Fuel Flow at Throttle Position?
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Mopar_Tony_SF1
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Fuel Flow at Throttle Position?
Does anybody have a document like the below that explains fuel metering by throttle position for CBX carbs? This will help with tuning.
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Mike Nixon
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carb tech
Larry and Tony -- There really isn't a reliable chart like the Mikuni tuning chart for CV carbs. The closest you can come to it possibly is the Factory Pro tuning procedure, which is pretty good. The pros like to think of it in terms of throttle grip percentage, but even then it varies from carb to carb, even among CVs. Some CVs like the CBX don't (stock) lift their slides until quarter throtle or more, while others lift much sooner. Also, there are "two jet" CVs and the better "3 jet" CVs, and several more variations, even a CV Honda made once that was a mechanical slide carb to a certain throttle position and then a CV carb after that! So you can see there is no set graph that would work.
Larry, on the jet tech, it's a lot more complex than that. First, both Mikuni and Keihin make jets that are of two systems (each). That is, both companies make flow rated jets and diameter spec'd jets. Few hard and fast rules here even, beyond the basics, but on the Keihin side, 90 percent of the round headed jets are marked in 100ths of a millimeter, while Keihin hex headed jets are virtually all flow rated. Simarly, Mikuni has the two types also, and in the old days it used to be small round was diameter and larger round and hex were flow (three shapes, two specs), but there is some question on that as well among us old timers. Incidentally, many folks say (and even Dynojet says) that their jets for Honda street bikes, while numbered by diameter and matching Keihin round street bike jets in most regards (including being 100ths of a millimeter), apparently many claim they are different nonetheless. (More than you wanted to know, I think...) They are in fact different, and some say that the differences make them flow different, but I have not noticed it. It can't be much. The differences are, a different taper into and out of the diameter (the reason jets are called "jets" in the first place), and the slightly taller head. Both can make Dynojet jets meter differently than their stock Keihin counterparts. The taper difference, well obviously, but the height difference due to pressure in the jet.
Jets for Brit and American carbs can be found that subscribe to the two flow systems also, but the majority of them use the diameter system, for example S&S and RevTech with their 1000ths of an inch diameter rated jets. An interesting note is that the flow rated jets are used on race spec carbs more often than not for one simple reason. When making changes to jetting, a diameter based jet would need to be converted to area, then the new jet's increased area calculated and the corresponding possible effect on flow determined, whereas the jets that are already numbered by flow have done all the calculations for you. :) In other words, a 10% change in a diameter based jet will not result in a 10% change in flow but one much larger, whereas theoretically at least you can do this with a flow based jet.
I talked to a fella today who was asking me to convert a set of carbs from one bike to use on another bike. Stock OEM carbs. Stock carbs are all compromises. And their parts are not available in variations. For example, Mikuni's needle jet. This is an elaborate part, with complex machining, not simply a hunk of brass with a hole in it. And it happens to determine 80% of how the carb works. So if it is not right for the engine the carbs end up on, and Mikuni doesn't offer variations in the particular style the part is made in (race carb needle jets are grouped in series), then you're stuck unless you want to machine your own part.
I'll be quiet now... :)
Larry, on the jet tech, it's a lot more complex than that. First, both Mikuni and Keihin make jets that are of two systems (each). That is, both companies make flow rated jets and diameter spec'd jets. Few hard and fast rules here even, beyond the basics, but on the Keihin side, 90 percent of the round headed jets are marked in 100ths of a millimeter, while Keihin hex headed jets are virtually all flow rated. Simarly, Mikuni has the two types also, and in the old days it used to be small round was diameter and larger round and hex were flow (three shapes, two specs), but there is some question on that as well among us old timers. Incidentally, many folks say (and even Dynojet says) that their jets for Honda street bikes, while numbered by diameter and matching Keihin round street bike jets in most regards (including being 100ths of a millimeter), apparently many claim they are different nonetheless. (More than you wanted to know, I think...) They are in fact different, and some say that the differences make them flow different, but I have not noticed it. It can't be much. The differences are, a different taper into and out of the diameter (the reason jets are called "jets" in the first place), and the slightly taller head. Both can make Dynojet jets meter differently than their stock Keihin counterparts. The taper difference, well obviously, but the height difference due to pressure in the jet.
Jets for Brit and American carbs can be found that subscribe to the two flow systems also, but the majority of them use the diameter system, for example S&S and RevTech with their 1000ths of an inch diameter rated jets. An interesting note is that the flow rated jets are used on race spec carbs more often than not for one simple reason. When making changes to jetting, a diameter based jet would need to be converted to area, then the new jet's increased area calculated and the corresponding possible effect on flow determined, whereas the jets that are already numbered by flow have done all the calculations for you. :) In other words, a 10% change in a diameter based jet will not result in a 10% change in flow but one much larger, whereas theoretically at least you can do this with a flow based jet.
I talked to a fella today who was asking me to convert a set of carbs from one bike to use on another bike. Stock OEM carbs. Stock carbs are all compromises. And their parts are not available in variations. For example, Mikuni's needle jet. This is an elaborate part, with complex machining, not simply a hunk of brass with a hole in it. And it happens to determine 80% of how the carb works. So if it is not right for the engine the carbs end up on, and Mikuni doesn't offer variations in the particular style the part is made in (race carb needle jets are grouped in series), then you're stuck unless you want to machine your own part.
I'll be quiet now... :)
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Terry
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Mike, a little off topic here, but you should meet this tuner at the Factory Pro dyno shop in San Rafael named Raul. I'll bet you guys would have a lot to talk about. Some years back I removed the very restrictive air boxes and installed K&N pod filters on the stock carbs and opened up the headers (gutted the megaphone mufflers) on a 1,360cc OHC Suzuki V twin (1993 1400 4-speed shaft drive Intruder) and took it to him to set up the carbs to go with the intake and exhaust mods. Talk about not idling well! All he wrote on the work order was 'Blueprint the carbs' and promised I wouldn't have to bring it back for tuning once he set it up. He was right. The bike is still killing off unsuspecting Harley riders in southern California. I traded it to my brother for his Prolink. It didn't corner worth a damn but in a straight line it sure would pull your arms off having almost 100 pounds of torque!
It ain't the destination, its the journey...
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Mike Nixon
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,) I also forgot I was dealing with CV carbs... 
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Mopar_Tony_SF1
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Great info Mike. Now the dumb questions from me ...
Generally speaking, will shimming the needles help with a 1/8 ... 1/4 throttle lean flat spot?
What are your general jetting guidelines for running Denco 6-6 pipes on a stock 79 motor, with stock air box with K&N filter. I'm at stock jetting now, and of course it is super lean from idle to about 1/2 throttle.
Generally speaking, will shimming the needles help with a 1/8 ... 1/4 throttle lean flat spot?
What are your general jetting guidelines for running Denco 6-6 pipes on a stock 79 motor, with stock air box with K&N filter. I'm at stock jetting now, and of course it is super lean from idle to about 1/2 throttle.
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Mike Nixon
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jetting
Tony -- "Super lean..."? I wouldn't expect that. There aren't many exhausts that will result in that condition, but a lot of air filter configurations will. There are more capable hands than me aboard, but I would go 75 to 77 on the primay main, no more than 102 on the secondary main, and leave the needle alone. Make sure the pilot screws are at the obligatory 2 to 2 1/2 turns out, and make doubly sure the ignition timing is not retarded and if anything, a smidge advanced.