By Steve “Brewdude” Garn
How many times have you been on a forum or at an event and someone is complaining how bad their bikes is running. I get calls and emails all the time asking what carburetor jets do I need? They start out saying I have a 42mm carb and it is on a 1988 Harley. It is running poorly and someone said it needs to be re-jetted. So, what size do I need?
If you have asked others on a forum in the past and actually did the changes that they suggested, who doesn’t even know the history of your bike and its condition, I doubt that your bike is actually running much better or it may be running worse! What this means is what changes have been done to your bike. Have you changed the exhaust? Does the exhaust have baffles and if it does what size? What ignition are you using? What about the ignition timing? What air cleaner are you using? How many miles are on the engine? Are the engine parts (pistons, cams and heads) stock or are they aftermarket? What altitude do you live at?
This is only a partial list that will determine what size jets you need. If you go too lean you will burn something up, or if you go too rich, it will cause spark plug fouling and engine parts caked with carbon deposits. Running too rich also washes the cylinder walls down and causes rapid piston ring wear. Some get their jetting close and then just ride it. Again, you could be causing engine damage!
So, how do you know what size jet for your bike? The best way is on a dyno. Most think that a dyno just shows how much horsepower an engine has. This is far from the truth. A dyno not only shows the horsepower and torque of an engine, but will also show how well the engine is tuned. A well-tuned engine will have a smooth slope up the horsepower/rpm graph and then flatten off and then will show a decline of horsepower. The graph should not have any sharp peaks and drops in power throughout the dyno run. The dyno also will show the air/fuel mixture of the engine. It is important to graph the air/fuel mixture at different throttle openings. This will ensure the proper mixture at any throttle opening and will make the bike pull at any rpm with different throttle openings. If it was a perfect world, the optimum air/fuel mixture would be 14.7(air)-1 (fuel). But that is if the mixture of air and fuel is consistent and no fuel droplets. For performance, most tuners will have an air/fuel mixture of 12.5-1 to 13-1.
It is important to get someone who runs a dyno and knows how to describe the results to you. I will explain why this is so important. I prefer a shop that is in to building not only strong street engines, but great race engines. These shops use their dyno on a regular basis and can explain what changes are needed. I use the dyno at Hawaya Racing. The shop is owned and operated by the Vicker family and are known for their many National Championships they have achieved with their bikes and the engines they build for many racing teams. When we hooked up my Yamaha RD350 land speed racing bike, there is one feature we didn’t use which is the air/fuel mixture. Why? 2 stroke engines with the oil/fuel mixture will get inaccurate readings and also ruin the oxygen sensor in a couple pulls. I had already gotten the engine close in air/fuel mixture by doing several runs here on the Brew Farm, and did several spark plug readings. Spark plug readings are how the spark plug looks after running at certain speeds. For high speed use, it is important to shut the engine down at high rpm and then check the color. A proper air/fuel mixture will be a tan/light brown color. There are many other indications that spark plug readings will show. The spark plug companies have great pictures showing the many things that could be happening in your engine.
Back to my dyno run on the first pull and a couple spikes of loss of power. The spark plugs showed it was a little rich in mixture. A rich mixture means too much fuel and a lean mixture means not enough fuel. I adjusted the power jets on the Lectron carbs and the next pull the bike had a smoother horsepower curve and added 4% more horsepower. I leaned it up just a little more on the power jets and turned the slide mixture needle up a ¼ turn. The next run the curve was smooth and had an increase of 13% more power over the initial run. The engine pulled smooth and sounded much better.
Another reason to use a shop that uses the dyno on a consistent basis is that the next time I do some changes on the bike (which I am doing now) I can then run it on their dyno and compare graphs. All dynos will get different readings! Some dynos are tweaked by some to show higher horsepower readings just so some can say their bike has so much horsepower. It is more important to use the same dyno anytime you make changes so you can truly see how much more of a percentage of power you have gained.
The next week I took the bike to the Ohio Mile and the engine ran great. After a run, I would remove the spark plug and check the mixture. The plug looked great and was a light brown. The time I spent down at the Hawaya Racing shop (hawayaracing.com) gave me much peace of mind running this bike at high speeds for the first time. Not only will your engine perform much better and last much longer, but it will now be a safer bike to ride because of a smoother and stronger pulling engine.
