What is it?
 

The adjustment knob on top of the carb next to the intake manifold. It rotates 180 degrees... is it an external main jet adjustment?
I got this on EBay with several other items in a 'lot' auction. I don't need it but I'd like to know more about it for a posible re-sale.
The only numbers on it are 27021-71A (on the manifold) and B83B-QC15 (which is a total mystery) above the float bowl on the choke cable side.
I was also wondering... in what year did the single throttle cable get replaced with the double throttle cable?
And did any of the Ironhead Sportsters come with a 42mm carb?

http://i296.photobucket.com/albums/m...dCarbAC004.jpg

http://i296.photobucket.com/albums/m...dCarbAC002.jpg

http://i296.photobucket.com/albums/m...dCarbAC013.jpg

B83B is a Keihin butterfly carb from a 1979 or 1980 Sportster. The plastic-looking thing is attached to the pilot screw. I have never seen one before - i expect it is a home-made apparatus to make adjustments possible without using a screwdriver.

The dual throttle cable started in 1981.

You have to measure inside the venturi at the most narrow point. It is a little tricky to do because of the tight spring on the throttle plate. I cut out a piece of cereal box or kleenex box cardboard 34mm wide and another 38mm wide; insert and see which fits best.

The Keihin butterfly carbs on late model IronHeads 7? to 1985 were all either 34mm or 38mm. I am not familiar with the other brands, but i would expect 42mm to be much too big for an IronHead Sportster.

That is a very clean, good looking carb.

It is a mixture limiter, and certainly not home made.
I have seen numerous.

Standard butterfly Keihin (-77 ->) with AMF-Harley-Davidson "brand" before that (74->) with Keihin "brand" . That what I have seen... not sure about year when "brand" was changed.

Thanks guys, I really appreciate the info. From what I understand so far... the external mixture limiter is for 'fine tuning' the idle mixture for a smooth idle... I think.

Yes, you adjust until the idle is smooth, but it is for more than just a smooth idle as it sets the correct mixture of air and fuel at idle. There is another screw on the carb for setting the idle speed, usually to about 1000 RPM.

Anyway, here are my notes on the process for setting the pilot screw and determining what is the correct slow jet size ...

Setting The Pilot Screw on Your IronHead

If the carb is old and dirty the pilot screw passage may be gummed up such that you will not be able to "gently seat" the pilot screw reliably. If this is the case remove the carb from the bike and clean it up. Some guys try to do carb work with the carb in the bike. IMO this is a very bad idea.

In the pilot screw passage there should be, in this sequence: screw, spring, washer, o-ring. [Occasionally POs have installed these parts in the wrong sequence; remember that the purpose of the washer is to protect the o-ring from being damaged by the spring]. The screw usually comes out easily. The other parts may require some work. The best technique is to stick a pipe cleaner in the hole, twist it around, and, like magic, out come the other parts on the end of the pipe cleaner.

EDIT: Some carbs, noteably 1966 to 1978 Sportster carbs, do not have the o-ring and washer in the pilot screw passage.

1. You need to have easy access to the pilot screw, easy enough to reliably judge "screw it in until gently seated". Loosen the front fuel tank mount bolt; remove the rear fuel tank mount bolt; prop the rear of the fuel tank up on a piece of 2X4; on some bikes this will not be necessary.

2. With the engine cold [so you do not burn your fingers] turn the pilot screw in clockwise until it is gently seated. Count the number of 1/4 turns as you do it; write the number down. Back it out to the original setting. You may need to return reliably to this setting after experimenting.

The "normal" starting point for this process is 1,1/4 [according to the 79 - 85 FM] or 1,1/2 [according to usual practice] turns out.

EDIT: An old HotXL magazine article recommends for Keihin butterfly carbs between 1/4 and 1,1/4 turns out. My experience is that this works best. If you are more than 1,1/4 turns out your pilot jet is too small.

3. The engine must be at full warm up. It will have very hot parts; to avoid burned fingers have a well lighted, comfy place to work.

4. Set the engine idling at about 1000 RPM. You want it to be idling at the slowest speed that is consistent with a smooth idle so that you can hear or feel slight changes.

5. Turn the pilot screw in clockwise until the engine idle becomes worse; tending to stall. Count the number of 1/4 turns as you do this. Then turn the pilot screw out counter clockwise until the engine idle gets good, then becomes worse, tending to stall. Count the number of 1/4 turns as you do this.

6. The best setting for your bike will be somewhere between these two settings. The FM says to use the leanest setting [most screwed in] consistent with a good idle quality. Some guys say to go between the two settings.

It should be between 1/2 and 1,1/2 turns out from gently seated. If it is not within this range you should change the slow jet.

EDIT: I follow the advice from the old HotXL mag article - set it between 1/2 and 1,1/4 turns out.

7. You may have to adjust the idle speed.

8. Make sure that the spark plugs are clean, then ride the bike. I usually go for a half hour or so ride outside the city, then ride home with the last 10 minutes at city riding speeds [so that it is on the "idle port", not the "idle transfer ports", the "mid range port", or the "main jet" [see carb diagrams in FM]]. Hopefully the plugs will come out a nice medium gray or tan color. If they are too dark you can screw it in another 1/4 turn; too light screw it out 1/4 turn, and try the ride again.

9. If they are really light or really dark the problem is not with the pilot screw setting. For example, too light might mean an intake or exhaust leak, and too dark might mean the pilot jet is too large [among other possibilities].

10. My experience with this process is that after making a change i have to clean the plugs and go for a good ride [say, an hour or so] before i can trust that the new results are reliable.