Thoughts on this Monza 225 seize

[eden]

Plug also looks in the pic to be glazed which indicates the motor suddenly got hot melting the normal deposits.
An 8 plug in a tuned motor is ok for normal riding but for fast running a 9 or 10 would have ran cooler.
I always use at least a 9 and always a 10 for long motorway stints.

[paulnobodyimportant]

Just to give an update and feedback for others. There were suggestions and I did think the same, that the barrel or piston may have been slightly oval. Although the barrel and piston were sent to Langcourts, who re-plated and honed to suit. Now from looking at the barrel, the nip marks appeared directly above the transfer ports. So I put the now cleaned piston back into the barrel and checked with feeler gauges at this point and it was no less than 3 thou, so OK. Did some other random checks around the piston at different heights and still within tolerances. Checked piston diameter with digital vernier, but a bit all over the place, but that could be due to me fumbling etc. So took it down the road to a precise engineering company that I know and they confirmed that the bore and piston matched, with no sign of ovalness, (new word).

With confidence, cleaned piston, piston ring grooves and barrel, gapped piston rings to 0.25mm and re-built top end. If possible, I normally don't use gaskets, instead I use a silicone product, which in the past have all worked. This time, I needed some more and after reviewing, used this one (V-tech Copper RTV Silicone Instant Gasket Maker 700F), which I've got to say worked brilliant in that it was easy to apply and seemed to set well around my exhaust tail pipe etc. But before completing top end, left silicone overnight and then did inner tube air test; no leaks.

Put it all back together and put main jet up from 128 to 132 on my 30PHBH with a 30 power jet. Note, before it was tweaked it had run on a 130 main jet for 3 years with no problem. I have also upped the oil from 3% to 4%.

So for the last 2 weeks I've been going to work on it, on account of being in between cars, and it runs great. Still plenty of kickback as it goes into the power band, but I've not gone full WOT yet as I'm still not totally sure why it went in the first place. Probably put another 200 miles on it and then see.

Also, just read Eden's bit in Jetset, I don't know how you do it Eden, but it was a great read.

Thanks

Paul

[HxPaul]

Surely a plated barrel with 3 thou clearance is a lot,MB suggest 0015 to 002 thou for a plated barrel.

[Grumpy225]

Note, before it was tweaked it had run on a 130 main jet for 3 years with no problem. I have also upped the oil from 3% to 4%

if it was running good for 3 years with a 130 on 3% oil it makes no sense at all to change it. You are introducing new problems and will make the whole process harder to trouble shoot. Go back to a known good state and determine what caused the failure.

I suspect that after that sustained wide open run the carb couldn't keep up. Basically it got starved for fuel. I would make sure you have a large enough float valve in the carburetor.

[eden]

Ye it could have been just down to the float bowl running low at full bore if the float valve is too small.
Have you checked fuel flow from the carb with it in position?
What I mean by that is checked it by just taking the bottom off the bowl with the carb still on the manifold.

Regarding jetset. I'm glad you enjoyed it, I did waffle on a bit too much though lol

[phil23fair]

Note, before it was tweaked it had run on a 130 main jet for 3 years with no problem. I have also upped the oil from 3% to 4%

You may have increased the lubrication by changing the %age oil, but you have actually weakened your mixture because you have now got less petrol to the air coming in.