Carb balancing tip

[jchesshyre]

Just a reminder of the importance of flicking the throttle a few times after each adjustment when balancing the carbs, in order to settle the throttle linkage. I sometimes switch the engine off for the snapping to avoid irritating the neighbours I checked mine today and for a few goes, after I'd got it spot-on on the gauges, I snapped the throttle and the reading was then off by a bit (though still within 20 mm Hg). After a few repetitions I've got it absolutely perfect

[VTRDark]

Yep and also if you adjust your TPS this affects the balance.

[Cadbury64]

I might just be a bit dense but how could the TPS affect carb balance? I don't doubt that it will affect throttle response, but it won't change the act of balancing the carbs.

The TPS is an electrical device that (I assume) is used to slightly alter the ignition timing depending on the position of the butterfly. Whereas the carb balance is simply balancing the engine vacuum of the two cylinders by adjusting the butterfly on one carb.

[VTRDark]

It's all part of the same mechanism. When you adjust the the carb sync screw it opens and closes the butterfly as does twisting the TPS, so one counteracts the other. By how much depends on how far out one or the other is but it's worth baring in mind.

[Cadbury64]

Great explanation, thanks. I did not consider that the TPS is on the adjustable butterfly. What you were really saying is that synching the carbs can change the TPS setting.

[jchesshyre]

I thought it was the job of the throttle stop screw, not the TPS, to set the resting position of the throttle linkage. Doesn't adjusting the TPS just move the position of the sensor relative to the throttle linkage but not affect the resting position of the linkage? Or are we talking about a fractional amount, which of course is important on these bikes? Still, I don't see how this would affect the carb balance as the linkage moves as one unit and I thought only the synch screw set the throttle plates relative to each other?

[Cadbury64]

I think we are all talking around the same thing here.

The throttle cables and throttle stop are connected to the front butterfly. The linkage connects from there to the rear butterfly via the synchronising screw. The TPS sits on the rear butterfly shaft.

What that means is that anytime the rear butterfly resting position is changed, the TPS idle resistance changes too. That means if you change the idle screw, or change the synchronisation between the carbs or both, the TPS idle resistance will shift a little too. Darned if I know how much of a difference a small change would make but I recall getting my TPS to 500 ohms was fairly delicate.

[CheckCharlo]

Hi guys!!
I just got my vtr going recently and was looking for this information. My understanding of sequence now is,
1 Adjust idle, it may cut out but close enough is ok.
2 Adjust schronisation screw to balance carbs.
3 Adjust tps to 500 ohms.
Should be good after this.