SIMON CRAGG Posted May 28, 2019 Share Posted May 28, 2019 I have just purchased a second hand, unrun, older style SC / APC .61 two stroke. It has a large carb fitted, (16.8mm), which I know is the correct carb for the age of the engine. The newer .61 is smaller @ 14.4mm. It is not possible to get the engine to run really "rich", which makes me think it is getting too much air, and hence SC then went for a smaller bore. I might try putting an insert in the bore to see what effect this has, The engine runs perfectly and has new bearings, but I am interested to discover why it has such a large carb fitted!. Edited By SIMON CRAGG on 28/05/2019 21:42:11 Quote Link to comment Share on other sites More sharing options...
Bob Cotsford Posted May 28, 2019 Share Posted May 28, 2019 My guess would be that the large bore was an attempt to record the highest BHP possible but that would have been at high rpm at the expense of poor throttling, and might have made it hard to tune with large (quieter) props due to the low intake velocity. The smaller bore would give a more flexible engine better suited to real world use. No doubt there would be other changes to optimise the engine for the smaller carb such as revised timing, porting and silencer volume. Quote Link to comment Share on other sites More sharing options...
TartanMac Posted May 28, 2019 Share Posted May 28, 2019 Probably mass manufacturing. One carb for 61-108 maybe. 16.8mm is that the effective choke area ? Quote Link to comment Share on other sites More sharing options...
SIMON CRAGG Posted May 29, 2019 Author Share Posted May 29, 2019 Posted by TartanMac on 28/05/2019 22:45:43: Probably mass manufacturing. One carb for 61-108 maybe. 16.8mm is that the effective choke area ? Its the OD of the outside of the carb intake at its widest point. I think you are right Bob, I did read somewhere else that these engines were better with a smaller prop as well. Interesting how engines have evolved, I will do some experiments with inserts. Quote Link to comment Share on other sites More sharing options...
Nigel R Posted May 29, 2019 Share Posted May 29, 2019 A few thoughts - The OD of the carb spigot doesn't tell you very much - how big is the hole in the centre of the barrel? Typically a 60 would be around 9 or 10mm. Much bigger would call for a pump. If you can't get it to run rich, that sounds like something is amiss, I would suggest there is a blockage in the fuel system or the nipple area of the carb. Quote Link to comment Share on other sites More sharing options...
SIMON CRAGG Posted May 29, 2019 Author Share Posted May 29, 2019 All ok now, and turns a Master 13x6 @ 10,600 easily. Combination of minor faults: Dragging / corroded main bearing. Leaking cylinder head. Liner not in straight. Needle valve O ring perished. Next stop the flying field! Quote Link to comment Share on other sites More sharing options...
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