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Your carbs should be balanced at least annually. Lots of things upset the sync of the carbs and on twin cylinder engine, good carb balance is essential for good performance.
Ideally you should check the balance every time after replacing the sparkplugs or adjusting the fuel air mixture or fitting different air filters.
The throttle cables stretch over time this also affects the carb balance.carburettor balance will affect the response, smoothness, mileage, performance and running temperature of your engine.
carburettor synchronisation consists of adjusting each throttle valve for carburettors or fuel injection throats so that they pass as much fuel-air mixture as all the others. This balances the load carried by each piston.
If one carburettor opened further than the others that cylinder will run hotter than the others. The other carburettor will also supply a richer mixture and mileage will suffer.
The actual mechanics of carburettor balancing vary a lot. Some bikes are balanced by adjusting the relative length of the throttle cables while others have a system of levers with adjustment screws but they all adjust the relative position of the throttle slides or throttle butterflies of each carburettor to each other.
To balance the jinlun 125cc 11 or 13 as there is no vacuum take of point on the inlet manifold like the 250cc bikes the only way to balance the carburettors is either by ear with a piece of tube (takes a lot of experience and good hearing ) or to take measurements at a fixed throttle openings at the throttle butterfly valves (takes a lot of experience and a lot of fiddling about) or to use a simple manometer on the rear belmouth openings of the carburettors .
the Gunson Carbalancer
the gunson carbalancer consists of a bung a pipe an a manometer with a simple crocodile clip on the back to hold it in a convenient place to take a reading
To use the gunson balancer you are going to need to remove the rubber "Y" piece between the air filter box and the carburettor as in the pictures above, you might be able to just disconnect the rubber "Y" piece from the back of the carburettors without removing it fully but i haven't tried this as the air box was already removed on my bike to enable me to take the photos.
To balance the carbs you need to run the bike till its warm and running with no choke.
Raise the tick over to a fast tick over to give a stronger reading , its just a mater of placing the rubber bung in each carb and taking a reading comparing the two and adjusting the balance with the screw as pictured between the carbs to get the same reading from each carb.
Don't worry about what the reading is on the scale only that they are both as close as you can get them the same.
when your adjusting the balance its important not to press down on the adjusting screw as it moves the whole linkage and gives a false reading, so a light touch is needed, also the adjuster only needs to be moved a fracton at a time .
I must admit i found the carbalancer a bit hit and miss at first you need to make sure your pressing the bung firmly against the back of each carb to get a good seal and a true reading, although the carbalancer got the carbs balanced better than i could get them balanced by ear it takes a lot of fiddling to get them well balanced.so be prepaired to spend a little time to get the readings just right. you might have smaller hands than me and be able to do this without removing the fuel tank but i found it a lot faster and easyer with the tank of , if the bike starts to run out of fuel just turn the engine off reconect the tank and fuel pipe restart for a few seconds then stop the engine and remove the tank , restart and try again the bike will run for a good 2 min just on the fuel in the carbs.
Jinlun JL 250-5 /2
Balancing the carbs on the 250 models is a lot easier as jinlun in there wisdom decided to fit vacuum take of points on the inlet manifolds.
to balance the carbs on the 250 i used a morgan carbtune pro
Note.. use a medium threadlock to stop the vacuum screws coming loose on reassembly
The carbtune pro is not cheap at over £60 with a case but it makes balancing carbs so easy, if you took your bike to an average bike workshop they would charge you £40 - £60 to balance the carbs and as I've got 3 bikes it was a worthwhile investment for me.
simply unscrew the screws in the vacuum take offs and insert the adapter that comes with the carbtune pro, connect the rubber tube to the adapter and the other end to the top of the carbtune pro do the same with the other vacuum take off on the other side of the bike. The difference with the carbtune is that it can read the vacuum in both inlet manifolds at the same time.
start the engine and look at the two columns that you connected the rubber hoses to.
(Courtesy of our friend's over at Pnoc)