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  #91  
Old 18th February 2022
Yamalol Yamalol is offline
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Quote:
Originally Posted by Ferrous Head View Post
You seriously need to research carb tuning theory if that's going to be your approach.

You cannot "smell" a rich or lean condition for the main jet.

The main jet does not come into play until about 3,000 rpm.

Guessing and just "leaning out" the main jet could well leave you with a holed piston.
What would be your suggestion then? I can't smell it running rich at idle, but it smells rich to me when I roll the throttle all the way open when the bike is stationary. Obviously I can't "smell" it running rich when I'm riding down the road, but it clearly doesn't like that shot from the pump, which is entering the combustion chamber either right before, or as the main jet is coming in to play. Sounds like a rich condition to me, but always happy to learn.

Last edited by Yamalol; 18th February 2022 at 22:44..
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  #92  
Old 18th February 2022
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I pulled this directly from the Bendix tuning instructions:

With the idle speed and mixture adjusted, high speed mixture can be adjusted by running engine up thru first or second gear to 4500 rpm or above. Overly rich mixtures respond by breaking up or running roughly at high rpm. To lean out the main jet, turn the main jet adjustment needle approximately ½ turn at a time until the engine responds by pulling cleanly thru high RPM ranges in first or second gear.
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  #93  
Old 18th February 2022
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OK. What they are talking about here is putting the bike under load above 4,500 rpm.

This is NOT the same as blipping or holding the throttle open while the bike is stationary.

The correct sequence for tuning the carb is, main jet, intermediate circuits, idle circuits.

This can be done 3 different ways.

The first is a "plug chop" which almost no one does correctly. It's time consuming and requires a suitable place to perform it.

Results are better than guessing.

The second way is to just "guess". Which is what far too many people do.

My 68 CH uses a 68 main so your should too. (not really - just joking) And it "feels" good when it is running.
Results MIGHT be perfect but that's unlikely. More likely is the result will be acceptable to most (some) people. You will make less power and use more fuel

The third way is to actually use tools to measure what you are doing and then adjust the carb to it's optimum settings.

Many consider this last approach to be akin to using torque wrenches or calipers on your engine. A very specialized approach not needed by Ironhead owners.

We can guess better than we can measure, right ?
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  #94  
Old 19th February 2022
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I have had good luck tuning following the S&S method, which is reverse of what FH suggested. Without an AFR gauge it is a good approach. The details may differ with a Bendix, but not the theory. I do realize Bendix main and intermediate are related, so there will be some back and forth using this method. Start with stock jets and tune from there.

1- Verify carburetor is set to stock settings:
a- Idle Mixture Screw, 11/4 turns from lightly seated.
b- Idle Speed Screw, ½ turn clockwise from engagement point.
c- Accelerator Adjustment Screw, two turns counter-clockwise from seated.

2- Start bike, bring to operating temperature.

3- Set Idle Speed Adjusting Screw, clockwise to increase rpm, counter-clockwise to decrease rpm. Idle rpm range should be 950 to 1050 rpms.

4- Adjust idle mixture by turning Idle Mixture Screw slowly clockwise until the engine runs poorly. Slowly turn the screw counter-clockwise until it runs smoothly—if you go too far the engine will start to stumble.

5- With engine idling, turn Accelerator Adjustment Screw clockwise until it lightly seats. Snap throttle open—engine should stumble. Turn screw counter-clockwise ¼ or ½ of a turn at a time, until engine responds to throttle twist with smooth, quick response.

6- Ride motorcycle in various rpm ranges, and then try to maintain a consistent 40- to 50-mph. If the engine is popping or sneezing in the air cleaner it can indicate a lean condition. If you notice stumbling or sputtering it can indicate a rich condition.

7- Low rpm operation is controlled by the intermediate jet (#11) if coughing or popping is experienced below approximately 3000 rpm’s the intermediate jet must be replaced with a larger sized (richer) jet. If the engine is sluggish or does not want to idle smoothly a smaller (leaner) intermediate jet needs to be installed. Always readjust the idle mixture (#2) and idle speed (#1) screws after making a jet change.

8- To test the Main Jet, do a roll-on from 50-mph to 70-mph in 3rd gear. If the engine back fires or breaks up in the carb, increase the Main Jet size .004”. If the engine is flat or will not accelerate, decrease the Main Jet by .004”. [If the bike likes 7/8 throttle better than full, the main is too small]

9- Since 2004 S&S Shorty carbs have been fitted with an adjustable main jet air bleed (#13). Changing this jet to one of a larger size will delay the signal to the main jet, therefore aiding with high speed tuning as described in step 8. This also aides with tuning an engine that is fitted with an exhaust system that is not intended for high performance.

10- NOTE: Drag or straight pipes can prevent you from obtaining optimum carburetor performance.

11- If the S&S teardrop air cleaner is being replaced with an aftermarket air cleaner be sure to remove the auxiliary bowl vent screw (#14). Never use a velocity stack on a street driven motorcycle! Poor throttle response will be experienced.

12- Always be sure to attach the hose to the overflow tube fitting (#6) and route it towards the back of the engine.
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  #95  
Old 19th February 2022
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The problem is that there is a fairly large range between "too lean" and "too rich".

The bike can run "fine" when just a bit too rich or just a bit too lean.

Yes, it will stumble if your too far off in either direction. But it's not going to br at the "optimum" just by trying to stay between the two extremes.

People have no problems whatsoever buying tools to time their engine to within one degree of optimum.

But ask them to do the same for their carburetion and no, they would rather guess at it.

Somehow, "near enough is good enough" at this point.

You won't guess at crankshaft end float. You won't guess at main-shaft end float. You won't guess at the finished bore size. You don't leave tappet adjustment to chance.

People strive to make their engines mechanically as near to perfect as they can,

Then leave one of the two most important parts o f the puzzle to a "near enough is good enough" approach.

I just don't understand it.
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  #96  
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Well, at least as far as I know, if I wanted to use an AFR gauge on my bike, the only way to do that would be to weld a bung on to one of the pipes and install an Wideband o2 sensor. I totally understand your point about accuracy, but I can't imagine too many folks are doing that.
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  #97  
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The guys who run dyno's will have a sensor that is inserted into the tailpipe. No bung required.
I do have a bung on my race bike, just behind the collector.

But you can just run a bung on one cylinder (exhaust pipe) as we run both cylinders from a common carb.
So, if you use a spare header pipe to do the tuning you can also use it for other bikes later on.

Once you have it dialed in, the carb settings won't change. The engine wants what it wants and you will know that.
When I say they won't change, it means "not much". I might allow for altitude changes and air pressure changes when I'm racing, but mostly it's not necessary to change anything.

I've been playing with carbs on race bikes now for half a century. 50 years.

I can't tune an engine by ear or by the seat of my pants.

I can get close, but that's just close.

I learned a few years back that there is more power to be had and a sweeter running engine by doing it right.

Again, this is a one time procedure. Unless you change something on/in the engine you don't need to keep repeating it.
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