Thoughts on this Monza 225 seize

[paulnobodyimportant]

This current issue is a continuation of the topic below. I've probably found my own answer, which I will come to below. So the point of this thread is to confirm my thoughts and let other learn from my mistake.
http://scooterotica.org/forum/viewtopic ... 32&t=34355

Story since rebuild from above thread.
Top end re-built with Langcourts re Nikasil barrel and honed to match the new super Monza oiston that I sent.
Piston to bore clearance was 0.003 thou. No base or head gasket and squish at 1.1mm. 3% synthetic oil and number 8 plug
Jetted rich and then dynod at 150 miles. And when I say dynod, we had quite a few runs, with only subtle changes to get a reliable air to fuel ration right across the range. I
Then on the way home from dyno, on a cold night, with a clear road I thrashed it for 30 minutes, but as I rolled off the throttle coming into Skipton, I lost power instantly, so pulled in clutch and rolled to a stop. Kicked over and no compression, so once again, rang a friend with a van. I know that rolling off then throttle from a long distance run can cause this problem,, but from the dyno curves, the mixture at this rev range, was well within safe limits.

First thing when I got back home was to check the plug. As you can see below, its a great color, which confirms the jetting was OK. Last weekend, I stripped the top end, but before going too far, did an air leak test using the Grandpa and Coaster inner tube test and confirmed no leaks.

As you can see from the photos below, it was a type of heat seize to either side of the conrod and opposite the transfer ports. The rings were locked in at these two pints. Fortunately, the lining is still intact in the barrel and the smear on the piston is just that and has cleaned up well enough to re-use. Will re-build this weekend with new rings and new small end bearing, but am concerned that it may happen again.

However, when doing the initial top end re-build, I forgot to check the ring gap. Just done that this morning, with the old rings and getting around 0.15mm, which is way too small. So my thoughts are, ring gap too small, is the main reason.

Any thoughts would be gratefully appreciated?

Plug Colour




Barrel, bad photo, but no scoring in real life




Smears on piston and locked in rings

[eden]

Did the dyno you used have an eddy brake?
If the dyno can't simulate drag it's useless using it to set carburettion because when the motor is running under load which it would be in 4th gear giving it some it will run much leaner than it would at the same throttle position and rpm on a non eddy brake dyno.

Dyno jet operators would disagree for obvious reasons but the above statement is factual!

If it was an eddy brake dyno, did the scooter have egt and cht guage showing temp ranges while running on the dyno? If it did then did the operator run the engine at high rpm for long enough to determine if the temps were creeping up which they obviously were.
Motors sieze up as you roll off the throttle not because the jetting is too lean as you roll off but because they are running too lean at the more open throttle position but because the motor is making power it prevents the over heated piston from locking up in the bore, as you roll off the throttle the motor isn't making enough power to keep the piston moving in the bore so it nips.

[paulnobodyimportant]

Eden. Don't know if it was a eddy brake dyno and as you suggest most dyno operators would not agree with your statement. If that's the case, how many of our well known dyno operators have such a machine. In the past I have used Chiselspeed and Dan and had no problems, but do you know if they have eddy brake dynos.

As for the egt and cht gauges, none fitted on this scooter so couldn't measure these temperatures.

Your suggestion about the high revs in 4th gear stopping it from seizing sounds interesting. I would have thought that if the top end was too hot at full wack, through being too lean, it would have seized solid and I've been behind a couple of scooters where they did lock up at 60mph plus and their spark plug was grey if not white. Or if not seized at full pelt, the action that you described would, I have thought, ragged the barrel and piston. Not disagreeing with you, just trying to get my head around it, but thanks anyway.

Will up the main jet when its back together, gap the rings correctly and do some WOT chops.

Thanks

Paul

[eden]

Surely ridding at 60mph they were not wide open running at peak power! Siezing at part throttle is quite different from Siezing at shut off after riding at peak power for a length of time.

PM have an eddy brake dyno as does a dyno place not far from me. There ain't many of them about.

[paulnobodyimportant]

Actually, when I say 60mph plus, I was trying to be realistic. We were all on TS1's and riding at full throttle for at least 15 minutes, so it may have been nearer 70mph, or others may report that speed at 90mph. But it was scary watching a scooter slewing with smooke pouring from his trear tyre. Fortunately in both cases, I was riding staggered, so i just shot past safely.

[Scoots]

With the seize lining up perfectly with the transfers could you suggest that at full throttle the transfer stream is what is actually keeping the piston cool and preventing seizure (or detonation)?

As you then shut off the piston is no longer cooled by the mixture flow and causes the seize.

This is information gleaned from Frits Overmars and the Aprilia project where he confidently stated he would run full throttle for 6 hours with no chance of a seize but the bike would seize within seconds when going to 20% throttle or below.

None of this is easy to replicate on a "normal" dyno as you simply go up and down the gears/ rev change in most cases therefore you get a false sense of what is happening. You never hold the bike at full throttle (with a load on like Eden says) for 30 minutes so its impossible to replicate.

This is one of the major issues with setting up on Dynos......but thats a whole nother conversation

[eden]

Actually, when I say 60mph plus, I was trying to be realistic. We were all on TS1's and riding at full throttle for at least 15 minutes, so it may have been nearer 70mph, or others may report that speed at 90mph. But it was scary watching a scooter slewing with smooke pouring from his trear tyre. Fortunately in both cases, I was riding staggered, so i just shot past safely.

Had their clutches failed too?

[paulnobodyimportant]

Don't know Eden.

The first one ended taking his whole scooter for an engine rebuilt at Chalkies, never heard the outcome, but it was a high bill on account of needing a lot of work. I'm guessing that he may have wrecked his spider and fried his plates. But since then (about 5 years ago) it runs great; well that's when he forgets and fills up with diesel. He will never admit this, but that is what we think. Last time was on our way to Brid last year after we filled up on the A64.

The other chap I don't know, but we were racing behind him on the way to Cleethorpes a couple of years ago. We stopped and it was locked up and after a while left him waiting for recovery

[Spanish Fly]

Could that piston be dodgy? Machined incorrectly? i.e not as elliptical as it should be. Might be interesting to throw a micrometer across it.

SF

[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.