Firstly, if you don't want to read all of this or can't quite grasp it all, that's ok. Just look at the first section. Any more fuel than that, and you need to sort out some other issue (or you have a turbo on that beast, take pics. And video!).
Secondly, I've had a fair amount of experience tuning/jetting carbs on Keihin CV carbs on 3 different types of bikes and have learned several things. At the time I'm rewriting this for VN750, this includes 3 personal VN750s of mine and maybe a couple dozen other bikes. This includes all combinations I've seen of stock/Emgo/K&N/Moxi/unfiltered velocity stack intakes and stock/degoated/h-piped/straight piped/drilled muffs/swapped baffled muffs/unbaffled mufflers which are basically just big cans. Please keep that in mind.
I hope this guide can help a lot of people. Since a certain member passed away some time ago, we lost a fair amount of experience with tuning. I picked up where he left off, dug deeper into the peculiarities of CV carbs, and have run a TON of tests to give people a good place to start. Being on this forum less often, I'm not as available as I was to answer the questions I've had on tuning individual bikes. Hopefully this helps
Finally, I adapted this write-up from another model of bike. If I've made a mistake somewhere or missed anything that needs to be changed for VN750s, just let me know.
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I've moved this section to the top for ease of use.
Again, I haven't been able to test a CX500 since I just got one as a project, but I recommend starting here:
+3% Intake pods only
+3% more open mufflers, H pipe, or crossover removal
+3% (in the high range) Removing H box and AGGRESSIVE
straight piping (for some bizarre reason, the Vn750 lingo is the "goat's belly" instead of a crossover, resonator, or Honda/Kaw's term "power chamber")
If you're doing only pods, or pods and swapping mufflers, you might get away without rejetting at all. Every bike is different, but most people who rejet are dumping WAY too much fuel in.
Jet flow rate chart (approximate):
Size # Area From stock
35 .0962 - 12.5 %
38 0.110 (stock)
40 0.126 + 15%
42 0.142 + 29%
130 1.327 - 3.77 %
132 1.379 (stock)
135 1.431 + 3.8%
138 1.485 + 7.7
140 1.539 + 11.6
142 1.595 + 15.7
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Point 1: Understanding consequences of intake/exhaust changes
This almost invariable changes the intake flow and dynamics of the carbs, and since they rely on air velocity for atomization and also vacuum for operating the slides it can be noticeable. Often there is a loss of power or driveability especially in the low range, or sometimes in the transition from the pilot circuit to the main (especially on a Keihin CV style carb). You can compensate for one or the other, but it's hard to hit both just right since the air intake isn't the same as they were designed for.
In any case, there will be a difference, whether you gain or lose power at different RPM or throttle position. It's "choppier" because a longer intake has a smoothing effect on the air flow, and a joined intake has even more smoothing effect on the air pulses like exhaust scavenging in a 2-to-1 or 4-into-1 pipe vs independent "drag" pipes. When you ditch the air box, you lose those. Since the new intake is like a short tube header, it's often going to flow more smoothly and efficiently at higher RPM and have less gain or even a negative net effect on the low end. That means a larger % change in air at the high end than the low end. This means a larger demand for jet sizing at the high end vs. the low end.
Point 2: Overjetting, and how size does matter
Most people tend to go overboard. It's better to be too rich than lean and burn a valve or piston. However, I keep seeing people recommending a huge size increase that isn't necessary. People end up with more power, but those who look deeper will find they're running rich so they're getting slightly suboptimal performance and definitely poor MPG. This also builds up a lot of soot and carbon from poor combustion! Running rich can also blue pipes or afterburn/overrun in the exhaust as much as going lean (this is the popping sound some people wrongly call "backfiring"). Ultimately, there are too many factors on any one bike even compared to other "identical" ones so you really need to do a plug chop or EGA to know what's going on.
Keihin jet sizes increase at a linear rate. Fuel FLOW does NOT increase at that linear rate. Why? Keihin numbers represent diameter in 100ths of a mm. Fuel flow (roughly, if you don't take into account the inconsequential difference in friction) is going to increase MORE than that because it's based on cross-sectional area. A 1" garden hose will flow MORE than twice as much as a 1/2" hose. By the way, Keihin sizes are rounded such that 72 is actually 72.5 (tenths of a mm?) and 78 is 77.5, which is why sizes always fall on 2, 5, 8, 10s.
Point 3: Calculating fuel flow with examples
Skip this and just use the charts if you like. I've spent a lot of time working with jet tuning/sizing on a few different types of bikes, mostly the Kawasaki Vulcan VN750. Generally by going to pod filters and different mufflers, people need 5-7.5% more fuel.
Let's compare a 78 to a 90:
In linear size and diameter, that's 15.38% larger. (Well, considering it's actually 77.5 vs 90, it's a bit different but let's go with 78)
78 = 0.78 mm diam, /2 for radius, then do pi * r^2 = 0.4778 square mm cross-sectional area of fuel flow. Keep in mind that it's a circle, not just squaring the measurement. It makes a bit of difference.
