Jetting CV Carbs

[glider.]

Jetting a CV carb is easy, but the jetting only accomplishes part of the job. You must raise the needle in it's holder to bring in the fuel circuits earlier for added performance.
When adding two #4 washers to the needle in a CV carb, slide them up the needle from the bottom and reinsert the needle in the holder (slide). This will bring in the fuel circuits a bit sooner because it raises the needle up from it's original position on the tapered end and allows the fuel circuits to richen the mixture earlier in the transition as the slide lifts.


Slide Detail

1. Slide spring

2. Spring seat

3. Jet needle

Spacers (washers) go under Clip on needle ( #3 )
Factory needles will have a fixed button on the top and not a clip.

 

[MadddHatter]

Didn't see any forums on fuel injection only on carbed engins. My question is is anyone familiar with Cobra FI2000R fuel tuned. After installing a K&n Air Intake and V&H straight shot pipes on a 2003 Fat Boy Anniversary Edition, would this particular tuner be suitable and would anyone have any pot settings for the best results

Thanks

 

[joel]

You may try contacting the manufacturer of that product to let them know the type of modifications, exhaust and the air cleaner kit you have on it to see if there is a bass line settings to start with.

 

[tourbox]

You can contact Dobeck Technologies also. They use to be a sponsor here and gave a discount to members or if you were a Veteran. I used one of their products on my 2012 Ultra and was pretty easy to tune. They will also give base line setting & easy to work with.
Their web is http://www.dobeckperformance.com/
tourbox

 

[French Peasant]

Jetting a CV carb is easy, but the jetting only accomplishes part of the job. You must raise the needle in it's holder to bring in the fuel circuits earlier for added performance.
When adding two #4 washers to the needle in a CV carb, slide them up the needle from the bottom and reinsert the needle in the holder (slide). This will bring in the fuel circuits a bit sooner because it raises the needle up from it's original position on the tapered end and allows the fuel circuits to richen the mixture earlier in the transition as the slide lifts.


Slide Detail

1. Slide spring

2. Spring seat

3. Jet needle

Spacers (washers) go under Clip on needle ( #3 )
Factory needles will have a fixed button on the top and not a clip.

Does it mean the main jet never closes completely?

 

[dolt]

Does it mean the main jet never closes completely?

The attached diagram of the tuning elements of the CV carb and how the come into play. This should answer the question.

 

[French Peasant]

According to this chart, the needle effect kicks in at ¼ to full out, due to taper.

Lowering the clip position should let the needle feed more fuel into the carb right from ¼ at least, and then the main jet would take over just as much sooner (than ¾). Giving you more acceleration sooner over ¼ to ¾ band (+/= clip position).

Which means fuel bleeds thru the main jet into the venturi in effect from ¼ to WOT, regulated by the tapered needle.

Or am I wrong?

I don't have a clip on my needle, it is closed when down (below ¼ throttle).

I know it for a fact because when I open the throttle ⅛ with mixture screw seated the engine stalls.

Do you know for a fact that a clipped needle doesn't let fuel sip below ¼ (at idle?) or are you just reading a diagram ?

Can someone please provide explanation on the "needle diameter curb" in this diagram.

Thanks for your inputs

 

[dolt]

According to this chart, the needle effect kicks in at ¼ to full out, due to taper.

Lowering the clip position should let the needle feed more fuel into the carb right from ¼ at least, and then the main jet would take over just as much sooner (than ¾). Giving you more acceleration sooner over ¼ to ¾ band (+/= clip position).

Which means fuel bleeds thru the main jet into the venturi in effect from ¼ to WOT, regulated by the tapered needle.

Or am I wrong?

I don't have a clip on my needle, it is closed when down (below ¼ throttle).

I know it for a fact because when I open the throttle ⅛ with mixture screw seated the engine stalls.

Do you know for a fact that a clipped needle doesn't let fuel sip below ¼ (at idle?) or are you just reading a diagram ?

Can someone please provide explanation on the "needle diameter curb" in this diagram.

Thanks for your inputs

I think you have a basic understanding of the CV fuel flow based on the provided chart. I will answer the question about fuel flow below 1/4 throttle with a question for you. If fuel was getting pas the main below 1/4 throttle, how could you control idle rpms with the A/F adjustment screw?

With regard to the "needle diameter curb", the peak of that curve is at a partial throttle position where the needle is moving out of the bore of the main jet to full throttle where the needle is at the highest point and no longer in play so the curve falls off.

You have to look at this curve in a dynamic situation, it 's not a snapshot of any particular throttle position but does show the function of the individual parts of the carburetor from idle to full throttle and can be used as a tuning aid, particularly when tuning on a dyno.