Topic: Results are in from leak-down test and they're not good :(

[Tim.]

So, my local shop did a leak down test for me today to try and identify the source of my burning oil.  The engine had fresh .5mm over pistons/rings put in last spring along with having the cylinder head re-done with new seals etc.

This spring, it started smoking something fierce.  So, here are the results - anyone with experience with leak down testing, I'd appreciate your feedback.

40 psi in

#1 held 25 psi losing 37.5%
#2 held 30 psi losing 25%
#3 held 35 psi losing 12.5%
#4 held 32 psi losing 20%

They told me 20% or less loss was a reasonable result.  Further, they identified the leaking air as escaping from the crankcase via the oil filler port, indicating a ring problem in #1 and #2.

Does all this make sense to people here?  Sounded reasonable to me.  Perhaps I didn't winterize as well as I thought on #1 and #2.

So, I have 2 sets of cylinders and pistons (matching with pistons in the right holes) that I'm going to take to their recommended machinist to have checked out, and will replace the existing cylinders/pistons.

Not sure if there's any chance of rescuing the current set of pistons/cylinders.  It would be nice, as they were brand-new a year ago, but I don't mind going back to a stock set either.

Thoughts/suggestions would be welcomed.

[clarkjh]

Keep us posted on the piston/ring/cylinder condition when you take it apart.  I have the same kit, it would be nice to know what part(s) failed.

James

[madbunny]

well, i'm a throw a monkey wrench in this whole affair.
back when i was having all the trouble with my electrical system i discovered i also had a leaking float needle seat on my number 1 which was running rich and burning oil.
i could kick the motor over by hand, hot or cold.
since i've sorted out the electrical issues, fixed the flooding number 1 and a few other non-internal probs the bike no longer burns oil out of any pipe and requires a pretty decent kick on a one minute hot restart indicating to me that ring pressure is holding up pretty damn good again.
for what it's worth, it may be unconventional issues?

[Tim.]

Forgive my lack of understanding, but how does a carb problem cause you to burn oil?  Keep in mind the leak down test pressurizes each cylinder and measures the loss of pressure, and by listening to where the air comes from (exhaust = bad exhaust valves, air box = bad intake valves, oil filler = bad rings) you can determine where the pressure is being lost.

I'm not jumping to do anything just yet.  It is running rich, and I'm replacing the carbs imminently to help deal with that.  My old carbs are just plain worn out, and I have a nice rebuilt set of much lower milage ones to put on.  If this solves 90% of my problem, I won't be doing any major work on the engine any time soon.

[clarkjh]

As Terry would say;
No worries Mate.

Mines not going back together for a bit yet (Spring 07).  I want my wrenching to be regular maintenance not major work. 

James

[TwoTired]

Forgive my lack of understanding, but how does a carb problem cause you to burn oil? 

Running rich washes the oil off the cylinder walls, increasing wear on rings or cylinder whichever is softer.  The blowby soot deposits onto the rings and piston lands and can make the rings stick in the piston grooves.  Then the rings no longer press onto the cylinder walls with vigor and oil gets by into the combustion chamber; causing oil smoke.

Cheers,

[MRieck]

Unfortunately those leak numbers are awful. 20% leak is only acceptable on something with a gazillion miles on it too. Yours should be in the 5% to 7% (at the most) range considering you just rebuilt it. I've seen cylinder wash with the constant oil burning but not on stuff I've built. I wonder if the bores where really round with those numbers.

[pmpski_1]

Running rich washes the oil off the cylinder walls, increasing wear on rings or cylinder whichever is softer.  The blowby soot deposits onto the rings and piston lands and can make the rings stick in the piston grooves.  Then the rings no longer press onto the cylinder walls with vigor and oil gets by into the combustion chamber; causing oil smoke.

extreeeeeemely interesting. And my wife says I just waste all my time on the internet.

[HondaMan]

TinTin:
Is this a stock 500, or a big-bore kit that you're adding the .5mm overbore into? (I don't know what you're riding, sorry..). 

One thing I have had quite some trouble with on big-bore kits is: ring end gap issues. Often the "recommended" endgap is not correct for the ring type (or whatever reason). This can result in distorted (or even broken    ) rings. One case in point: a 500 that the customer installed the Action Fours 600cc kit on in 1973: he used the stock Honda endgap numbers and 3 of 4 compression rings fractured in a month of riding time. Another: my CB750K4 basket case was abandoned by the PO because of oil burning: #3 cylinder had fractured top (2) rings and there was only .002" to 0.000" on the other 3 pistons' top rings. I didn't measure any further rings than that: it told the story.

At your rebuild, what endgap did you use?

[Tim.]

Unfortunately those leak numbers are awful. 20% leak is only acceptable on something with a gazillion miles on it too. Yours should be in the 5% to 7% (at the most) range considering you just rebuilt it. I've seen cylinder wash with the constant oil burning but not on stuff I've built. I wonder if the bores where really round with those numbers.

Yeah, I'm not impressed either to be sure.  I'm buying my own leak-down tester.  I'm not sure I trust anyone anymore with the bike.

I have two sets of cylinders/pistons.  I'm going to have them checked out and with luck, will install them later on this fall.  In the meantime, I'm going to install another nice set of carbs I have, and see if I can fix the running rich issue a bit.  I'm not going to spend tons of time tinkering, as I'm obviously into another rebuild (or an un-rebuild) this year.  Will take the opportunity to completely tear the bike down and repaint the frame and rebuild the bike from the ground up.  I'm not splitting the cases though, as the bottom end of the engine seems good - no transmission issues - just the freakin' top end driving me nuts.

