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Pro Tuning on a Shade Tree Budget


95 and Earlier Model
CV Carburetor Modifications

If you are an owner of a stock late model Harley-Davidson 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. Here is what you need to do to create your own basic 'Stage 1' engine.

If you have a 1996 or later model CV carburetor and do not want to change the OEM needle, additional jetting information has been provided to Motorcycle Performance Guide that should improve the performance on these bikes. If you have one the these later model bikes, go to the 96 and later CV carburetor modification page


Fundamentally, what you 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. A word on pipes. Straight pipes may look and sound good, but this is not the best setup for a street performance engine unless you are willing to give up Torque and useable HP. Straight pipe will not give horse power readings greater than other pipes and the power band created by this type of exhaust system is very narrow. It will not win you any stoplight Gran Prix's against a similar engine with well designed pipes such as SuperTrapp, Python, Rich Products, Bartell, Vance and Hines, or even the H-D slip-ons. You make the decision. Follow the pack or make it Fast.

bulletStep One for getting more air into the engine is to first get more air into carburetor. That means buying a high-flow air filter element like those made by K&N. The Screaming' Eagle brand is as good as they get, but most after-market companies also offer them. K&N now makes a replacement for the stock air filter. If you use this filter element, you will need to add holes in the plastic air cleaner cover. Do this by drilling a series of 1/4" to 1/2" holes in the plastic cover. This will improve the air flow to the filter element.
bulletStep Two is to make changes to the stock Harley-Davidson CV carburetor. That's what these instructions are all about.

This step-by-step instruction are a consolidation of articles and information from Mark "Tusk" Amos, Charlie "Hacker" Powers and Steve 'V-Twin Cafe' Mullen. These techniques were used in the V-Twin Café Stage One Basic Engine Development. It's easy and it works. The techniques described here were gleaned from:

bulletThe Big Twin High Performance Book by William Denish, Crystal Publications, ISBN 0-9640115-0-6 (BUY THIS BOOK!),
bulletThe Harley E-mail Digest
bulletPersonal experience
bulletAbout 20 articles from various bike magazines written over the past 5 years.

Do not under-estimate the ability of the stock 1990 and later Keihin CV carburetor to produce good power. The stock CV carburetor has a venturi diameter of 40mm (1.575 inch). Properly tuned, a CV carburetor is capable of supporting 80+ horse power in modified engines. A well tuned, stock cam late model 1340cc ( 80 CID) can develop up to 61 horse power with a modified CV carburetor. Look at the V-Twin Café Stage Three Round One Engine development. The engine combination listed here produced 81 HP with 85 Ft.Lbs of Torque. The same engine combination with a Mikuni HSR-42 produced 85 HP. This is not a significant difference for a street engine. Review the Dyno run results for yourself.

Parts needed:

bullet27094-88 Main needle ('88 XLH part)
bullet27116-88 Main jet (#165)
bullet27170-89 Pilot or "Low speed" jet (#45)

Tools needed:

bullet1/8" bit (.125 inches)
bullet1/16" (or approximate) bit
bulletHand tools for removing and disassembling carburetor
bulletGood quality fine flat metal file (i.e. Nicholson)
bullet400 or 600 grit emery paper

Remove the carburetor 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 carburetor completely and go to a work bench. I usually leave the choke cable hooked to the carburetor and disconnect the pull-handle end, taking the entire choke cable/carburetor assembly with me. Your call.


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 carburetor 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 carburetors 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 carburetor and needle. Now back off about 3 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.


Remove the top of the carburetor (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 carburetor upright remove the top and spring, then the slide/diaphragm 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 hanging 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 carburetor 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 smoothes 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 carburetor 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.


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 over-tighten when reinstalling: good and tight is better than stripped and loose. Note: be sure to use jets numbered for the CV carburetor, and not for the older butterfly carburetors, as the numbering system is different. Use the HD part numbers listed above to be sure.


