GAWD so much to learn. What is Re-Jetting?

[matcom]

Ran across a V-Star 650 site and have been reading. The following caught my eye....

Rev limiter points 1st 30-32 mph, 2nd 50-52 mph , 3rd 70-72mph, 4th 90mph this is a good indication that the bike is jetted correctly.

"JETTED"?????

now, mine isn't broken in but I find myself in 5th gear at about 40 mph currently.

I don't even know if the above is even realted to shifting so if anyone can explain to me what i'm reading from a novice standpoint it would be much appreciated. :D

:shrug:

[ThomWV]

1. OK, let me explain a little bit - in fact maybe a lot

First off what they are telling you has nothing to do with 'jetting'. It sounds like an odd set of things to put together to me too because jetting refers to something you might do to your carburetors but when they are talking about a rev limiter then you're talking about computer control and you normally associate that with fuel injection.

At any rate a rev limiter is just that, it is a device that limits the maximum RPM your engine can turn. Some rev limiters hold the RPM at the maximum and some of them temporarily shut the engine down until the safe RPM is reached. It can be useful if there is a christmas tree in front of you and a useful nuisance for spirited riding. What they are telling you is at what speeds you would have to be traveling in each gear for your rev limiter to be activated.

For whatever its worth my Harley's rev limiter (my bike is injected), which is an integral part of the ECU, shut things down at 5,500 RPM in standard form. I have mine increased to 6,000 via another device which also allows me to modify my fuel delivery curve and my spark advance curve. Very handy device. Did I tell you that I absolutely love fuel injection? You can do one hell of a lot more tuning with a computer than you were ever able to do with a screwdriver.

As to the jetting here we to to the long story. I want you to think about your engine in a different light for a moment. Instead of thinking of it as a power source I want you to think of it as an air compressor. Think of it as sucking in air, compressing that air, and then releasing that air back into the general atmosphere. Now to make power we are going to add gas to the air and we're going to burn it, but for now we only want to think about air flowing through that compressor. Your task is to get as much air as possible to flow though your compressor and you can make great improvements in the factory system because you are not held to the same legal restraints they are.

The air has to come into the compressor through some sort of manifolding and through a filter as well. The gas will also exit through some sort of manifolding and also through a muffling device of some sort so that the noise of operation is not objectionable.

So the engine is designed at the factory and it has some sort of stock intake and exhaust system and they are all matched to each other for good efficiency and also to meet various regulatory requirements which are placed on manufacturers. But now you own it and maybe you want things a little bit different.

When the factory designed that engine they put together a package that met a lot of different requirements and when you change any one of a number of the components it has an effect on all of the others, So when you decide that the restrictive and expensive air cleaner that came on your bike might better be replaced with a washable high flow unit from K&N it will have an effect on how much air gets into your compressor. In fact more air will get in but the amount of fuel being provided will stay the same so now you will have an engine that is running 'lean'. The same thing will happen if you add a more efficient exhaust system, the increased flow of gasses through your compressor will make the flow of gasoline relatively less and once again, you have a lean condition. So if you make modifications which increase the flow of air you will also have to increase the amount of fuel delivered - hence larger jets in the carburetors or changes in the duty cycle of the injectors.

You see in a carburetor the control of the supply of fuel is controlled by the size of the hole in a part called the jet; bigger hole means more fuel can be delivered. In a fuel injection system the amount of fuel being delivered is dependent on how long the injector is held open. Injectors are either on or off, there is no meaningful in between. With today's engines the amount of time the injector is held open is controlled by the engine's ECU, which will also control its spark. So if you can take control of the ECU you can control how much fuel gets to the engine with great precision.

The reason all this matters to you is that engines that run lean are running hot. That is why so often when you see someone put a new set of pipes on a bike or maybe a new intake system the very next thing that happens is their pipes burn to blue, gold, and purple. Its the pipes overheating causing them to discolor. If the bike had been 'jetted' to run richer it would not have blued the pipes. The point is that if you change one thing to incresase the flow you have to change other things too and rejetting is one of those things unless you want to fry the engine. Running lean is a no-no. One easy way to spot it is this, if you are coasting down hill with the throtle shut off does the bike pop? If it does its running lean.

[matcom]

2. I admit I don't understand ALL of your explanation but..

I understand enough to know that the information is so valuable to me that I've bookmarked it :D

once again, THANK YOU!

:yourock:

[ThomWV]

3. Basically it works like this

If you change either the intake or the exhaust it allows the engine to flow air more efficiently (hopefully). When they do that the original carburetor jetting becomes inadequate. So you put in larger jets so that the fuel flow is in sync with the new air flow. If you don't the bike will run lean. Now the problem is that all new bikes run lean. We have the EPA to thank for that and we have cleaner air thanks to it too. Unfortunately that EPA mandated quality standard that results in lean running engines means that even stock the engines now days run hot. Heat is the deadliest enemy of our engines.

Rejetting is relatively simple if you've ever been inside a carb, not all that hard if you haven't. With injected systems you can buy a device that alters either the input from sensors on the bike or the output from the ECU but the effect is to give you control over adding or subtracting fuel flow. With the one I use I can adjust fuel flow at various throttle settings and at every RPM from idle clean up to the Rev Limiter kicks in. It gives me total control over the injection up to the service duty limit of the injectors.

Let me explain something about injection systems. I'll tell you a little bit about MAPping while I'm at it. Because an injector is basically either On or Off the only way to control how much fuel flows out of an injector is by how long it is kept open. There is an upper end limit to how much they can flow and that comes when you are pumping so much fuel into the engine that the injector is open virtually all the time. Fuel burning drag bikes can be like that at full throttle but you'd never see it on a street bike. What one of the controllers does is put a screen up on your computer with what looks like a spread sheet on it. RPM is on one axis and in ever cell there is the ability to either increase or decrease the stock ECU fuel flow settings. That spread sheet is called your MAP. It is specific to your engine and the modifications that have been made to it. There are hundreds of them available for free. You just look up your bike, the intake system you chose, the exhaust you chose, and hundreds of combinations of various after market parts you might be using. So you find the MAP for your bike and you download it and try it out on your bike. If you like it then good enough, if not you can tweek it yourself. I spent months getting mine right and I know as well as anyone its not exactly right yet. However with the device installed I can take my bike to a shop with a Dyno and they can dial it in with about a half a dozen passes after the baseline is established. They do it by running the bike through something you would not want to be in the room to witness.

Got to go, picking up the bike in town and bringing it back home tonight. Tell you about the new light later.

Thom