Needle / Main Jet Relationship

[Mr. Jiggles]

I know that the needle jet covers the throttle positions between the pilot and main jets with significant overlap for each. However, the needle jet draws fuel exclusively through the main jet (correct?). To what extent does the main influence the richness of the needle jet? One way of looking at this is that between 1/4 and 3/4 throttle the amount of fuel needed by the needle jet is always less than the maximum that the main can deliver, even if the main is relatively small (stock size for example). Does anyone know if this is indeed the case?

Any input / discussion would be most appreciated.

Thanks,

Jeff

[daveo]

Re: Needle / Main Jet Relationship

I know that the needle jet covers the throttle positions between the pilot and main jets with significant overlap for each. However, the needle jet draws fuel exclusively through the main jet (correct?).

Indeed it does

[quote]To what extent does the main influence the richness of the needle jet? Jeff{/quote]

Actually it's called the jet needle(the main in this case being the needle jet)and it depends on the diameter and taper of jet needle and the height of the needle in the jet, some needles may be adjusted for height by means of a circlip and slots on the needle, hope that helps a little

[Mr. Jiggles]

The jet needle and the needle jet are two different parts - the jet needle is what is attached to the vacuum slide while the needle jet is what the needle slides into and the main screws into.

I'm aware of the clip adjustment but regardless of that the needle jet still must depend on the main for its fuel and my question is about this dependency.

I guess what I'm looking for is the basic principle / general theory.

Thanks,

Jeff

[daveo]

The jet needle and the needle jet are two different parts - the jet needle is what is attached to the vacuum slide while the needle jet is what the needle slides into and the main screws into.

I'm aware of the clip adjustment but regardless of that the needle jet still must depend on the main for its fuel and my question is about this dependency.

I guess what I'm looking for is the basic principle / general theory.

Thanks,

Jeff

The needle jet totally relies on the main for fuel flow, it's job is to mix air and fuel emulsion

[Mr. Jiggles]

Thanks, Dave.

Here's a hypothetical. You've successfully adjusted your jet needle (via the clip position) so that 1/4 to 3/4 throttle is the perfect richness. Then you test out WOT and find that it's too lean and you increase the size of the main jet. Does this mean that you'd need to re-adjust your jet needle setting for lower throttle positions? If so then this would mean that the main jet size does impact how the needle jet operates.

The reason I'm asking this is that I"m thinking about where the fuel for the needle jet (which the jet needle modulates via position and taper) comes from, which is the main jet. But is it exclusively through the main jet? If so, is it possible that the main jet might be rate-limiting between 1/4 and3/4 throttle (maybe yes but only where the area of overlap occurs between needle jet and main jet circuits)?

Sorry - I guess I should have gotten the Sudco manual.

Jeff

BTW, that's a beautiful cutaway / saggital section of a CV carb.

[daveo]

Thanks, Dave.

Here's a hypothetical. You've successfully adjusted your jet needle (via the clip position) so that 1/4 to 3/4 throttle is the perfect richness. Then you test out WOT and find that it's too lean and you increase the size of the main jet. Does this mean that you'd need to re-adjust your jet needle setting for lower throttle positions? If so then this would mean that the main jet size does impact how the needle jet operates.

Yes you may,but, if it's not bogging or stumbling, I'd leave it, better rich than lean, some call this 'moving the flat spot around' but I've always been able to tune so there's nothing noticeable

The reason I'm asking this is that I"m thinking about where the fuel for the needle jet (which the jet needle modulates via position and taper) comes from, which is the main jet. But is it exclusively through the main jet? If so, is it possible that the main jet might be rate-limiting between 1/4 and3/4 throttle (maybe yes but only where the area of overlap occurs between needle jet and main jet circuits)?

Yes, just the main, and indeed they may be 'rate limiting', that's where you would want to adjust needle diameter to compensate

Sorry - I guess I should have gotten the Sudco manual.

Jeff

Good Reading!!

[Anonymous]

You adjust the main first at WOT, then adjust the needle, not the other way around.

[KEITH KRAUSE]

Hi.
Yes, all fuel that enters the needle circuit must first pass through the main jet.
Yes, between 1/4 and 3/4 throttle, the amount of fuel passing through the needle circuit is less than the main can provide.
At idle, fuel is regulated by the pilot jet. As the throttle is opened, SOME fuel is drawn through the main and then through the needle circuit, while some is still being drawn through the pilot jet. This is the first overlap, as the throttle continues to open the fuel will no longer be regulated by the pilot and the needle circuit will control fuel regulation. You can change the "timing" of this overlap by raising or lowering the needle, or changing the pilot jet size. With the throttle at about 1/4 to 3/4, the needle circuit controls fuel regulation, drawing fuel through the main first. As the tapered needle rises, the opening between the needle jet and jet needle increases, allowing a steady increase in fuel flow. At about 3/4 throttle, this opening equals the size of the main jet. This is the second overlap or transition. Depending on the taper of the needle and main jet size, this transition between circuits can be seamless or too abrupt to fuel flow. You can also change the timing of this overlap by raising or lowering the needle and changing the main jet size. For example, if you increase the main, the main jet will take over fuel regulation at a later throttle opening. This will change the timing of the transition. The trick is to have the correct fuel/air mixture at all rpm's. If the mixture is too rich, combustion will be slow. If the mixture is too lean, combustion is erratic, leading to higher engine speed and pre-detonation of the sparkplugs. To me, the overlap effect is the most difficulty in jetting. It needs to be as seamless as possible. As the throttle continues to open, fuel regulation will now be controlled by the main. However, it is my opinion that even at full throttle, the needle circuit still has an effect on the main. The main regulates fuel flow at this point, but the needle still presents an overall restriction to fuel passing from the main to the carb throat. A needle that is raised higher in its jet (and higher above the main) will be a little less restrictive to the fuel entering the carb throat. This will result in a richer mixture, at least temporarily. For example, my own testing experience: I did a gradual roll on from 60mph to near 130 mph with good results. No bogging, etc. Still, I decided to raise the needle 1/2 step. No other changes. Under identical conditions, I again did an exact same roll on from 60 mph. The bike bogged noticably and even as I reached full open throttle, the bike still could not burn the mixture well. I did not hold the throttle open any longer to see when it would clear itself. In this case, raising the needle had a richening effect on the main at full throttle, at least temporarily. I put the needle back and the bike pulled strong again.

[Mr. Jiggles]

Thanks for the info, guys.

Jeff