Carbon Caked on PIston Crown

I have a metric Vulcan V-Twin. Kawasaki replaced the pistons and wrist
pins at about 20k miles. At around 40k miles, I had the heads off and
found a cake of carbon covering the crown of both pistons. I scraped
it off. The bike ran very well for a while after the clean up.

At 100k miles, leak down tests showed a 15% loss of compression. I
pulled the engine and again, found a cake of carbon on both piston
crowns. I lapped-in the valves (with helpful directions from this
group), replaced all the valve oil seals, had the cylinders honed,
installed new rings, and removed all the carbon from the crowns. When
the job was done, I rode the bike 11,000 miles and burnt a quart of oil
every 2,000 miles. I did a leak down test that showed around a 2%-3%
compression loss, well within the acceptable range.

I'm of the opinion that although the static leak down test is very
good, the oil is obviously being burnt during high-speed engine
operation.

I have been surfing the net for possible causes and trying to skull out
the problem. Here are the some of the suspects:

1. The 50 horse power engine may be straining at highway speeds
against the windshield wind resistance plus 250 of rider and packs on
long trips, throwing off engine timing causing oil to be pulled around
the rings.

2. The crankcase breather tube used to run into the air intake stream
to the carbs, where the crankcase gases would be burnt away. Now the
tube runs to a filter element mounted on the bike frame. I'm wondering
if excessive crankcase pressure is "pushing" oil into the combustion
chamber as the piston moves downward?

3. There's just too much piston "rock" allowing oil to creep around
the rings at high speeds.

Are any of these a possible cause of the carbon build-up? Are there
other possibilities? Has anyone had this problem with a 4-stroke
engine, and if so, how did you solve/manage the problem?

TIA for any fixes, tips, suggestions, or comments.

Manjo 1995 1500 Vulcan V-Twin

 

Try making a road draft tube instead of running the crankcase breather
to one of those little filters. A road draft tube goes vertically down
behind the engine and the end is cut at a 45 degree angle with the long
side of the tube forward. This creates turbulence which sucks vapors
out of the crankcase.

That sounds good. Piston rock is one of the reasons why Ducati Corse
engines are only good for 30 hours of operation.

You might try leaning up the idle mixture a bit. Then the engine will
burn oil vapors as fuel instead of allowing them to cake on the piston
tops.

 

FB,

Thanks for the tips and ideas. The draft tube and leaning the idle mix
will be done shortly.

On piston rock: is there a Ducati fix for the piston rock? If not,
should I look at a rebore and new pistons/rings, or perhaps
new/different pistons?

TIA for your reply.

Manjo

 

Maybe the rider should should be a collector and enjoy "looking" at his
Bostrom signature Ducati Corsa race replica instead of actually riding
it and making the V-twin engine turn 12,750 RPM in an attempt to equal
the 180 horspower of a Japanese I-4 design.

Maybe slipper pistons with thinner piston rings?

The ever increasing horspower requirements have caused engineers to
develop slipper
pistons with very little skirting to reduce friction.

Thinner piston rings weigh less, so the acceleration and deceleration
forces at the top and bottom of the stroke is reduced. I calculated
that the Yamaha YZF-R6
subjected its piston rings to 4000+ g's at 15000 RPM. And that machine
has a stroke ofless than 1.5 inches.

Bigger displacement short stroke machines are naturally going to rock
their pistons more, even though they don't turn as fast as an R6 or a
Ducati 999...

 

You didn't run it in on synthetic oil, did you? You need to use
dead dinos (changed frequently if it's a totally new engine) when running
in bores/pistons or they'll never wear enough to seal properly.

--
Ivan Reid, Electronic & Computer Engineering, ___ CMS Collaboration,
Brunel University. Ivan.Reid@[brunel.ac.uk|cern.ch] Room 40-1-B12, CERN
GSX600F, RG250WD "You Porsche. Me pass!" DoD #484 JKLO#003, 005
WP7# 3000 LC Unit #2368 (tinlc) UKMC#00009 BOTAFOT#16 UKRMMA#7 (Hon)
KotPT -- "for stupidity above and beyond the call of duty".

 

SNIP<<

No synthetic oil used in this engine. I've used Castrol 20W50 for a
while.

For this last rebuild there was just a rehoning of the cylinders (no
rebore) and new rings. I ran the bike for about 500 miles starting at
an average of 40 mph and then increasing the speed each 100 miles to
60-65 mph over the last 100 miles. I then went on a long road trip
where the average daily mileage was 300 miles with most miles at higway
speeds (65+ mph).

The top end rebuild before this last one, I ran the bike hard and soft
(40 to 60 mph) for the first 500 miles. I still got carbon caked on to
the piston crowns. So I figured I'd break in the rings "soft" to
"hard" this time. Obviously, no reduction in carbon build-up.

Manjo

 

FB,

I have the crankcase breather hose running directly downward tied to
the frame. Before this, I ran the bike to test the pressure at the
tube alone and at the crankcase filter. At the tube, the pressure was
obvious, but seemed to lessen as rpms increased to 3000. With the
breather tube attached to the filter, I could hardly feel any pressure.

I took the bike on the road and ran it at 25 to 80 miles per hour over
50 miles. The engine ran smoother and seemed to have more power. I'm
taking the bike on a longer ride today. I'll then check the piston
crowns for carbon build-up. I'm hoping the filter has been restricting
the flow of air OUT of the crankcase allowing pressure to build too
high and therefore forcing oil up and around the piston rings on each
down stoke.

I've also leaned out the idle mix by half (2 turns out versus 4).

Thanks,

Manjo

 

piotor,

Thanks for the heads up. I don't think there'll be a problem. The
crankcase breather port is located at the front of the engine. I took
the hose and just ran it as far down the LH frame tube as I could
pretty much parallel to the breather port. It's positioned to the LH
side of the rear tire, but a good 25-30 inches form the rear tire. If
there is any vapor accumulation, it should be on the crankcase. I'll
keep an eye out to see how much is there.

Manjo

 

Some older Kawasakis used to run the breather tube so it would drip on
the drive chain.

 

So did my old Yamaha XS650B!

Chains lasted for ages. Such a simple and neat idea.

 

Making a virtue of a ...hmm... "idiosyncracy".

--
Ivan Reid, Electronic & Computer Engineering, ___ CMS Collaboration,
Brunel University. Ivan.Reid@[brunel.ac.uk|cern.ch] Room 40-1-B12, CERN
GSX600F, RG250WD "You Porsche. Me pass!" DoD #484 JKLO#003, 005
WP7# 3000 LC Unit #2368 (tinlc) UKMC#00009 BOTAFOT#16 UKRMMA#7 (Hon)
KotPT -- "for stupidity above and beyond the call of duty".

 

Heh. Well, yes, and why not.

"Oil mist comes out of the breather? Hey, let's make a chain oiler!"