He then checked it on flat surface on one of his lathes. Both checks showed that the head was not warped enough to explained the problem I have. He does not have the equipment to pull a vacuum. If I get bored one day, I may plug the oil return port, put as much oil as possible on the top of head, and sit the head on a hot plate (~300F) to see if any oil seeps through to the combustion chamber. That should tell me if I have a hairline crack that opens up due to thermal expansion.

 

[cjpayne]

This may sound barbaric or crude, but could you use an old fashioned, clean, cast Iron skillet, to transfer the heat?
How about one of those thick glass baking tray's used in ovens? My wife has one that's used for baking rectangular shaped cakes. About 3in high.
Just a thought.

 

[allenp42]

You're reading my mind......

 

[allenp42]

I know it's been a while, but I finally got around to finding the source of the problem. I put the head in a pot with the oil return plugged and with a lot of oil around the valve guide. Heated the head to ~300F and made sure oil was all around the valve guide. Let it "cook" for about 15 minutes. There was not a hint of oil on the top of the head. I checked the original head gasket (OEM Honda) and noticed a slight burr (on the head gasket itself) near the 16mm o-ring for the oil supply so just to rule out a "bad" head gasket I tried a new head gasket. Worked like a champ. No more oil in the chamber and no more smoke.

At this stage, I don't care if the gasket was bad out of the package or if I did something when I installed the head. It's fixed, and I appreciate the help.

 

[Gregg]

Initially, I just installed new rings. The ring gap was ~0.2mm. Yes, I staggered the rings. Went one step further - I install a brand new OEM honda piston, rings, wrist pin, retainers and a NOS Honda Cylinder. Even after this I had the same problem.

Checked the crankcase vent while on the bench - Not plugged. Even has a new vent hose.

Not running rich - No odor or even a hint of running rich. The smoke is blue in color and smells like burning oil.

New exhaust seal - Actually tried a 2nd seal - No difference

I have had the head off 2-3 times. Each time there appears to be oil coming from the oil supply cavity across the head gasket. Another visual - More oil on the top of the combustion chamber near the exhaust valve than on the intake. I did try a 2nd set of new o-rings on one of the many tests. No help.

The 79 head works perfectly

I have a new exhaust valve the diameter measures the same on my cheap micrometer (~5.43mm)

I have not checked the movement of the valve in the guide yet. By feel, it does not appear to be close to the service limit (0.1mm) specified in the service manual (will try to rule out the "feel factor" get a definitive measurement). I have not checked (yet) to see if the head is warped, or has a hairline crack that opens up when warm. However, I feel certain it is not the piston, rings or cylinder -

I am not an expect by any stretch, but I am leaning toward: warped head (will confirm); small hairline crack somewhere, bad head gasket, or valve guide.

I’m having the same nightmare of a problem 1986 Z 50. Doing the same thing. I’ve had to head out several times. Change those several parts. It still leaks between the big O-ring between the cylinder in the head. Leaks oil into the cylinder.

 

[Gregg]

I’m

Initially, I just installed new rings. The ring gap was ~0.2mm. Yes, I staggered the rings. Went one step further - I install a brand new OEM honda piston, rings, wrist pin, retainers and a NOS Honda Cylinder. Even after this I had the same problem.

Checked the crankcase vent while on the bench - Not plugged. Even has a new vent hose.

Not running rich - No odor or even a hint of running rich. The smoke is blue in color and smells like burning oil.

New exhaust seal - Actually tried a 2nd seal - No difference

I have had the head off 2-3 times. Each time there appears to be oil coming from the oil supply cavity across the head gasket. Another visual - More oil on the top of the combustion chamber near the exhaust valve than on the intake. I did try a 2nd set of new o-rings on one of the many tests. No help.

The 79 head works perfectly

I have a new exhaust valve the diameter measures the same on my cheap micrometer (~5.43mm)

I have not checked the movement of the valve in the guide yet. By feel, it does not appear to be close to the service limit (0.1mm) specified in the service manual (will try to rule out the "feel factor" get a definitive measurement). I have not checked (yet) to see if the head is warped, or has a hairline crack that opens up when warm. However, I feel certain it is not the piston, rings or cylinder -

I am not an expect by any stretch, but I am leaning toward: warped head (will confirm); small hairline crack somewhere, bad head gasket, or valve guide.

having the same dam problem 86z50

 

[kirrbby]

I’m

having the same dam problem 86z50

Have you replaced the head gasket? What are you torqueing the cylinder studs to? Are you SURE oil is getting into the cykinder from the head gasket??...and not a bad valve seal...ring gaps...etc???

 

[Gregg]

Have you replaced the head gasket? What are you torqueing the cylinder studs to? Are you SURE oil is getting into the cykinder from the head gasket??...and not a bad valve seal...ring gaps...etc???

I am sure the oil is getting in through the head gasket. When I took it apart the third and fourth time. Same area every time. The oil channel that runs down the stud seals. But the bigger o-ring is we’re it’s leaking oil to the cylinder. The head that I have is not original. I’m pretty sure that’s my problem. I’m just going to order 88 cc big bore kit with head. And a high volume oil pump. Now I see why they’re so expensive. To buy a running one. I have three different distance from three different companies and they’re all different. The only one that would work if I had the correct cylinder head. Is the one from TB parts

 

[Gregg]

Have you replaced the head gasket? What are you torqueing the cylinder studs to? Are you SURE oil is getting into the cykinder from the head gasket??...and not a bad valve seal...ring gaps...etc???

And yes, I am torquing my head bolts 9 foot pounds.