Running two carbs

GoldenMotor.com

Slogger

Member
Sep 8, 2014
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nohio
I believe it was Honda, but in the old days there was a 4 stroke enduro bike, a big thumper, that had a carb with two throats, a small one for idle and plonking around, a big one that opened via vacuum at high revs.
It didn't last, so I guess it wasn't such a good idea after all.
A tiny single cylinder won't gain anything but lower intake velocity and bad fuel atomization. Try a bigger carb if you aren't flowing enough air. Too big and it will stumble and miss at slow speeds.
The carbs we have are pretty good. I like the SHA clone I put on, it works very well.
No faster than the NT, though.
 

2door

Moderator
Staff member
Sep 15, 2008
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Littleton, Colorado
I can't find the thread but there was a recent one on this subject.
There were several opinions offered but the general consensus was that there is little to no benefit except for the 'cool' factor.

You didn't tell us what engine you have but if you're talking about a Chinese 2 stroke, China Girl, then you probably aren't going to see any performance increases but it will look great. Expect some tuning issues in addition to the expense. Nevertheless, if you try it, post some photos and the results of your experiment.

Tom

EDIT: Found it > http://motorbicycling.com/showthread.php?t=57465
 
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M_a_t_t

New Member
Jan 22, 2015
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Indiana
Yea i heard of the benefit of 4barrel carbs on like a v8 was wondering if it sorta woulda worked the same. another question though. would it be possible/not fabricating a bunch of custom pieces to make one of these a two cylinder like splicing two together?
 

Slogger

Member
Sep 8, 2014
544
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nohio
Actually........ no.
It would be a pointless exercise. If you're working with a chinagirl engine, the carb size isn't limiting your horsepower. It's volumetric efficiency is bad, being piston ported it can only make good hp in a certain rev range, and getting that output is more a problem of exhaust system volume and length, port timing, intake tract length and size, compression, cylinder scavenging and ignition curves.
Spend 500 on one of those high output 50cc 2 strokes over at DAX.
High revving power right out of the box. About 9 hp, I believe, 11 or 12,000 RPM. It comes with what looks like a simple NT carb on it.
See my point?
 

Davezilla

New Member
Mar 15, 2014
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San Antonio Texas
Yup.. Slogger's right... My Morini engine just has a 14mm Dellorto SHA on it and it does just fine. there is an upgraded manifold and reed box to run a 19mm, but it's not needed for the 50cc and 4hp it puts out. The 9hp Dax KTM clones also run a rather small carb, I think it's also a 14mm SHA or SHA clone.

There's a lot more performance to be had by opening up the ports, timing them properly for your riding style, and adding a pipe that's tuned for your riding style and your engine's power curve to be had compared to tring to run 2 carbs on a single cylinder.
A larger carb will help your midrange and top end out, but the rest of the engine needs to be set up for it or it'll only disapoint.
 

Davezilla

New Member
Mar 15, 2014
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San Antonio Texas
Yea i heard of the benefit of 4barrel carbs on like a v8 was wondering if it sorta woulda worked the same. another question though. would it be possible/not fabricating a bunch of custom pieces to make one of these a two cylinder like splicing two together?
I do like the way you're thinking tho... I remember back about 6 months there was a guy who was designing a case and crank to make a 2 cylinder China Doll, but I think somewhere in his progress or designing phase he decided to go with better quality cylinders and parts.

the only reason anyone would run 2 carbs on one of these would be if space was too limited to run a bigger single. these things do wake up pretty good once the engine is ported and has a good pipe, then if ya add an 18 to 21mm carb after all the other stuff is done you'll like it.

another thing on running or deciding to design a twin cylinder for a bicycle, it's been done by someone in here with an old washing machine engine, but with the price of some of these 60 to 125cc 2 stroke RC model engines and the excellent power output they have as well as being very compact, it would be cheaper and easier to get one of those and fit it in a bike frame. I'm thinking it's either OS or DLE that makes a 110cc opposed twin that puts out around 8hp and only costs a few hundred bucks. One of those would need to be detuned and the ports re timed so they would have nice low end torque and midrange power, right out of the box the throttle is more like a light switch... it's either on and making a lot of power or off idling with not much in between.
 

Theon

New Member
Jan 20, 2014
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FNQ Australia
I like the dual pocket bike carbs on my soft tail.
They open progressively ( one cable is a little tighter than the other, so I guess second carb may not fully open?), But It gives a very nice low down, and plenty up top.
Wasn't that difficult to tune, but obviously more than just one carb.
Looks cool, and fits in the tightest of frames.
Worth a go in my opinion.
Biggest problem is the crappy pocket bike carbs, lol.
 

Goat Herder

Gutter Rider
Apr 28, 2008
6,237
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N.M.
19 TO 21MM Carby on these motors all day long for me. Remmember a smaller carb is considerably easier to tune tho.