Carby: main jet size and needle height relationship

[oldskool]

I have dropped needle to the lowest setting by raising circlip to highest slot.
The main jet size is #158, air screw is at 2 1/2 turns.

My question is:
If I replace the main jet size to #155, which slot does the needle circlip go to give it a similar setting as #158/1st slot?

Or is this one of those unexplainable trial and error things?

[TripleZee Dyno]

Or is this one of those unexplainable trial and error things?

Exactly. Suck it and see.
Generally speaking the needle position and the main jet affect different areas of operation but there is some overlap. choose the best main for full throttle/top end power, adjust the needle for good part throttle roll on. But it will all be a lot of experimenting to find what combination works the best.
cheers

[oldskool]

Exactly. Suck it and see.

I was almost there. I figure on going one main size smaller (#155) and return needle clip back to 3rd hoping that should be more or less about where I left off with the #158. I could throw a dice I guess?

[F5 Dave]

Not really, you are still thinking of it more like the pilot vs air screw adjustment which act on the same area.

You totally have to think about these things as throttle position dependent. Think of the main working WFO jetting. Get that right & then worry about where/what the needle is. Esp with std needle.

Forget the revs, just think about how it runs when you crack it wide open, which on a small 2 stroke is often.

(With carb swaps & large needle changes above is all bets off).

[Viscount Montgomery]

OK, dropping the needle down you're leaning out the midrange. Why? Plugs carboned? If so, which circuit is rich, lo circuit or hi circuit? Done a few plug-chops to find out? Leaning out the main jet won't have much if any effect on a rich lo circuit, also, still running rich with the needle dropped maximum low doesn't sound kosher, sounds like float level too high or the pilot jet's too large or an air-jet is stifled. Then again the emulsion tube could be worn oval inside, doesn't take much to be worn.. All this assuming the air filter's not grunged..

[F5 Dave]

Buddy, no disrespect but you are talking like diagnosing a CV carb. That is why I said you have to think about it not in rev range, but throttle application. It is std fare for MX bikes (assuming we are tuning what is in his list) to be too rich on main from std & no one to get around to changing them despite running like a sputterbus "2 strokes are all like that etc etc"

[The Pastor]

hey f5dave, do u know what to do when you put a larger main jet in, which works well for open throttle but then the low end is too rich?

[TripleZee Dyno]

I was almost there. I figure on going one main size smaller (#155) and return needle clip back to 3rd hoping that should be more or less about where I left off with the #158. I could throw a dice I guess?

Dont get too carried away with all the techo stuff. Essentially it comes down to this,
Make a change
Is it better or worse?
If worse you are going in the wrong direction (dont need a degree in techo to figure that out)
If better change again until it doesnt make a difference or gets worse.
The biggest hassle is figuring out if its better or worse (if its not obvious)
You need to test the same way, same conditions, and have some way of quantifying improvements, eg speedo or stopwatch. As long as you test consistently you will get there.
Thats all we do when dyno tuning, the dyno just allows us to control the test conditions, otherwise I would be riding bikes up and down the road to test them.
good luck

[F5 Dave]

hey f5dave, do u know what to do when you put a larger main jet in, which works well for open throttle but then the low end is too rich?

So was it ok before? Or was likely always that way.

Possible for it to be too rich at closed, lean at half throttle & lean again on full. Different jets control the different ranges as one jet can't properly control the altering conditions. If the carb is straight pull on the slide (like a KX would have) then the jets are dependent on throttle position, float level should be set to somewhere near std & left there.

On CV carbs (on most 4 stroke roadbikes) it is another (more complicated) story altogether.

There are a heap of articles on the web.

You say 'low end', but this doesn't indicate if you are thinking in terms of throttle position.

Don't go crazy moving several jet sizes. Dyno makes it easier to test esp on bigger bikes & likely have jets can use for test rather than buy heaps.

[The Pastor]

So was it ok before? Or was likely always that way.

Possible for it to be too rich at closed, lean at half throttle & lean again on full. Different jets control the different ranges as one jet can't properly control the altering conditions. If the carb is straight pull on the slide (like a KX would have) then the jets are dependent on throttle position, float level should be set to somewhere near std & left there.

On CV carbs (on most 4 stroke roadbikes) it is another (more complicated) story altogether.

There are a heap of articles on the web.

You say 'low end', but this doesn't indicate if you are thinking in terms of throttle position.

Don't go crazy moving several jet sizes. Dyno makes it easier to test esp on bigger bikes & likely have jets can use for test rather than buy heaps.

Thanks dave, im talking about CV carbs on a road bike. (250cc)

If i get a dyno with AF done, can people look at that and make some educated guesses as what to be done?

[F5 Dave]

A decent dyno operator will have some real good clues. Before you start though make sure you are working from a good baseline. New plugs & airfilter.
Check carb balance ports sealed.

Strip & clean carbs, blow through all jets, take out where possible esp Pilot jets. check with torch, a blockage here will be a drama at low revs, don't poke wire through them.

Check float level which can have quite an effect at these revs (note no longer talking directly in throttle position on CV carbs). Could give them a minor tweak so the tag pushes the float more (ie float level lower when in use = higher level when upside down). Check emulsion tubes for ovality, a small amount will cause lots of drama. Search around excellent but disorganised Factorypro.com website.