Intake & Exhaust

Intake & Exhaust

Airfilters

Tests were done using SAE J726C Test Method.
As you can see, K&Ns are great for airflow, which is what they were designed for. Their original application was on racing engines, where airflow is important and ultimate engine life was of little concern. They are not as good at filtering as paper or oiled foam types.
Rating: 5-best --> 1-worst

Individual Filters vs Factory Airbox

Individual filter obviously improve air flow but, there are a couple of things that you should know before you commit to removing the factory airbox.
First, apparently this is a problem experienced with most all pod type filters. When I first purchased my FJ it was fitted with individual filters. I took them off to clean them. When I put them back on the far left filter fell off while I was riding. The frame curves in at the point where the filters should sit. When combined with a short lip on the intake side of the carburetor it makes keeping individual filters on the FJ near impossible.
Second, Due to the confined space in which the filters must sit the inside two get much less air flow and are exposed to a lot of heat. This makes tuning difficult.
I am not saying by any means that fitting individual filters on the FJ is a terrible idea. I just think that before the owner goes through all of the work to get the factory airbox out of the frame that they should have all the facts.
It has been suggested that a third option is possible. Cutting & gutting the factory airbox to contain individual filters is an interesting idea especially if you were to open it up a bunch. Ram air? Sounds like a fun project to play with during the winter months.

Jetting Kits

Dynojet defines the differences between kit types this way. I have found it very simple to use this as rule of thumb when dealing with such things.
Stage 1 - stock airbox, stock pipe, K & N insert air filter
Stage 2 - stock airbox, aftermarket pipe, K & N insert air filter
Stage 3 - K & N individual filters, aftermarket pipe
Tip: One should avoid using the drill bit offered with some jetting kits. Once this is done it can be very costly and a damned hassle, to go back once this is done. Ask me how I know.

Replacement Carburetors

Don't bother with the 40mm constant velocity GSXR 1100 carbs. I tried that one myself. Rick Tomasic, one of Canada's top tuners, and I, spent four hours on the dyno with an exhaust gas analyser tuning the carbs. They were pretty close but the needles need to be changed to a different and unknown specification. On the road, the 40mm CV's produced significantly less power both down low and on top compared to the Mikuni RS 38's. The only area that seemed to produce okay power was around 6000 RPM. The bore spacing between carb #2 and #3 needs to be made smaller
Why did I want to replace the Mikuni RS 38's which produced lots of power with constant velocity carbs in the first place? Driveability!
The RS 38's have a nasty habit of sticking open at all throttle openings when you try to back off. Granted, you'll need to run a throttle return cable and very stiff return springs but the problem will persist. Subtle throttle adjustments, such as needed in the city, are almost impossible. Lastly, anything below 4000 RPM is almost unusable.
I ended up purchasing Kehin FCR 39mm carbs and have been extremely satisfied. They have rollers on the throttle slides so they don't stick, have excellent sub 4000 RPM response and produce as much power on top as the RS 38's. They are available pre-jetted for the Yamaha XJR 1300 and fit the FJ 1200. The only problem is that they retail for almost $2000.00 Canadian-Ouch! If you are tight with a certain dealer you can do much better though. Brian, owner of Ultimate Cycle in Stoney Creek Ont. also put a set on his 148 HP FJ 1200 and has been extremely happy. Jon, if you want any other info, you can give me a call at 519)-641-4123
In summary:
Stock CV 36mm carbs - great drivability and power down low but limited top end
Mikuni CV 40 mm GSXR 1100 carbs - soft down low and on top but OK midrange and good driveability
Mikuni RS 38mm flatslides - Lots of power over 4000 RPM but miserable driveability
Kehin FCR 39mm flatslides - Lots of power everywhere and almost as easy as CV carbs to live with.
[Clint Komenda]

Using Mikuni Flatslides with a Fuel Pump

Our friend and hero Mr. Jarosz ordered a Pingle high-flow fuel petcock and a set of 38mm Mikuni flatslides to use this season in his campaign to go faster. He asked if anyone had any advice on the matter. His post yielded the information above and this bit of advice from Jon Cain:
I have a set of RS36 Mikuni flatslides on my 89 FJ and the stock fuel pump works fine. Check the carbs you plan to use to make sure that they are set up with the correct float valves. Later FJ's come with 1.5 valves but most flatslides come with 2.8 valves. The 2.8 valves are for gravity systems and are too big for a system with a pump. My carb vendor recommended that I use a regulator with the stock pump and the 2.8 valves. I didn't want to add a regulator so I moved the 1.5 valves from my stock carbs to my new flatslide carbs and they no longer leak a drop. I tried to run with no pump but there was just not enough flow with less than a totally full tank to keep the carbs supplied at speed. Your new high flow petcock may fix that problem. I also was surprised to find that the float levels were way off on my brand new flatslides. [Cain]

Nitrous Oxide Systems

Called "the juice" by racers and riders everywhere nitrous oxide kits are the best way to get a monstrous increase in BHP with little effort. Nitrous will seriously tear a motor up doing damage to the top end. It is advisable to rebuild the top end with some seriously stout parts before you install a kit.

How to Build a Bike You Cannot Ride

Every piece of information that I have seen on this indicates that the results are shoulder wrenching. Once the motor is built to the point of being scary by itself you slap one of these 2600 dollar kits on top of it. The end product makes more BHP than a stock big block V-8. Sound fun? Then there's probably a 12 step program for you or you can click the picture for information on how to contact Mr. Turbo.
A recent inquiry has discovered that Mr. Turbo no longer makes a kit for the FJ. They will put one

together for you by special request during their off season. Strangely enough, they consider the winter time the off season for motorcycle modification.

