Fundamentally, what we want to do is get more air into the engine and allow more exhaust to get out. To allow more exhaust to escape the engine you'll need to punch the baffles out of your stock exhaust system, or get a set of better pipes. Your call. Step one for getting more air into the engine is to first get more air into carburator. That means buying a high-flow airfilter. The Screamin' Eagle brand is as good as they get, but most aftermarket companies also offer them. Step two is to tune up that CV carburator. That's what this page is about.
This step-by-step instruction comes to us from Mark "Tusk" Amos (tusk@iofc.com) and Charlie "Old Hacker" Powers. Your humble webmaster has performed this very mod on his charished '96 Dyna Glide Convertible. It's easy and it work. The techniques described here were gleaned from:
Parts needed:
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Tools needed:
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The mod works well for all types of Harleys, but the starting values of the needle and jets here are for the big twin. Sportsters will require slightly different jetting, but part numbers appear at the bottom so you should not have too much trouble adapting to your particular motor.
Underneath you will find a slotted screw. Turn this screw clockwise until GENTLY seated...jamming the screw down too tight will ruin both the carb and needle. Now back off about 2 full turns. This gives you an approximate starting point for tuning, and once we have it tuned you will need to secure the screw with a dab of silicone, but not yet. Don't forget! They don't sell those ANY of the idle mixture screw parts separately. Tuning details to follow in the IDLE MIXTURE ADJUSTMENT section.
Looking at the bottom of the slide where the needle was hangin out, you will see a second hole off-center from the needle hole. This is the vacuum hole, and needs to be CAREFULLY drilled to 1/8". Be careful to make a clean straight hole and keep shavings away from the rest of the carb and blow them out of the slide. Remove any burring that may have occurred.
Now it is time for the critical part for a true craftsman. The front bottom edge of the slide needs to be chamfered or "radiused". This is the edge OPPOSITE the side the vacuum hole was on (the front). Use a fine flat metal file to CAREFULLY smooth this edge from the 90 degree to a "rounded" 45 degree. This smooths the airflow. DO NOT file too deep or you will cut through the wall of the slide creating a hole, thus ruining the slide. DO NOT file the flat parts which contact the carb body as this could cause improper operation. You may need to take some fine (400 or 600) emery paper and polish the chamfer a bit if the file has left grooves or roughness.
Comments from Cheezie (cheezie@ezl.com): "One side note of observation I made recently about needles in a CV carb... I noticed that the needles for the 1996 models has changed... don't know if this will have any bearing on these mods at all... might not hurt to check this out further... I did note that the other parts associated with the needle (slide and spring seat) didn't change... don't know if this has any relevance or not."
A damaged diaphram will have symptoms of the motor being able to idle well but not being able to take ANY throttle. If you think you may have a damaged diaphram, check for any pinholes with a bright light behind the rubber, pulling on the edges to stretch the diaphram a bit. If you find any, you must replace the entire slide/diaphram assembly and start over on the drilling and chamfering.
If you follow the owner's manual you will certainly foul your plugs, so use your head.
Letting your bike idle for 15 minutes to warm up is not always desirable, and your bike should be running well enough for a CALM test run around the block. This will speed up the warmup time and also give you a little bit of a feel for the improvement of the mods. DO NOT blast off and ride wheelies down the street! Your Evo is as sensitive to the warmup time as you have always heard, so be gentle the first few miles. I would go around once to get the temp up then stop to tweak the idle and idle mixture. Word of advice: find a little screwdriver you can use to adjust this screw BEFORE the motor is hot, as fumbling about trying to use the wrong screwdriver will probably result in burned knuckles if you are not careful.
Once you are happy with the setting, you might want to apply a small amount of RTV silicone to the idle screw hole to hold the screw in place. You could otherwise lose the screw after a while due to vibration, causing the bike to run poorly and not idle, and causing you to tear your hair out in frustration since you will not find the part listed as a replacable part (remember there was a plug over it?).
