Check leaking outlet valve with piece of paper at exhaust pipe?

[Chris.R.]

Does any of you know this little "trick" - to hold a piece of paper at the exhaust pipe while the engine is idling. If one of the outlet/exhaust valves does not seal properly, this paper is pulled back towards the pipe, like a non-return valve? Can any of you confirm this? I hope I bring this across properly..

BTW, greetings from Africa/Cape Town...am pretty new in the Forum...

[djadams]

sounds like it'd be tricky to get a decent "reading" out of this, unless the problem's immense. Far better to borrow a compression tester if you can.

[j]

compression tester will test the valves and piston rings. easy wat take tank off munch out the center of a spark plug fit ailine to it and fit back in cylinder an listen in air box, exhaust, header tank and engine breather if you thin piston ring worn give it a squirt of engine oil this seals the rings. done this several times on minis and heralds.

[ladder106]

On a one cylinder engine with one straight pipe I could see how it would (in theory) work.

But tuned exhausts use negative pressure waves in the system timed to the cam and valve opening. Even a plain straight pipe will exhibit this (although most likely not at the right time to be helpful)

So.....with exhaust system complexity and mutliple cylinders firing (particulatly the staggered firing order of the AT) I'd think it would be a very poor indication.

Better to use a Leak-Down Tester...at least that's what we Yanks call 'em. As described on previous posts....compressed air injecting into cylinder at TDC evaluate for leaks and then trace leaks to intake, exhaust or crankcase.

[Chris.R.]

Thanks guys, will try that. I used a compression tester when I bought the AT, but wasn't sure how much oil to use to seal the rings? Do you then crank the engine once or twice BEFORE you put the tester back on? Because I thought if I put in +-2ml of oil this will give me a false reading if I put the tester on straight away (since liquid is not compressable..).

[djadams]

Do a compression test without adding any oil first. That will tell you if you have a problem. If it comes up as low compression you can try adding oil in an attempt to tell if it's rings or valves, but if you've got low compression the engine's gonna have to come apart anyway.

Do you have a reason to suspect a valve sealing problem?

[Chris.R.]

Do you have a reason to suspect a valve sealing problem?

Yebo, after 97000km and now a slight lack of power when trying to go over 120km/h (whereas the oil consumption is ok +-0.5L/1000km) and in my opinion strong vibrations of the engine, I think it's time.

I did check the compression on a warm engine and it gave me 800kPa for the front, 1000kPa for the rear cylinder...should be between 1100 and 1500.

[Chris.R.]

UPDATE - I opened the beast..and lo & behold the outlet valve DID NOT seal properly! Also the head of the valve stem and the tappet screw showed some decent wear. The other valves are fine.

Also, the cam chains were stretched to their limits and will be replaced (does any of you know a supplier in South Africa which is cheaper than Honda?).

As we opened the case, a friend found the oil pump drive chain and the oil pump shaft too loose, he had his alarms bells going and I didn't have webaccess at that stage...as I afterwards read the chain always seems to be loose I will leave it. Funny that "Haynes" does not mention anything about this chain!?

Well, so to be safe "I split the cases"...and found the oil pump "used" but in its limits. I just don't have another R1700 for that now. Shall check the oil pressure after re-assembling and praying over my @...

All other accessible bearings etc "looked and felt good". Any advice as long as I have the case open?

Stay tuned...

[mal]

Have a good look at the gearbox output shaft & bearing where the drive sprocket goes onto , well worth replacing if worn

[Chris.R.]

"I just wanted to do an oil change" I kept saying when I stood in front of my disassembled engine...

But, she is running again!! And how smooth, compared to when the rear exhaust valve was blowing (she had felt like a thumper..). Now she is smooth, and pulls alright-unspectacular. Also, the paper effect has changed - it's not sucked in anymore! (you should have seen the amount of paper in my pistons, haha...)

Ok, maybe to sum up what I did during this 98'000km "service" and how it came to that:
- compression was low (800kPa rear, 1000kPa front, warm)
- oil consumption was ok (0.5L/1000km not high speed)
- she didn't pull much further than over 120km/h
- felt "lean" at higher revs (like when your petrol is getting low/shortly before switching over to reserve)
- awful vibrations

We found: rear outlet valve was burned. Pistons & barrels in excellent condition, just honed/roughened them a little! The carb settings were ok according to the sparkplugs.

We did:
- replace rings, pins & clips
- replace 1 burned valve, cut & grind all seats
- valve springs all excellent (length)
- replaced valve stem seals
- set valve clearance on "maximum"
- replaced timing chains (over the limit)
- I split the engine, and went deeper and deeper..
- checked oil pump (within limits)
- wondered about slack of oil pump drive chain (left it..)
- "gear box" sprockets looked good as well
- output shaft bearing felt ok
- replace clutch thrust bearing, clutch still good, no grooves, new cable
- replaced rear wheel bearings (swing arm still to come - I had enough at this stage..)
- gave the drive chain a good clean

That should be it. I will still need to run her in 100% as she still is a bit "tight" in higher revs. But so far so good! And no bolt/nut left over...