SOHC.co.uk Forums > CB750
Bogging down.
brodsord:
Ok thanks for this Trigger, I’ll see if I can try and find out some info on this.
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K2-K6:
As the others have already posted, you can really chase your tail on this sort of stuff.
Basic problem is not that the engine will burn more fuel, but that the original airbox / intake etc cause more vacuum that pulls the correct amount of fuel from the float bowls. Going to pods gives you less vacuum in the carb venturi for the same engine speed, and consequently pulls less fuel up from the float bowls, hence the weak mixture.
As a test / see, you could lift the needle in main jets by dropping the circlip two notches, setting the float height a couple of millimetres higher, turning the idle jets in by 1/4 from standard setting and using resisted spark plugs with resisted HT caps as well.
It would push it toward where you need to be and at least see if it improves without significant cost.
Edit;- also make sure the plugs are at their minimum specified gap too.
K2-K6:
A question Brodsord, which number and type of spark plug are you currently running?
Trigger:
--- Quote from: K2-K6 on May 12, 2020, 08:27:09 PM ---As the others have already posted, you can really chase your tail on this sort of stuff.
Basic problem is not that the engine will burn more fuel, but that the original airbox / intake etc cause more vacuum that pulls the correct amount of fuel from the float bowls. Going to pods gives you less vacuum in the carb venturi for the same engine speed, and consequently pulls less fuel up from the float bowls, hence the weak mixture.
As a test / see, you could lift the needle in main jets by dropping the circlip two notches, setting the float height a couple of millimetres higher, turning the idle jets in by 1/4 from standard setting and using resisted spark plugs with resisted HT caps as well.
It would push it toward where you need to be and at least see if it improves without significant cost.
Edit;- also make sure the plugs are at their minimum specified gap too.
--- End quote ---
If you use resisted plugs and resister caps that with give a weak spark ;)
K2-K6:
--- Quote from: Trigger on May 12, 2020, 08:54:15 PM ---
--- Quote from: K2-K6 on May 12, 2020, 08:27:09 PM ---As the others have already posted, you can really chase your tail on this sort of stuff.
Basic problem is not that the engine will burn more fuel, but that the original airbox / intake etc cause more vacuum that pulls the correct amount of fuel from the float bowls. Going to pods gives you less vacuum in the carb venturi for the same engine speed, and consequently pulls less fuel up from the float bowls, hence the weak mixture.
As a test / see, you could lift the needle in main jets by dropping the circlip two notches, setting the float height a couple of millimetres higher, turning the idle jets in by 1/4 from standard setting and using resisted spark plugs with resisted HT caps as well.
It would push it toward where you need to be and at least see if it improves without significant cost.
Edit;- also make sure the plugs are at their minimum specified gap too.
--- End quote ---
If you use resisted plugs and resister caps that with give a weak spark ;)
--- End quote ---
It's commonly given that way but it doesn't look true to me.
Raising the resistance builds more voltage in the coil before the spark will start with breakover ionisation across the plug gap. It actually raises the energy deployed at the plug but elongates the spark duration, which more effectively ignites leaner mixtures.
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