Topic: Strange smell to exhaust and low compression. HELP?

[lrutt]

This really is strange  - I just hope rebuild no2 will sort the issues!

Just ordered a flex hone kit! Got the rings back on the right order (but was surprised to see the first signs of wear on the rings?? Some of the black colour of the cast iron rings has worn off and they have only been in the engine 48 hrs!
I just mic'd the bores again and with a vernier caliper I am getting readings of 55.78 - 55.92 on all 4 bores. So I judge that as very little wear?

Incidentally while stripping down I checked compression again with the rockers off. (ie all valves closed) still got very low readings around 40psi so suspecting my tester is off. Put my thumb over the plug hole and cranked the starter. Very little puff but masses of suction nearly pulled my thumb into the chamber!!
This all seems to indicate that getting the rings in the Wrong order might be significant! Could be preventing compression but allowing suction..
At this stage I suspect its all caused by a film of oil getting up the bores into the combustion chamber and then continuing to burn in the exhaust outlet (this would explain the blue flame I saw and also the cordite/carbon smell around the outlet). Guessing I have a good enough compression to ignite the  fuel/air/oil mix but it burns just that bit longer than the ignition stroke. 
There was a fair bit of oil/ carbon around the exhaust outlet from the head but the valves look ok and no sign of oil around them.

So I suspect the rings may have been changed at some point over the last 40 years and someone got them in the wrong order!
I then replicated their mistake!

Unless the unthinkable......... a mistake on the original Honda production line??? It is a very low serial number...

Beginning to think I need to invent internet "smell - o - vision" to really sort out this problem.

do you have the throttle cracked wide open when you're doing the compression test? And are you using the e start or kick start. with e start and wide open throttle. unless you have a big time burnt valve you should get more than that.

Did you do the test as I had mentioned above, ie when the head was off, pouring some gas into the ports and checking for leaks around the valve seats???

[lucky]

QUOTE:
"I checked compression again with the rockers off. (ie all valves closed) still got very low readings around 40psi so suspecting my tester is off. Put my thumb over the plug hole and cranked the starter."

If the rockers are off and all valves are closed how can it get any compression??

The cylinder develops compression by sucking in air for a certain number of degrees of rotation through the intake valve. Then it compresses that air.
If the exhaust valve is closed how can the compressed air escape?

No wonder your finger almost got sucked in!!!

Can you dial 911 with one hand on your cell phone?

I have never found a wedding ring in a sump yet! LOL...lol

[raymond10078]

If you are using a compression tester with a long/large hose - you will get low numbers.

My compression tester - a Craftstman unit - gives good numbers when measuring car engines - where each cylinder is larger than our whole engine.  On some bikes, with the small chamber volumes, the volume of the hose can alter the results substantially.

I've seen this discussed before - possibly on this site.  Someone even went as far to determine the volume of the compression tester hose (in cc), and then calculated the expected result when the compression tester volume is added to the engines compressed chamber volume.

Regardless, on bikes, the best way to get the most accurate numbers is to use those short rubber tipped testers - which I personally do not like nor own . . . .

[Kingscorpion]

Hmm A compression/ tester debate!

With the valves all closed she should create compression and the only conceivable way out for this is through the spark plug hole.
So with my thumb over it should at least blow out a little on the compression stroke right?

Any way I am using a long hose Laser tester with the right adapter so this might explain a little loss of reading. But not that much.

Engine now too stripped to repeat the test so lets hope the rring mix up has fixed it.

Just checked the ring to piston clearance and .005 feeler just slips in on the top ring and is still a tiny tad tight on the second ring so I judge no problem there.

Re checked the bore with the caliper and I am getting 55.78 - 55.85 readings on all 4.
Pistons measuring out at 54.67 - 54.78

Again all the valves look seated and set perfectly. Checked the head for flatness with straight edge and its A OK.

Gonna strip the head tomorrow and replace the seals.

It must be the rings! Maybe this engine was never run in properly its got 1584 miles on the clock and everything seems bang on spec.

Just smokes and smells like most 40 yr olds.

[TwoTired]

True Synthetic oil can make and odd smell when burned.  It could be the base oil itself or the additives blended with it that create the odd smell.

In a truly unfair labeling allowance, some synthetics are simply a step more refined crude oil than actually created molecule by molecule.

Was your synthetic oil PAO?

