120/120 jetting?
Posted: Thu Jul 03, 2014 6:23 pm
Hi everyone,
I've been working on the carbs on my NC30 that I bought a couple of years ago but haven't used in almost ten months.
The exhaust on this bike has a removable baffle with the "road legal kitemark" stamped on it, so it is either "quite free-flowing" (baffle in) or "extremely free-flowing" (baffle out). I hadn't opened the airbox or carbs before, so I was surprised to discover that the previous owner (who only used it for trackdays) had the following setup:
Standard air filter
No sub air filter fitted
Slide holes drilled out to 3.05mm
~1.5mm washers on the needles
120 main jets in the front and the rear carbs
Despite all this weirdness, the bike used to run pretty well - I got over 50mpg(!) all the time and decent riding experience, also tested top speed at slightly more than 130mph on the speedo.
I also did the valve clearances and found that a lot of them were out of tolerance (all of which were too tight; obviously nobody bothered to check it, bike's on +30,000 miles now) and the number 4 cylinder had a Denso sparkplug where all the rest were NGK. Now I am putting it back together with all NGK plugs and correct valve clearances.
I'd like to hear from those in the know about the airbox/carbs, about whether I'd be better off to reset everything to the recommended settings. That is, should I:
(1) put the main jets to 118/120 or maybe 120/122,
(2) put the 1mm needle washers instead of the 1.5mm,
(3) put the sub air filter in.
Also if anybody knows anything about the 120/120 jetting setup - is this likely to be something that people have used for a reason, or is it just some random decision by a previous owner? I don't actually know why the front and rear jets should be different sizes, but all the setups I've read about say slightly smaller in the front than the rear.
I've been working on the carbs on my NC30 that I bought a couple of years ago but haven't used in almost ten months.
The exhaust on this bike has a removable baffle with the "road legal kitemark" stamped on it, so it is either "quite free-flowing" (baffle in) or "extremely free-flowing" (baffle out). I hadn't opened the airbox or carbs before, so I was surprised to discover that the previous owner (who only used it for trackdays) had the following setup:
Standard air filter
No sub air filter fitted
Slide holes drilled out to 3.05mm
~1.5mm washers on the needles
120 main jets in the front and the rear carbs
Despite all this weirdness, the bike used to run pretty well - I got over 50mpg(!) all the time and decent riding experience, also tested top speed at slightly more than 130mph on the speedo.
I also did the valve clearances and found that a lot of them were out of tolerance (all of which were too tight; obviously nobody bothered to check it, bike's on +30,000 miles now) and the number 4 cylinder had a Denso sparkplug where all the rest were NGK. Now I am putting it back together with all NGK plugs and correct valve clearances.
I'd like to hear from those in the know about the airbox/carbs, about whether I'd be better off to reset everything to the recommended settings. That is, should I:
(1) put the main jets to 118/120 or maybe 120/122,
(2) put the 1mm needle washers instead of the 1.5mm,
(3) put the sub air filter in.
Also if anybody knows anything about the 120/120 jetting setup - is this likely to be something that people have used for a reason, or is it just some random decision by a previous owner? I don't actually know why the front and rear jets should be different sizes, but all the setups I've read about say slightly smaller in the front than the rear.