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Ironhead Sportster Motorcycle Talk (1957-1985) For all those that wanna talk about Ironhead Sportster Motorcycles

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  #11  
Old 22nd June 2022
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Quote:
Originally Posted by needspeed View Post
^^
Actually Ferrous is right. If the tolerance is plus 1/64 and minus 1/64, that's a range of about .030.
Yep. That's right.

I wasn't thinking....
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  #12  
Old 22nd June 2022
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The adjustable float height system Doc has developed would give you a dynamic method for determining the float height as opposed to the static setting with a 30 thou tolerance.

Like adjusting your low speed jet while the engine is running instead of just 11 -1/2 turns out (Yeah I forget what it really is).

But how does the correct height in the float bowl affect how much fuel is drawn through the idle or intermediate circuits ?

The transition period between circuits on a carburetor are always the difficult part.

Ideally they transition would be seamless. As the next circuit started to supply fuel the first circuit would reduce it's supply.But with fixed size jets of course that not really possible. It's here that FI wins hands down.

We use tricks like air bleeds to help with this but it too is done by a fixed jet size and one size does not fit all.

What I'm tiring to understand is how the height of the fuel in the float bowl affects the transition from idle to inter on these carbs.

Because the FSM doesn't really show how fuel passes through the various circuits the intermediate circuit is a black box to me.

I'm thinking the idle passage tube along with the vent tube act as an air bleed. But I don't know, I'm only guessing at this point.

The only thing I can think of is that the height of the fuel in the idle tube affects the strength of the signal required to draw that fuel up. I just can't see how, that's all.
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  #13  
Old 26th June 2022
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This is the before and after inlet valve and seat assembly of the S&S MGAL moded for methanol that I implemented on the KHK after talks with our good doctor and done either prior or after our visit to the Salt Flats:



So, not only adjustable, but twice the diameter of the std one, so 4 times the area...

Gene, on the DCs or S&S carbs, the capillarity effect actually draws gas or fuel higher in the iddle tube than the real level in the float chamber...
So I would say that the level is not as critical to the iddle as it is for main jet running.
However, set up of iddle screw and small opening of the butterfly is essential to allow a smooth transition from intermediate running (the small 3 or 4 holes immediately in front of iddle passage in the carb throat) to main jet running.
If float level is not right, all sorts of bad behaviour will ensue!!!

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  #14  
Old 27th June 2022
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Quote:
(the small 3 or 4 holes immediately in front of iddle passage in the carb throat)
I will have to go and have another look. I'm only aware of one hole in the butterfly block connected with the idle passage tube.

I know the Tilly's have 3 holes there and they need to be sized correctly or starting and idle will be "problematic".

I haven't run a DC as a race carb, On the Super B's I do run I use a main jet in the air bleed opening to help adjust the transition.

I know the L Series is based on the DC but is it just a bigger version ? Or are there a lot more differences regarding internal passages ?

Part of what got me wondering about this is that my engines run fine after I turn off the fuel supply (or run out). I turn off the fuel and allow the bowl to empty rather than turn the engine off. So there's no fuel sitting in the bowl while the bike is resting. I've been "caught" with those little waxy balls in the past.
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