What I’ve Learned Different About Carb Jetting
#1
05-17-2020 | 05:21 PM
Thread Starter
|
Member
Joined: Jun 2016
Posts: 32
1st Gear Member
What I’ve Learned Different About Carb Jetting
What I’ve Learned About Carb Jetting
1. The needle/needle jet clearance affects the idle mixture because some fuel is pulled up past the space between the needle shaft and the needle jet at closed throttle. When the needle jet gets wallowed out the tuner can be driven crazy trying to correct the idle mixture because the extra needle jet source is throwing the idle mixture off. But this normally happens only on 250cc engines and above.
2. Since the idle jet, air screw, throttle slide, needle, needle jet, and main jet are almost always overlapping in their effects there is a dire need to use a jetting calculator to figure out how to get perfect jetting thru the whole throttle range. Also because anything within 5% of ideal jetting feels OK to us (although that is a big 10% range) although it isn’t perfect.
3. The most important factor in jet sizing is the carb bore area. Second is the design of the exhaust pipe. Air velocity also has some influence. On one story on a forum a guy jetted a carb right for his bike and then put it on two other engines, one being twice as big. He said the jetting felt right for each engine.
4. The hardest part about jetting is getting the right needle in it, although most people just leave in the original needle and just change the clip position. They usually settle on a jetting compromise, something that isn’t too bad but isn’t perfect. I’d rather do it right and find the perfect needle for it by using a jetting calculator.
5. Triple taper needles are usually terrible. They almost always screw up the jetting curve to be too lean mid throttle. Keihin only uses single taper needles and Dellorto and Mikuni VM only uses single and dual tapered needles. Single or dual tapered needles do a better job of maintaining a linear response of jetting throughout the whole range of throttle opening. The exception to that rule is the Keihin PWK 28 which has a selection of needles with tapers that don’t go lower than 2.5 degrees which is still too much. See my latest video on how I had to find a 1.5 degree Mikuni needle to substitute for it. That gave a near perfect jetting graph.
6. The needle height should only be adjusted to start “kicking in” (increasing the flow area at the needle jet) around 1/4 slide opening. It should never be used to adjust the mid range jetting at the expense of minimal throttle jetting. That’s what other needles are for, to get the taper right to match what the engine wants for mid range. If you raise the needle too much then it kicks in before 1/4 throttle and makes it too rich there. If you lower the needle too much then it kicks in too late and around 1/4 slide open the jetting is too lean.
7. Places that sell needles couldn’t give a flying **** about you knowing all about their needles so you can make an informed decision. They just want you to buy as many needles as possible like shooting in the dark till you find something you like. I offered to the three main sources to measure their needles for them free of charge and they all turned me down. I had to buy a few of their non-standard needles to measure them myself to put the data in my jetting calculator.
8. Jets-R-Us supposedly sells “genuine” Mikuni needles but two out of three that I measured had their taper angles way off from what their letter codes said they should have. RESULTS ON MEASURING TMX NEEDLES : 6GDY12 should have angles 1.75/1/6.5 degrees but I measured 1.15/1.83/3.7 degrees. 6DGY04 should have 1/1.75/6.5 degrees but I measured 1/1.83/3.45 degrees. Their 6CEM3 was pretty close to the .75/1.25/3.25 degrees it should have. Now I only buy from Niche Cycle or Sudco.
9. Carb sizing isn’t as easy as using the common formula that we have seen on the internet. There is a range from high velocity racing carb sizing to medium velocity street/trail carb sizing and so there is no one formula for all since it’s all about max air velocity and what type of riding you do. A free calculator (spreadsheet) can be downloaded from www.dragonfly75.com/moto/carbsizing.html
10. Different engine oils with different viscosities mixed at different ratios all had the same flow rate thru carb jets in experiments I did. I wanted to know if those differences affected the needed jetting. A 25:1 ratio has 96% gasoline and a 50:1 ratio has 98% gasoline so the 2% difference is hardly anything. But I didn’t chck any high viscosity oils at lower ratios than 25:1 so there is a possibility that would make a jetting difference.
11. Correct main jet sizing is a combination of 1) spark plug color (although type oil and gas used affect that also), 2) carbon pattern on top of piston (piston should be clean near the transfers) and black everywhere else, 3) lack of carbon underneath the piston crown (carbon there indicates it is getting too hot), 4) how the engine runs (including exhaust note).
12. If you have to size the main jet too big in order to keep the engine from seizing then you should consider lowering the compression or retarding the ignition or increasing the stinger diameter. “Too big” is noticed by the engine response and exhaust note seeming too rich.
