Lifan 150, Mikuni VM24 jetting

[longjohn66]

hi all, hope you are all well and complements of the season

though id let you know how i was getting on with the 150, it blew up
seemed to suffer coil bind and dropped the exhaust valve, anyways got a new 1 under warrenty.

im now having , i think is jetting issues so i am hoping for some advice.

the bike starts fine, idles fine, and even blips fine, however when you wind the throttle on without load past 1/4 turn it makes a wrrr wrrr wrrr sound and seems to be when it hits the second circut and wont rev.
i have a mukuni 24mm push fit carb,engine has been ported and it has a z40 cam and springs, the outer rotor is set to just past middle (advance), valves are set to 0.005, jets ive tried are 114,110,107.5,100,80, also tried needle settings from top to bottom, the 80 seems to allow it to rev further before it wrr wrr wrr's. im at a complete loss

failing your help im going to have to buy the carb that comes with the engine

eagerly awaiting replys

longjohn

 

[longjohn66]

jets used are round stubby threaded type

 

[Gawain]

Assuming its not mechanical or electrical i would look at fuel starvation. Blocked tank breather/ fuel filter/ fuel hose or possibly float level too low or floats flooded with fuel. Good luck.

 

[longjohn66]

i hope its none of those, brand new engine with correct loom and large cdi, connected as the digram on the dax site with out lights, green to green with one kill wire, pink to red and the plug connected and the other kill to the orange wire, coil pack is earthed to the bike with blue wire on coil, loads of blue spark , even tried a new ngk cr7hsa plug, plug is very black ( rich)

 

[longjohn66]

float is just past level?

 

[Cactus Jack]

Yes ... the fact that it runs better with a small #80 jet points to the float level possibly being too high ...

Another thing is that you could be copping valve float or bounce ... which can happen quite easily if the engine is revved high under no load ... When the load comes off an engine the cylinder pressure drops ... You should NEVER rev your engine too high when it's not under load and that includes knock the bike down through the gears to slow down ... BAD NEWS ... as the valves can quite easily hit the piston and bend the valve stem ... especially the inlet valve ...

Cylinder pressure helps force the valves shut ... that's why we used to "trumpet'' or "Tulip" our XR valves ... IF the inlet valve is floating , bouncing , or simply not sealing correctly , reversion pulse waves will travel back thru the carb and they can cause the engine to run unnaturally rich ... Just think about it for a second ... the intake air picks up fuel as it passes the jets ... then if it gets blown backwards by combustion pressure leaking past the intake valve ... it goes back past the jets in reverse picking up fuel ... then reverses again to head back into the cylinder , picking up even more fuel as it passes the jets again for the THIRD time ... the symptom is the engine hits a wall like it does if revved when the choke is left on ... ie it can "stutter" ...

If your intake port has been hogged out by someone who doesn't know what they're doing ... then the lack of velocity can ALSO create the same effect ... a port that flows good one way ... will also flow just as well in reverse ... Hogged out ports are known as "Lazy" ports ... and they can suffer from reversion because there is no velocity ramming to hold the charge in the cylinder with a long duration , high overlap cam ... When ports are done right up to 125% cylinder filling can be achieved ... via velocity ramming and super efficient cylinder scavenging ... With a high overlap cam you can use a stepped header , reverse cone megaphone exhaust to accelerate the intake velocity in order to achieve a super charged cylinder ... (anything above 100 % is super charged) ...

Yo' ... Merry X-Mas to all Mini Riders members ...

 

[longjohn66]

ports have been set 24mm shouldnt be too big? there is a tapper on the porting, valves were lapped, so they should seal good.
how do i set the float level as ive never done that before?

many thanks for the replys

longjohn