needle shim

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tony.mon
Posts: 15943
Joined: Wed Jul 25, 2007 10:46 pm
Location: Norf Kent

Re: needle shim

Post by tony.mon »

tastethefloor wrote: Thu Jan 07, 2021 7:27 am
tony.mon wrote: Wed Jan 06, 2021 6:17 pm Floats are not adjustable, and almost never fail.
Check your fuel tap diaphragm first.
sorry why the fuel diaphrag ? can be related with a rich mixture?
Yes. It's an easy thing to check.
It's not falling off, it's an upgrade opportunity.
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tastethefloor
Posts: 107
Joined: Wed Oct 26, 2016 8:45 am
Location: Modena (Italy)

Re: needle shim

Post by tastethefloor »

tony.mon wrote: Thu Jan 07, 2021 6:24 pm
tastethefloor wrote: Thu Jan 07, 2021 7:27 am
tony.mon wrote: Wed Jan 06, 2021 6:17 pm Floats are not adjustable, and almost never fail.
Check your fuel tap diaphragm first.
sorry why the fuel diaphrag ? can be related with a rich mixture?
Yes. It's an easy thing to check.
please tell me why, honestly I don't see any connection between a rich mixture and the diaphragm
MK_WF
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Joined: Mon Aug 31, 2015 6:02 pm
Location: Wolfenbuettel/Germany
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Re: needle shim

Post by MK_WF »

That's easily explained, if you remember the functionality of a CV carburetor.

The butterfly valve is coupled to your throttle cable, i.e. a twist on the throttle openes the valve a bit.
The carb bore is still pretty much closed by the slide, so the region in front of the slide has a pressure lower than the surrounding (also referred to as vacuum). And it is coupled with the area above the slide diaphragm.
So this vacuum acts on the diaphragm and pulls up the slide against the spring force.
The slide opening equals the vacuum before/after the slide until an equilibrium position is reached.

This slide position as a reaction on engine air flow and throttle position determines if its running rich or lean as it determines how far the needle is diving into the needle jet. Changing the clip position also influences this.
The diaphragm area/state of maintenance, spring stiffness and slide holes influence that.

So if the diaphragm is damaged the slide position is way off (usually the slide doesn't get up far enough then which ends up in a much to lean condition).
Other possible issue is hardened rubber which slightly adds to the spring force which also creates a slightly to low position causing lean behavior.
Bye
Martin
jchesshyre
Posts: 590
Joined: Wed Jun 15, 2016 9:09 am
Location: Chester, Cheshire

Re: needle shim

Post by jchesshyre »

Also, petrol can get sucked directly into the inlet manifold through the vacuum hose if the fuel tap diaphragm is holed. However it's the rear cylinder that'd be rich from this since it's the rear manifold that the vacuum hose connects to.

Btw, stock setup is one shim washer under each needle. But the rear needle has a marginally richer profile. I'd expect the washers do serve a function otherwise why would Honda have put them there?
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