commando 850 sulking

OK had a great day at the historical race meet at Lakeside Raceway and some 350 entrants (I was not competing) there were a number of nortons and manx's.
There was a nice 850 with maney barrels , and crankcase plus a fulauto head.
I think mine decided to sulk on the way home.
You know when you feel something is just not right but you just can't put your finger on it.
It didn't have that crisp sound that Nortons have.
When it was idling it didn't have that pop pop feeling/sound and the left muffler while firing was not that pressure you get when you put your hand over the muffler.

Bike starts ok and no smoke from the exhaust and ran at highway speed ok.

Pulled the valve covers off and both valves had clearance.
Did a compression check and found the left pot was 95-100psi and a dribble of oil and it went up to 120psi.
Right pot 135.

The question is somewhat mute as whatever the cause I will have to pull the head off.
Don't want to as it runs ok but it's not going to get better.

I'm guessing if a valve was burnt then the oil would not make a lot of difference.
Can't see any evidence of a blown head gasket and unless it was a small leak (could be) then the oil trick should make little difference either.
Could be rings but no evidence of smoking but that is not the definitive answer.

Guess I will know more when the head comes off but any comments from the group would be appreciated.

Cheers
Peter R

Did you use a differential compression tester? You really need to go that route before you pull it apart.

Onder said:

Did you use a differential compression tester? You really need to go that route before you pull it apart.

Click to expand...

Definitely the best plan, as you can listen at the air filter and muffler outlet for signs of whistling during the test, indicating leaky valve(s).
Your oil test pretty much nails the rings as the culprit, however. As for the lack of oil smoke, the upper compression rings could be failing, and yet still have functional oil control ring, at least enough to ward off any obvious oil smoke issues. Either way, it's going to need the top end pulled, as you've already determined. 'Sorry, bro.

Nathan

Duh, better also open the rocker boxes for escape sound$. After trying what might apply to a V12 Jaguar to direct shops cost estimates by leak downs, its pretty much useless if not possibly dangerous damaging to bother with on Norton that still need to pop er open to deal with it. In Trixies grit ground rings blow by case, air leak sound were only heard out R intake rocker box, which if following normal engine logic, eliminates rings and indicates valve seat head work needed but once wrench open > air path found was under head gasket into intake rocker oil drain hole to out to ears. Best thing I learned by the leak down pressurizing was finding the absolute functional geometric balanced TDC Zero crank position to mark. Best wishes on 2nd step in leak down > moving a wee bit off center to check the bore area ring flutter/wear actually occurs.

Count on having to fit new pistons etc. My 850 kept going, and didn't blow smoke, and nor did the oil tank or engine have that smell that blow by causes. It however pumped oil out the seems. When it did stop, the l/h piston had nor effective compression rings and the ring lands had been chewed through right to the oil rings. Even then, it did not blow smoke nor make any noises. With new pistons and a valve grind it now goes like hell. There were some slight marks on the bores but not enough to cause a problem. rebuilt engine real quick them belted it down the road. [ I didn't deglaze or hone the bores either - see previous threads somewhere ]
Dereck

Thanks for the info
Might see if I can get a hold of a leak down tester before I start the pull down just to satisfy myself.
It's a bit harder to check the rings when the head is off without pulling the barrels off, unless I make a plate to seal the cylinder.

OK got a hold of a leak down gauge, it took a while to work out how it works as it had no instructions.
I set the compressor to about 100psi, dialled up to "set" position on the right hand gauge the left gauge was only 20 odd PSI but I'm guessing what you want is a manifold reference with no air leak.
Ran the engine up to temp and then attached output to sparkplug hole and got 25% mid green band on both cylinders and not evidence of leaking could be heard.

So while engine was hot tried compression gauge
About 10 kicks with open throttle.

Left side 110psi and right 140psi.

So what does this all mean?
Leak down say all good
Compression say left pot down.

Stuffed if I know might just keep riding.

Although at one stage when I was mucking around with a squirt of oil in the cylinder I'm sure when I was manually rotating the motor both plugs out I could hear a hissing sound in the left pot plug hole. It was enough for me to check if I still had the air connected.
Maybe crankcase pressure coming up past the rings.

Very frustrating and it most likely going to be one of those simple thing I never thought to check.
I even checked the timing in case it was out.

My gut is telling me compression rings or sticking ring.

OK over to you experts.

bluemax said:

OK got a hold of a leak down gauge, it took a while to work out how it works as it had no instructions.
I set the compressor to about 100psi, dialled up to "set" position on the right hand gauge the left gauge was only 20 odd PSI but I'm guessing what you want is a manifold reference with no air leak.
Ran the engine up to temp and then attached output to sparkplug hole and got 25% mid green band on both cylinders and not evidence of leaking could be heard.

So while engine was hot tried compression gauge
About 10 kicks with open throttle.

Left side 110psi and right 140psi.

So what does this all mean?
Leak down say all good
Compression say left pot down.

