'91 FJ1200 won't rev past 5k rpm under load PLEASE HELP!

[Pat Conlon]

Good deal.
While you wait on the jets, why not take your coils off and bench test them (primary and secondary) to rule out a dodgy coil.

I just thought of something....

Re:Jetting, a DavidR pointed out the main air jets are fixed on our USA carbs, yet the JDM specs show the main air jet to be a #45 size jet...so the 3 questions are:
1) Even though the USA main air jets are fixed, does anyone know the size?
2) Does the OP need to take out the JDM #45 main air jets and install the size used in our USA bikes?
3) In lieu of buying new main air jets, perhaps just drilling out the JDM jets to the correct size used on our bikes?

[FJ_Hooligan]

If the bike won't rev past 5K then it's either a rev limiter built into the ignition or an air flow limit. 

Adding more fuel via changing jets will not solve your problem.

[FJ_Hooligan]

Just for reference, drilling an AIR jet larger will allow more air into the circuit and thus make it leaner.

The DJ kit includes a #144 air pilot jet which is smaller than the stock #155 to richen up the idle mixture.

[Pat Conlon]

David, do you think the OP will be ok with the JDM #45 main air jet and the 112.5 and 155 jets?

I was curious how the JDM #45 jet compared to our fixed main air jets.
The idea was to get his carbs back to the un restricted jetting.

Of course there is still the issue with the smaller JDM 32mm intake boots choking down the air flow.
A set of new 36mm boots should solve that issue:
http://rpmracingca.com/proddetail.asp?prod=36Y-13586-00-00%2C36Y-13596-00-00&cat=39

Does anyone have a wiring diagram on the JDM '91 FJ? I wonder if there is a sensor wire running from the ignition box to the speedometer or Tachometer the OP could cut.
I know of a sensor wire which sends a tach signal (or is it from the coil?) to activate the fuel pump, and deactivate the fuel pump when no tach signal is present (safety shut off)

[FJ_Hooligan]

David, do you think the OP will be ok with the JDM #45 main air jet and the 112.5 and 155 jets?

I was curious how the JDM #45 jet compared to our fixed main air jets.
The idea was to get his carbs back to the un restricted jetting.

Pat, I have no data on that, only a guess.

If the air jet and main jet are somehow a "matched" pair, then that would probably mean the #45 air jet is smaller than the fixed "unrestricted" air jet that is used with the 110 - 112.5 main jets.

The #45 air jet being smaller should tend to richen up the a/f mixture in the emulsion tube even more if larger main jets are installed.  

The main question is, is the air jet a significant source of air for the overall needle jet circuit or is it more a source of bleed air to help promote atomization of the fuel as it's pulled up the emulsion tube and into the carb throat?  I know on other bikes DynoJet included glue-in inserts to change the size of the main air jet.  I would think this has an effect on the idle quality and throttle response at small throttle openings but that effect would diminish as more throttle was applied.  

The JDM carbs are intentionally jetted lean.  I can believe the factory would also limit RPM to a safe range to prevent that leanness from overheating the engine at higher revs.  But, that also brings up another option where if they are going to restrict the red line, why not just leave the carbs the same as unrestricted models?

If there is no 5000RPM rev limiter on a JDM, then do the smaller diameter intake manifolds provide enough restriction to limit the achievable RPM?  Doing a little math shows the 32mm intakes have a flow area that is ~80% of the 36mm manifolds.  Is that significant enough to restrict the engine to a red line that is almost half of an unrestricted model?  

I do have a somewhat related data point based on experience with my FZ1.  Hoffman was visiting me and we were out riding.  He was on the FZ and at an intersection he pulled up to me and said the FZ would NOT rev past6000 RPM.  Since it was my commuter bike I never rode it with much aggression and never revved it that high.  We swapped bikes and sure enough at about 6K the motor would just stop accelerating.  If I continued to twist the throttle more the bike would actually start to slow down.  

