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Carburetion 101
- Thread starter MrYeats
- Start date
Nice find MrYeats. I enjoyed reading that. I don't agree with all of the information provided, but in general it was accurate.
I have to point out that the carbs described in this PDF are NOT CV carbs. This will cause confusion for those not familiar with the differences. For example, the idle/pilot circuit described uses the screw to control air, and turning it out will lean the idle. In our CV carbs, the pilot screw controls a mixture of air and fuel, and turning it out will enrichen the idle. Exactly opposite operation.
I'd highly recommend that those unfamiliar with carbs, or those not interested in learning about different types of carbs, do NOT read this PDF. It will only confuse and frustrate you when you go to work on your XS400.
Dave
I have to point out that the carbs described in this PDF are NOT CV carbs. This will cause confusion for those not familiar with the differences. For example, the idle/pilot circuit described uses the screw to control air, and turning it out will lean the idle. In our CV carbs, the pilot screw controls a mixture of air and fuel, and turning it out will enrichen the idle. Exactly opposite operation.
I'd highly recommend that those unfamiliar with carbs, or those not interested in learning about different types of carbs, do NOT read this PDF. It will only confuse and frustrate you when you go to work on your XS400.
Dave
Lego
XS400 Addict
Good to know Dave, today i printed it @work ,for a goodnight read.. I think picture Fig.7 can be usefull also for us with CS carbs, when trying to tune them 
MrYeats
XS400 Addict
I am interested to know more about this as I am getting ready to do carb jobs on 2 yamahas with BS28's and BS38's.
Yeah, well, Fig. 7 can be safely ignored. It is hopelessly wrong. Anyone who thinks that the Pilot circuit has any real effect at or beyond 1/2 throttle (probably less than 1/3 throttle in reality), is only fooling themselves. The size of the jets and passages in comparison to the throat of the carb should make that plainly obvious without much consideration.
The Main Jet on CV carbs only becomes a factor above 3/4 throttle AND high (7500+) RPM.
Like I said, an interesting read, but not applicable to our CV carbs.
Dave
The Main Jet on CV carbs only becomes a factor above 3/4 throttle AND high (7500+) RPM.
Like I said, an interesting read, but not applicable to our CV carbs.
Dave
MrYeats
XS400 Addict
So, let me know something I am in the dark about. The pilot needle. When it is put in its place, I notice that a tiny bit of it protrudes into the venturi area when it is screwed all the way in. What is a good starting point to set that needle?
I think you are referring to the Pilot Screw. If so, then the general suggestion is to gently seat the screw, then turn it out 2 turns. This should get the bike running, but will NOT be the correct idle mixture adjustment. This will require warming the bike up with a 20+ minute ride, then adjusting for strongest idle. If the Pilot Screw requires more than 4 turns out for a decent idle, the Pilot Jet is too small and should be replaced with a larger jet. If less than 1 turn out, the Pilot Jet is too large and should be replaced with a smaller jet.
HTH,
Dave
HTH,
Dave
MrYeats
XS400 Addict
I can understand that and it is a good place to start. Thanks.
