Post by wayne on Nov 17, 2012 20:36:21 GMT -5
I was contacted a couple of weeks ago by Bill, another forum member.
He discovered that there was no ball in the accelerator pump circuit of his carby and not only replaced it, but made some interesting modifications based on his experience with other pumped carbies.
As that pump seems to be designed to assist specifically in the transition from primary to secondary, Bill's modifications may help. But as his bike is some distance to completion, he was just bouncing ideas.
The pump action is quite curious. If you have it on later spec, you're squirting fuel against a closed butterfly from at least 13 degrees of throttle movement before the butterfly starts to open. Early spec, you're still doing it for at least 6 degrees. And I haven't found either spec to make much difference. Perhaps why is below.
I thought that I'd ride an otherwise perfectly running bike with the pump disconnected to see what happens.
I rode about 100 transitions through the "zone" between 2,500 and 3,500 rpm with a fully warmed up and 100% hesitation free bike using all stock jets.
Amazingly, of those hundred odd transitions, there were only 3 where a real hesitation came back (as if ignition is switched off and then back on again). There were at least 5 where NO symptom at all was seen. And of all the rest, the bike only exhibited a subtle flat spot (ie a slight lag in the engine's normal rate of acceleration).
I even did a hard acceleration from 1500 rpm in 1st gear. No hesitation, just a slight flat spot. Given the apparent importance of that pump, WTF ??
If someone asked me to ride that bike for an opinion, I'd hop off and say "not perfect, but rides quite well, I wouldn't touch it".
It seems like everything else in that carby, it all adds a tiny bit but no one thing controls or defines the smoothness of the transition.
Like the enrichment mechanism. I disabled it and set it up by manual activation and it took ages to convince myself that it actually does do something.
Just for interest...............
He discovered that there was no ball in the accelerator pump circuit of his carby and not only replaced it, but made some interesting modifications based on his experience with other pumped carbies.
As that pump seems to be designed to assist specifically in the transition from primary to secondary, Bill's modifications may help. But as his bike is some distance to completion, he was just bouncing ideas.
The pump action is quite curious. If you have it on later spec, you're squirting fuel against a closed butterfly from at least 13 degrees of throttle movement before the butterfly starts to open. Early spec, you're still doing it for at least 6 degrees. And I haven't found either spec to make much difference. Perhaps why is below.
I thought that I'd ride an otherwise perfectly running bike with the pump disconnected to see what happens.
I rode about 100 transitions through the "zone" between 2,500 and 3,500 rpm with a fully warmed up and 100% hesitation free bike using all stock jets.
Amazingly, of those hundred odd transitions, there were only 3 where a real hesitation came back (as if ignition is switched off and then back on again). There were at least 5 where NO symptom at all was seen. And of all the rest, the bike only exhibited a subtle flat spot (ie a slight lag in the engine's normal rate of acceleration).
I even did a hard acceleration from 1500 rpm in 1st gear. No hesitation, just a slight flat spot. Given the apparent importance of that pump, WTF ??
If someone asked me to ride that bike for an opinion, I'd hop off and say "not perfect, but rides quite well, I wouldn't touch it".
It seems like everything else in that carby, it all adds a tiny bit but no one thing controls or defines the smoothness of the transition.
Like the enrichment mechanism. I disabled it and set it up by manual activation and it took ages to convince myself that it actually does do something.
Just for interest...............
