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Tuning the Genesis Carb
Like most machines designed for extremely high performance, the Genesis carb works amazingly well when properly adjusted, and moderately well, at best, otherwise. Note: In my opinion, this carburetor is as forgiving as many others, if ill-tuned. Once it's tuned well, though, it should need no ongoing attention for normal street riding.
In fact, here's another area where the Genesis Series IV carb really shines: All normal adjustments can be made by turning knobs on the outside of the carb! No need to disassemble anything to try a different main-jet; the Power-Jet handles that, and it's adjusted with a knob near the carb's top. No need to change clips or washers for the needle; the metering needle is easily adjusted from the carb's top. It's a tuner's dream.
First, check the adjustment settings on the carb. Make sure they match the Spec Sheet that came with it. In case you have no spec sheet, the settings for the Road Star should start somewhere near the following:
- Pilot-Fuel: 0 to 1.5 turns. Tip: Mine is (and several others are) set to 0 turns out.
- Pilot-Air: 2 to 4 turns out
- Needle Height: 12 clicks up/out from bottom. Note: If you have an earlier type needle adjuster screw, with 4 clicks per revolution (instead of 8), set the needle to 6 clicks up/out from bottom.
- Power-Jet: 18 clicks out
Every bike will need unique settings. Exhaust pipes, engine mods, coil and/or ignition module changes, spark plug choices, and even altitude and ambient temperature can demand different carb adjustment settings.
All settings are measured as either clicks or as full-revolution turns out from a fully screwed-in position. As you fully seat (screw in) each adjuster, be very careful not to twist with any more pressure than needed to feel the adjuster bottom out. The adjusters are brass -- except for the needle -- and the adjuster seats are aluminum. Both are soft metals, and easily damaged. Use extreme caution.
Next, connect a fuel source to the fuel hose. If you install your gas tank as your fuel source, access to some of the adjusters will be limited, and you may need to remove the tank several times as you fine tune the carb adjustments for your bike. But that would also be necessary for adjustments to ANY carb.
Tip: Some Genesis owners mount their stock gas tank, then when they want to make a needle adjustment, they remove only the tank's rear mounting bolt. The carb's needle adjuster -- on the top-plate -- can easily be reached by lifting the rear of the tank a bit. Just be careful; the engine will be hot!
Tip: I use a 1/2 gallon test-tank made by Summit Racing (See the Build a Cheap and inexpensive Aux Fuel Tank article for details). I welded a mount for it so I can bolt it to the bike's frame for testing. This gives me full access to all adjustments. Others have modified 1-gallon plastic gas cans, and some have even modified 2-liter soft-drink containers. Just be sure to keep an eye to safety. See a photo of my mount, below.
Now it's time to start the bike. I know; the breather has not yet been installed. Don’t worry.
Pull the carb's enricher (choke) knob all the way up, and then -- without touching the throttle handle -- press the starter button. If the bike doesn't start, try again with the choke off. If still no results, pull the choke up again, plus place your hand momentarily over the carb throat while you press the starter. Then take your hand away, and continue cranking the starter.
If it just won't start, it's time to stop and think things through...
If it looks or smells like the carb is allowing too much gas, you need to try to reduce fuel and/or increase air. Unless the Pilot-Fuel screw is already set to zero, try adjusting it in 1/2 turn or so. Or try adjusting the metering needle in (down) 4 clicks (1/2 turn) or so. Or try increasing (screwing out) the Pilot Air 1/2 turn. If still no go, adjust more.
Of course, if you suspect it is too lean, do the opposite. Also, if you have a too lean condition, check for air leaks at the junctions between the carb, spacers, manifold, and engine.
If all this fails, you may have the wrong needle and/or nozzle for your bike's set up -- or you may have a problem unrelated to the carb. Ask for help on the forum of this site or other popular forums that discuss the Genesis carb. Also, don't forget P.S.I. ; they built the carb, and are extremely motivated to increase its popularity. They can be a tremendous resource.