[Tim.]

TinTin:
Is this a stock 500, or a big-bore kit that you're adding the .5mm overbore into? (I don't know what you're riding, sorry..). 

One thing I have had quite some trouble with on big-bore kits is: ring end gap issues. Often the "recommended" endgap is not correct for the ring type (or whatever reason). This can result in distorted (or even broken    ) rings. One case in point: a 500 that the customer installed the Action Fours 600cc kit on in 1973: he used the stock Honda endgap numbers and 3 of 4 compression rings fractured in a month of riding time. Another: my CB750K4 basket case was abandoned by the PO because of oil burning: #3 cylinder had fractured top (2) rings and there was only .002" to 0.000" on the other 3 pistons' top rings. I didn't measure any further rings than that: it told the story.

At your rebuild, what endgap did you use?

The kit I used was a complete pistons/rings/pins/clips/gasket kit made by ART.  It's a 555cc kit for a 500, which ended up being 0.5mm over stock on the 550.

This is the kit I bought, from the same seller on eBay:

http://cgi.ebay.com/ebaymotors/Honda-CB500-Four-BIG-BORE-555cc-Performance-Rebuild-Kit_W0QQcmdZViewItemQQcategoryZ35595QQihZ012QQitemZ220008648803QQrdZ1QQsspagenameZWDVW

Depending on the condition of the cylinders/pistons I have, I might have one of the cylinder sets bored to match the 59mm pistons I got with the kit, if they're still good in my engine.

Bottom line, I have three sets of cylinders and pistons.  Surely I can make one good set out of them    If I have to drop another $120 on another kit, well, for the money the kit can't be beat (unless of course it's crap quality).

I'll post some pics of the cylinders/pistons I have for your visual inspection.  I got them from a guy who races 550's.  He had boxes and boxes of parts, including a few NOS Yoshimura kits, which he wouldn't part with.  He's building not one but two Rickman 750's in his basement at the same time, and has a vintage racer 550 and 750 in his garage, along with many other bikes he's building.  I trust the guy obviously, and the parts he gave me weren't worn out raced bits - he's getting out of the 550 stuff, and had these laying around.

[Tim.]

One more question - can I use a stock head/base gasket on a 0.5mm bored cylinder/piston?  That's the one concern I've always had about boring out the engine - no real source for replacement gaskets.  I'd expect the stock ones would be ok, but I'm not 100% sure.

[MRieck]

You didn't answer HondaMan's question about the end gap. Did you measure them?

[Tim.]

I didn't do the install unfortunately.  I had taken the bike in last spring to a reputable shop to have the carbs built and the bike tuned.  I had no time to do the work myself.  As a result of working on the bike, they determined I had problems in the cylinders.  This was after the carb rebuild, reinstall and in their attempt to tune.

They offered to tear the top end down and reassemble / retune for 3 more hours labour, which I decided was worth the gamble to find out what was wrong.

So, upon tearing in, they found broken rings in #3 and general wear in the rest.  I sourced the .5mm kit on eBay, and had them replace those parts as part of the 3 hour deal we made.  They farmed out the machining of the cylinders once I had the pistons in-hand.  At the same time I had a machine shop that specializes in valve jobs clean up the head.  They replaced the seals with some aftermarket ones I had from a gasket kit, and lined a couple of the exhaust guides with bronze liners.

After the assembly, it didn't smoke and ran well.  It really only started smoking this spring.  Maybe I messed up winterizing somehow and didn't oil up the cylinders enough.  It's a mild though humid winter in Toronto.  At best it was 4 months of non-running.

Not sure what damage could have been done to the cylinders in that time, but perhaps enough.  I can't imagine rings are stuck or anything, but will attempt to get the bike running better from a fuel mix standpoint to at least make it as good as possible under the circumstances.

So long story short, I didn't measure the end-gaps as I didn't do the work.  Maybe I'll just build a new engine on the bench in the meantime.  Not sure I want to run the risk of building an engine with an unknown bottom end though.  I will likely pull the engine this fall and rebuild on the bench.  At the same time I'll strip the rest of the bike down for a proper refresh.  I've got bronze swingarm bushings and other goodies to put on, and the frame could use a fresh paint.

[nickjtc]

From this very forum, a 'how to' build your own tester:

http://sohc4.us/node/42

[crazypj]

I have experience with leakdown testing.
20% may be OK for old V-8's but anything over 10% is bad in the bike world.
It sounds to me like the rings either were not fitted righ in the first place (how many miles did you do last year? ) or, when the shop was trying to get carbs fixed they did the 'Italian' tune up. ( rev it until it clears up- I hadnt heared of this until I moved to US)
Is engine still in pieces? If so you need to inspect pistons, if the rings failed due to over revving the ring lands will probably be damamged, the new rings can 'tilt' in the grooves. What sort of damage was on the original ring set? Can you get/do you have any pictures?
What is surface finish of cylinder? Rust wear would only show above the areas pistons were covering (if it was light surface rust, anything deeper is real obvious) Even if you left bike outside for 4 months I wouldnrt expect oo much deterioration, we had bikes in storage where vandals had broken all windows out of buildings that ran a year or so later. If carbs were really rich it could cause fuel to wash oil off cylinder walls and give excessive wear?
Good luck with it.
PJ