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CB750 rejetting... not a new topic

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martin_uk:
I will be going through the delights of finding best set up on my 836 soon.

Carburation is a complex issue and acheiving best set up will vary depending on preferred use.

The main reason electronic fuel injection is so good, is the ability to vary fuel delivery to achieve the best fuel ratio, using an oxygen sensor in exhaust to provide a constantly changing mix to suit the varying throttle openings.

If you are using pods or open intakes, mixture will weaken and will need to be richened.

Worst case is a very weak mixture that causes piston overheating and can burn a hole in it, so better to err on richer side, especially at high sustained revs !

mgy66:
Thanks Martin, I actually went through redrilling up by 50micron the pilot and the main jets due to big bore. In addition, I have reset the air screw to 2.0.

I am checking the spark plugs and response (still have a slight gap so probably still too rich).

Actually, I also went through buying a infra-red thermometer. Did anyone tried that?

The idea is to measure the heat at the exhaust connection with the engine. I have set the air screws to get 180C but not sure if that is the right temp. I can see that it definitely reach the 220 which I am trying to avoid. I have actually noticed that despite having all the air screws at 2.0, one exhaust was running at 220C!

I think the OEM book provides temp indication for the oil but not so much for the engine. In all case the temp measure is very quick and repeatable without having to open up the spark plugs which can on the long term also damage the alu threads. Any idea what should be the combution temp?

Cheers,
Marc

martin_uk:

--- Quote from: mgy66 on June 04, 2015, 08:20:33 PM ---Thanks Martin, I actually went through redrilling up by 50micron the pilot and the main jets due to big bore. In addition, I have reset the air screw to 2.0.

I am checking the spark plugs and response (still have a slight gap so probably still too rich).

Actually, I also went through buying a infra-red thermometer. Did anyone tried that?

The idea is to measure the heat at the exhaust connection with the engine. I have set the air screws to get 180C but not sure if that is the right temp. I can see that it definitely reach the 220 which I am trying to avoid. I have actually noticed that despite having all the air screws at 2.0, one exhaust was running at 220C!

I think the OEM book provides temp indication for the oil but not so much for the engine. In all case the temp measure is very quick and repeatable without having to open up the spark plugs which can on the long term also damage the alu threads. Any idea what should be the combution temp?

Cheers,
Marc

--- End quote ---

Depending on the carb, the air screw/mixture screw may weaken mixture as you unscrew as it controls air not fuel. The PD carb fitted to late SOHC 750's is the opposite  ::)

Weak mixture causes high exhaust temperature.

Without cooling fans you are limited how long you can run statically before overheating occurs.

An infra red thermometer is good for giving rough idea, but engine revs increase is the best indicator for adjusting at idle, with maybe a slight richening to help transition from idle/slow circuit on take off.

Spitfire:
When I fitted a 900cc kit to my CB750K1 I upped the main jets one size, raised the needle one notch and ran without a filter in the airbox worked great (plus a pretty noisy Dunstall 4 into 1).

Cheers

Dennis

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