An Approach to Rejetting BUBF's

After you've had a chance to read the proceeding section on CV carb principals and theory, you'll be familiar with your carb's "anatomy" and may want to explore rejetting your bike for optimum performance.  This is not the definitive statement on carb rejetting, but it will give someone new to the subject a starting point.  

You need to consider rejetting your bike whenever there's been a major change to the air intakes (like installing K&N filters which allow more air to pass to the carb), or adding drag pipes.  Both modifications will lean out the air-fuel mix.  Installing larger jets increases fuel to the carb to restore the 14 parts air to 1 part fuel optimum mix.  

In all other cases, the stock jets will take you through 95% of your riding requirements.  If you do sense a loss of power, or rough running and no changes have been made to the pipes or air intakes, rejetting is not the solution to the current problem.   

Getting started.  Yes, you DO start rejetting with the main jets and work "backwards".  Since the main jet effects all speed ranges after idle, you need to get the main jets right first, then tune the mid-range and low end.  And it is possible that YOUR carb setting will be different from another rider as you tune your carbs to your riding style and performance preferences.  But to have a starting point and reference, here's a link to 40 bikes that have been rejetted for air intake mods AND third party exhaust pipes.  VROC posts and suggestions

All tuning should be done on a fully warmed up engine because that's the way you usually ride;  the spark plugs should have been previously checked and cleaned; the gas tank should be at least half full;  a new or recently installed fuel filter;  the air intake filters checked for holes, and cleaned; and all air intake and manifold connections tight and secure;  the battery fully charged;  and the carburetors synchronized for optimum results; re-synched after a major jet change; and a final carb sync when you are done.  A STOCK BUBF A starts with 112/115 main jets and a #42 pilot (the B's starts with larger mains 130/132, and #42 pilot jets; and the C's start with 118/118, #42, I believe). 

This information has been collected from several web sites, VROC News Group, and from personal experience.  I suggest visiting the Factory Pro web site http://www.factorypro.com/ for their carb tuning page.  Although aimed at higher RPM sport bikes, the principals are good for Vulcans Keihin CV carbs.  Since most BUBF's don't have a tachometer, don't worry about references to RPM's.  High speed or top end main jet sizes are determined by full open throttle performance.  Low and mid-range tuning is determined mainly by how the bike feels to you as you roll the throttle. 

Factory Pro mentions float adjustments for low end performance.  I have not seen/read a lot about the carb floats having a lot of effect on BUBF performance unless the floats are way out of spec.  I'm guessing that Factory Pro is talking about the Mikuni CV carb which may be substantially different from the Keihin CV.  If someone knows the difference, please let me know so I can update this page.   You can easily check your float level with a short piece of clear plastic fuel line.  The process is described in your service manual and the Clymers manual.  It's worth the time to check float levels, if only to eliminate a possible problem area. 

A good general purpose starting point for rejetting after an ear-shave and new pipes is #140 main jets, and #45 pilot jets 2-3 full turns out.  You can then increase/decrease jet sizes as required to fit your riding style.

HIGH SPEED JETTING

High speed performance (50 MPH and up) is controlled by the main jets.  To make sure you have the right main jets, take your bike out to an open highway where YOU can safely (and legally) open the throttle ALL the way  in fourth or top gear.   Roll that baby full open and let the bike get to top speed.  Again, do this only if you are experienced and comfortable running at top speed, and you are not in danger of injuring yourself or someone else.

A.  If at top speed or before, the bike runs rough or begins to hesitant and buck, your main jet is TOO BIG.  The mix is too rich (more fuel than the optimum 1 part fuel to 14 parts air) and you are getting an uneven burn and poor performance.  Install smaller main jets and take the bike for another test run.

B.  If before top speed the engine is running smooth, but has no power, seems to be running hot, and you can't push the bike to top speed, then your main jets are too SMALL.  You are running too LEAN a mix (less fuel than the optimum 1 part fuel to 14 parts air) for maximum engine performance.  Put in LARGER main jets and take her out for another test run.

MID-RANGE OR MID-SPEED TUNING

Your carburetor needle controls mid-range performance.   The needle tapers down to a  point that fits into the main jet.  The needle is lifted upward (along with the slide) by the carb diaphragm as the air flow increases through the carb.  As the needle moves upward it is withdrawn from the main jet, allowing more fuel to mix with the increased air volume.  

All that is needed to modify mid-range performance is to place (or remove) one or more small, thin washers between the diaphragm and the head of the carb needle where it's held in the diaphragm.  Adding washers effectively RAISES the tapered needle further out of the main jet at all engine rpms and allows more fuel to mix with air to create a richer mix.  Removing a washer effectively leans out the air-fuel mix.  

There's usually no need to replace the stock needles since they have proven over time to provide good performance and good gas mileage.  If you do use a third party needle for increased performance, then anticipate lower gas mileage (in the 28-32 MPG range, versus the 36-42 mpg you can get with stock needles).

