Tuesday, 30 November, 2021

Harley CV Carb Tuning Guide


harley-cv-carb-tuning

Harley CV Carb tuning isn’t voodoo, and it’s not rocket science either. If your motorcycle suffers from hesitation and performance issues, bad gas mileage, poor idle, or other symptoms of a poorly performing carburetor, you’re in the right place.

Harley CV Carb Tuning – Step 1: Cleaning

The first step to any good Harley CV Carb tuning procedure is a good carb cleaning beforehand. If you haven’t done so already, you’ll want to go ahead and get that taken care of.

Note:

If your current set has some miles on them, be sure to install a fresh set of spark plugs prior to tuning.

Harley CV Carb Tuning – Step 2: Vacuum Leak Elimination

The most common cause of hesitation and performance issues following a fresh rebuild or new carburetor installation is vacuum leaks. Make sure the vacuum lines (VOES, petcock, etc) are all hooked up, and in good shape. The seal between the CV carb and manifold is critical, and easy to get wrong. A small leak here will have a big impact on performance. Vacuum leaks will throw off jetting (requiring larger jets than necessary), and directly affect the mixture screw setting.

Before installing the carb, make sure the seal is installed fully on the intake manifold and apply a light coating of silicone grease to the inner surface. After that, firmly press the carb into the boot seal. the grease should allow it to slide into place smoothly. If you find yourself wanting to hit the carb with your hand or a mallet to get it seated, don’t. Simply remove the carb from the boot seal and try again.

To test for a vacuum leak, use a propane torch (unlit) and wave the gas around the manifold area while the engine idles. If there is a vacuum leak the gas will be drawn into the leak and cause the engine RPM to slow or fluctuate.

Warning:

DO NOT PERFORM THE ABOVE STEP NEAR AN OPEN FLAME.

Once you’re satisfied that a vacuum leak does not exist, you can proceed.

Step 3: Fuel Mixture

If you’ve modified your air intake or exhaust (K&N Filter, high flow air intake, performance exhaust, etc.) you’ll need to provide more fuel to compensate for the change in airflow. Here’s what the parts in those jet kits do:

1. Slide Spring
2. Slide Diaphragm
3. Vacuum Slide
4. Vacuum Port
5. Needle Jet
6. Main Air Jet
7. Main Jet
8. Slow Jet
9. Idle Mixture Adjustment Screw
10. Throttle Butterfly

Usually, stock Harley jetting is very lean from just off idle to about ¼-throttle. This is also true of all road-going bikes sold in America for the last quarter century. However – and this is important – at idle and above ¼-throttle the jetting is pretty good. Harley’s Keihin CV (constant velocity) carburetor is based on the basic Amal slide carb design from the early post-World War I era. And therefore, it shares similar parts which perform similar functions. Idle and just off-idle air/fuel mixtures are controlled by the idle jet which is fine-tuned with a screw. Both the jet size and screw setting are important.

Off-idle to approximately ¼-throttle mixtures are controlled by the straight-diameter part of the needle together with the inside diameter of the needle jet, in which the needle rides. This is the range that is too lean for best engine performance on stock motorcycles. Either the diameter of the straight part of the needle, or the inside diameter of the needle jet, must be changed to affect mixtures in this throttle range. Nearly all street riding is done within this off-idle to ¼-throttle range.

From about ¼- to ¾-throttle, the taper of the needle controls the main mixture. The kits normally have you raise or lower the needle with spacers or clips to fine-tune mixtures within this range.

The main jet takes over at about ¾ throttle and is essentially irrelevant below that opening.

Save money, don’t buy a jet kit if you don’t need one!

The 1988 Sportsters did not have an accelerator pump. Why is this relevant? The needle in these bikes (H-D Part No. 27094-88) is richer in the off-idle to ¼-throttle range, right where you’ll need a little boost in fuel. This will usually be enough for all but the largest aftermarket air intakes and/or bikes with cams. For those, you will need a jet kit or tuner’s kit.

The Harley part# may be discontinued, but they’re still available here or through CVP.

Determining Throttle Percentage

Knowing your throttle position percentage during a test ride will help narrow down tuning issues. To help, you can make a mark on your handgrip with the throttle closed (using chalk or masking tape and a permanent marker). Make a matching mark on the throttle housing. Now roll the throttle wide open and make another mark on the throttle housing. Now make smaller marks between those two marks to divide the throttle grip’s movement range on the housing into quarters. With this method, you’ll be able to approximate your throttle percentage at a glance whenever you notice a hiccup or stumble and adjust your jetting accordingly.

Step 4: Idle Mix Screw

I will assume you’ve already drilled out the anti-tamper plug that covers the idle mixture screw. I won’t go into how to accomplish that here.

Bring the bike up to operating temperature. Make sure the choke is pushed in (off). Turn the idle speed down using the idle SPEED adjustment screw (the screw behind the throttle cables on the CV carb) until the bike is only barely able to stay running, maybe a little higher than that. 800 RPM is a good RPM if you have an accurate tach (No tach? Get a clip-on one for tuning purposes here). With the RPM lower than normal, you will be able to see the effect that adjusting the mixture screw has.

The idea is to turn the mixture screw in until the idle speed drops, then turn the screw back out until the idle speed drops again, and set the screw in the middle of those two points. What you are doing is making the mixture leaner, so lean that the engine does not like the mixture and runs poorly, then you make the mixture richer until it doesn’t like that, and the perfect setting is half-way between the point where is falls off either way.

With that done, go for a quick test ride. If you notice a flat spot in the RPM range, adjust your needle position up with one of the included shims, or clip, and try again.

Note:

If you find that all of the appropriate needle positions have different issues, a different needle may be the solution.

The original CV carburetor in 1988 lacked an accelerator pump commonly used on most carburetors. To compensate for the lack of an accelerator pump and to eliminate lean flat spots, the factory produced the N65C needle whose length, diameter, and taper positions were designed to alleviate the off idle hesitation.

The NOKK needle and NOKV needle are also popular choices. My general impression is that the needles aren’t terribly far apart. However, the NOKK seems to have a slight advantage in overall performance & usability.

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