We are all pretty much stuck with E10 gas. Ethanol leans out the mixture. CBX's seem pretty lean on the slow side of the circuit. With stock airbox, pipes, etc is anybody have any luck with fattening up the bottom end.
Below is an example of on a 79 going up to 70's on the primaries. Seems like an even better idea with E10 gas.
viewtopic.php?f=20&t=7596&p=58589&hilit ... +65#p58589
Jetting For E10 Ethanol Gas
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Mopar_Tony_SF1
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oldschoolcarbs
Re: Jetting For E10 Ethanol Gas
We've been enlarging the slow jets a tiny bit for a while now with nothing but good results.
The one thing to keep in mind is that the mild lean-ness in the idle circuit helps considerably with cold starting and thus richening the circuit CAN lead to harder starting. It becomes more important to have a well tuned carburetor and lay off the throttle COMPLETELY. A properly tuned carb should start with ZERO throttle but there's always the stubborn few who grew up thinking that you have to give it a few twists for the accel pump to juice things up prior to spark. Once that habit is broken we're having engines catch before the crankshaft has turned 180 degrees, which is literally on the first cylinder to get gas, compression, and spark at the same time.
You can also get aftermarket jet needles that will work fine in the CBX, with the added benefit that they're movable-clip type. This way a person can make seasonal adjustments without having to modify the needle holder. Plus you can richen the range from 1/4 all the way to redline in one easy step.
Oldschool
jack (at) oldschoolcarbs (dot) com
The one thing to keep in mind is that the mild lean-ness in the idle circuit helps considerably with cold starting and thus richening the circuit CAN lead to harder starting. It becomes more important to have a well tuned carburetor and lay off the throttle COMPLETELY. A properly tuned carb should start with ZERO throttle but there's always the stubborn few who grew up thinking that you have to give it a few twists for the accel pump to juice things up prior to spark. Once that habit is broken we're having engines catch before the crankshaft has turned 180 degrees, which is literally on the first cylinder to get gas, compression, and spark at the same time.
You can also get aftermarket jet needles that will work fine in the CBX, with the added benefit that they're movable-clip type. This way a person can make seasonal adjustments without having to modify the needle holder. Plus you can richen the range from 1/4 all the way to redline in one easy step.
Oldschool
jack (at) oldschoolcarbs (dot) com
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Mopar_Tony_SF1
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Re: Jetting For E10 Ethanol Gas
Interesting, I've lived at sea level all my CBX life and always found a little fatter on the idle/ slow circuit helps the CBX when dead cold. A hair richer when hot did not make starting difficulty. Maybe something weird going on with the RVP of boutique bay area fuels...
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oldschoolcarbs
Re: Jetting For E10 Ethanol Gas
+. We have a veritable smorgasbord of fuels. You don't realize how different they are until you line up a bunch of baby food jars filled with samples from a handful of stations. Colors range from clear to amber to jade green. What we find, which is admittedly rare owing to the difficulty of tweaking CBX carbs but not at all rare among the other DOHC engines, is that some perhaps well-intentioned but nevertheless inept mechanic operated according to the theory that if a little is good, a lot must be better. I've seen slow jets hogged out to 50 and more, and mains (primary and secondary) upped from stock to 5 or 6 sizes over.
65/102 to 77/145 and hogged out slows? Gimme a break. And the new owner has no idea why it was abandoned all those years ago.
An increase in jet size has a proportionally larger effect than folks realize. Where 70 is fine, 72 may be a bad bet. It might run well at home but as soon as you try to climb into the mountains you're bogging. Try explaining this to someone who hasn't a clue as to how carburetors even work in the first place and you appreciate all the more encountering someone who does! (WINK)
I live at sea level but near a 2700 foot mountain so we get to test our pet theories on the road. You're on the money--70 is the ideal size.
65/102 to 77/145 and hogged out slows? Gimme a break. And the new owner has no idea why it was abandoned all those years ago.
An increase in jet size has a proportionally larger effect than folks realize. Where 70 is fine, 72 may be a bad bet. It might run well at home but as soon as you try to climb into the mountains you're bogging. Try explaining this to someone who hasn't a clue as to how carburetors even work in the first place and you appreciate all the more encountering someone who does! (WINK)
I live at sea level but near a 2700 foot mountain so we get to test our pet theories on the road. You're on the money--70 is the ideal size.