SOHC.co.uk Forums > CB750

throttle hangs

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neat street:
thanks again, issue got worse yesterday, so will strip carbs again and place in the untrasonic bath, then re-build. just in case of small particles in the carbs.
apreciate all comments, thank you, John

K2-K6:

--- Quote from: McCabe-Thiele (Ted) on October 05, 2024, 08:58:38 PM ---
--- 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.

--- End quote ---

While I can see the reasoning, I think this deserves expansion to understand. 

The amount of fuel added to the air going under the slide at idle us adjusted by the routine that Honda gives for setting airscrew idle mixture.  In other words it can't be wrong if its set correctly. 

It can go wrong if set badly, or IF the idle circuit, both jet and screw controlled air passage, change after its been correctly set.

In effect, you can't get too much gap under the slide as it should always be proportionately accompanied by the correct amount of fuel. This is exactly what the idle circuit is for.

I intend discussing in carb thread, when I get to this bit

That Honda routine specifically targets fuel air ratio delivered by the idle circuit against the set position of the slide. If that setting then becomes inappropriate, the mixture, for that cylinder, moves out and away form how you've set it and usually lean with the fuel jet orifice being compromised. So you do get too much air, but not from overall position of slide, but failure of delivery fuel jet to maintain flow at defined rate.

To make clear , and I'll shout this bit  :)  THERE'S NO REQUIREMENT TO SET THE AIRSCREW EQUALLY ..... THE HONDA METHOD VERY SPECIFICALLY TARGETS PARITY OF COMBUSTION AND ACCEPTS DIFFERENT SETTINGS FOM CARB  TO CARB ON THE SAME ENGINE.

If you carry out the setting routine to the manual description, it will "show" you if there's any impairment in that specific circuit by needing a setting that just doesn't fit the expected.
It does this by you making observations of the fuel air ratio as you turn the screw and listen to the combustion.

That must be one of the most under used parts of their manuals .... but so important in setup.

Everyone goes straight to carb synchronization, missing the substance and reasoning contained within this little routine that they give.

Sesman:
John. Just a small point, but when I cleaned my carbs (several times in the US bath) and using two cans of aerosol carb cleaner, I found one of the pilot jets to be blocked by a stubborn deposit. I had to resort to cleaning with a piano wire and hey presto problem solved. I’m not saying this will resolve your issue, but it’s worth paying very close attention to the jets.

Tony.B:
My experience (many years ago) was simply getting the idle screws adjusted correctly, after everything else has been checked and you're sure the throttle slides are spot-on lifting equally.
No substitute for using vacuum gauges, but a Carbtune can help, it's better than nothing. As said above, screwing the screws out the same amount is only the starting point.
Good luck, hope it's sorted soon.

neat street:
Thank you all for your valued comments, unfortunately I have been working away so unable to get back to it, but really appreciate your help.
thank you, John

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