90 = 0.90 mm diam, = 0.6362 mm^2.
.6362 / .4778 = 33.15% more flow. So +15% size is actually +33% fuel!
So a 112 is 0.9852 mm^2, and going to a 120 is 1.1310, for a difference of 14.8%.
So when you're rejetting to a 90/120 you're going up TWICE as much on the primary as the secondary even if it's almost the same number of sizes increased (5 vs 4).
Point 4: Choosing jet sizes, bringing it all together
Jet sizes are going to depend on the change in AIR flow efficiency of the engine. It's one big air pump (not just a gullepump
). How much of a jet increase do you need? Exactly proportional to the % of power increase you expect to get. Intakes generally are less than 5%, so it's no surprise that people who rejet at 10% or more just for pod filters notice they are running very RICH. Exhaust is what, maybe in the area of 5-10%? From my experience, both together are more in the neighborhood of 8-12% total.
That's just average across the power band though. Generally people are going to have independent pods and a more open exhaust, both favoring air flow at the TOP end of RPM. The increase is NOT the SAME % across the entire RPM range, so depending on what you do you could actually lose performance and MPG at low to mid range, or you might gain just a touch at the bottom but a lot more in the mid and high range. That is what you should (generally) be jetting for, keeping in mind that a hair rich is best at WOT to avoid overheating heats, pistons, valves.
So why recommend 90/120 on a CX500 when it's +33%/+15% at the low/high range? Why #40-42 pilots on a VN750 when that's +15-30%? That goes against everything we know. Even if the top end is good, a lot of people are too rich at 90/120 on the CX and 40/140 on the VN750. I'll make an educated guess that it's from the oversized pilots.
So what's a good jet size? I tested a lot on other bikes, and the best running ones were below the (high) usual recommendations. Guess what, they usually were best around 5% increased cross-sectional area
(not jet number size). For a Vn750, that turned out to be only 1 size increase on the pilot jet dialed back but 2 sizes on the large, Or stock low jets with the idle screws turned out instead. That gave around 10% increase with a lot of people who only did the intake mod running a little rich, which gels perfectly with all of the above.
Minor point on shims/transition fueling
I have two things to add to my write-up here Firstly, the transition (sub)circuit, and secondly shimming.
The passages that feed the transition circuit further complicate jetting on SOME constant velocity (slide) carbs. Not all CV's have this, but the VN750 does. From my understanding, the pilot jets meter this fuel flow as well, however unlike the pilot circuit, its flow is NOT affected by the pilot screws. This means sizing up your pilots will increase your transition circuit even if you close the screws (for example to compensate for the huge 15% jump from #38 to #40s).
Secondly, shims often cause more problems than good. They can in some
cases help to "smooth" the transition from pilot to main circuits on modified bikes, but they are not required and often are counter-indicated!
They can and do
affect the pilot circuit even just at idle! Shims were the sole cause of poor idle and hot start problems on 2 of my 3 personal VN750s and some others I worked on
Here's a chart that shows how different factors affect carburetor tuning in general. This is not a VN750 specific chart and does not take into account the peculiarities of our transition and pilot circuits.
Where to start: VN750s
How much power do you REALLY get by bolting on intake/exhaust on any stock fuel injected motorcycle or car for that matter? Generally a little below estimates if you're lucky. I really cannot imagine someone cranking 1/3 more power out of a bike by bolting on an open exhaust and pod filters. It can't happen, so there just isn't that much of an air increase to necessitate the wasted fuel.
1. Start over from stock.
2. #38 pilots with opened screws, or for pods AND pipes AND low altitude consider the BIG 15% jump to #40s, but be prepared to dial your pilot screws way back to compensate for that excessively rich mixture
and SKIP SHIMS at this point.
3. Now tune your high end to eliminate any stuttering at WOT under high load and speeds. I've found that most people are best with stock jets for just the ear shave, #138s for intake and exhaust and only occasionally #140. Most people don't need more fuel. Only very rarely
and I mean once
have I seen a VN750 that actually needed #142s to avoid a lean top end and that's because it did best with #38 and zero shims. I'm still not sure it didn't have another unknown problem because #142 is an excessive
16% more fuel.
4. Re-check your idle and throttle response. It should purr like a kitten even below 1000 RPM and tear up the highway at 100+mph at WOT without skipping a beat. If not, clean your carbs (take them APART and MANUALLY clean everything) and do a full tuneup. Then try again. I don't know of a VN750 with forced induction, so you do NOT need 40/142s or #42 pilots with any mains. If those run better, you have something WRONG with the bike to fix FIRST.
5. Shim only if you have a rough transition. I had one bike that would "surge" a bit only at a 55-65 mph constant speed due to slightly mistuned transition fueling. This is a case where you'd add 1 shim to each needle and test before going to 2. Shims will screw up your low end and idle if it isn't done right.