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Discussion Starter · #1 · (Edited)
Problem Fixed!
See thread for details...
Please excuse what may be a novice question, but I have scoured the interwebs, manuals and jj and just seem to find the answer I'm looking for.

I'm used to slide carburetors which allow me to jet and adjust for the low, mid and main circuits but without a needle the simplicity is confusing to my problem.
The bike: 81 ironhead
carb: stock keihin butterfly w/andrews accelerator pump.
pipes: xlr radius cycle shack straight pipes

I bumped up the pilot from 68 to 72 and the main that was in the carb is a 175.
Anyway, when rolling on the throttle everything seems kosher enough albeit maybe a bit lean in the middle. When you lay it on anymore than a slow roll it starts to break up in the middle before it's all main. Haven't tested the main as the motor is fresh as fresh so I'm just concerned about getting the middle straight. On a slide carb I adjust the needle position to fatten it up and hit it.

Anyone want to clue me in on how to adjust the mid-range on these or should I just slap an even fatter main into it and call it a day?
 

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I think any more than three turns out and you should bump it up.
I run that carb on an 84" with an 90 intermediate, and I'm more turns out than I'd like but I don't know if they make a bigger jet.

I did the choke plate removal mod, some tricks with the accelerator pump too...speaking of which, is yours working?
 

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Discussion Starter · #4 ·
The accelerator pump is working well and I aimed it at the y in the throat. I haven't flushed the pump arm with the bottom of the bowl as I fear it won't push as much into the engine. As far as the screw it's at 3 turns which in my understanding is pushing too rich. If it was <1/2 turn I understand it's too small.

I'll probably flush the rod but I still don't understand what is driving the mid-range. It seems that on the throw of the throttle, it's breaking up when the accelerator pump is all used up.
 

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Can you muscle your way through the breaking up? If so, does it clear up once it's on the main jet circuit?

Unless I'm mistaken, adjusting that screw out is letting more fuel in. I actually have a tool-free knob on there so I can adjust while running.

Anyway, try dialing that thing out even more; granted it's not how you want to run it, but if the bike runs better it tells you which direction you need to go.

What do your plugs look like?
 

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If you've ever seen a dyno chart on straight pipes - what you're describing is the grand-canyon sized power drop at about 2700 RPM. It's a huge hole - then the motor runs like a bat outta hell afterwards.

A set of baffles, torque cones or the washer trick will reduce the hole - but won't get rid of it. The only way I was able to get rid of it on mine was to go back to stock pipes with the crossover.

And...the Keihin is dialed in with the same jets as you - and 1-1/2 turns out on stock pipes. No hole in acceleration.
 

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Discussion Starter · #8 ·
A power drop would be a flat spot in acceleration (which I understand should be there with drag pipes). A studder or "sputter" on acceleration would be a lack of fuel which is where I am at.
I do have the next size up pilot which I can throw at it but I feel like a hack if I can't exactly tell you why I did it; which is why I'm asking how the mid-throttle is supplied. I haven't tried muscling past it; I'm a bit of a limp wrist on rebuilt motors with 15 miles on them.

I think your on to something doc if the middle circuit is supplied by both the pilot and main jets. Honestly a 175 is awfully fat on stock and I could get a bigger one, but I'm thinking I'll get better or more consistent results out of the pilot for now. The plugs look a little dark but the rings are seating so I won't hold them on accuracy.

I also noticed their are 3 spring positions on the accelerator pump actuator. Mine was set to have the strongest amount of tension (force) on the accelerator rod but I can go up to 2 steps lower in tension. My thinking on this is to actually limit the amount of force the pump can project the fuel into the manifold with as oppose to having it take longer to completely empty the circuit and consequently raise the point in the throttle when the pump is used up. (aka It used to shoot it's "load" by 1/4 throttle but with lower spring tension it now takes until 1/2 throttle to shoot it's "load")

Sorry for a lot of mumbo jumbo; you can see I just want to fully understand this type of carb.
 

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Discussion Starter · #10 ·
Thanks doc; that's the best version of that article I have seen and the pictures are great. I've also read a couple of old ez rider mags that have shown some of the same info plus flushing the accelerator pump rod to the bottom of the float bowl and plugging the pin hole in the bottom of the float bowl that allows a small percentage of fuel to return to the bowl instead of getting shot through the accelerator pump jet. I will do that tonight.

I'm still feeling like somebody in this huge pool of talent may have a some more insight on how these crazy simple carbs work.
 

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Discussion Starter · #11 ·
So I did all the rest of the mods I have read about and even figured out why the carb was leaking sometimes, while other times it was not. Started it and still didn't fix the problem. I was starting to sweat about cam timing and intake leaks etc.

Here is what I found:
carb leak: jp carb kit came w/2 float needles. I had tried both the spring loaded on non-spring loaded ones and both worked but only sometimes....

I checked one against the oem harley needle and noticed it was cast from a different style body. I checked the other and same thing. When I placed the needle into the seat, the 3 sided aftermarket needle had a lot of side-to-side slop. The 4 sided oem needle did not. Problem is the aftermarket ones with "slop" are not consistenly matching the rubber tip into the oriface which is causing the carb to overflow.

As far as the missing, consider it like this. On a slide carb you adjust the needle to have more or less fuel in mid-range. On the butterfly you do not.

While I was watching the bike from a squat checking for intake leaks I saw a small lick of flames come out the tips of the exhaust after a couple of seconds of breaking up.

It occured to me that in one article I read on these the gent suggested adding a screw to the already existing hole in the accelerator pump pulley. At the time he never explained why I should do this other than control of the pump so I just thought more is better and this thing came like this so whatever.

After witnessing that, I decided to fire it up again and stuck a really fine flat screw driver between the pulley stop and the pulley to stop it at maybe just shy of 1/4" of opening completely.

Bang; that mofo reved solid and clean! It's times like this I feel like I could win any street fight or get voted in for president. Now I just have to get a small screw and adjust it and that shit is done.
 

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Discussion Starter · #14 ·
Fuck yes I was! I ran from the garage fist pumping like a motherfucker and wanted to tell somebody. Realizing my wife wouldn't care I jumped on here to close this thread in case anybody else ends up searching here with the same problems.

Doc I gotta say that is a bit too scary to think about.
 
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