Tuning Keihin CR33 Carbs

TomU · Jun 18, 2023

jamesp said:
Do you only use one sensor?

Yeah, only have one sensor. I welded two bungs, but the one on the drive side was welded at the wrong angle and interferes with the frame. It’s no big deal since the carbs will be set up the same.

jamesp · Jun 18, 2023

TomU said:
Yeah, only have one sensor. I welded two bungs, but the one on the drive side was welded at the wrong angle and interferes with the frame. It’s no big deal since the carbs will be set up the same.

TomU said:
I have one sensor on one side for my GTO. Most people use just one, but I have seen them for every cylinder on a race car.

maaseyracer · Jun 21, 2023

acotrel said:
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.

I have tested this the cross over does not change anything on the tune. The muffler does. Lean on an air cooled engine is different than lean on a water cooled engine. While 14 or even 15:1 is possible for a lean cruise on a water cooled engine, that is running very hot on an air cooled engine. I have been warned by someone I would consider an expert tuner that 13.5:1 is as lean as you want to go on an aircooled engine unless you have an oil cooler, and the ability to monitor temps.

The general rule of thumb for tuning on the dyno (which has some caveats) is, time for torque and jet for horsepower.

acotrel · Jun 21, 2023

I do not believe the crossover does anything to help performance, but it might reduce noise.
The Japanese seem ti have their road bikes well-tuned. But with air-cooled motors using petrol as fuel, heat will always be a problem. Commando barrels are usually made from cast iron.
If I had a road-going Commando, I would not tune the motor for more performance - I would tune the gearbox. The motor is probably adequate for all purposes. With close gear ratios, not as much power is needed.
A Suzuki GSX750L has six gears close ratio - that is a convincing argument in anybody's language.

maaseyracer · Jun 21, 2023

In my tuning quest, I kept running up against a wall with sputtering around either 1/4 or 3/4 throttle depending on how I jet, with the bike acting like it had too much fuel then revving into the power. I spent the last week with air fuel meter trying to figure out what was going on. Finally the bike started having issues starting when hot, which I know can be an issue with the Tri Spark ignition.

Well after just shy of 10 years my Tri Spark bit the dust. I picked up a Pazon and I can see some design advantages with the Pazon. I installed the Pazon and suddenly my jetting is wayyyyyyy off. I have gone back to the stock settings that JS Motosport ships these thing with except I have gone a richer main jet. I have some fine tuning to do, but this is really close.

Current Jetting:
68 Pilot
YY6 Needle (3rd position from bottom)
140 Mains
230 Air

Tuning Keihin CR33 Carbs

TomU

1972 Roadster

jamesp

maaseyracer

acotrel

maaseyracer

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