MkIII Mystery Cylinder

Heeding advice from a local Brit bike mechanic, I poured a few cc's of Marvel Mystery oil into both cylinders and let it sit for a week before attempting startup from years of not running. When I went to suction the oil out - the left cylinder returned what I poured in while the right cylinder was dry.

Both pistons were roughly even so I don't think the oil disappeared into the exhaust. The other bothersome observation is oil leakage around the mystery cylinder's head gasket.

Don't have a compression gauge on hand and aim to get this running before making major efforts. Any thoughts on the mystery cylinder? The motor turns over freely. Any risks in starting up as is? Yes, a compression gauge is on my list of next tools…

Taking it in stride since I bought the bike not running and no history from the previous owner.

Thanks,

Andy --

Quick check:
Take spark plugs out
Press one thumb over a plug hole
Kick engine over
Repeat for other side
Analysis:
If you can discern a compression difference using this rough and ready method then you have a real problem.
Ta.

The only danger would be if that cylinder has a leak at the gasket, firing it up may make that side run excessively lean and tend to heat it up and possibly collapse a piston crown or burn an exhaust valve. What may be just a leaky head gasket could turn into a more expensive repair. If it were mine, I'd remove the head, turn it over and fill the combustion chambers with fuel and see if it leaks into any of the ports faster than it can evaporate. If the valves hold fuel and there's no apparent damage to the cylinder walls, check the head and cylinder top for flatness. If that's all good, torque the head down on a new gasket and THEN try to fire it up. If fuel leaks quickly into any of the ports, you're looking at freshening up the valves, guides and seats before you bolt it back together. And if you have to do all that, you might as well pull the cylinder, check the rings, ring gap and piston clearance. If all that shows good, put it back together with everything going back to the same place as before, being careful not to mix up any parts. Just my opinion, but doing a total top end rebuild is still cheaper than blowing it up.

Hi ag12680.
I re-read your post and was concerned by "...Both pistons were roughly even so I don't think the oil disappeared into the exhaust...". Both pistons should actually be level and below the height of the exhaust/valve.
I suggest that you have your brit bike mechanic assess it and quote.
Ta.

Thanks Danno. Agreed. I don't need to be in a rush... Will pull it apart, do it right and know it's right.

Ta, I've rebuilt several American and Japanese engines (that run), but you would not get that impression from what I wrote... Fair comment...

Thanks again,

Andy --

Well guys, the gauge says 30 PSI for the left and "0" for the right cylinder. Put a bit of oil in both sides - same results…

The bike shows 7,800 miles and the PO put on a 2 into 1 exhaust, Boyer and an oil cooler as the only mods I see. The carb for the right cylinder left the needle down when I pulled the slide out.

Obviously, I'm on a much longer path now… but this is a complete, unmolested MkIII (externally) so based on other's stories here it will be worth it.

Have a good bit of studying and research to do this being my first British project, but am curious as to what these clues say among those here much more knowledgable than I.

Thanks..

Andy --

With the addition of oil in the cylinders there should have been at least a marginal increase in psi.
I suspect stuck open or bent valve(s). Perhaps back off tappets gap and lever rockers to see if the valves move freely in their guides. If so, not stuck so then suspect bent.
Ta.

ag12680 said:

Well guys, the gauge says 30 PSI for the left and "0" for the right cylinder. Put a bit of oil in both sides - same results…

Click to expand...

This is a compression test, of each cylinder ?
You don't say what year or model, but no matter.

Compression testing is best done with the engine hot, with the throttle WIDE OPEN.
If done cold, with no throttle, you might get a lower reading - but not that low.
Does it feel like it has compression when you 'kick' it over ???

I stripped an 850 Commando, where the rings were stuck in their grooves - engine had been standing for a few years.

Drain the sump, and see what comes out.
Some marvel oil, and sump oil. And hopefully no engine parts....

Thanks Ta. I'll explore the valve situation as you suggest next.

Rohan, It's a '75 MkIII 850 Electric start that I bought "as is" not running with no available prior history. Will drain and screen the sump as well.

Thanks guys,

Andy --

Hehehe another tenderfoot begins learning lore and Commando worship rituals either piece meal like above or for a really road worthy Commando > full Sacrifice of blood/sweat/tools/time/money down to the crank and back out. Ask your self just how far away you might be comfortable on a pig in a sack and ease of rescue. If bore has a rust pit or slight scoring then it will smoke till surface and rings renewed, for instance. If not ready to hop on and go might be damaging to dangerous in a number of areas. But hey it is such a simple machine dive in, what could go wrong? Let the forum count the ways...