Choke Knob Screw

Yamaha in it's infinite wisdom does not offer this screw as a replacement part. It is small enough that the most coordinated person on the planet could lose it on a good day. In order to replace it from OEM's one must order the entire choke cable assembly.
The dimensions of this screw are: M2.5 X 6mm, long
It is recommended that one look at a sewing machine shop or their local hardware store for the replacement. The sewing machine shop is the best call because apparently you can find allen headed replacements there which allow you to remove the choke knob without removing the gas tank. (Whoo Hoo!) While you are there purchase several of them. If they want more than $2 USD for a package of four screws run, don't walk, to the next store.

Replacement Exhaust Systems

This is one of the most popular replacement items. The factory exhaust is very heavy weighing in at about 30 pounds. It is not difficult to find a replacement that weighs half of that.
When we start talking about model year availability as related to exhaust one should keep in mind that what really changed was the location where the muffler section of the pipe was mounted. The header section should be the same though all the model years. Given that this is only a minor change it would be possible to hang an exhaust for a 93 on an 84 with a bit of good old fashioned ingenuity.

Reviews of UK & US Exhausts on the FJ Modifications Page.

[What's New] [About The Webmaster] [Accessories] [Brakes & Suspension] [Buyers Guide]
[Cosmetic Mods & Repairs] [Rims & Tires] [The Gallery] [The Links]
[Project FJ]

BACK TO FJMODS SITE

Airfilters
	Tests were done using SAE J726C Test Method. As you can see, K&Ns are great for airflow, which is what they were designed for. Their original application was on racing engines, where airflow is important and ultimate engine life was of little concern. They are not as good at filtering as paper or oiled foam types. Rating: 5-best --> 1-worst

Replacement Carburetors
Don't bother with the 40mm constant velocity GSXR 1100 carbs. I tried that one myself. Rick Tomasic, one of Canada's top tuners, and I, spent four hours on the dyno with an exhaust gas analyser tuning the carbs. They were pretty close but the needles need to be changed to a different and unknown specification. On the road, the 40mm CV's produced significantly less power both down low and on top compared to the Mikuni RS 38's. The only area that seemed to produce okay power was around 6000 RPM. The bore spacing between carb #2 and #3 needs to be made smaller Why did I want to replace the Mikuni RS 38's which produced lots of power with constant velocity carbs in the first place? Driveability! The RS 38's have a nasty habit of sticking open at all throttle openings when you try to back off. Granted, you'll need to run a throttle return cable and very stiff return springs but the problem will persist. Subtle throttle adjustments, such as needed in the city, are almost impossible. Lastly, anything below 4000 RPM is almost unusable. I ended up purchasing Kehin FCR 39mm carbs and have been extremely satisfied. They have rollers on the throttle slides so they don't stick, have excellent sub 4000 RPM response and produce as much power on top as the RS 38's. They are available pre-jetted for the Yamaha XJR 1300 and fit the FJ 1200. The only problem is that they retail for almost $2000.00 Canadian-Ouch! If you are tight with a certain dealer you can do much better though. Brian, owner of Ultimate Cycle in Stoney Creek Ont. also put a set on his 148 HP FJ 1200 and has been extremely happy. Jon, if you want any other info, you can give me a call at 519)-641-4123 In summary: Stock CV 36mm carbs - great drivability and power down low but limited top end Mikuni CV 40 mm GSXR 1100 carbs - soft down low and on top but OK midrange and good driveability Mikuni RS 38mm flatslides - Lots of power over 4000 RPM but miserable driveability Kehin FCR 39mm flatslides - Lots of power everywhere and almost as easy as CV carbs to live with. [Clint Komenda]



Using Mikuni Flatslides with a Fuel Pump
Our friend and hero Mr. Jarosz ordered a Pingle high-flow fuel petcock and a set of 38mm Mikuni flatslides to use this season in his campaign to go faster. He asked if anyone had any advice on the matter. His post yielded the information above and this bit of advice from Jon Cain: I have a set of RS36 Mikuni flatslides on my 89 FJ and the stock fuel pump works fine. Check the carbs you plan to use to make sure that they are set up with the correct float valves. Later FJ's come with 1.5 valves but most flatslides come with 2.8 valves. The 2.8 valves are for gravity systems and are too big for a system with a pump. My carb vendor recommended that I use a regulator with the stock pump and the 2.8 valves. I didn't want to add a regulator so I moved the 1.5 valves from my stock carbs to my new flatslide carbs and they no longer leak a drop. I tried to run with no pump but there was just not enough flow with less than a totally full tank to keep the carbs supplied at speed. Your new high flow petcock may fix that problem. I also was surprised to find that the float levels were way off on my brand new flatslides. [Cain]



Nitrous Oxide Systems
	Called "the juice" by racers and riders everywhere nitrous oxide kits are the best way to get a monstrous increase in BHP with little effort. Nitrous will seriously tear a motor up doing damage to the top end. It is advisable to rebuild the top end with some seriously stout parts before you install a kit.

Replacement Exhaust Systems
	This is one of the most popular replacement items. The factory exhaust is very heavy weighing in at about 30 pounds. It is not difficult to find a replacement that weighs half of that. When we start talking about model year availability as related to exhaust one should keep in mind that what really changed was the location where the muffler section of the pipe was mounted. The header section should be the same though all the model years. Given that this is only a minor change it would be possible to hang an exhaust for a 93 on an 84 with a bit of good old fashioned ingenuity.
Reviews of UK & US Exhausts on the FJ Modifications Page.