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

[FJ_Hooligan]

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.

Pat,

A #48 (0.076 inch dia) is too big and a #49 (0.073 inch dia) is slightly loose, it will wiggle just a tad.

That is if you trust a set of number drill bits from Harbor Freight......  :-)

[Pat Conlon]

Thanks David.
Pardon the stupid question: On the #'s used for the Mikuni jet sizes, are those jet #'s equivalent to the #'s used on the drill bit sizes, or are they different?

Never mind, I think I've answered my question:
The larger the Mikuni jet # the larger the jet diameter.
However, it appears with your above information on the drill bit #'s the larger the drill bit # the smaller the diameter. (#48=0.076 and #49=0.073)

[oldktmdude]

Thanks David.
Pardon the stupid question: On the #'s used for the Mikuni jet sizes, are those jet #'s equivalent to the #'s used on the drill bit sizes, or are they different?

Never mind, I think I've answered my question:
The larger the Mikuni jet # the larger the jet diameter.
However, it appears with your above information on the drill bit #'s the larger the drill bit # the smaller the diameter. (#48=0.076 and #49=0.073)

Pat, the Mikuni jet size is the actual size in millimetres (hole diameter) i.e. jet size 112.5 = 1.125mm diameter.

[Pat Conlon]

Thanks Peter. I used to know that........

So, if the JDM carbs have a #45 main air jet, (0.45mm) that has has an opening of  0.018" diameter

In our unrestricted carbs, David can get a loose fit in the main air jets with a 0.073" drill bit which is ~ 1.85mm, this tells me that Nigel (OP) needs to open up his JDM main air jets if he wants to run our larger main jets.

...or if his JDM main air jets are removable, he's looking for a #185 or 190 air jet?  (if my math is correct)

Going from the JDM #45 main air jet to a #185 air jet sounds a bit much, however, on the fuel side, going from the JDM #87.5 main jets to our 112.5 jets is also a big jump.

Makes sense, the logic being: More fuel flowing thru the carbs needs more air to make it work.

Nigel might as well replace his JDM #135 pilot air jets with some #155's which we run.

[aj52]

Thanks Peter. I used to know that........

So, if the JDM carbs have a #45 main air jet, (0.45mm) that has has an opening of  0.018" diameter

In our unrestricted carbs, David can get a loose fit in the main air jets with a 0.073" drill bit which is ~ 1.85mm, this tells me that Nigel (OP) needs to open up his JDM main air jets if he wants to run our larger main jets.

...or if his JDM main air jets are removable, he's looking for a #185 or 190 air jet?  (if my math is correct)

Going from the JDM #45 main air jet to a #185 air jet sounds a bit much, however, on the fuel side, going from the JDM #87.5 main jets to our 112.5 jets is also a big jump.

Makes sense, the logic being: More fuel flowing thru the carbs needs more air to make it work.

Nigel might as well replace his JDM #135 pilot air jets with some #155's which we run.

Hi Pat
The set of carbs with the aftermarket exhaust has 112.5 main jet and 157.5 air pilot jet. The other jet,think it is called pilot jet is 42.5.
I don't know where main jet sits to check that. Is like I said,the bike ran very well with that setup and fuel consumption was also way better than the JDM set with OEM exhaust.

Hannes

[Pat Conlon]

Were these JDM carbs you converted?

[aj52]

And the other thing was when I put the ABS's carbs with the aftermarket exhaust,from about 4500rpm,it blew fire out of exhaust.

[aj52]

Were these JDM carbs you converted?

Both my bikes are JDM. I bought the one with the aftermarket exhaust in March. Don't know if you will recall it,but it is the carbs that some idiot used thread seal tape at the valve seats.
That is why I put the ABS's carbs and exhaust onto it. ABS is work in progress.
Where are the main air jet located,then I can check tomorrow what size it is

[Pat Conlon]

Main air at 9 o'clock.

Pilot air (removable) at 1 o'clock

[aj52]

Main air at 9 o'clock.

Pilot air (removable) at 1 o'clock

Thanks Pat,will check tomorrow.
I didn't know there is also a jet in that hole.
The ones at 1 o'clock are 157.5

Hannes

[Pat Conlon]

And the other thing was when I put the ABS's carbs with the aftermarket exhaust,from about 4500rpm,it blew fire out of exhaust.

If you put larger fuel main jets in and are still running the small (#45) JDM main air jets, I could see this happening.
As DavidR posted above ^^^  more air is needed to be introduced into the carb to properly atomize the extra fuel (from the larger jets).
Sounds like you have a lot of raw fuel running through your engine. 

[aj52]

That was what I thought after I saw the flames coming out Pat
Can't use the standard JDM carbs with aftermarket exhaust,it needs to be jetted

[RPM - Robert]

FJ12IRL you also need to know what main jet needle and emulsion tube is in your carbs. If they are different that what we have in the unrestricted models, just changing jet sizes might not resolve the issue.

The needle and emulsion tube are both numbered and need to be identified

[FJ_Hooligan]

And the other thing was when I put the ABS's carbs with the aftermarket exhaust,from about 4500rpm,it blew fire out of exhaust.

If you put larger fuel main jets in and are still running the small (#45) JDM main air jets, I could see this happening.
As DavidR posted above ^^^  more air is needed to be introduced into the carb to properly atomize the extra fuel (from the larger jets).
Sounds like you have a lot of raw fuel running through your engine.  

BIG CAVEAT here.  The main jet doesn't really determine the fuel flow until the needle is pulled most of the way out of the needle jet (emulsion tube) probably beginning somewhere around 3/4 throttle.  At anything less than that and the needle position and shape are controlling the fuel delivery in the main circuit.

If you never get to 3/4 throttle, you could run without a main jet and possibly not even notice.  Except that the main jet holds the needle jet in place.  :-)

Which carb circuit/jet has the most influence/control on fuel delivery is determined by throttle position, NOT RPM.

I can cruise on the freeway at 80 MPH with less than 1/8th throttle.  The size of the main jet has ZERO influence at that throttle position.  On the otherhand, attempting to accelerate from, say, 50 to 100 MPH using WOT means the main jet has LOTS of influence.

And the carb circuits have a lot of overlap.

[Pat Conlon]

I hear ya David.
The goal of my exercise is to get the jetting in FJ12IRL (Nigel's) JDM carbs close to what we have.
He still has air flow to address along the that mystery ignition interrupt.

Besides, I've been taught that getting the main jet correct is the first step in the tuning process.

http://www.factorypro.com/tech_tuning_procedures/tuning_carbtune,CV,high_rpm_engines.html