198 Mugello

[dave411]

Hi all,just getting the cylinder prepared for rebuild.I have matched the inlet and exhaust ports to the manifolds,now I am wondering about the crankcase.You can see in the pictures the black areas are where the cylinder fits in the crankcase,should I remove these or leave as is?









I am really not sure about this,as the way I am thinking is that 1,yes it will give a bigger and better fuel flow,but 2, it will decrease crankcase compression as I am removing part of the crankcase and making it larger.What are your thoughts?

[rosscla]

Whether you'll actually notice the difference is debatable, but anything that improves the flow will help. It will slightly lower the primary compression, but you're only removing a few ccs of metal so probably not by much.

Don't forget to put the mag housing in and blend thtransfer cut out on that too, fit it with studs on and scribe it so you always put it back in the same place.

I'd be a bit careful with the drive side one looking at how you've marked that up as it looks a little close to the edge. This can crack.

Two rules -

Measure twice cut once.
You can take it off but you can't put it back.

[dave411]

Yes,it is a bit close isn't it.Didnt know about the mag housing so thanks.Will check that later.

[olliewtf]

Re crankcase compression. I had this conversion on the lambretta street racers page on facebook. I tried to find the conversation, but as it was over a month ago, life got in the way
Basically I asked whether the large transfer ports we find on high hp barrels, to a point on the mugello too, lowered crankcase compression in a negative way, and in the context of porting my own barrel (sr barrel, so very conventional port set up) was it worth me widening the passages if it meant a loss of compression. Now there are plenty of other factors which shouldnt be overlooked, for example the impact of the port time area, which my basic understanding tells me is pretty crucial in finding more power, and the angle of the transfer entry in the barrel. Ofcourse more obviously the timing of the transfers also, and their relationship to the exhaust timing.

The reply made my head hurt.

I didnt fully understand it, but the gist of it I will try my best to relay.

The answer was essentially no. In Laymans terms, and this really is not the full picture, lambrettas have very high crankcase compression, and what typically can happen is that the mixture is shot out of the transfers and much can get lost out of the exhaust port. Enlargening the passages allows for more mixture to enter the combustion area at less velocity, increasing its chances of remaining there.
Now I understand theres plenty of flaws in what ive just said, and unfortunately its a topic I dont fully understand, but I did come away certain that my idea, similar to yours, of retaining as much crankcase pressure as possible, was false, and in fact the opposite was true.
Now this ofcourse always works in relation to everything else thats going on.

So. erm. yeah... check out MBs tech pages also, he covers transfer feeds and timing, and my overarching impression from reading that was to leave them well alone! so not necessarily contradictory, but more parts of the puzzle.

Once again id like to reiterate that Im no expert in this, just showing some of my findings which may / probably are wrong.

hopefully someone can go into a lot more depth, but trust me, it will make your head hurt.