Ironhead Keihin carb…mixture screw

[Ferrous Head]

OK. Carbs 101.

I think I told you earlier on that almost any carb can be made to work. And that's true. But there are limitations.

If you do your carb selection carefully you'll begin with ventiri size. Size is not the only determinate of flow which is what we really want to know. Dome carb designs are better than others when it comes to flow. Butterfly carbs (S&S Tilly, Linkert etc) have an obstruction in the air flow, the butterfly (valve) itself.

Other designs, round and flat slide carbs lift this valve out of the way at WOT. Albeit at the expense of "camels" through the venturi and the needle. These things were addressed in "Smoothbore" versions and Amal's RN (Renate Needle) carbs.

What that means is for a given size, say 40mm one type of carb may flow more than another at WOT.

Why are we concerned about this ?

Well, an engine needs a certain amount of air to run at it's peak performance. So the first step in determining what carb you SHOULD run is, how much air flow does my engine need to run correctly at WOT ?

You can then make a valid comparison to available carbs to see which ones will work and which ones won't.

NOTE: You don't want to just grab the biggest carb you can. Because velocity of the air through the carb is also very important. At part throttle openings the flow rate falls off and if it falls below a certain point fuel mixture doesn't happen correctly.

So, ideally what you want is the carb that flows just enough at WOT.

Well, unless your willing to sacrifice some power in the top end.

You'll notice trials type bikes have carbs that are clearly too small for the engine. But they don't give a damn about top end performance I digress.

Bigger engines (capacity) requires more air flow.. That's exactly why the S&S came into being. Drag racers were being hampered by the small venturi the Linkert offered. A bigger Linkert was needed.

So, to begin with, your Mikuni VM 38 ihas a 38 mm venturi. The S&S has a 45mm venturi.

Once you get past the initial selection you have to "jet" the carb for the entire rev range. Each "circuit" ( dile, inter, main etc) will be controlled in some fashion. Mostly different sized jets. These jets provide a "metered" amount of fuel flow at given pressures.

It's up to whoever fits whatever carb to whatever engine to determine the correct jet sizes/settings.

You CANNOT just pull a carb off the shelf, from the pile, off eBay and expect it work correctly. It's not going to.

How do you determine which jets/settings are the right ones ?

Well, that's up to you. You can:

1. Ask your mates what they are running.
2. Guess
3. "Seat of the pants" testing
4. Plug "reading".
5 Do it properly.

4 of those methods will give you a result of some sort. The 5th will get it correct. (As good as it gets)

[Ferrous Head]

Edit (of sorts)

All of the above is of course just my opinion. Others may disagree.

I base it on 40 odd years of road racing different bikes. But I could still be wrong.

[Ferrous Head]

Oh.

(After reading Jim's Post)

I use a simple "diagnostic" tool. Abnout $60 off eBay. A "fake" S*S super E. Buy a new one for your model of bike. It will come jetted for a stock engine. It will work well in most cases.

Drop it on and tun the bike with it.

Have a look at what jets are in there. If the S&S doesn't have the same numbered jets as yours change them over. (or leave the fake on there)

[JB in NC]

Originally Posted by Ferrous Head

Edit (of sorts)

All of the above is of course just my opinion. Others may disagree.

I base it on 40 odd years of road racing different bikes. But I could still be wrong.

Your point about correct venturi size is often overlooked. Case in point, I had a '76 FXE that came to me with a Super E, and it ran fine. In the course of tinkering with it over a number of years, I found the correct Keihin for it at a swap meet (for $10) and rebuilt and installed that as an experiment. The bike ran just the same with the smaller carb, because its venturi size was adequate for a 74 cu.in. motor.

Jim

[Ferrous Head]

S&S will tell you the Super B (E) will work on engines up to 90 inches.

I think they're a bit optimistic. Nut they will certainly wok on an 84 incher.

And some engines need more. And some less.

Stock IH's flow around 155 CFM (This is from my memory, so grain of salt). The ones on my race engine flow 155.
The Super B is rated around 168-170 (again. my memory).

So with stock heads a Kehein flowing 125 will be fine. The Super b is in fact too big.

To get optimums everything needs to be correct. near enough will also work, but if you want to go racing it's never good enough.

Street performance really follows race performance as well. Don't expect any old carb with any old settings to work correctly.