TwoTired just replied to someone else's woes with the PD carbs, and rather than hijack his thread, I wanted to start my own from the same point.
The engine wheeze with throttle roll on is usually a lean condition. Mechanical slide carbs open their slides at the twist of the throttle. And this causes the carb throat vacuum to drop. The loss of vacuum drastically reduces the fuel flow from the jets, starving the engine from fuel. The engine essentially runs out gas. When the slides are returned to make the opening smaller the vacuum goes back up, fuel flows and the engine starts making power again.
Later 750 carbs (PD style) have an accelerator pump to squirt fuel into the carbs during a throttle twist, significantly improving throttle response for the operator.
Earlier carbs depend on an over rich idle setting, and initial throttle valve rich setting to give smooth acceleration under load. It will always wheeze when snapping the slides full open, unless the carb setting are way rich, and then you will probably have spark plugs that carbon foul frequently. When the earlier carbs are set up correctly, the engine should respond predictably with up to one half total sudden throttle twist.
-Two Tired
My 650 is having the same lean thing go on. The accelerator pump is working fine. I turn the throttle just a little to raise the rpms, and the engine wheezes a little until the accelerator pump squirts, and like the flick of a switch, the rpms jump up to 2500, 3000 and after that initial wheeze she's smooth as can be. The slow circuit jets are as clean as they can be (go ahead, ask how I know) and I've noticed a huge improvement as far as riding performance on replacing my air cutoff diaphragms. I'm just wondering what "the deal" is with the mixture screws.
I know (now) with PD carbs they govern mixture. Out= more air
and fuel, in = less air
and fuel. This really is only like finding a 'sweet spot' where 'Yes, she idles smoothly' or 'Nope, runs like poop.'
Is there any way (without actually modifying these pressed-in jets) to enrich the idle circuit? I'm at 2 1/2 turns out right now, and it seems to be a (the?) sweet spot.
The cam chain is a bit loose right now, also, so that may be having an impact on the mixture, inasmuch as cam timing is concerned, but by how much?
I also remember TT (er, I think it was him) saying that when properly set, these bikes should (theoretically) be able to idle all day without fouling the plugs (though overheating would be another matter, of course) so would this lean idle risk damaging the plugs, or would it have them just hot enough to keep the plugs clean?
