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Several people have tried putting the mod 50 front end on both stock 49's and 56s blocks. You need the complete mod 50 package to make them work. Mod 56 motors with the stock carb set up can make 100 to 105 hp at the crank. Only problem is they last about 10 hours running time before you need to refresh them.

Turned out that it was only running on 2 cyl, no spark on number 3 so I replaced the CDI box which fixed the spark on number 3

Its now revving to 7200rpm and 77kmph which is still no where near where it should be, I need 9000 and 120kmph or so.
Almost sounds like its missing under full throttle and load?
Any ideas, sounds like its missing spark under load?

Common problem when you have too much carb. The motor gets more air and not enough fuel.

Something doesn't add up if your turning a 23 pitch prop 7200 and only getting 77kmph, that's a lot of slip, not a 23 pitch prop, or not the 15:28 gear ratio. That kinda rpm should put you up to at least 70 mph if right gear ratio and pitch prop

X2
Don't know what kind of tach you are using but maybe check with another tach to see if you get the same reading. I once bought a stinger that had 2 cylinder gears in it. They will fit and are much more common so if someone smoked a set of gears in the past, they may have replaced them with the wrong ratio.

Yep turns out its got the 2.4:1 gearbox ratio in it. and the prop was only 9 3/4 diameter which I have found is way to small.
Anyone know the cheapest way to get a lower ratio box? I have read 2 cyl of the same era run the same boxes with lower ratios? anyone know?

If you want the 15:28 ratio it's either a sst60 gear case or a 15" fishing nitro from a 75 stinger/hustler, the later being the cheaper. The two cylinder never came with the 15:28 ratio the case is the only thing they share in common.

Something still doesn't add up a small diameter prop with that gear ratio should have turned huge rpm to the point of blowing up the gear case or the motor.

You should be looking for a 12" to 12.5 diameter prop, probably in the 22-23 pitch range, if you want pure top end a 24-25 might be better suited with the potential power of your motor.

I think I'd be hooking up with one of the Aussie racers that can come help you out

OR, keep it how it is and get a 12x27 prop and see how it goes! lol. If you got 6800, and say 10% slip, that's like ~65mph no?
Gotta be innovative!

I'm kind of joking though, i don't think the pinion gear in those gears is very strong