Piston/cylinder scuffing

[JayDee24ca]

I have had pretty poor performance on this G5 on the 3 miles or so I have run it, so today I thought I would do a leak down test on it, particualrly the right side as I had a devil of a time getting the carbs dialed because of crap performance on that side. Turns out I had a 15% pressure loss on that side (zero loss on left side), and I could hear the air rushing out the exhaust. So, time to pull the cylinders off.
The piston is extremely scored on the side closest to bike centerline, and the cylinder is scored so badly on that side that the spigot has discoloured from the heat.
So, its time for a new set of works. But my question is, what causes this to happen? I haven't measured rig gap or small end bearing lash, but the rings LOOK okay (but I know I will replace them of course) and the wrist pin has longitudinal slack, but no lateral slack at all.
Thanks
John D (a sucker for a pig in a poke it seems)

[moto-uno]

  Your problem is the exhaust valve and or seat is badly burned ( and or pitted ).
  Unless you find the plug and cylinder thick with black ( and somewhat shinny) deposits , I'm not
  staying up nights worrying about some scoring in the cylinder ( unless pics show bad scoring ).
Peter

[JayDee24ca]

 I should mention that the valves appear to be seating properly; I tested the inverted head for leakage using gasoline, and had no seepage. At first I couldn't figure out what you were getting at, but it makes sense that as I could hear the air through the exhaust then the exhaust valve must be leaking. I'm not sure what to make of it now that I have seen zero leakage from the inverted head. It sounds as though it couldn't hurt just to lap the valve seats.
JD

[BikeMike]

Hi JayDee24,

I had the same issue on my Convert.  The right cylinder exhaust valve and head gasket were leaking.  The piston is scored but luckily the cylinder is still ok.

[JayDee24ca]

Seeing we are sharing dirty pictures, here are a few shots of the cylinder, spigot and piston from the G5. The piston in particular is worse than can be seen in the photo.
JD

[JayDee24ca]

moto-uno your diagnosis is correct. following are a couple of photos of the rhs exhaust valve and its seat. Lapping it in had no effect, it is deeply pitted and beyond redemption. I will be having a head rebuild done, guides and valves, both cylinders.

[oldbike54]

 Thanks for bringing this full circle 

 Dusty

[moto-uno]

  Sorry to hear this , make sure who ever is doing the valve guide replacement thoroughly bead
blasts the guide section in the ports or there will be a hell of a mess in the head guide holes after removal.
If you're really lucky the shop would mill the port side of the guide completely off before removing
them.  Peter

[JayDee24ca]

Thanks for the advice Peter. I am going to see if there is anyone in the Kelowna are that is reputable for Guzzi head rebuilding. I had terrific success with heads sent south to Randy Long in PA a few years ago, but the world is a bit different place now, isn't it? So I will try local. You have anyone in Van that you would recommend?
JD

[Don G]

Call up the local engine re-builder/ high performance engine shops, ask them if they do K-Line valve guide relining, cost effective and no buggering about removing and replacing guides. If they ask ,tell them 8MM is what you need.  DonG

[BikeMike]

Valley Speed Machine Shop in Kamloops

[Rough Edge racing]

 Some of the photos look like four corner seizure, excessive heat from detonation.

[JayDee24ca]

Some of the photos look like four corner seizure, excessive heat from detonation.

Can you expand a bit on that? I'm not familiar with the term "4 corner seizure".  I know nothing of the history of the bike, but there is some evidence of neglect/abuse for sure. What leads you to think detonation? The excessive heat is seen in the discolouration of the spigot, but I thought that was due to the piston rocking and hitting that spot. Any insights?
Thx JD

[Rough Edge racing]

 Four corner seizure is when both thrust surfaces of the piston are completley scored...This caused by heat expanding the piston due to detonation, overheating or in the case of a new engine, tight piston fit..Does a bad exhaust valve cause the cylinder to heat up more than usual ?Or does the cylinder run cooler because because the combustion is poor ?
 

[mondtster]

Some of the photos look like four corner seizure

The one photo that looks like a four corner is from a different poster, so I presume a different bike. The OP's looks to be simply wiped out on the thrust side of the piston...

At a minimum, that head needs a valve job. I'd also replace the piston and clean the cylinder up.

[JayDee24ca]

A set of gilardonis are on the way from Steine Dinse, and the heads will be rebuilt in Kelowna at Pandosy Machine with new components. All this likely at the cost of a peaceful marriage of 47 years....
JD