So I found this interesting.

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So I recieved my new 34cc OBR engine yesterday with a WB-1107 carb. I e-mailed them for the carb settings. The low should be set at 1 1/8th turns the High should be set at 1 3/4 turns. When I bought the Rovan it has a 36cc with a WB-1107 . Recommended settings1 1/8 low and 2 3/4 turns high. Wonder why the Rovan motor is run so rich ? This is along with a 25:1 fuel ratio where as the OBR is 28:1 for the fuel .
 
So I recieved my new 34cc OBR engine yesterday with a WB-1107 carb. I e-mailed them for the carb settings. The low should be set at 1 1/8th turns the High should be set at 1 3/4 turns. When I bought the Rovan it has a 36cc with a WB-1107 . Recommended settings1 1/8 low and 2 3/4 turns high. Wonder why the Rovan motor is run so rich ? This is along with a 25:1 fuel ratio where as the OBR is 28:1 for the fuel .
Well you can run 35:1 or 28:1 in anything. I'm assuming obr wants you to run 28:1 b cuz it performs better and that's what they made the 9.4hp on. Rovan wants you to be safe, also rovan probably tested the motor on that. I'd run 28:1 on any motor becuase it's less of a headache when it come to fouling spark plugs, but I'm not a expert. Not sure on the carb thing tho.
 
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I find the carb setting more interesting than the fuel ratio . 1 full tun on the high is a big difference but when you combin it with the fuel ratio it's a real surprise . If you watched the OBR break in vid when he started backing off on the high needle he was worried about starving the engine and seezing the motor. He started at 1 3/4 . Rovan is at 2 3/4 . As I was adjusting high settings the motor would start running like crap if I got around 2 turns. This is the same carb for both engines . The 36cc is putting out 4+ hp where the 34 is putting out 9.4 . As you lean the 36 out it should be building HP and it's not .
 
I find the carb setting more interesting than the fuel ratio . 1 full tun on the high is a big difference but when you combin it with the fuel ratio it's a real surprise . If you watched the OBR break in vid when he started backing off on the high needle he was worried about starving the engine and seezing the motor. He started at 1 3/4 . Rovan is at 2 3/4 . As I was adjusting high settings the motor would start running like crap if I got around 2 turns. This is the same carb for both engines . The 36cc is putting out 4+ hp where the 34 is putting out 9.4 . As you lean the 36 out it should be building HP and it's not .
Yeah that's weird, I have no idea. I'm really interested in this
 
Wouldn't the idea be the same for a bbc and sbc? Like putting a 600cfm carb on lets say a 350ci and a 454ci. You can tune the 350 to run damn good on that 600cfm but the 454 would be starving for fuel on the horsepower side. Sure it would get better fuel mileage with the 600cfm bit performance wise it would need either larger jets or even a bigger carb. Even tho the 36cc is only 2cc bigger, wouldn't the similar principles apply?
 
Different porting in the engines reed vs piston port would be my guess. Also I would assume chamber design, transfer ports and port timing would have something to do with it as well. I'm sure someone on here that builds them cannexplainnin greater detail.
 
It's all about the air fuel ratio. The AFR for a gas engine is 14.7 to 1 for complete combustion. I'm going to use grams as the unit of measure to explain as best I can. If the engine draws in 14.7 grams of air, 1 gram of fuel needs to be metered in. Compared to a gram of 25:1 fuel, a gram of 28:1 mix has 12% more gasoline. So less/leaner setting on the OBR is needed to maintain 14.7 to 1 AFR.

OBR engine - 28:1(more gas per gram) + small displacement = less turns open on the high speed screw.

Rovan engine - 25:1(12% less gas per gram) + higher displacement = more turns open on the high speed screw.
 
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