In the stock BUBF A, the rear needle is thinner than the front needle.  This will richen the rear cylinder mix at all rpm's.  This is also why the stock BUBF A rear cylinder main jet is larger than the front main jet (115 versus 112).  It is my minimal understanding that this is done to reduce rear cylinder overheating.  If someone has a better explanation, please send it along so I can update this page.

Mid-range tuning is accomplished mainly by how the bike performance feels to you.  If there's a smooth increase in power as you roll the throttle, then you're there.  If there's a slow response or there's no power, then the mix may be too LEAN and you may need to add one or more washers to richen the mix.  If the bike accelerate with power, but feels rough, you may be too RICH and need to remove one or more washers to lean out the mix.   Now take the bike out for a test spin.  

LOW OR SLOW SPEED JETTING

Now you can tune for low end performance.  If you are getting a poor jump or hesitation off the line, or a lot of backfiring on deceleration, then you need more fuel through the pilot jet.  You increase fuel flow through the pilot jet by turning the pilot jet screw OUT a half turn on each carburetor.  It's best to start at 2 1/2 turns OUT on the pilot screws and then increase the turns OUT 1/2 turn at a time, and go for a test run.  Do this until you get the performance you want and/or there are no back fires on deceleration.  If you go more than 5 or 6 FULL turns out, you will probably have to install a LARGER pilot jets.  Stock BUBF pilot jets are #42.  The next step up, which is usually more than sufficient, is a #45 pilot jet.  Larger pilot jets are usually needed if you have removed the stock air intakes and surge tank and installed K & N filters which dramatically INCREASE air intake volume (see  Ear Shave write-up for details). 

CHANGING JETS 

After turning off the fuel form the tank and draining each carb bowl, use a 7mm wrench to remove the two bolts holding on the carb bowl to the bottom of each carb. Unscrew and remove the main jet which is the lowest jet protruding from the bottom of the carb. Install the new main jet. A good trick is once the jet is broken loose, use a 1-inch piece of vacuum line placed over the main jet to screw it out. Your fingers usually get in the way here, but the mains can be removed and started by hand after some practice! The pilot jet is next to and above the main jet. When reinstalling the carb bowl, be careful the rubber seal along the inner edge doesn't leak gas. 

NOTE!: Do yourself a favor and get a 7mm 'L' shaped ratcheting flat blade screwdriver

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The 'L' Shaped screw driver will be very useful for removing and tightening the jets. A regular small screw driver doesn't have the leverage to break the jets free easily or tighten them up well (IMHO).  Borrowed from Wiredgeorge's Ear Shave write-up.

The needles can also be accessed without removing the carbs from the engine, but the process requires moving the fuel pump to one side, and taking off the black rubber air ways that connect the carb air intakes to the surge tank.  The needle is under the black plastic carb top covers which are held in place by 4 screws.  There's a spring under the cover that forces the diaphragm, needle and slide downward.  Be careful the spring does not get away from you when you take off the top.  Be even more careful to make sure the diaphragm is fully seated in the groove around the top of the carb when putting the top and spring back into place, or you will have an air leak that will cause very poor running in that cylinder.  Keep an eye on how all the parts fit together as you add/remove a washers.  

There are several excellent write-ups in the VROC Archives on opening up the carbs to access the jets and needles.  Do an archive search on rejetting.  Read each one.  They all contain valuable information.  If you find better or clearer BUBF rejetting instructions or want to add or clarify this write-up, please e-mail me your suggestions and comments by clicking here to access the Feedback page. 

And always ride safe!

GENERAL PROBLEM SOLVING TIPS:

If you have poor power at small throttle openings and a surge at wider throttle then you may be running lean.  Quick ways to see if you are running rich or lean ;

APPLY CHOKE AFTER WARM - UP TO ARTIFICIALLY  RICHEN THE MIXTURE-
If you bike runs better with the choke on even after a few minutes of warm - up, your original mixture might be lean in the lower rev range. This test does strange things to the upper rev range, so don't use it at highway speeds.

REMOVE THE AIR FILTER COVER  ARTIFICIALLY  LEAN OUT THE MIXTURE-
Temporarily remove the air liter cover (to INCREASE air to the mixture), and go for a test ride. A well tuned bike (where the air and fuel mix is right on) will run pretty badly (i.e. mixture too lean) when you do this , but if the mixture was originally too rich, the bike will run better.

If your bike's performance has a hitch or hesitation at certain speeds and you can't tell if it's rich or lean "stumble", leaning out the mixture is the easiest way to find out - if the mixture was originally too rich, the bike will run better immediately.  If the mixture was originally too lean, your bike will run worse immediately (Use the tuning techniques listed above to adjust the low, mid- and high speed ranges).

REMEMBER THAT NOTHING IS CERTAIN.
It's possible, however unlikely, to have rich running after installing an exhaust system, or lean running even after you've richened the main jet. KEEP AN OPEN MIND and keep on experimenting.

contributor - anonymous