The jet needle is supposed to control from 1/2 to 3/4 throttle I believe. At 1/2 throttle my 351 is running a little over 9000 rpm. Is this correct. Would a better way to tune the jet needle be via RPM?

 

Let me clarify: I'm trying to get a plug reading and holding 1/2 throttle at 9000 rpm for a mile or so doesn't make sense.

 

Stock carb? The CV carb somes into the main jet much sooner than slide carbs.

You've found one of the reasons that reading plugs for jetting is a bad idea.

 

This may sound odd, but plug readings on pump gas are awfully hard to do. Better ignitions, combustion chamber design, and the type of fuels we run for the street make it very difficult. You almost have to make different kinds of runs on the bike and discern the difference in throttle response throughout the range. That may sound vague, and it does perhaps require some experience at detecting these changes in response, but in the end it is the most common way to achieve satisfactory results. In many automotive applications there are different sensors that can be used to detect fuel mixture and such, but a single cylinder motorcycle with a single carb can usually be tuned with rudimentary efforts. Also the fact that there is a very large body of tuning and jetting setups revealed here and on other forums for this particular bike, you shouldn't have to start from scratch. You may be trying to overanalyze your jetting.

 

 

The chart misplaces the 1/3 and 1/4 marks - they should be reversed. 1/4 < 1/3 < 1/2

What is the "straight dia?" Is this the throttle body diameter, or a measure of diameter while the slider is opening?

A lot of my recent around-town commuting has been at 1/3 throttle or less (going easy on the new knobbies on the slab and trying to check mileage under easy conditions, which sucketh at about 45 mpg). If I understand this correctly, this kind of riding uses primarily the slow jet, very little of the needle, and even less of the main jet. I can tell the slow jet is too large (#38) & rich and needs returned to stock (#35).

 

Don't some jet needles have a short straight section before they start to taper?

 

Okay - so you're saying that's the straight diameter portion of the needle? Makes sense. Thanks. But then does that mean if my put-put mileage sucketh, I may have the clip in too low of a position, and moving from 3rd to 2nd from top could help? (I also know...one step at a time when adjusting jets, but getting to that carb is soo easy...NOT).

 

Help me understand carbs better....

How do you determine if you need to go bigger or smaller on the pilot jet? How do you determine the number of turns for the fuel screw?

When raising the needle by moving the clip, how does that change the richness or leanness?

From the diagram, what's the throttle valve cut away?

On a side note, I've heard that people are drilling the slide bigger. What hole is this? What does it accomplish?

Thanks for your help

 

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