JAP engine

Im helping a friend who has obtained an old Ransomes Multimower that has a JAP side valve engine with WIPAC ignition and an AMAL 379 carb. (model 4/3?) It hasnt run for decades and had no spark. Amazingly i got a new set of points (15gbp)and the needle jet (5gbp) from L&S engineering near Walsall. It now fires on ether spray. I cannot get the old needle jet or the float valve out so am looking for a substitute carb. I dont understand how a big engine can use such a small carb. The inlet tube is 14mm OD. Even a 50cc moped clamp-on carb is too big. But my question is, if i can rejet a small two stroke carby body with the new needle jet (and needle that i can get from AMAL) might it work? Any suggestions appreciated.

Apologies - responded to you other thread before reading this one.  Maybe AMAL carb was an alternative for lighter duties.
If you can get the required bits from L&S then there are a couple of carbs on fleabay.
AMAL 379 was used on Villiers F12 and J.A.P Model 80 so maybe someone has one.  

If wristpin is around maybe he can offer some advice. I seem to remember a similar issue on the old lawnmower forum.

Many thanks for the manual,

https://www.ebay.co.uk/itm/266715226107?customid=&toolid=10050

My JAP 4/2 engine uses a Zenith 24T-2 carb.

I have a JAP 5 on the bench being restored at the moment, it also came from a Ransom mower, it has an Amal 391/10 carb and also a governor . The carb does look small in comparison to the engine