Topic: can somebody explain "hunting"?

[grumpy]

I don't get it.

If the carbs are out of adjustment shouldn't it just run rich or lean?
Why do they hunt/surge?
It seems like it should still run at a constant speed for a given throttle position - just with the wrong mixture... 

[dancaudill]

im with you  on this one...ive never got a straight answer to that question

[Hush]

I always understood hunting to be when the carbs were so far out of whack that one might be lean and another rich, all this of course cured by carb synchronising.

[TwoTired]

If carb 1 is set for a 1000 RPM idle, carb two is set for 1500 RPM, ,Carb 3 set for 1250, and carb 4 set for 1300, what rpm will it idle at?

Answer: they fight for dominance.  When they get what they want, others try harder till they get what they want.  They trade off for who is dominating and when.
There is only one crankshaft.  So it hunts, or is at least, erratic.


Want more technicality?

The carbs draw fuel from the jets based on throat vacuum.  If one slide is open more than the other, it has less vacuum, more air and less fuel.  Lean tends to create more heat and a bit more power, raising the RPM.  More RPM increases the carb throat vacuum, drawing more fuel and only a little more air, as the carb slide hasn't moved.  Therefore, the mixture gets richer, the cylinder runs a little cooler and less power is created, slowing the crankshaft.  The cycle repeats.  When other carbs are added and involved, the imbalance is magnified. Slides that are open more than others, run leaner (wanting the crank to go faster), whereas the carbs with lower slides run richer (wanting to go slower).  The battle for dominance goes on, unless they work together as a team, and that is what synchronizing accomplishes.

But, I could be wrong...

[MJL]

If carb 1 is set for a 1000 RPM idle, carb two is set for 1500 RPM, ,Carb 3 set for 1250, and carb 4 set for 1300, what rpm will it idle at?

Answer: they fight for dominance.  When they get what they want, others try harder till they get what they want.  They trade off for who is dominating and when.
There is only one crankshaft.  So it hunts, or is at least, erratic.


Want more technicality?

The carbs draw fuel from the jets based on throat vacuum.  If one slide is open more than the other, it has less vacuum, more air and less fuel.  Lean tends to create more heat and a bit more power, raising the RPM.  More RPM increases the carb throat vacuum, drawing more fuel and only a little more air, as the carb slide hasn't moved.  Therefore, the mixture gets richer, the cylinder runs a little cooler and less power is created, slowing the crankshaft.  The cycle repeats.  When other carbs are added and involved, the imbalance is magnified. Slides that are open more than others, run leaner (wanting the crank to go faster), whereas the carbs with lower slides run richer (wanting to go slower).  The battle for dominance goes on, unless they work together as a team, and that is what synchronizing accomplishes.

But, I could be wrong...

This would be my answer as well. My 650 did that, back when I didn't know what the heck I was doing with carbs. It doesn't happen at higher speeds, but at or near idle it's very annoying.

[JLeather]

An air leak can also cause hunting.  At idle, the air leak is sufficient to lean the mixture out a lot.  As the engine leans it revs up, and now the air leak is smaller in relation to the air entering the carb and it goes back to near correct mixture and idles back down.  Rinse and repeat as nauseum at a stoplight.

[Bob Wessner]

What about an intermittent elec. problem along the ignition path? Just a thought.

[jtb]

If carb 1 is set for a 1000 RPM idle, carb two is set for 1500 RPM, ,Carb 3 set for 1250, and carb 4 set for 1300, what rpm will it idle at?

Answer: they fight for dominance.  When they get what they want, others try harder till they get what they want.  They trade off for who is dominating and when.
There is only one crankshaft.  So it hunts, or is at least, erratic.


Want more technicality?

The carbs draw fuel from the jets based on throat vacuum.  If one slide is open more than the other, it has less vacuum, more air and less fuel.  Lean tends to create more heat and a bit more power, raising the RPM.  More RPM increases the carb throat vacuum, drawing more fuel and only a little more air, as the carb slide hasn't moved.  Therefore, the mixture gets richer, the cylinder runs a little cooler and less power is created, slowing the crankshaft.  The cycle repeats.  When other carbs are added and involved, the imbalance is magnified. Slides that are open more than others, run leaner (wanting the crank to go faster), whereas the carbs with lower slides run richer (wanting to go slower).  The battle for dominance goes on, unless they work together as a team, and that is what synchronizing accomplishes.

But, I could be wrong...

Great analysis, TT.

[BobbyR]

Now that you know the thoery of why this may be happening:

Check for and eliminate all air leaks.
Tune up the engine including plugs and timing.
Synchronize your carbs.

Ride on

[grumpy]

OK I get how being out of sync can cause this.

But in the tech manual it says hunting can be caused by improper needle settings. That's what I really don't get. I can see how having one needle out of whack would do it - same as being out of sync - but a suggested fix is to raise/lower all needles.
 What's up with that?
If all needles are too high then all are equally rich - if all too low then all are equally lean. No?
Assuming no worn needles / clogged jets...