Go to www.dragonfly75.com/moto/carbtuning.html to read how I tune carbs to perfection.
1. The needle/needle jet clearance affects the idle mixture because some fuel is pulled up past the space between the needle shaft and the needle jet at closed throttle. When the needle jet gets wallowed out the tuner can be driven crazy trying to correct the idle mixture because the extra needle jet source is throwing the idle mixture off. But this normally happens only on 250cc engines and above.
2. Since the idle jet, air screw, throttle slide, needle, needle jet, and main jet are almost always overlapping in their effects there is a dire need to use a jetting calculator to figure out how to get perfect jetting thru the whole throttle range. Also because anything within 5% of ideal jetting feels OK to us (although that is a big 10% range) although it isn’t perfect.
3. The most important factor in jet sizing is the carb bore area. Second is the design of the exhaust pipe. Air velocity also has some influence. On one story on a forum a guy jetted a carb right for his bike and then put it on two other engines, one being twice as big. He said the jetting felt right for each engine.
4. The hardest part about jetting is getting the right needle in it, although most people just leave in the original needle and just change the clip position. They usually settle on a jetting compromise, something that isn’t too bad but isn’t perfect. I’d rather do it right and find the perfect needle for it by using a jetting calculator.
5. Triple taper needles are usually terrible. They almost always screw up the jetting curve to be too lean mid throttle. Keihin only uses single taper needles and Dellorto and Mikuni VM only uses single and dual tapered needles. Single or dual tapered needles do a better job of maintaining a linear response of jetting throughout the whole range of throttle opening. The exception to that rule is the Keihin PWK 28 which has a selection of needles with tapers that don’t go lower than 2.5 degrees which is still too much. See my latest video on how I had to find a 1.5 degree Mikuni needle to substitute for it. That gave a near perfect jetting graph.
6. The needle height should only be adjusted to start “kicking in” (increasing the flow area at the needle jet) around 1/4 slide opening. It should never be used to adjust the mid range jetting at the expense of minimal throttle jetting. That’s what other needles are for, to get the taper right to match what the engine wants for mid range. If you raise the needle too much then it kicks in before 1/4 throttle and makes it too rich there. If you lower the needle too much then it kicks in too late and around 1/4 slide open the jetting is too lean.
7. Places that sell needles couldn’t give a flying **** about you knowing all about their needles so you can make an informed decision. They just want you to buy as many needles as possible like shooting in the dark till you find something you like. I offered to the three main sources to measure their needles for them free of charge and they all turned me down. I had to buy a few of their non-standard needles to measure them myself to put the data in my jetting calculator.
8. Jets-R-Us supposedly sells “genuine” Mikuni needles but two out of three that I measured had their taper angles way off from what their letter codes said they should have. RESULTS ON MEASURING TMX NEEDLES : 6GDY12 should have angles 1.75/1/6.5 degrees but I measured 1.15/1.83/3.7 degrees. 6DGY04 should have 1/1.75/6.5 degrees but I measured 1/1.83/3.45 degrees. Their 6CEM3 was pretty close to the .75/1.25/3.25 degrees it should have. Now I only buy from Niche Cycle or Sudco.
9. Carb sizing isn’t as easy as using the common formula that we have seen on the internet. There is a range from high velocity racing carb sizing to medium velocity street/trail carb sizing and so there is no one formula for all since it’s all about max air velocity and what type of riding you do. A free calculator (spreadsheet) can be downloaded from www.dragonfly75.com/moto/carbsizing.html
10. Different engine oils with different viscosities mixed at different ratios all had the same flow rate thru carb jets in experiments I did. I wanted to know if those differences affected the needed jetting. A 25:1 ratio has 96% gasoline and a 50:1 ratio has 98% gasoline so the 2% difference is hardly anything. But I didn’t chck any high viscosity oils at lower ratios than 25:1 so there is a possibility that would make a jetting difference.
11. Correct main jet sizing is a combination of 1) spark plug color (although type oil and gas used affect that also), 2) carbon pattern on top of piston (piston should be clean near the transfers) and black everywhere else, 3) lack of carbon underneath the piston crown (carbon there indicates it is getting too hot), 4) how the engine runs (including exhaust note).
12. If you have to size the main jet too big in order to keep the engine from seizing then you should consider lowering the compression or retarding the ignition or increasing the stinger diameter. “Too big” is noticed by the engine response and exhaust note seeming too rich.
Go to www.dragonfly75.com/moto/carbtuning.html to read how I tune carbs to perfection.
Last edited by jaguar57; 05-19-2020 at 06:28 PM. Reason: additions
Thread
Thread Starter
Forum
Replies
Last Post