Well, the right side exhaust valve was too tight. Compression went to a whopping 25 when I backed it off. The rest had enough lash to close.

Looking through the intakes with the carbs off I can see leakage from the valve seats.

hobot, I get the fork in the road here and have been through these sacrificial marches before. With this marque I sense there is an uncommon degree of folklore around both the brilliance and fallibility of the design.

These machines reveal things about ourselves. So far, I'm looking forward to it and the sharing of the experience here...

ag12680 said:

hobot, I get the fork in the road here and have been through these sacrificial marches before. With this marque I sense there is an uncommon degree of folklore around both the brilliance and fallibility of the design.

These machines reveal things about ourselves. So far, I'm looking forward to it and the sharing of the experience here...

Click to expand...

While I will never be able to write poetry anywhere close to the missives of Hobot, I can attest that a Commando is a magical machine
which can test the souls and bankbooks of us mere mortals. But, should one be willing to learn the deep lore and pay the high fees, the
other side is quite worth while. But beware of the similarities to the Bataan Death March. Not everyone survives a Commando. Certainly,
the money invested will be much more than the market value of the ensuing machine.

Combats have tested my manhood constantly and mostly win to the point of infantile screaming into the woods blindly in middle of the night, a number of times. I am not cut out for this kind of fun but got in over my head past point of no return with dead Roman Genral daddy telling me I never finish what I start , feeling it would never end and guess what after 15 yrs I was right, ugh. The old school UK guys call the ideal a *fully fettered* Commando, a almost mythical state - if manhood enough pysically financially and will power and luck of fate to steal time from rest of life.

My Peel got taken out by stuck throttle after less than a year of use, my Trixie Combat was retrieved just before Hurrancane Katrina flooded river side shop and everything worked once fluids changed and fresh gas but 3 wks later deer took out forks and whole L side and about killed me, so 3 yr later got both us recovered so timidly took on commute, with no obvious reason to take down to the crank first, she siezed about coasting throttel 50 mph d/t alu gasket intruding on oil slotted taboo pistons till one came apart and took out most of engine. So ask yourself How Lucky do ya Feel Punk.

Have ya ever been so frustrated ya wanted to shot the shit out of an engine, hehehe I do but Commandos are just so cute I resisted on them. Oh yeah do have to put up with coco for combats hobot while he lasts..

You guys are an artful and encouraging lot… Sounds like a couple of software development projects that got away from me resulting in a fair amount of wailing and gnashing of teeth.

Thanks for the entertaining, yet cautionary perspective!

[video]https://www.youtube.com/watch?v=Ws513Esq74k[/video]

Now why did you have to go and do that Steve. I fear for LABs sanity.

The MK III is notorious for having soft cams, mine failed on the way to the Rally last week. Include replacement in your rebuild plans.

TWH
INOA 2036

MIIII also known for it sprag gear for which I suspect a whole mythical brand Spragthrope is based on, all named after dog breeds, if that means anything. Do note L.A.B. thought with so much of forum time taken up by their ongoing unique issues a whole separate forum area might be provided but decided against it for lack of puzzling entertainment for the rest of us.

L.A.B. might know where the shown on sidewalk is coming form this one?
https://www.youtube.com/watch?v=AjaiWNi7qzA

hobot said:

Do note L.A.B. thought with so much of forum time taken up by their ongoing unique issues a whole separate forum area might be provided but decided against it for lack of puzzling entertainment for the rest of us.

Click to expand...

Ultimately it would be Jerry's decision-not mine, but tread carefully, as Jerry finally saw the error of his ways and got himself a MkIII.

hobot said:

L.A.B. might know where the shown on sidewalk is coming form this one?
https://www.youtube.com/watch?v=AjaiWNi7qzA

Click to expand...

If I'm deciphering that particular piece of hobotese correctly, :roll: then I think what you mean is the shot outside what was Alfredo's Restaurant and Cafe, Essex Road, Islington, London. (No idea what that has to do with MkIII's though?):
http://www.classiccafes.co.uk/Best.html


As it looks now, apparently-not that I've ever been there.
http://www.meatpeople.co.uk/


Perhaps what the forum really needs is a separate hobot section? :roll:

The discrimination of why there isn't a separate MKIII section is blatant. (better put a smiley face down) I may be an old man, but I don't need to advertise it by riding a MKIII.
I didn't open this door but I sure will walk through it. There has been recent lists of difference and there are many. Certainly the P11, with it's own section, is so much more relatable to normal Commandos.
So, Hobot, in my opinion, carry on for it's not you who needs to tread lightly, it's me.

"Damn the torpedo's, full steam ahead!" ( not :| )