Tuning Keihin CR33 Carbs

I got a new toy tool. I picked up a relatively inexpensive Air Fuel Ratio meter on Amazon along with a small 12v battery to help me tune my MK3 after the engine rebuild. I could skip the 12v battery part if I was wired with a negative ground, which is a project for another day. I am using the MK3 crossover with a O2 sensor bung welded into it. Once tuned will ditch the cross over. I have plans to weld a bung to each of my pipes so I can tune in each individually.

Prior to the rebuild I had zero issues with the jetting that was supplied by JS Motorsport. However, after going with a larger cam (stage 2), and 9.6:1 compression, RH10 port conversion sleeves, and a more open exhaust, I had some need to refine the jetting. Changes were relatively minor.

Idle jet moved down from 68s to 60s. Needle jet went from a YY6 to a YY7, clip is 3rd position from bottom of the needle, I kept the 130 mains, and I moved the air jets one size leaner to 240s. Idle screw is roughly 1 turn out, and closer to 3/4 on the other side.

Idle sits roughly at 12.5 to 13:1. WOT is around 12.5:1, if not slightly richer. I have no throttle stutter coming off idle and the pull feels great.

This is probably the easiest way I have found to tuning a main jet without looking for the perfect road and looking over your shoulder constantly.

maaseyracer said:

I got a new toy tool. I picked up a relatively inexpensive Air Fuel Ratio meter on Amazon along with a small 12v battery to help me tune my MK3 after the engine rebuild. I could skip the 12v battery part if I was wired with a negative ground, which is a project for another day. I am using the MK3 crossover with a O2 sensor bung welded into it. Once tuned will ditch the cross over. I have plans to weld a bung to each of my pipes so I can tune in each individually.

Prior to the rebuild I had zero issues with the jetting that was supplied by JS Motorsport. However, after going with a larger cam (stage 2), and 9.6:1 compression, RH10 port conversion sleeves, and a more open exhaust, I had some need to refine the jetting. Changes were relatively minor.

Idle jet moved down from 68s to 60s. Needle jet went from a YY6 to a YY7, clip is 3rd position from bottom of the needle, I kept the 130 mains, and I moved the air jets one size leaner to 240s. Idle screw is roughly 1 turn out, and closer to 3/4 on the other side.

Idle sits roughly at 12.5 to 13:1. WOT is around 12.5:1, if not slightly richer. I have no throttle stutter coming off idle and the pull feels great.

This is probably the easiest way I have found to tuning a main jet without looking for the perfect road and looking over your shoulder constantly.

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This is how I tune my carbs and one of the reasons I changed from positive to negative earth
Then fullauto pointed out that you don't need to!
I only have a single channel Koso
I marked up the twist grip so I knew what circuit of the carbs I was on and then adjusted the needle position/pilot air screw/slide cuttaway / main jet
My mate uses a twin channel with a universal homemade datalogger made from a ford throttle position sensor that clamps to the twist grip

baz said:

This is how I tune my carbs

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What AFR or Lambda are you targeting at idle and as you transition to WOT?

maaseyracer said:

What AFR or Lambda are you targeting at idle and as you transition to WOT?

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From memory I think it was 13 or 13.5 I was chasing at idle
I found the optimum 14.7 almost impossible to get to
But what it did flag up was how far from optimum the standard setup carbs (standard 32 mm concentrics) were especially needle position and cutaway I can't remember what the transition to wot was but I remember it was initially very rich
I now have a single 40 mm TM flat slide fitted
It runs remarkably well but one day I'll fit the afr gauge to see what it's doing

baz said:

From memory I think it was 13 or 13.5 I was chasing at idle
I found the optimum 14.7 almost impossible to get to
But what it did flag up was how far from optimum the standard setup carbs (standard 32 mm concentrics) were especially needle position and cutaway I can't remember what the transition to wot was but I remember it was initially very rich
I now have a single 40 mm TM flat slide fitted
It runs remarkably well but one day I'll fit the afr gauge to see what it's doing

Click to expand...

I am new-ish to air cooled engines, but that seems on the lean side without water cooling and or fuel injection.

“Once tuned will ditch the cross over. I have plans to weld a bung to each of my pipes so I can tune in each individually.”

