New Carb Tuning

Hey guys!

I'm new to motor biking and I just assembled my first motor bike to get me around campus. I bought an 80cc 2 stroke from boygofast on ebay.

My question is, how do you tune the carburetor? I searched on the forums and have found some very helpful posts, however I'm still a little foggy on some things.


When would you move the retaining ring on the needle inside the carb?

Moving it towards the back end of the needle would do what? Moving it up closer to the point of the needle would do what?

When I tested my bike, I found that it would just not idle at all. As soon as I engaged the clutch, the motor would die out instead of idling unless I gave it a little throttle to keep it alive. I found a post with someone that had the same problem and just adjusted the idler screw.

I've heard a lot of terms like "lean" and "rich"... what do they mean? I heard that having the idler screw in too far can cause damage.

Bottom line is... I'm deathly afraid of trying to tune my engine and somehow causing it an early death.

Any suggestions with the carb?

The benefit of the idler screw is to lift the cylinder, which will allow a bigger gap between the cylinder and the throat of the carby. Do you have the standard carby? Assuming you have this carby, an easy way to inspect the "gap" is to remove the black air filter, and look straight through the throat of the carby. Inspect the gap and you don't want "NO GAP". This is a general cause of bogging.

So remove the filter, inspect the throat and adjust the idler screw until u have a reasonable gap for fuel/air to run through.

I'm not entirely sure about the idler screw being all the way in damaging the carby, but Ive had no problems doing that.

The needle rides in the main jet and blocks the main jet hole when it sits lower in the jet. Block jet means less fuel, leans out the mixture (less fuel to same air ratio)

Using this information, you should be able to figure out how the needle clip setting affects mixture. If not, form a circle with your left index finger and thumb. Using your your right index finger, put it into the circle that was formed with your left hand. The hole represents the main jet through which fuel flows, the right index finger represents the carb needle that limits or allows the correct flow.

Haha, ok thanks guys... I really appreciate your help. This forum is a wealth of information. I'm going to fine tune my carb settings today and see if I cant get that engine to purrrr.

I'll let you guys know how it goes

tuning the carb

hey guys just some info to get you started. The mixture screw adjusts the airfuel mix, and is different to the idle screw. Incorrect adjustment will cause overheating, seizure and possibly a hole in your piston. To set, screw the mixture screw in fully then back off 3 whole turns. Warm the engine, set a high idle. Slowly screw the mixture screw in till you can hear the revs climb. This setting is slightly lean, back the screw off till the revs stabalise and your done. Hope this helps, Ben.

Creamed,
There is NO mixture screw on a standard HT carb. There is only an idle screw on the left side of the carb that pushes up against a beveled section of the carb lifting the carb slide assembly which effectively is the same as pulling the throttle very slightly for increased idle rpm.

ahh sorry skyliner, my 70cc was shipped with the 'upgraded' jap carb with the remote choke and idle/mix screws. These are becoming more common it seems. Goes a bit quicker than the old carby and doesnt fourstroke at WOT. Ben

iT'S SIMPLE
THE HIGHER THE CLIP ON THE NEEDLE THE LEANER IT RUNS IE LESS FUEL
THE LOWER THE CLIP IT RUNS RICHER IE MORE FUEL
tHE ABOVE INFO ONLY ADJUSTS THE FUEL AIR MIXTURE RATIO!
tO ADJUST THE REVS IE IDLE SCREW, ITS THE SCREW ON THE LEFT SIDED OF THE CARBY(YES LEFT IS WHEN YOU SIT ON THE BIKE!) SCREW IN FOR HIGHER REVS AND UT FOR LESS REVS AND NO DAMAGE IS CAUSED BY SCREWING IN TO FAR!
DONT USE THE MANUAL FROM ZBOX AS FAR AS THE CARBY IS CONCERENED THE GUY WHO WROTE IT IS A TOOL NO COMPREHENSION OF A SIMPLE WORKING CARB OR ITS PARTS AND AS FOR 500KM RUN IN WHAT THE? THATS CRAZY ONLY NEED THREE 30MIN RUNS AT 1/2 THROTTLE, THREE RUNS AT 3/4 THROTTLE AND THE MOTOR IS MORE THAN RUNIN KEEP IN MIND THAT REVS SHOULD NEVER SIT AT SAME SPEED ALWAYS DROP TO LOW REVS THEN ACCELERATE HARD UP TO DESIRED REVS

tuning

Ok, I have the cheap carb that came with the BGF "80cc" kit. The thing ran terrible until I replaced and re-gapped the plug (small help) and then WAY better when i leaned-out the mixture. I put the needle clip on the top groove, thereby cutting the fuel in the mixture down when i am running anything less than WOT. Now, all I have to do is stop the 4-stroking at WOT. I'm gonna try some acetone...

Wow! I tried 1 1/2 capfuls of acetone (garden variety hardware store kind) and am pleased with the results. The 4-strokeing is nearly gone and my top end is way smoother and faster. I did seem to lose a little hill-climbing power, though, but the trade-off is nice.