02/18/2022
How to tune a 2stoke carb in 8 steps.
I've struggled with this topic for quite some time but am now confident enough to say I have figure out the proper tuning order. It took me many years to figure this out which is why I'm sharing to try to save you time and frustration.
Below is how to properly tune a 2 stroke, high performance or not. This information has been culminated from multiple manuals like Mikuni and others to get to this point. Have 1 used sparkplug (installed) and 1 new plug ready for testing.
Step 1 : Start from the pilot jet and work your way up, not the opposite. Starting from the main jet will not work. Start with the used plug, bring your bike up to operating temperature. 250-350 degrees F. Once at operating temp, kill the engine and swap a brand new plug in.
Step 2 : Start the engine and do nothing but let it idle till its at operating temp. Try not to blip the throttle at all and keep the RPMs at 2k or less. Just enough for it to idle without dying is ideal. You might need a fan or something to keep your engine cool but since I use RDM Tiki heads I had no issues and did not need a fan.
Step 3 : Kill the engine and check the plug. This is the critical aspect and starting point as you are deciding the pilot jet size based on the plug color. If needed adjust the pilot jet based on if it looks rich or lean. You can repeat this process a few times to get where you need to be.
Step 4 : Once your plug is close to where you want it, now is the time to adjust the pilot air screw. This will give you 0-1/4 throttle response. The air screw should ideally be 1.5 times out from the furthest setting in but not more than 3 turns out. If you are outside that range your pilot jet needs to be readjusted.
Step 5 : To adjust the pilot air screw start at 1.5 turns from fully in (clockwise). Turn the screw counter clockwise until you hear the RMPS start to change. Mark that point. Now turn the screw back clockwise till again you hear the RPMs change, all the time counting how many times your are turning the screw. Now you have 2 points, max and min. Take the half point between them and that is your pilot screw setting. (Example: 2 turns out till RPM changes, 1 turn in till RPM changes. 3 turns total so 1.5 turns between both extremes is the ideal setting.
Step 6 : Now the fun begins. Take the bike around the block and judge how its running from 0 - 1/2 throttle. You need to know what it sounds like to be rich and 4 stroking. It will be hard to tell what lean is until the engine blows so hopefully you are starting rich.
Step 7 : Adjust the needle c-clip based on your judgement of the throttle response at 1/2 throttle and the sound. Higher c-clip is leaner and lower c-clip is richer.
Step 8 : Now finally is the time to adjust the main jet. Starting out big and working down is much safer than the contrary.
Key things you need to know to adjust:
- what does 4 stroking sound like
I hope this helps you get your tune right. I have been building motor bicycles since 2010 and always had problems tuning. Following this procedure all my tunes come out perfectly now. Im hitting 60mph @ 14k rpm on an Athena which people have told me is not possible.
Best wishes
