Bonneville America Forums Forums Technical Forums Performance Mods Stupid Question...

Ok, I'll be the first to admit this question solidifies my handle as "The_Chimpanzee" as having some merit...

I know they sell rejet kits out there with different size jets, which can turn into a bit of a guessing game when trying them out. I also know that Dyno[ing] a bike essentially does the same thing, only it sounds as though Dyno[ing] a bike may be more precise, and eliminate the need for guessing. So my [dumb] question is this: Does Dyno[ing] a bike eliminate the need for a jet kit? When you dyno a bike do you use the stock parts within the carbs (jets, shims, etc...)?

Seriously, I feel like an idiot even asking. Thanks for the help in advance.

- The [one true] Chimp

Think of a dyno run as a guide to where improvements may be made. Lot's of us re-jet based upon the seat of our pants, so to say, AND by looking at the spark plug color and observing throttle response, observing exhaust color etc. The Dyno, graphs out your power band so you or a more learned one can guide you on jet sizes, need for needle shims, etc.

Does Dyno[ing] a bike eliminate the need for a jet kit? No and Yes. If you are there, yes, if you aren’t then no. When you dyno a bike do you use the stock parts within the carbs (jets, shims, etc...)? Again yes and no. Generally when you are tweaking your ride it is because you have changed something. Pipes, air filter, air box elimination, BIG BORE. Even if your bike is stock, the use of the dyno will most always show you that improvement may be made, in air mixture setting or jets, or needle/shims or a combo of all.

So if ya want to dyno your bike, be prepared to re-jet (buy some socket head cap screws for the carb bowls and tops). Re-jetting guides exist and most after market pipes come with a jet kit. This forum has suggestions. So if you have changed something re air intake or exhaust (pipes) then rejet according to the guidelines that others have laid down, but do be prepared to re-jet when you ride to the dyno shop. That is to say have some jets in smaller and larger sizes available so you can swap them out and re-dyno. Have some shims too? Have some new plugs also…

PS: There are no stupid questions!

Dyno-ing you bike ma eliminate the need to buy the whole jet KIT by just buying indevidual JETS for the bike, or needles, shims, etc. Knowing which to get is a LOT cheaper than those whole kits that cost a lot, ya know?

Would it be advantageous to have many different jets, shims, etc... in the event I decide to make future alterations to the bike? I would think that I may want to continually fiddle with the bike throughout its existance. Thoughts?

- The Chimp

Chimp...

If you're running a "stock" setup, you really have no need to re-jet. If you de-install the AI system, then you've changed (slightly) the air flow to the combustion chamber & may require some change to the Main, & possibly, Pilot jets.

Basically, if you make any changes to the engine's air flow (input: air box &/or filter mods)(output: after-market exhaust), you will need to re-jet. Like Moe says, this will require some trial &/ error testing of different jet combinations.

The Dyno is only a tool to test & report the results of your changes.

A dyno run will tell you the power and torque your engine is producing. Many (but not all) dynomometers also have an exhaust gas analyser that will tell you what your mixture is. With that, you can make an informed guess as to which way you need to go and how far with your jets, but you will find that it's going to take a few trys to get it exact. When you get the mixture closer to ideal, the engine will run better and that will change the air flow, and that will change the mixture so there is little chance of getting it exact on the first try, but you will probably not need more than 3 sizes to get it right.

I suppose I will buy a kit then, and bring it in for some Dyno testing. I'll be interested to find out how poorly the bike is tuned stock.

How much more power (bhp) do you think can be attained by simply removing the AI, Snorkels and modding the airbox, adding straight pipes, etc...?

I have a feeling it could be rather substantial. When I slapped on thoose JC Whitney shorties, it seemed to make a huge difference in how hard it pulled. I can only imagine how much better it would perform once the bike could breathe some more air. At the time, I just didn't want to have to rejet or run the risk of bluing the headers from those shorties. Plus, those bastards were ridiculously loud - too loud!!! The crappy baffle that they install in them rots, and was just really slappy (a clappy harsh sound). Meh...

- Lo Chimpancé

I'd say check out the performance guide on this site to see who else has the same or similar setup to what you PLAN on having, then see what they are running for jets. Then, like someone said, get 3 or 4 jets in the neighborhood of that size. Now you're talking 20 bucks instead of a hundred or whatever they want for the kits...

Good call on that! I'll definitely check that out...

- The Chimp

With properly tuned pipes, matching jets and all the rubber bits out of the airbox, 68 - 75 HP at the crank can be reasonably expected. Add another 5 to 8 if you rip out the airbox altogether.