Tip: I bought a larger enricher (choke) jet. I swapped out the 0.015" jet for a 0.022" jet. P.S.I. thought it would help with easier starts in St Louis' cold weather. To replace the enricher jet (a.k.a. Pickup Tube), you will need to remove the float bowl. The float bowl is easily removed with a single, nut at the bottom-center. Just be careful of the float assembly inside, as it is only held in place by the friction of a tiny o-ring. Turns out that the larger enricher jet was too big, and I re-installed the original one.
Once you're started and idling, the first thing is to adjust the idle to a proper level -- not too slow. The adjuster is on the carb's left side (facing the installed carb). Let the bike warm up a bit before you do this -- enough so that the choke is not needed, at least.
Next, adjust the Pilot Fuel and Pilot Air mixture. It appears that most Series IV carbs, installed on Road Stars, need very little, if any, pilot fuel. Try shutting yours completely off. Just be careful you don't bottom the adjuster out very hard at all; it is brass, and easily damaged.
Pick a pilot fuel/air combo where the idle is strong and smooth, and yet there isn't any visible exhaust smoke when you nudge or blip the throttle just above idle.
Finally, it's time to go for a ride. The purpose of this ride is two fold:
- Warm the engine thoroughly.
- Begin adjusting the metering needle height. This is the most important adjustment for getting the Genesis to perform its best. Note: for top performance, the Genesis carb will work best when set between 8 and 14 clicks (4 to 7 quarter turns). Tip: To adjust the needle height, simply raise the slide fully by twisting the throttle grip while simultaneously pressing down on the needle adjustment knob on the top of the carb.
Note: If your carb seems to work best when adjusted outside the 'sweet-spot' range described above, the carb will likely work even better with a different needle or nozzle. Call or email P.S.I. for assistance.
Once the mid-range throttle has been adjusted completely, you can move on to the final adjustment: the Power-Jet. This adjustment is similar to changing 'main jets' on most carbs, but far easier. Turning the Power-Jet adjuster knob 4 clicks (1/3 turn) equates to one Mikuni main jet size change. The Power-Jet adjuster is located near the top, but on the front (face) of the carb.
To adjust the Power-Jet, you must either take your bike to an experienced, reliable dyno tuner, or you must do some high speed testing.
High speed testing has been around for a long time. Attempt this method only if you are skilled and comfortable riding in such conditions, and you have a place to perform this test without breaking the law. This method is potentially dangerous. Perform it at your own risk. To do high speed testing, I suggest the following:
First, thoroughly warm your bike via a 20 minute ride. Next, go to your testing track or un-congested roadway designated for legal high-speed driving. Then accelerate the bike to about 55mph in 4th gear. Tip: If you have a smaller than stock, front, drive pulley, you could start at around 50mph.
Now twist the throttle to WOT (wide open throttle), and accelerate to 75 or 80 mph (the faster the better for testing purposes, as long as it is not near the bike's 4th gear top speed). Tip: Since the Genesis carb has no accelerator-pump, you can proceed to the next step immediately.
When you hit your chosen speed, quickly back off of the throttle 1/8th turn. While doing so, pay attention to the RATE of continued acceleration. In other words, the bike will still be speeding up since it has plenty of power for higher speeds than you are going. Remember, your throttle position is still quite aggressive. Here are the possibilities and their meaning:
- The bike increases its rate of acceleration -- In other words, it feels like it has a bit MORE power with the throttle eased a bit. In this case, the Power-Jet is set too lean and you need to screw the adjuster out (counter-clockwise).
- The bike feels like you have done nothing different -- In other words, it continues accelerating at the same rate as when the throttle was wide open. This is ideal. It indicates that the Power-Jet is set very close to perfect.
- The bike looses some power -- In other words, you may still be accelerating, but at a slower rate. Unless you are nearing your bike's top 4th gear speed or you reduced the throttle by more than 1/8 turn, the Power-Jet may be set too rich. Try adjusting the knob in (clockwise).
Make several test runs, as follows:
- Keep adjusting the Power-Jet OUT until you experience conditions 2 or 3 above. Note: You may not ever experience condition 3, even with an overly rich setting.
- Then adjust the Power-Jet IN until you barely experience condition 1 above.
- Then adjust the Power-Jet back OUT one click.