gearing question

[fortunatotredici]

im having trouble with my gearing and would appreciate any advice my cut down engine is cast iron barrel. 240 liquid cooled, rd reedblock 26 mm amal with a 60 mm crank using 2 mm packing plate its got a li 150 gear box think its running 17x 47 sprockets exhaust is a leovinci with manifold welded to barrel . the thing is it pulls well in 1st 2nd and in third it accelerates like a train but when i drop it into 4th its like hitting a brick wall any ideas on best setup ive got 46x15 , 46x18, ,47x17 and 47x18 combinations thanks in advance

[wack 63]

If the pipe is the snail type it is designed to rev up to 10k which is why it goes like f**k in the lower gears then drops dead in 4th.Either try 15 x46 or change the pipe but it may still struggle with 5.2 ratio as a mates' GP 125/imola did.

[Muttley McLadd]

How big is the exhaust port?

[fortunatotredici]

Gear Box is Li 150 Italian pipe is a modified Leo Vinci it's more or less straight thru I'm stuck with it really as manifold is welded to cylinder suppose this is a drawback for having a LC cylinder so it's difficult to use another exhaust. will take a pic of pipe and post it on here from what I can remember it's a fairly big exhaust port but the manifold is forward facing

[Adam_Winstone]

A mate's Jap 200 TS1 was over ported for road use and that needed to pull 5.7, anything more and it too couldn't pull 4th. Whilst people would immediately think that it was the wrong ratio... it was very much the right ratio for the big carb and fairly revvy pipe when combined with his state of tune. What you need to do is to forget cc alone and pay attention to what the porting and pipe require but do make sure that other factors are not causing the issue.

Adam