Replace the float bowl and the slide, and install the new needle and related hold-down hardware, then the spring and top. CAUTION: some diaphragms are a bit flimsy and are easy to get mis-aligned and pinched in the top edge. I used a VERY small dab Hylomar applied around the lip of the carburetor body to hold mine in place while I installed the top. The diaphragm 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 diaphragm causes it to stretch and can make re-assembly tricky. In addition to Hylomar, try using the cap to position the diaphragm. It will not only allow you to evenly push the diaphragm into the groove but you can "wiggle" the top and feel when the diaphragm 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 diaphragm 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 diaphragm, check for any pinholes with a bright light behind the rubber, pulling on the edges to stretch the diaphragm a bit. If you find any, you must replace the entire slide/diaphragm assembly and start over on the drilling and chamfering.


Assuming everything went OK, you are now ready to reassemble everything and test your handiwork. Be careful not to over-tighten the choke cable mounting nut as the plastic will break fairly easily. Warm the bike up to full operating temperature. You may have a little trouble keeping her idling, but one thing to remember is you now have a carburetor 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 warm-up time and also give you a little bit of a feel for the improvement of the modifications. DO NOT blast off and ride wheelies down the street! Your Evo is as sensitive to the warm-up 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.


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 carburetor, you have your idle mixture right. If it backfires through the carburetor ("carburetor 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 replaceable part (remember there was a plug over it?).

Cheezie's comment: That welsh plug is actually over it because the EPA doesn't want you tampering with it at all... it 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 silicone on that hole for the carburetors I've modified... using silicone is just a feel good thing for the owner.


The details of carburetor tweaking and plug reading is a very involved subject, so you may want to refer to a higher authority after this. If you do not have access to a Dyno facility, here is a very basic guide that will get the adjustments close.

Go run your bike while it's at operating temperature. 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 HD part numbers.

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 lean. If it hesitates or the top speed is poor (i.e. less than 80 MPH) you are too rich. Adjust your main jetting accordingly. Use your common sense and seat of the pants feel and you will get close enough to do plug reads.

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.


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, ignition 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 carburetor like an Mikuni HSR-42 to realize the full performance potential, but the CV with these modifications works very well on a stock or near stock motor.


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.



These modifications 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 modifications, but I would not know who that would be ;-) Also, the authors nor their descendants 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.


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Modification, Installation, Maintenance and Tuning Index  will help you find most of the information you want on one page.

How to get Professional Tuning Results at home
Testing the Innovate Motorsport LM-1 portable air fuel meter

Profession Tuning on a Shade Tree Budget

Veypor VR2 Data Logger and Instrument Panel
Video Installation and Demo
Purchase VR2

Engine Performance
How to Build a
TC96 2007 Engines
TC88 70HP Stage1  
TC95 128HP Stage 3
TC95 100HP Street
TC96 2007 Stage 1/2
EVO 64 HP Stage 1
EVO 74 HP Stage 2
EVO 82 HP Stage 3
EVO 95 HP Stage 3
883 to 1200 Upgrade
Shovelhead Modifications

New EFI for EVO and TC

Performance Gallery
Horsepower Gallery
Evolution 80
Twin Cam 88/95
Evolution Unlimited
Sportster Unlimited
Drag Strip Gallery
Land Speed Racing Gallery
CV Carburetor
Modifying the CV carb
Tuning a CV carb
Selecting a cam
Install a TC 88/95 cam
Install a Big Twin cam
Install Sportster cams

Camshaft Specifications
Twin Cam

Exhaust Systems
EVO Exhaust Testing
TC Exhaust Testing
Khrome Werks AR100 test
Making Drag Pipes Work

Shop Manual
Carburetor Troubleshooting
Finding Manifold Leaks
Cylinder Heads
Pistons and Cylinders
Belt Drive
Shop Manual Appendix
$20 Bike Lift
Plug Wires
Spark Plugs
Engine Tuning
Nitrous Oxide
Motor Oil
Stutter Box
General Information
WEB Links
Buy Books and Manuals
Performance Calculations
Estimate Horsepower
Estimate 1/4 Mile Time
Estimate Top Speed

Engine Displacement
Exhaust Length
Gear Ratios
Air Density

The Nightrider Diaries
The ramblings of a genius a, a madman and something in between.

Where is Sifton Cams?

Autocom Active-7 tested

Harley-Davidson EFI
-EFI basics explained
-EFI modifications explained

183 HP, 2 carbs, 2680cc

Copyright 1997-2006  Stephen Mullen, Oldsmar, FL -+-