Does the 1-3/8” pipe have room for the sensors to be screwed in without blocking the exhaust exit? Thanks.

jamesp said:

“Once tuned will ditch the cross over. I have plans to weld a bung to each of my pipes so I can tune in each individually.”

Does the 1-3/8” pipe have room for the sensors to be screwed in without blocking the exhaust exit? Thanks.

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On my bike I only had one bung welded in one downpipe
Not ideal I know
The bung is around 25 mm diameter with an 18mm thread
It has to be within 200mm of the exhaust port on my one
And only just projects inside the downpipe

maaseyracer said:

I am new-ish to air cooled engines, but that seems on the lean side without water cooling and or fuel injection.

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I used mine mainly to tune out some rough running
I've had times when at different throttle openings I knew the bike wasn't right
But never sure if it was lean or rich or what it was doing
Once the twist grip was marked up I could hold the throttle in those positions and check what the guage said

jamesp said:

Does the 1-3/8” pipe have room for the sensors to be screwed in without blocking the exhaust exit? Thanks.

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Plenty of room, the Sensor does not enter the inner diameter of the port by much at all. At the same time I am not sure I would want to leave the sensor on full time, it stands out and is fairly tough to position inconspicuously.

maaseyracer said:

Plenty of room, the Sensor does not enter the inner diameter of the port by much at all. At the same time I am not sure I would want to leave the sensor on full time, it stands out and is fairly tough to position inconspicuously.

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I was thinking of installing them before the muffler at an angle that faces under the frame.

jamesp said:

I was thinking of installing them before the muffler at an angle that faces under the frame.

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On the Mk3 I have the center stand and the side stand are totally in the way. As is the primary. I felt like I had to angle them down more than I was comfortable with.

maaseyracer said:

On the Mk3 I have the center stand and the side stand are totally in the way. As is the primary. I felt like I had to angle them down more than I was comfortable with.

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My Mk2a is in pieces, so hard to tell right now, but probably has the same considerations. I have asked Paul at Viking Exhausts to take a look at it. I know he is also open to suggestions. Maybe the spark plug method is not so bad, after all.

Interesting thread. I too have a wideband installed. Temporary, because you don't need it once you get the carbs dialed in. It helped that i had an older set of pipes i could sacrifice. Also, the sensor blocks a lot of the airflow, so you wouldn't want it there permanently

What i don't know is what is the optimal fuel map. I'm just guessing and using the plugs to confirm my guesses

baz said:

I found the optimum 14.7 almost impossible to get to

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Optimum? That seems lean to me

maaseyracer said:

On the Mk3 I have the center stand and the side stand are totally in the way. As is the primary. I felt like I had to angle them down more than I was comfortable with.

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My sensor location

Tom, thank you for posting your sensor location. I have been staring at the primary side this entire time and my brain was stuck there. As for Air Fuel Ratios I am targeting 12.5:1 as that is what we used to tune Honda DOHC car engines to at wide open throttle. I am working on small efforts to see how lean I can safely take it. I am not sure I want to venture very far past 13:1 in the lean direction. Below 12:1 it gets too rich in a hurry.

Fuel air ratios mean nothing to me, even though I am an industrial chemist. If you are tuning for performance, best power is just slightly on the rich side of too lean. I would never tune a motor then change the exhaust system afterwards. The exhaust system, ignition advance and comp. ratio need to remain constant while you adjust the jetting. The main jet size is usually irrelevant, as long as it is too rich. Your main concern is your needle jet size and the shape of the needle. When you seek performance, the slightest bit too rich on the needles will take the edge off the motor.
If your motor misses when you ride the bike - stop and raise the needles one notch.

My friend is a champion in an historic racing class. He uses an oxygen sensor to get the jetting right, then while his bike is on the dyno, he advances the ignition timing until he gets maximum torque.

A motor will not usually miss if it is jetted too rich , it will just be sluggish. If it is jetted too lean, it will usually let you know by missing - do not ignore it. A cold motor will usually miss until it is warm.

I've found with Keihin's and particularly FCR's over richness of the main jets can cause both bogging and stutter/missing issues from right down from 1/8 throttle opening which still has a influence of the main jet..

TomU said:

My sensor location

View attachment 107655

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Do you only use one sensor?