Stuffed if I know might just keep riding.

Although at one stage when I was mucking around with a squirt of oil in the cylinder I'm sure when I was manually rotating the motor both plugs out I could hear a hissing sound in the left pot plug hole. It was enough for me to check if I still had the air connected.
Maybe crankcase pressure coming up past the rings.

Very frustrating and it most likely going to be one of those simple thing I never thought to check.
I even checked the timing in case it was out.

My gut is telling me compression rings or sticking ring.

OK over to you experts.

Click to expand...

In the absence of any experts chiming in... here's my thoughts...

If nothing is showing up on the leak down test then surely this means that rings, valves, etc are all sealing fine?

Maybe the difference in compression has always been there, uneven machining of cylinder head and or difference in piston deck height etc?

If there are no other symptoms... I'd be tempted to keep riding it until either a) you become convinced all is well or b) the cause makes itself more identifiable.

OK I've done everything I can think of.
The bike start second kick as usual and idles sub 1K however it does not have that crisp Norton exhaust dance at idle.
If I put my hand over the right muffler and good exhaust flow hand gets a good blast.
Do the same for the left and its chalk and cheese and while the pot is firing the exhaust gas is feeble.
Gets a bit better when I lift the rpm.
So the compression test tells me there is an issue and the leakdown tells me its OK.
My jury says pull the head and fix.
I think the compression tester is putting more pressure on the rings etc than the leakdown tester is and the higher test pressure is showing up the problem.

Its just a shame to pull down an engine that starts and runs OK, doesn't blow any smoke or use any oil.
But I wont be happy until its back to its normal self.

Any last things to check before the tear down?

Cheers
Peter R

Maybe a cam lobe going away so less valve lift to fully charge at idle. Ugh on your no win decisions, ride till can't or dig in again before it breaks.

if it were me, i'd ride it until winter solidly sets in and then take it apart (unless you have a second bike)- it sounds like there is a minor problem but nothing catastrophic that suggest it must be looked at yesterday

seriously, Fast Eddie has the best advice

Maybe the difference in compression has always been there, uneven machining of cylinder head and or difference in piston deck height etc?

If there are no other symptoms... I'd be tempted to keep riding it until either a) you become convinced all is well or b) the cause makes itself more identifiable.

Click to expand...

All good comments
I did a compression check a year or so ago when I thought I might have damaged a valve and both sides were the same 135psi from memory.
I thing that would discount an imbalance in combustion chambers that were pre existing.
I had not thought about cam lobes.
How does one check the cam while still in the motor, certainly worth a check.
Be interested in how to do it.

Oh I'm on the other side of the globe and it is our summer season, actually we don't get much of a winter so it riding 24/7 365days.
Yes could dust of the Triumph T100R or the 18s AJS but its not the same is it.


Cheers
Peter R

When my two stroke lawn mower played up for a day and a half , I hurled it down the bank by the reaining wall .
Two weeks later I hurled it back up and it started second pull and did the lawn in 45 minutes , about half the usual time .
If beating it with a stick dosnt work , this should shock it into submission .

How old are your plugs, have you changed them, when you were at my place yesturday it seem to me to be a faulty plug when you took off or your left carbie manifold has a small leak, I don't think it something wrong with your motor as its not blowing any smoke at all and as soon as you got the revs up it sounded OK to me.

Did that clamp nut fit your milling machine?

Ashley

Thanks Ashley,
Plugs are good, running iridium plugs, yes took off yesterday with the enhicher still on.
It took me about 1/2 a K to realise but it was still off song.
I stripped and cleaned the carby in desperation, at least the carby is cleaned.
Yep that clamp fitted see PM.

cheers
Peter

i'd still try fresh or known good plugs and rule out any and all possible ignition system or carb issues issues first

OK here is the definitive answer.
I stopped pussyfooting around trying to be all technical and went back to my gut feeling.
I could not get the same answer twice with the leak down gauge, sometimes it said no good sometimes it was good.
So I got the shop air at 80PSI and stuck it in the left pot at TDC and it whistled out the crankcase breather in the oil tank like a hurricane.
Put it on the right side and just a little whisper.
This is consistent with the compression gauge readings I've been getting all along.

So whatever it is it need pressure to play up.

Candidate are broken compression ring but good oil ring or head gasket to pushrod tubes blown.

My money is on the rings.

Watch this space for the next exciting episode of why the 850 got the sulks.

Cheers

i'd forgot I'd looked in here prior so was just the sulking term that drew me in again. Leak down instructions usually say to start with 100 PSI but less often works as you found the hard way. To extend your entertaining sulking efforts redo with oil in jugs to get sense of how bad rings are, other wise just skip right into wrenching for real. Once you do get it all working well take new baseline readings to monitor condition over the long haul.

Sounds like my bike, pre- the most recent set of rings. That cured things but good. No, I didn't replace the pistons nor have I regretted that.