HIGH RPM'S

[XSLeo]

The needle jet is a snug slip fit and the main jet holds it in place. No you don't need to change it unless it is worn. The needle and needle jet are a matched set, so if you replace one it should be with the same part.
The Canadian bikes came with an adjustable needle that you can use but it's best to use the Canadian needle jet to go with it.
On changing the air intake or exhaust you might need to rejet to match. Testing is the only way to do it right. You make a change and test, make a change, test. Don't make two changes and test, you won't know which change made the difference.
The carb guide explains this.
Leo

 

[gabecustom]

okay thanks! that puts me at ease a little. my next concern is that i have a 78 xs650sf and the carbs are bs34's, as i have been researching i have found that the bs34's were an model year 80 and later. where as 79 earlier carb jetting was different. will this affect my tuning, having a later year carb assembly?

 

[weekendrider]

gabe when you change/tune the carbs you do it for the carb not the motor.
The motor doesn't know if the gas is coming from 34's, 38's or a 55gal drum.
But it will know if it isn't being delivered in the correct proportions.

 

[gabecustom]

i didn't change the carbs, bs34's were on the bike when i got it. let me put it another way.. will the stock jetting in bs34's give me problems on a 78 XS650sf engine?

and by the way a motor receives power from an external source, and then converts it into mechanical energy. an engine on the other hand creates power, through internal combustion or the pressure formed from other substances.

 

[XSLeo]

As weekendrider said the engine don't know or care what carbs you use.
No the stock jetting won't hurt the engine. As a matter of fact the stock jetting is a good place to start. Test it, make one change, test. That's the way to tune, just one thing at a time.
Leo

 

[gabecustom]

okay, thanks, that's all i was wondering. didn't know if stock jetting from one carb to the next differed. and sorry about any miss communication along the way. Got it on tuning one step at a time..

 

[XSLeo]

Yes, the jetting specs changed about every two years on the BS38's. Your BS34's stayed the same except for the float. The first year had a brass float and the rest a plastic float. The brass and plastic floats have a different buoyancy so the float level spec differed. The brass speced 27 mm the plastic 22 mm.
The stock jetting was 132.5 mains with 42.5 pilots. With an idle mix screw setting of 3 1/4 turns out.
Right now I have a set of 82 BS34carbs on my 75. I have the pilots up one and the mains up one but I need to try two up on the mains. It runs well but a little lean on the 3/4 throttle up. It doesn't pull guite as well as it could. I had a set of 76-77 carbs on it that was one up pilots and two up mains and they pulled much stronger.
They didn't transition from idle to midrange and from mid to wot as nicely as the BS34's, but pulled stronger in the upper ranges.
With a bit of tuning you will get those BS34's working very well.
Leo

 

[gabecustom]

thanks for the info Leo! much appreciated!!

 

[uknowme2goog]

TTT...Any other ideas?