When we got back home the first thing I did was change the original fuel filter thinking it might be clogged after ~50 Kmiles.  I couldn't shake any debris out of the old filter, and it looked fine.  A road test showed the bike still nosing over at 6K.  I pulled the seat to look at the air box.  That's when I noticed about 4 inches of an old tank bag strap had been sucked into the airbox intake port.  I had changed from a strapped tank bag to a magnetic bag a few months earlier and hadn't bothered to remove the strap from under the seat.  That strap is about 1 inch wide.  So at high air flow it was getting sucked up against the air filter and blocking approximately 4 square inches of the intake area.  I have no idea what the actual intake area is, but the loss of 4 in^2 was significant enough to essentially limit the bike to a 6K redline.

And that's how I know that lack of revs is an airflow issue rather than a fuel issue.  Once I removed that strap, the bike freely accelerated past 6K up to red line.

[FJ_Hooligan]

I probably should avoid such absolute conclusions. 

If the fuel mixture is off bad enough it will affect an engine's ability to rev also.  So it's not just airflow.

[FJ12IRL]

Hi all, from my research I have discovered that the Japanese Domestic Market 4CC FJ 1200 is restricted to 180kph (112mph) and around 90bhp by
1. Airbox restriction in the horn by about 50%
2. Electronic limiter in the CDI igniter box
3. Restriction in the speedometer.
4. Reduced diameter of carb rubbers.

To counter these limitations I have fitted an ECU, instrument cluster and the airbox from a 3CV model.
So It came as a surprise to me to find out that there are restrictions in the carb jetting too.

She starts, idles and pulls well up to around the 5k mark and that is where the main jet takes over from the slow running jets.

So I can only assume after removing the restrictions the smaller main jets must be at fault.
Unless the coils are faulty.. but then how is it she idles sweetly and runs lovely up to 5k?

My head is about to fall off...

                                                                                                                                       

[FJ_Hooligan]

Are you still using the 32mm intake manifolds?

[oldktmdude]

   Make sure that the main jet washers are in place. I just had an experience, fixing a problem with an FJ, that would not rev past 6,000rpm.
The main jet washers were missing and the main jets were above the fuel level in the float bowls. Just a thought.
   Regards, Pete.

[Pat Conlon]

......My head is about to fall off...                                                                                                                                    

Stay with it Nigel.

What you're going thru is going to help the next guy.
I fully intend to save all your information and findings in the Files for easy future reference.

As you can tell, we don't see JDM FJ's very often.

David, thanks for taking your time in sharing your knowledge with us. I learn something every time you post.

Hey Randy or Robert, can you do us a favor? Can you grab a carb and see what drill bit size fits the fixed main air jet in our USA carbs. Again, I wonder how that size compares to Nigel's #45 JDM air jets.

Nigel, can you see if your JDM main air jets are removable?

Cheers

[aj52]

I have no problems with both the FJ's to go to the red line.
It is only cutting out at 180,when the limiter kicks in.
I don't think there is a rev limiter.
The one with the aftermarket exhaust is more lively than the ABS one.
It is jetted differently than the ABS.

Hannes

[FJ_Hooligan]

It is only cutting out at 180,when the limiter kicks in.
I don't think there is a rev limiter.

Please explain these two statements.

There's a limiter but it's not a rev limiter?  Then what kind of "limiter" is it?

[aj52]

It is only cutting out at 180,when the limiter kicks in.
I don't think there is a rev limiter.

Please explain these two statements.

There's a limiter but it's not a rev limiter?  Then what kind of "limiter" is it?

Both of them goes to the red line in 1st and 2nd gear,in 3rd when you hit 180 on the speedo,something interrupts that the bike doesn't want to pick up speed. Feels like the bike is surging,at about 175 the bike pulls again till 180 and start surging again.
When the ABS is finished,I am busy with upside down forks,I will then be able to see if it is the speedo that has something to do with it.

Hannes

[FJ12IRL]

Are you still using the 32mm intake manifolds?

Yes, I am.. but I think I'll mod them out to 36mm

[Pat Conlon]

FYI I ping'ed Randy....
He will be able to get an answer for us next week on the size of the USA unrestricted main air jets.
Fractional drill bits won't work, he needs his dad's numbered drill bits.

Stay tuned.