Compression:
If the valve can't work, air entry (to be compressed) enters the cylinder through the spark plug hole.
If you plug the hole with your finger when the piston is at top of stroke (and no other valve action), your finger becomes the air control valve, and the cylinder can create vacuum only and no pressure.
If you plug the spark plug hole with your finger, and the piston is at bottom of stroke, the pressure will likely will blow your finger away from the hole (releasing air), until the piston reaches top of stroke, whereby it can only create vacuum until your finger is ingested.

Pressure, or vacuum is created by the piston changing the volume that is containing the gas.

The CB550 has a 9 to 1 compression ratio.  The cylinder volume at the bottom of stroke is nine times more than when the piston is at top of stroke.
(Formula is: swept volume + compressed volume to Compressed volume.)

The piston itself displaces 136cc during its travel.  This means the final compressed volume is about 15 cc for 9 to one comp. ratio.  If your tester adds 10cc at the spark plughole, you reduced the comp ratio to about 6 to 1.
There is a FAQ entry to explain this.

If this can't be understood clearly, I'd recommend using a leak down tester instead of a simple compression tester.

Cheers,

[Kingscorpion]

Should of thought of that! Synthetic oil!
Yes I have semi synth in it which would account for the smell.
From all I have seen theres clearly oil coming up the bore. And my only clear memory of an oil burning engine is from way back when a VW bus blew a gasket producing clouds of blue smoke and a smell like a chip pan fire (french fries) 

That must be it  -synthetic oil burning due to rings/seals problems.

OK back to the rebuilding.

Thanks for the copious and informed advice folks!! Much Appreciated!!!

[Hush]

"Until your finger is injested" love that line Two Tired.

[Kingscorpion]

Ok  - heres the news...

Honed the bores with a three legged wetstone kit on a speed controllable had drill. Very easy to get a nice cross hatch with just a few strokes of this and minimal spring pressure required.
Rebuilt the head with new stem seals and noticed quite a build up of hard residue behind three of the exhaust valves. Quite difficult to scrape this stuff off and I suspect its partly what is causing the sulphur/cordite smell. Guess its been preventing the valves closing so I lapped the valves gently to get a good fit.
Have to say it was difficult to find a valve spring compressor to fit the tight clearance of the head.  (another one for the collection)
Anyway after all the above it started first time and runs perfectly with absolutely NO SMOKE!
First time I have really heard the bike running properly and probably all due to it sitting in a the original owners home being started and run weekly but never ridden until he got too old to do that! The broken ring I suspect occurred when I first started the bike after its long time laid up (so long in fact as to reduce the remaining petrol in the tank and carbs to red coloured varnish/gum -that smelled like old paint!
All looks good now and the compression tester is right back up there at 160psi!.

[Hush]

Nice, seems the valves were your stinky problem.

[Blackandsilver]

I had that problem exactly once. Looked like the bike was running on water the exhaust was white and really weird smelling. The top piston rings had been installed upside down. Glad you got yours going!


Win some lose some, it's all the same to me.

[trueblue]

Glad it's running well, now to put some miles on it

[dave500]

yeah you think you had trouble?my wife thinks i smell strange!dont be afraid to up the rpm,itll keep those valves clean seriously,people that have an old bike or car and fire it up now and again over years can actually do more harm than good,with cold running and incomplete combustion deposits etc,it results in acids in the oil and exhaust system moisture.

[Kingscorpion]

Thanks all  - Its been an experience but all the symptoms add up - smell, moisture,smoke etc. Definately one to note. The ring order is vital. Lesson learned - dont just duplicate what you find in the engine!

Kind of weird though that the rings in what appears to be an untouched motor with less than 2k on the clock were installed wrongly.
Its obvious when you think about it  - the thicker cast iron middle ring is a heatsink/buffer/seal between the compression ring and the oil scraper. I guess that installed in the wrong order there is a good possibility of igniting an oil mist in the cylinder. Hence the smoke etc.
So to recap the CORRECT ring order on a CB500/4 redone with NPR rings is:
Top: Chrome edged ring with R mark uppermost
Middle: Black cast iron ring with R mark uppermost
Bottom: 3 piece oil scraper ring

Now I recall from other posts that the CB500/4 K0 would originally have had a one piece oil ring? Can anyone confirm this?
If so would indicate that someone had been in my engine previously and changed (badly) the rings!!!

Alls sweet now apart from the brake light stops working when the headlight is switched on . ......

[trueblue]

Don't assume that the number on the clock is original, either that or it could be 102k.