Looking for a Racer's Perspective on Carbs - CBR Forum - Enthusiast forums for Honda CBR Owners

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Old 06-09-2018, 01:48 PM
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Join Date: May 2018
Location: Pacific Northwest
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Default Looking for a Racer's Perspective on Carbs

Hi All!!! I'm new to the forum and new to CBR's. I bought an '89 about 3 weeks ago. It's a straight Track Bike that had been used competitively for quite awhile. I'm going through the bike, needed to deal with rust in the tank, old gas and really dirty carbs: Keihin VG20A.

She ran, did a cold start, warmed up without any unusual noises but we had to put the choke on and nurse it through the lower RPM's (dirty Slow Jet) and then again on the upper RPM's (Dirty Mains). No worries really. Ran it through the gears on the rear stand so the trans works, no noises.

Where I'm Going With This;
The bike has a K&N Air Filter, Air Box Mod, full headers topped off with a Vance & Hines can. What I noticed in the process of taking the carbs apart and cleaning them were the stock jets; #35 Slow Jet and #105 Main Jet. I pulled the #1 Plug and it is a really light Tan (lean). Of course my initial thought is fatten it up. #38 Slow Jet, #108 or #110 Main, shim the needle and go from there. Or do I?

I've been shopping for jets and such, but two things happened. First, I saw a random post on Facebook about a 2-Stroke plug and it clearly being a tad rich. A person commented "Lot of hp there if you lean it down one jet size at a time. Coffee with cream color and wide open throttle pass on new plug." Oh yeah, lean is fast. Second, it dawned on me; This bike had raced a lot of seasons, appears to have been well maintained and, this kids dad never switched jets and ran it lean. Perhaps there is actually something to that in the "If it ain't broke don't fix it" theory.

Which leads me to believe that maybe I'm overthinking this. Thoughts?

This bikes main purpose, and likely only purpose, is for Track Days so I can learn to race, which is the ultimate goal. If it's a really strong runner, I might even race it. But, for now, she'll play in the greener more relaxed pasture and make me the only guy on my block with a race bike!

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Old 06-10-2018, 04:04 AM
drakito's Avatar
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Join Date: Oct 2006
Location: Southern Utah
Posts: 624

Reading plugs is harder to do than most think. A lot of people drive it around hard, then drive home and read the plug. That will give an inaccurate reading. You need to rod the **** out of it, then shut it down right then in the RPM range you are testing and pull them right away. It's best if you cut the spark plug shell so you can see the entirety of the porcelain

Reading spark plugs | Tuning Spark Plugs

If you absolutely need to retune the carbs:
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