SOHC.co.uk Forums > Anorak's Corner
Carburettors with more depth of discussion
McCabe-Thiele (Ted):
This is not really in-depth more anecdotal evidence.
"I have little experience with no formal qualifications on bikes, with that disclaimer heres what what I think.
I'm often wrong with a relatively short experience on only two Hondas in the last four years.
I am in my seventh decade of experience on a variety of cars and at the limit of my memory from bikes during the 1960s & 1970s. I do eventually admit when I am wrong.
My perception now seen through the prism of old age & the many posts on this subject is roughly as follows.
These bikes are fitted with four carburetors that at a basic level have two fuel supplies to the engine.
1) The first is what I think of as the idle jet circuit - this supplies fuel not just at idle but at significant amounts as you increase the engine revs with a sort of reducing influence as the main jets take over.
2) The second fuel supply is the main needle & jets that operate by the sliders.
Assumptions - there are no air leaks on the induction side with standard air cleaners, original jets & needles in good condition. When hot the open & closing throttle cable are working & not binding. The ignition advance/retard is working as it should & timing is spot on at idle & full advance settings.
A hanging throttle is confusing as if the sliders fully close when you shut off the throttle the engine rpm should fall relatively quickly back to a normal idle as no fuel is being provided by the main jets as the needle is effectively shutting off the fuel supply.
Contrary to logic I believe a hanging throttle is caused by a lack of fuel or conversely too much air supply - a weak mixture at that point as the revs should fall instead they take a while to drop from say 2,200 rpm to 1200 rpm.
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.
I'm waiting for incoming - my shields are up !
Just edited some poor spelling & sentence structure for clarity
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« Last Edit: October 02, 2024, 03:41:04 AM by McCabe-Thiele (Ted) »
Johnwebley:
This is really confusing me,
But that's easy,
I recently did some work on my 500,nothing to do with the carbs,
And fitted new carb to head rubbers as the old ones were hard,
Since then, I have had a hanging throttle,
It runs well when cold, but as it warms up ,if I blip the throttle it zooms up to about 4,000rpm,
It can be slowed by pressing the large nut in the linkage,
But why?
I haven't touched the settings,
Help and advice needed
I have the carb sync gauges connected, all read the same,
Sent from my SM-A546E using Tapatalk
McCabe-Thiele (Ted):
My suspicion would fall on what's been changed could be air being sucked in between the carbs & the engine I guess you have thought of that ?
Did you remove the alloy part of the inlets or just change the rubbers?
What large nut are you reffering to - the main throttle adjustment screw?
Johnwebley:
--- Quote from: McCabe-Thiele (Ted) on October 05, 2024, 04:46:51 PM ---My suspicion would fall on what's been changed could be air being sucked in between the carbs & the engine I guess you have thought of that ?
Did you remove the alloy part of the inlets or just change the rubbers?
What large nut are you reffering to - the main throttle adjustment screw?
--- End quote ---
I renewed the 4 rubber tubes between the carbs and manifolds,
The 4 large nuts,are In 2 pairs between carbs 1 and 2,and 3 and 4,
Part of the actuation mechanism,
Sent from my SM-A546E using Tapatalk
DomP:
It must be something to do with Herefordshire air John
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