Tech Tips

To Jet or not to Jet?

Written by  March 31, 2004

For years now, I have been asked the question, why do we re-jet engines? The answer I usually respond with is, did you change the exhaust or modify the engine in any way? Or, do you simply just want better performance?

Today’s patient is a 1999 Harley-Davidson Heritage Softail. This is my neighbor, J.O.’s bike that was featured in our March Readers and their Rides section. He purchased his bike from a gal that was going through an ugly divorce, so she did not have any documentation of the upgrades or maintenance records. I completed a visual inspection and determined that it has a stock 1340cc Heritage with a Screaming Eagle exhaust. We have now decided to open the wallet and take charge.

My parts list, for now, will be a K&N serviceable air filter, and I have chosen the Dynojet jet kit with Thunderslide. This will give a more efficient controlled burn and response. NOTE: These parts are not a cure for all drivability complaints. It is only recommended by Dynojet.

When you remove the stock exhaust and add an aftermarket system, the engine is dramatically lean. I have talked with many seasoned technicians and been told that Harleys run lean off the bat anyway. We must compensate with different Jets in the carburetor. This process can be extremely difficult or easy, depending on your skill level and tools. Let me start by saying if you are not equipped with the proper tools, I recommend having a trained, qualified technician perform this job. I have been blessed with a motorcycle lift and safety clean industrial carb solvent. This helps takes the guesswork and backache out of the process.

We rolled the bike onto the lift and strapped the bike to the table. We don’t want the bike to fall off! If you happen to have the Factory Service Manual, that is great! If you do not, there are several publications available for this upgrade. I removed the air cleaner assembly and noticed that slide in the carb was stock, which is what I expected. I also noticed that the air/fuel mixture has the original aluminum plug. All late model motorcycles have this plug installed at the factory when the bike was built. This will need to be carefully drilled out carefully so I can adjust the mixture. This is a very important step since this is the circuit that runs the idle mixture. This bike has been running extremely rich, so I will remove the carb and dissemble. This is not as hard as you may think. On this model of motorcycle, tank removal is not required. You can remove the carb quite easily by disconnecting two cables, the pull and the push cable. NOTE: You will have to adjust the cables for as much free play to remove. There will be a choke cable and fuel line as well. Please be sure to shut the fuel valve off, or you will spill gas when the fuel line is removed.

The air cleaner housing actually holds the carb in the intake manifold; you will notice that the carb is very loose when the housing is removed. Once the carb is removed, it is time to setup a place to disassemble and identify the parts. This is a great time to take a parts inventory of the jets that are currently installed. Having documentation is great so you can use the new parts provided in the Jet kit. Once you have recorded the parts, you can now disassemble the carb. Remove the mixing chamber top cover and the float bowl. Remove the float and float needle. With a 1/8” drill bit, very carefully drill through the aluminum mixture screw plug as you apply light pressure with your drill. When you feel the drill bit go through the plug, stop and pull out the drill bit. Take a short 5/16” machine screw and screw that into the hole you just drilled. Once you have done that, using a good pair of pliers, remove the aluminum plug. This will give you access to the air/fuel mixture screw. Turn the air mixture screw counterclockwise and remove the spring, washer and o-ring. Be careful not to lose the o-ring as this will make the bike run rich no matter what you do.

If you have only Carb Spray cleaner, spray every passageway and blow out each passageway with compressed air. If you have Carb dip tanks, you will need to take all the parts, carb body and float bowl, minus gaskets and let them soak in solution for a minimum of 45 minutes. After that time has passed, remove them from the solution and immerse in water. This will dilute the solution and give it a great clean look. With air, blow every passageway to ensure there are no blockages. One critical piece that can be overlooked is that you must have the float level correct. You can get correct float level info from your local dealer.

Once these steps have been completed, you are ready to start the rebuild using the jet kit. The following link takes you directly to Dynojet's installation instructions for this particular jet kit. The instructions are in PDF format, so if you don't have Adobe Acrobat installed on your PC, you can download a free Adobe Reader.

After you have installed the jet kit, it is time to reinstall the carb. While you're at it, it’s also a great time to lube the two throttle cables. When you have everything reinstalled, it's time to take a test ride. Our test ride revealed that we needed to drop the Needle clip position one notch richer due to midrange lean condition. The idle also had to be reset due to the engine running more efficient. Once I made this change, she ran like a charm!

If you decide to tackle this job on your own and you have questions, feel free to e-mail me and I'll be glad to help!

Article and photos by Dave Miller