Stock CV Carb Upgrade

If you are an owner of a stock late model Harley you may have noticed that it does make all that much power for all that displacement. There are many reasons for that fact, many of which you can't change. Luckily there are a couple things you can do to get some some significant increases in HP without laying out much cash.

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:

As Tusk says at the end, there is nothing really new here, but I've yet to see it posted to a web site for general consumption, so here it is...

Parts needed:

  • 27094-88 Main needle ('88 XLH part)
  • 27116-88 Main jet (#165)
  • 27170-89 Pilot or "Low speed" jet (#45)

Tools needed:

  • Drill
  • 1/8" bit
  • 1/16" (or approximate) bit
  • Hand tools for removing and disassembling carb
  • Good quality fine flat metal file (i.e. Nicholson)
  • 400 or 600 grit emery paper

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.

GETTING STARTED

Remove the carb per your service manual. You MIGHT get away with leaving the throttle cables OR the choke connected to the bike, but don't count on it. It would be much easier if you remove the carb completely and go to a work bench. I usually leave the choke cable hooked to the carb and disconnect the pull-handle end, taking the entire choke cable/carb assembly with me. Your call.

IDLE MIXTURE ADJUSTMENT

Before we *really* get started, you might as well remove the aluminum plug covering the idle mixture screw. Using about a 1/16" drill bit, turn the carb over and CAREFULLY drill a hole in the small plug just on the motor side of the float bowl. If the plug did not fall out when drilling, remove your drill bit and use the back side to pry it loose. Some carbs may require the CAREFUL insertion of a self-tapping sheet metal screw the get a good enough grip to pull the plug.

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.

SLIDE MODIFICATIONS

Remove the top of the carb (slide vacuum chamber cover) being careful to loosen the throttle linkage stop plate. There is a spring under the top cover so hold it with a finger until all screws are loose. Holding the carb upright remove the top and spring, then the slide/diaphram assembly (all one piece). If you look into the top of the slide you can see some plastic stuff holding down the needle. Pour this stuff in your hand as you will be replacing the main needle. Before reassembling the slide you will need to modify it.

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.

REPLACE JETS

With this done you can set the slide aside for now. Open up the float bowl CAREFULLY and replace the main and low speed jets. These are brass in aluminum and both are soft- use a light hand and the proper tool. Do not overtighten when reinstalling: good and tight is better than stripped and loose. Note: be sure to use jets numbered for the CV carb, and not for the older butterfly carbs, as the numbering system is different. Use the HD part numbers listed above to be sure.
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."

RE-ASSEMBLY

Replace the float bowl and the slide, and install the new needle and related hold-down hardware, then the spring and top. CAUTION: some diaphrams are a bit flimsy and are easy to get misaligned and pinched in the top edge. I used a VERY small dab Hylomar applied around the lip of the carb body to hold mine in place while I installed the top. The diaphram has the appearance of being "swollen" and seems to be too large to fit into the groove. The repeated up and down movement of the diaphram causes it to stretch and can make re-assembly tricky. In addition to Hylomar, try using the cap to position the diaphram. It will not only allow you to evenly push the diaphram into the groove but you can "wiggle" the top and feel when the diaphram is properly located. Later model CV's have a dowel in them at the cable guides...this can make matter's difficult at times.

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.

TESTING

Assuming everything went ok, you are now ready to reassemble everything and test her out. Be careful not to overtighten the choke cable mounting nut as the plastic will break fairly easily. Warm the bike up to full operating temperature. You may have alittle trouble keeping her idling, but one thing to remember is you now have a carb which is not as leaned out as the factory setup, so DO NOT follow the owner's manual directions for cold starting. Here is the new procedure: Pull choke out either half or all the way depending on ambient temp (below 70 degrees? Pull all the way out) and if the bike has been run in the past 2 hours or so (if the motor is totally cool, pull out all the way, otherwise half or none if the motor is already warm). Start the bike, quickly adjusting choke to reduce fast idle to a reasonable level. No more than 30 seconds later, push the choke all the way in and use the throttle to keep the bike idling while warming up. Some use the throttle lock for this purpose.

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.

ADJUSTING IDLE MIXTURE

With motor hot and at idle, turn the screw inward (clockwise) SLOWLY until the motor starts to falter. If the motor will not idle on its own when you begin this procedure, bump up the idle set screw until it will. Throughout this procedure try to keep the RPMS at or below 1000. Now having turned the screw inward until the bike falters, back it out slowly, making a mental note of the position on the clock. Turn outwards until the motor begins to run smoothly, then adjust idle stop screw as necessary to bring RPMs down, but not TOO low. Evo oiling systems need better than 700 RPM to work properly. Blip the throttle a time or two and observe the results. If the motor responds with a gratifying blast without backfiring through the carb, you have your idle mixture right. If it backfires through the carb ("carb farts") you will want to back the idle mixture screw out another 1/4 turn. Do not go too far, as too rich an idle mixture will certainly cause you many headaches and poor gas mileage.

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.

FINE TUNING

The details of carb tweaking and plug reading is a very involved subject, so you may want to refer to a higher authority after this. Here is a very basic guide.

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.

FINAL NOTES

Large changes to the low speed jet will probably require you to adjust the idle mixture again. Keep this in mind when choosing and applying the RTV. You should expect to end up with a richer setup if your bike is light and/or you are running exceptionally good flowing pipes (i.e. Thunderheader). There are so many variations here it is hard to predict, but the ranges listed here are for most normal applications. If you try these and your bike still runs like hell you may have some other problem (clogged fuel screen/filter, bad petcock, iginition trouble/electrical malfunction, timing wrong, fouled plugs, bad gas, etc. etc.) You may want to get a seasoned wrench involved if you are in doubt.

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.

ACKNOWLEDGEMENTS

Thanks to all the digesters who contributed to this and especially to the Old Hacker Charlie Powers for inspiring this work. There is really nothing new written here except it is in electronic form to be sent around the net. Don't ask me why yet another CV article was needed, but I thought it might as well be done.


ADDENDUM

Disclaimer: These mods are for off-road use only and are not approved by the EPA or DOT or Bill Clinton. Some people ride on the road with these mods, but I would not know who that would be ;-) Also, the authors nor their decendants nor their ancestors are responsible for any damages due to following these instructions. If you follow them correctly and do not screw up you will not have any trouble anyhow, but there it is.

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.


APPENDIX: Jet and needle part numbers

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
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