125cc Zongshen Engine/ Firehawk Clone

I just finished installing my 125cc Zongshen engine late last night on my Black Firehawk Clone and will be test riding today and carb swapping.The engine looks cool, Black powdercoat and Smoke Chrome case 1/2`s


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Intake port is 26 mm across
I Put a straight 24mm carb on it( Not tapered inside) and 24mm intake and itis too much carb for the motor. Bogs when throttle opened and wont run right.
This is the carb in the pic.
It may need jetted richer ?
I put on the 22 mm carb and intake supplied with the motor and it runs excellent.
Hit 60mph no problem, still needs broken in.
Air filter supplied with engine fits my 24m carb but is too big for the 22mm carb.


Well-Known Member
Consider yourself lucky, you've found out sooner, rahter than later, that lots of people run oversized carbs. You might be able to crisp-up the throttle response with a 24mm carb, but I doubt that you'd gain anything power-wise. Getting the fuel curve dead-on would involve pilot jet, main jet, needle profile and needle jet changes...a major can of worms. Even if you gain 1-2mph of top speed, acceleration would be weaker at any speed below top end and the driveability, in all likelihood, would tend to suck.

A 22mm carb is enough to support well over 10-12hp, and the motor doesn't make that much in stock trim. For anything but a race bike, you'll be happier tuning for torque, which your engine will produce in abundance.


New Member
Well put. I used to see guys running 21mm carbs on 40cc pocketbikes, and the guys were wondering why they were hard to tune. Its like putting a holley 650 double pumper on a ford focus, its just not going to create enough velocity to properly tune in.

TP, your ride looks killer. I just got a an CRF50 clone with a 72cc E22 head, and after I thoroughly trash it, i will most likely be doing the same type of swap. Looks sharp.
Thanks for the replies and cpompliments! I have found that a 19mm carb works good with the 108c and 97cc engines I have. I rode the 125cc bike to work Tuesday ( 40 mile round trip) , it was fun ! I just got a real CRF 70 last week and I am going to put the 108cc motor on it. It is fun overpowering the small bikes, Throttle response is good!


Yeah I'm sure these little ass motors can only burn so much gas at once - Not sure what size my 125cc came with but it was kinda hard to adjust fuel fixture - I bet a smaller carb would be better but I'm not sticking any more money in it ATM - All I know without seeing it is its a Mikuni 3 position carb with huge intake - The air filter is also bigger than the K&N on my 70cc - I'll have to check it out later at friends - Hope its written somewhere on it - BTW whats the smallest to biggest sized carbs used on these bikes?