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Sportster Motorcycle Air intake, Carburetor, EFI, Fuel, and Exhaust Problems, advice and/or how tos for Sporster and Buell motorcycle carburators, Electronic Fuel Injection (EFI), Air Intake, Fuel and Exhaust.

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  #31  
Old 4 Weeks Ago
John Harper John Harper is online now
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I really can't say what size your jets are, since the real orifice for the jet is actually further inside than the hole you can see in the photo. Can you post up a "head on" shot to compare actual orifice sizes? Maybe a picture taken from the threaded side of the jets?

Also, having the straight screw slot might lead to some burring on the inside of the opening of the jet in your picture. If the jet has some kind of special "contouring" I would think a burr would potentially interrupt any special flow characteristics. I have also heard roughly the same thing about the Keihin jets as IXL2Relax, but no evidence to verify the veracity of this "urban legend" about Keihin jets versus aftermarket. Take a close look at the Keihin 42 jet, look at the irregular opening it has. Even worse than the knockoff 45.

John
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  #32  
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Quote:
Originally Posted by IXL2Relax View Post
I presume, because I don't know exactly, that it means the physical shape of the orifice - which may very well involve chamfering...

I don't have any technical data - simply that 'I've heard' they are specifically contoured...
They are "contoured" in that the length of the orifice (hole) is regulated.

Keihin jets sizes are based on their orifice diameter. A 42 pilot jet has a .42 mm hole.

Mikuni jet sizes are flow ratings and the orifices are tapered.

FWIW the jets for the Holley 4 bbl carbs we ran were also flow rated. Flow on those being controlled by the diameter and length of the orifice both of which varied.

These are precisely reamed and gauged pieces. Run pieces of steel wire thru them to "clean" them (or check their size) and damage those carefully reamed orifices and you will change how they flow. After you judiciously "clean" one with the tip cleaner for your oxy-acetylene torch you might as well buy a new jet...
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Pull the carb apart, clean it really good with carb cleaner and use compressed air to blow out the ports. If that doesn't fix the problem at least you can cross it off your list of things to check and it's easy enough to do.

D
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  #34  
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news...rode today
#42 is good tunable, but I have to turn it out 3 full turns for good satisfying idle, and good throttle response - no coughing, runs good, stock OEM 45 will be perhaps the right jet, but now we´re happy with the result. Will see how it is when weather is warmer.

There is a taper part on the stock jet, which closes from the threads to the hole, this taper part is different on the 45 jet, will try to show you later in a pic.
It could be that this tapered area is not sealing the thread and fuel can pass thru + thru the jet hole - too rich

thx for all your comments and help
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20200609_183058 (2) by DéDé1963, auf Flickr
20200609_182628 (2) by DéDé1963, auf Flickr
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now this taper area...
20200609_182401 (2) by DéDé1963, auf Flickr


you shurely see the OEM one
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OK, so with the Keihin that mix screw is a fuel screw, so getting three turns out sounds about right for a stock 42. You needed to supply more fuel to get the idle mix right.

So, how is the transfer from idle to just off idle? Is there a stumble. Slow hand on the throttle and feel for it. If there is a stumble, then you may need a bit more to near 3.25 turns. Possibly the stumble will not tune out. You find that out test riding.

Basically you just proved that you do need a good 45 to tune it to where the idle will be in the two turn area. As long as that 45 doesn't make the transition pig rich.

So, to be more precise, the idle is handled with the screw, so within reason you can tune the idle good within a range of two or three pilot jets.

Where you will feel it and see the difference on plugs is in the transfer from idle to where the needle comes into play. It's a small part of the curve, so this is where people can easily be off in tuning and never know.

You are on the right track so at least you will not be one of them.
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no stumble from idle to off idle, runs strong, this bike is completely stock. The only added part is an K&N filter insert in oem ham can.
My friend ordered already an oem ''45 jet for spare.
But for now he´s happy how it is
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Quote:
Originally Posted by DéDé View Post
no stumble from idle to off idle, runs strong, this bike is completely stock. The only added part is an K&N filter insert in oem ham can.
My friend ordered already an oem ''45 jet for spare.
But for now he´s happy how it is
Then it's fine. The only issue that three turns out creates is the possibility that with less tension on the spring it can back out as you ride. I had mine at around 3 turns for a long time before I went to the 45, and it never fell out, or moved out of adjustment so you will most likely be just fine on that front too.
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Quote:
Originally Posted by wedge View Post
Then it's fine. The only issue that three turns out creates is the possibility that with less tension on the spring it can back out as you ride. I had mine at around 3 turns for a long time before I went to the 45, and it never fell out, or moved out of adjustment so you will most likely be just fine on that front too.
this is true, less tension with 3 full turns, I know
thx
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