carbs (view original)
How our carbs work
How the BS36 CV Mikuni carbs work:
The key to the CV carb is the vacuum controlled slide. It responds to the velocity (and vacuum) produced by the air flowing through the throat of the carb. CV stands for CONSTANT VELOCITY, and that's what the slide seeks to maintain. At a steady state throttle position, the slide is held at a position that results in a certain air flow velocity. During acceleration, as the throttle (and thus the throttle plate) is increased, more air is allowed to flow through the carb.
For more air to flow, it has to speed up. When it speeds up, it creates a higher vacuum which initially picks up more fuel (kind of like an acceleration pump on a Holley). The slide diaphragm also sees this increase in vacuum and reacts by pulling the slide further up, exposing more throat area to the air flow. The larger throat area causes the air flow to slow back down to the steady state (i.e. constant) value. The slide tries to keep the air flow velocity through the carb at a constant value under all conditions.
This is where Andy's reference to RPM comes in. Pinning the throttle allows the maximum air flow for a given RPM. Because you can't instantly accelerate from 1000 RPM to redline (at least not while riding, not counting missed shafts), the engine doesn't need a bunch of fuel when you stomp on it. As the engine RPM increases, more air is drawn through the intake, and the slide rises to maintain the constant velocity throught he carb. As the slide comes up, the needle exposes more of the needle jet and allows a proportional increase in the amount of fuel pulled into the flow.
If you do anything to an intake component which disturbs the speed of the air, the carb needs to be adjusted to compensate. The best example is air filters. A "less restrictive" filter will allow air to flow "easier" which results in a lower velocity. The slide reacts to a lower velocity by dropping to a lower position which reduces the carb throat area and thus speeds the flow back up to the desired "constant" value. You're now too lean because the jet needle is lower in the needle jet and not flowing as much fuel for the given amount of air passing through the carb.
DavidR.
More info: http://www.fjowners.com/index.php?topic=573.0
THE GUIDE TO RESTORING
*THE* FJ Carb cleaning guide can be found here.
Pictures for reference are here.
I'd like to combine them, but the PDF guide is not text editable by me. Maybe if someone has some OCR that would allow them to copy/paste, they could paste the text here, and someone could insert the pictures.
Another source: http://www.fjowners.com/index.php?topic=7730.0
If you plan on rebuilding your carbs, you should ABSOLUTELY purchase this 122 washers/rings/screws kit from RPM for $22.
Source for cheap diaphragms if yours are shot. [Update: according to Randy at RPM, these do not allow the slides to open up the last 4-6mm of travel, meaning your bike will run lean at Wide Open Throttle; otherwise, the bike seems to run fine, according to various testers.[1]]
Options for repairing your diaphragms include liquid electrical tape, or Spray-on Plasti-dip[2]. (The stuff you are looking for is at your local NAPA store. The part number is 765-2527 "Plasti Dip Spray-on Heavy Duty Flexible Rubber Coating.")
Alternate Carb Setups
From the FJmods site.
The standard FJ carburation, is designed for performance throughout the entire rev range. However manufacturers also have to make allowances for emissions, noise etc, to comply with laws and regulations. It is possible to enhance the FJ's performance, enabling it breathe more freely and thus increase the power. Often adjustments need to made with an aftermarket exhaust system . Manufacturers sometimes claim no rejetting is needed; they may be right. However, rejetting frequently restores much of the low to mid-range power often lost, when a free-flow exhaust system, designed primarily to increase the upper rev range, is fitted.
ALTERNATIVE CARBURETORS :-
I have no experience of these myself, however one of my correspondents (whom wishes to remain anonymous) was kind enough to send me this article of his findings.
Ok Barry . I will do my best not to be confusing. The carbs work great in conjunction with a few other upgrades. To burn more fuel efficiently you need a hotter spark . A set of Dyna or Accel coils (I have tried both) work well in this area. A good aftermarket pipe is also required (quality of the finished product aside, it must flow more than the stock unit). Removal of the air box is mandatory.(so better get used to those external air cleaners). The VH powerpack and Nology coils amps further enhance the effects of all of the above (big bore kit,porting, polishing etc. are an even bigger bonus). Without the above , the effects of carburettor enhancements are seriously compromised. Now provided you have the essentials and an ENGINE IN EXCELLENT MECHANICAL ORDER, you are the ideal candidate for a carburetor upgrade. I have tried two different carburettor manufacturers in three different carb sizes. They are as follows:
Mikuni RS38, Mikuni RS40, Keihin FCR39.
