Seizure on a cast barrel

[Strummer10]

Guys,

Just wanted to float this one for a few ideas on causes. Lambretta 150 barrel, bored out to a 175 cc on an otherwise standard set up using a Jetex carb through airbox. Bike was recently rebuilt with new bearings, seals, chain and piston and rings on existing 175 bore, but otherwise nothings changed from the original set up which ran fine.

The chap has however, had a few instances where the bike has died and he said it felt like the engine was just dying going up hills (but initially not locking up) almost as though someone had merely turned off the ignition. We therefore suspected fuel starvation and sure enough there was an issue of sorts in that the petrol pipe was bent under the fuel tap and again onto the Jetex carb. A side outlet tap and new petrol pipe was fitted (and he has added in-line filter) but it seems petrol flow is fine.

Since the above done it seized again today, but this time he said the back wheel locked up and he pulled the clutch in (I suspect it was a heat seize and that this happened before perhaps and he just didn't understand the mechanism of a heat seize cos he said it was just like before). When built we set the rings at Sticky's (0.2 I think it was or 0.02, anyway the minimum ring gap commented in the book) and timing was set at 19 degrees (as book states ideal for non-tuned).

I can only think it's either the ring gap is too tight (he run in for 100 miles the new rings) or maybe timing, any thoughts ? We are going to strip the top end to see what scoring is apparent and whether we can get away with a rub-down or its a new piston. I have said go Mugello, but he wants to stay cast ...

Thanks

[eden]

how much tolerance was given to the bore?

[Strummer10]

Not absolutely sure, but believe it was the tolerance quoted in the book for a steel barrel as we were religiously quoting the book at each stage. Wondering whether we could retard to 17 degrees but maybe it's just too tight in the bore, but must confess felt ok at the time...

[Adam_Winstone]

Lots of people would opt for 17 degrees as a starting point these days. Indeed, I've worked on a couple of 175s that were supplied with converted 150 heads where the compression was high enough that they would lock up on 19 degrees, with them then running without fault on 17.

Adam

[Strummer10]

Adam, maybe i'll try it at 17 then. Dependant on the scoring to the piston do you think we would get away with rubbing down the high spots. I know in the 80s I would just wet and dry the piston and carry on ? but with the science catching us up .......maybe this is no longer acceptable

[HxPaul]

I would personally have 0.088mm - 0.0035" ( 3.5 thou ) piston to bore clearance on a cast iron barrel.

[Strummer10]

Cheers Paul going to check Tuesday when we strip again.

[eden]

4 thou for me on cast barrels.

yes just take the ally out of the bore with emery paper and take the high spots off the piston.

[Chris in Margate]

"Died going up hills" ?
Is he running it in correctly?
Not trying to keep to 30mph in top gear or something daft and labouring the engine is he?
Personally and it's only my opinion I think 17 degrees BTDC is too far back. As long as he is using the highest octane he should be OK.
I have just rebuilt a 175 conversion for somebody and I was very insistent about the running in process. Don't labour the engine; let it rev if necessary rather than struggle in too high a gear; best petrol;; and keep checking spark plug and also smell it to see if it's running too hot.

[Donnie]

Certainly sounds like a fella that doesn't recognise the behaviours of a soft seize / full seize and doesn't know not to labour the engine. also agree re 4 thou clearance between piston to bore on cast.