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.



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).

Many thanks for the know who you are !




Jetting Kits -

Dynojet Kits -

Individual Air Filters - Most popular are those made by K&N and S&B.

Air Box Modifications - The standard FJ air box is restrictive, with an awful lot of air needing to pass through a relatively small opening. I've not done this 'mod' myself, but I know others have. So INFORMATION NEEDED

Nitrous Oxide - Haven't dabbled in this myself, so I can't speak from experience. By it's very nature, the power boost is short lived, as only a limited amount of gas can be carried. Probably it's usefulness is restricted to the dragstrip and some heavy 'streetfighting', if you're so inclined. Try PDQ (here in the UK)

Turbos - Serious performance requiring serious money. Only a few FJ's around with a turbo fitted, most of them seem up for sale. However if you're really contemplating this one try Mr Turbo To know what to expect, read Bob Shammas' article on his turbo FJ.