SOHC.co.uk Forums > CB750

throttle hangs

(1/3) > >>

neat street:
Hi All, quick question please, after the carb clean and rebuild on the 750 F1, when I open the throttle and shut, it hangs at arround 2000 revs for a few seconds or so before it drops back down,. any suggestions please.many thanks, John

K2-K6:
1/ Couple of reality  checks .... if you put one finger onto top of carb slide actuator (where the rubber boot covers) the move the "master" adjustment know to lower rpm, can you feel it dropping the slides ?  That's really to check if the slides are sitting too close to the base adjustment of that control knob. If there's not enough movement at that point, then you'll not be able to set low enough air volume through slides to competently shut the revs down concisely. If that were the case, then the slides will need to be dropped with their individual adjusters to facilitate the master tickover knob working within it's ideal mechanical range.

2/ what are the air screws set at  ?

3/ is it definitely, definitely,  definitely  :) running on all cylinders at idle ?  If not, and you've got even one not contributing, then you'll just keep looping around trying to set tickover without anything like a repeatable result.

4/ do adjustment of each idle screw, individually, result in changing rpm as you turn it ? Failure, even with just one, and it'll give that variation in steady tickover control and meaning there's some impairment present in that circuit.

neat street:
Hi and thank you for your reply
Master cylinder check and all working as it should.
air screws set bat 1 turn out
all pipes are (very) hot so not missing a cylinder
I will start to play with the idle screws today and see what happens.
thank you again, John

Sesman:
It is possible that during the carb cleaning some debris has settled in one or more of the idle circuits?

McCabe-Thiele (Ted):

--- Quote from: neat street on September 29, 2024, 09:08:26 AM ---Hi All, quick question please, after the carb clean and rebuild on the 750 F1, when I open the throttle and shut, it hangs at arround 2000 revs for a few seconds or so before it drops back down,. any suggestions please.many thanks, John

--- End quote ---

See the last paragraph of my post on this subject.

i.e.So how does the engine manage to maintain this hanging throttle?

I think it is as a result of a tad too much air passing through the intake under the sliders when the throttle fully closed, this draws fuel via the idler jet circuit as well as an unknown amount from the main jet in some situations.
This might be as a result of 1.2.3 or all 4 sliders being too wide open in the throttle closed position.

It's possible to think you have the air screw spot on when you do the bench sync by reducing the fuel flow by opening the air screw you can get an a false idle. When the engine is up to full running temperature this in my view points to the slider gaps being just that bit too wide. So it's all good when warm but after a fair run the idle gets too fast.

If when the main throttle stop screw is completely undone the sliders are fully closed then as no petrol can get through it will of course not even idle but stop.

I think the trick is getting the least amount of slider gap yet maintaining a tickover contolled by the air screw adjustment only - on a engine that is up to temperature.

Navigation

[0] Message Index

[#] Next page