Mikuni RS38&RS40: these are the more common carbs (cheaper) available. They offer power gains throughout the rev range. They are pretty straight forward to install.(although due to their large size it is a workout ). Jetting is also pretty straightforward (they utilize the standard Mikuni jets). They are also equipped with a choke, accelerator pump(tuneable),adjustable throttle spring. On the downside:- the throttle spring works best on the hard setting. (on the softer setting the butterflies remain open and must be closed manually, quite disconcerting when you are concentrating on riding ) but the throttle is VERY STIFF (a GOOD cruise control makes all the difference),and the choke is rather stiff (inconvenient,but you get used to it all).From a value\performance point of view : If more power\drivability is desired and your engine is HEALTHY they offer a great bang for the buck. The performance difference between the RS38& RS40 is negligible.(the bigger the carb the more potential for a bigger engine, when done up right). Now comes the POWER BABY.
Keihin FCR39:
WOW. These things KICK ASS. These carbs are light years ahead if the Mikuni's (new technology). They have no choke (don't really need one). The throttle spring is NICE and soft and always closes. They also have a fuel pump(tuneable). Jetting is also very straightforward (standard Keihin jets). They offer quite a difference over the Mikuni counterparts but are very expensive comparatively. I LOVE these things and after trying them out I just can't go back to the Mikuni's.
Some special notes: All of these carbs require external air cleaners.The Mikuni's are available for the FJ (last time I checked). The Keihin FCR's are not. (have to use the P\N for the XJR1200-1300). They all required a little fiddling with the throttle cables(some fitting required).They are a PAIN to slide into those intake manifolds (patience and persistence required,and lots of RUBBER CARE).All the carbs required a complete disassembly and a thorough cleaning and calibration (float levels,air screws, etc.). I haven't found one that works good out of the box (too long in storage probably).
More info: http://www.fjowners.com/index.php?topic=9677.0
Carb Tuning (especially after mods)
GREAT guide on tuning your carbs, especially after modification
http://www.factorypro.com/tech/carbtun.html
Their short version:
1. Have selected the BEST main jet for full throttle power (not just a "good" main jet - we mean "the BEST main jet" for power at high rpm).
That eliminates the common severe tweaking of the midrange and lower tuning ranges to compensate for a "wrong" main jet.2. Then - select the BEST needle height / clip position for power at FULL THROTTLE / MIDRANGE after selecting the BEST main jet -
That almost eliminates weird problems at cruise caused by tweaked needle heights that were required because the main jet wasn't correct…….3. Then, adjust the BEST Float Height for BEST FULL THROTTLE / LOW rpm (many Honda's excluded because floats are not adjustable) -
You should be able to apply FULL THROTTLE at LOW RPM in TOP gear without ANY misfire of bogging or stumble…….If you follow that order, you will have:
1. Best topend.
2. Best midrange.
3. Best low rpm power.Then - all you have left is dialing in the pilot circuit - i.e. mixture screw and pilot jet size - That's IT - Don't tweak needle heights and throw away full throttle midrange to try to fix a cruise issue!!! (Unless you want to!)
Jetting for altitude
The short info that you want, as an FJ owner:
Using a baseline as a 120 main jet (appropriate for a good pipe and a decent airfilter, a lil lean for pod users):
If you're going to go up more than 2000 feet in the mountains (or corrected altitude, if you run "well enough" at your home base), go down one jet size (2.5 in the nomenclature… 117.5 from 120, for example).
If you ride in weather 30 degrees colder than normal, go up a jet size (120 to 122.5, etc).
I just saved you six bucks for buying the little slide thing. You're welcome. Send me pizza and thai hookers.
In truth, if you're going WAY into the mountains, you're going to see some temperature changes that will compensate in part (it's f'king COLD). So jet it to pull hard and give good plug reads, and ride the thing. If you're racing, it makes more difference (but there's no correction for humidity, so you're really on your own, but you knew that because you're a racer).
More info:
http://www.fjowners.com/index.php?topic=573.0
Specs
Carburettor Type Mikuni BS36
Main Jet
All US, 1984 through 1987 UK 112.5
1988 and later UK 110
Main air jet 45
Jet needle (-clip position)
FJ1100 US 5FZ63-3
FJ1100 UK 5FZ62-3
FJ1200 US 5FZ72
FJ1200 UK 5FZ74-3
Needle Jet
FJ1100 Y-0
FJ1200 Y-2
Pilot air jet
FJ1100 160
FJ1200 110
Pilot jet
US 37.5
UK 1984 through 1990 40
1991 on 42.5
Pilot screw
US Preset
UK 1984 through 1990 2 turns out
1991 on 3 turns out
Valve seat size
US 2.3
UK 1984 through 1987 2.3
1988 on 1.5
Starter jet 30
Float height 22.3mm ± 1.0mm (0.88in ± 0.04in)
Fuel level
FJ1100 3.0 ± 1.0mm (0.12in ± 0.04in)
FJ1200 2.5 - 3.5mm (0.098 - 0.138 in)
—Source: http://fj1200.8m.com/Library/FJ_Tech.htm
Buy New Carbs
Carbs in rough shape? All of this sound like too much work, or a bit out of your mechanical aptitude? Just feel like blowing some money on something pretty? You can buy a brand new set of carbs here, at RPM, for about $1250 bucks.