Jetting question (new)

[Robert Haller]

What type of added power do you get when you go to larger jets...more top end, more bottom end, or across the board? I'm satisfied with my top end but wouldn't mind more bottom end.

[Jerry]

Basic layman's carb theory...

The goal for proper jetting is to get the correct fuel/air mixture for the rpm/load/throttle opening of your engine. Carburated engines have several, and often overlapping, fuel circuits in their carbs. In theory the ideal mix is around 14 parts air to 1 part fuel, or 14:1. Different circuits handle the fuel-air mix at different rpm ranges.


Changing to larger jets for any given RPM band may not give you more power unless the carbs are incorrectly jetted to begin with, that is, the fuel is already at the 14:1 ratio. If you have changed air boxes, exhaust systems, or made engine mode the jetting may need to be changed as the air flow characteristics of the engine are now different.

The Tech Tips section includes a couple carb tuning sites that go into the technical aspects of carb tuning, including how to see if you are jetted correctly or need to 'tweak' your carbs. One of the best articles is at:

http://www.factorypro.com/tech/carbtun.html

I would look at this site to gather a bit more info on the hows and whys of carb tuning and what you can expect to get out of your efforts.

[skids]

Re: Jetting question (new)

Something else to be aware of: Because you have a model F, changing main jets in model E and F carbs will also affect the lower rpm circuits in the carbs. The pilot jets draw fuel from the main jets because the bores below the pilot jets are capped (or should be).

Originally posted by Robert Haller
What type of added power do you get when you go to larger jets...more top end, more bottom end, or across the board? I'm satisfied with my top end but wouldn't mind more bottom end.

[Robert Haller]

Really just wondering what differences can be expected when you increase the airflow and correctly increase the jet sizes. I presume there's a gain in power, does it tend to be in the low end, top end, or both?

[Mike Hart]

the good news: it's pretty much across the whole range
(unless you went from stock pipes to 4-1 which in itself moves the power band up some)

the bad news: the motor is only getting a small percentage more air/fuel mix, a small % more power, you might notice it, you might not

On my Bandit, I had to increase main jet size from 102.5 to 150 - theoretically almost 50% more flow? - and gained about 15% in power and torque across the band, but esp. at ~4k where the stocker had a little flat spot, it erased that. Now that was noticable! But I can't honestly say I feel much difference in my XJ that went from 112.5 stock main up to 122.5. Sorry, no dyno chart for the XJ

[dragxs11]

Here is what I know Rob,
An aftermarket pipe and jet kit will bump you up across the dyno graph, if matched correctly. A little on the bottom, mostly on the top end. You put in a bigger pilot air jet for the bottom end, and a bigger main jet for the top end, as well as new needles to change the main gas regulation. Results are such that you can tell. Don't know about '79 sf carbs being different from my '80. I have no regrets in jetting mine.
Matt

[Mike Hart]

Matt, do you have a dyno chart of your xs?

Robert, mains alone might do the job fine. Several have done it without having to change pilot jet or needle position.

[dragxs11]

The kit came with a chart(stock, stage 1, and stage 3), thats all I've got. But I could tell the difference just riding it. The way it responds now seems to follow with what the chart shows. I'm waiting till I finish improvements before going to the dyno. It'd be nice to have one in the garage, huh. The only valuable data I will be left with by then is hp and torque improvements over stock with a list of mods. It'll be hard to tell how much each part does.