Cheezie's (cheezie@ezl.com) comment: That welsh plug is actually over it because the EPA doesn't want you tampering with it at all...has nothing to do with the screw falling out. Did the screw fall out when you drilled out the welsh plug? I'll answer for ya....Nope.
I haven't yet used any sillycone on that hole for the carbs I've modified...using sillycone is just a feel good thing for the owner.
Go run your bike while it's hot. Run up the gears, giving hard throttle and messing with roll-on performance. If it is lacking in this area, as in spitting or hesitating, increase the size of your low speed jet a couple of notches and try again. If the bike blubbers and/or coughs black smoke from the exhaust, go down a size or two. Below you will find the part numbers as stolen from the digest.
The main jet is another story. You must be in fourth or fifth gear and running fairly high RPM (3000+) then open the throttle all the way to the stop, noting the feel of the bike. Immediately let off about 1/8 turn and note the feel of the bike. If it seems to accelerate some when you let off the 1/8, your main jet is too rich. If it hesitates or the top speed is poor (i.e. less than 80 MPH) you are too lean. Adjust accordingly. Use your common sense and seat of the pants feel and you will get close enough to do plug reads. NOTE: You should not need to change the needle unless you feel confident it is really necessary.
Plug reading is as much an art as a science for us backyard wrenches, since high-dollar equipment is needed to REALLY do it up right, but for most street riders making sure your plugs are a nice tan color is good enough. Don't bother reading the plugs until you have done the fine tuning above, and then use new plugs (hopefully you started this whole thing with a new set also- and don't forget to check that gap!). Do some riding which exercises either low speed or main jetting and stop immediately and check the plugs for the most accurate reading. If your plugs are black you are too rich, which sucks your gas and performance but will not harm your motor. Too light or worse yet bone white you are too lean, and motor damage will soon follow if proper steps are not taken.
Hope this helps you. And remember, after you flow your heads and add a good cam, you may need another carb like an S&S to realize the full performance potential, but the CV with these mods works very well on a stock or near stock motor.
Fuel Supply: The following mods apply no matter whether you apply the mods above to your CV or if you change the carb out to a better one. Suggested by The Old Hacker once again.
You may want to drill out the gas cap. Turn over the gas cap and look at the bottom. The plastic molding SHOULD have 4 spokes but some of the bad ones had 8 spokes...8 spoke gas caps need to be replaced with a 4 spoke gas cap then the mods can be made. The one with 4 spokes has four pads or flat spots about 1/8 inch in diameter spaced around the outside of the inner molding ring. Using a 1/16 inch bit or smaller, drill a hole in the center of each of these pads clean though to the airspace below. Do not get too enthusiastic and punch a dent or hole in the top of your gas cap. If you do it is a good time for one of those stick-on gee-gaws (Live to Ride...). The result will be better gas tank venting now that you are using more fuel and probably riding harder and faster. There is currently some controversy about this subject so please read the Denish book for more info.
Fuel petcock: Due to EPA regs the stock fuel petcock on late model Harleys has very restrictive fuel flow. There are several alternatives but Pingel is far and away the best on the market. Excellent flow and very high quality construction and feel. The Old Hacker sez it's better than sex, so he frequently will go to the garage when he every now and then and turn it on and of a few times just for the feel of it. This is no kidding, folks.
Jet Needle (mid-range) in decending order Rich->Lean 27175-90 N72R Rich 27176-90 (CA Only) | 27178-90 N72S | 27179-90 (CA Only) | 27184-92 N86F | 27186-92 N86E | 27280-92 (CA Only) Lean 27094-88 '88 1200 Sportster (Recommended needle) Pilot Jet (low or slow speed) 27170-89 45 Rich 27171-89 42 | 27281-92 40 | 27117-88 35 Lean Main Jet (high speed) 27185-90 185 Rich 27114-88 180 | 27090-89 175 | 27115-88 170 | 27116-88 165 | 27152-89 160 Lean