How many times have you been on a forum or at an event and someone is complaining how bad their bikes is running. I get calls and emails all the time asking what carburetor jets do I need? They start out saying I have a 42mm carb and it is on a 1988 Harley. It is running poorly and someone said it needs to be re-jetted. So, what size do I need?
If you have asked others on a forum in the past and actually did the changes that they suggested, who doesn’t even know the history of your bike and its condition, I doubt that your bike is actually running much better or it may be running worse! What this means is what changes have been done to your bike. Have you changed the exhaust? Does the exhaust have baffles and if it does what size? What ignition are you using? What about the ignition timing? What air cleaner are you using? How many miles are on the engine? Are the engine parts (pistons, cams and heads) stock or are they aftermarket? What altitude do you live at?
This is only a partial list that will determine what size jets you need. If you go too lean you will burn something up, or if you go too rich, it will cause spark plug fouling and engine parts caked with carbon deposits. Running too rich also washes the cylinder walls down and causes rapid piston ring wear. Some get their jetting close and then just ride it. Again, you could be causing engine damage!
So, how do you know what size jet for your bike? The best way is on a dyno. Most think that a dyno just shows how much horsepower an engine has. This is far from the truth. A dyno not only shows the horsepower and torque of an engine, but will also show how well the engine is tuned. A well-tuned engine will have a smooth slope up the horsepower/rpm graph and then flatten off and then will show a decline of horsepower. The graph should not have any sharp peaks and drops in power throughout the dyno run. The dyno also will show the air/fuel mixture of the engine. It is important to graph the air/fuel mixture at different throttle openings. This will ensure the proper mixture at any throttle opening and will make the bike pull at any rpm with different throttle openings. If it was a perfect world, the optimum air/fuel mixture would be 14.7(air)-1 (fuel). But that is if the mixture of air and fuel is consistent and no fuel droplets. For performance, most tuners will have an air/fuel mixture of 12.5-1 to 13-1.
It is important to get someone who runs a dyno and knows how to describe the results to you. I will explain why this is so important. I prefer a shop that is in to building not only strong street engines, but great race engines. These shops use their dyno on a regular basis and can explain what changes are needed. I use the dyno at Hawaya Racing. The shop is owned and operated by the Vicker family and are known for their many National Championships they have achieved with their bikes and the engines they build for many racing teams. When we hooked up my Yamaha RD350 land speed racing bike, there is one feature we didn’t use which is the air/fuel mixture. Why? 2 stroke engines with the oil/fuel mixture will get inaccurate readings and also ruin the oxygen sensor in a couple pulls. I had already gotten the engine close in air/fuel mixture by doing several runs here on the Brew Farm, and did several spark plug readings. Spark plug readings are how the spark plug looks after running at certain speeds. For high speed use, it is important to shut the engine down at high rpm and then check the color. A proper air/fuel mixture will be a tan/light brown color. There are many other indications that spark plug readings will show. The spark plug companies have great pictures showing the many things that could be happening in your engine.
Back to my dyno run on the first pull and a couple spikes of loss of power. The spark plugs showed it was a little rich in mixture. A rich mixture means too much fuel and a lean mixture means not enough fuel. I adjusted the power jets on the Lectron carbs and the next pull the bike had a smoother horsepower curve and added 4% more horsepower. I leaned it up just a little more on the power jets and turned the slide mixture needle up a ¼ turn. The next run the curve was smooth and had an increase of 13% more power over the initial run. The engine pulled smooth and sounded much better.
Another reason to use a shop that uses the dyno on a consistent basis is that the next time I do some changes on the bike (which I am doing now) I can then run it on their dyno and compare graphs. All dynos will get different readings! Some dynos are tweaked by some to show higher horsepower readings just so some can say their bike has so much horsepower. It is more important to use the same dyno anytime you make changes so you can truly see how much more of a percentage of power you have gained.
The next week I took the bike to the Ohio Mile and the engine ran great. After a run, I would remove the spark plug and check the mixture. The plug looked great and was a light brown. The time I spent down at the Hawaya Racing shop (hawayaracing.com) gave me much peace of mind running this bike at high speeds for the first time. Not only will your engine perform much better and last much longer, but it will now be a safer bike to ride because of a smoother and stronger pulling engine.