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

[FJ12IRL]

Hi All,
I have spent the last 4 months restoring my 4CC Japanese Import FJ 1200. It had been stood out in the Irish weather for a few years so there was a load of corrosion all over it she ran when I bought it but the brakes were seized so I didn't drive it.
I now have it back to roadworthy condition with YZF600 front forks, fender and front and back wheels.
She starts and idles perfectly and pulls as she should up to 4.5-5k rpm but then starts misfiring as if its running out of fuel..
I've stripped the carbs and blown out all the jets and circuits (3 times), swapped out the fuel pump and fuel lines, changed the plugs, HT caps and coil leads and fitted a relay to the coils, all to no avail.
Also, I've fitted a new stock air filter, air box and ignitor box from an unrestricted 3CV model.
Can anyone offer any advice as to where the problem might be?
Thanks in advance.

[Pat Conlon]

The JDM (Japanese Domestic Market) FJ's were both power restricted and speed restricted as a condition of their sale in Japan. I have no experience with these models but I recall something about a restriction in the carb boots (smaller diameter) and a restrictive rev limiter in the tachometer.
Perhaps someone will chime in here.
I know the FJ guys over in the UK have come across this issue before. You may want to check in with them.
http://www.fjclub.co.uk/

Good for you on getting that FJ back on the road.   Cheers  Pat

[Harvy]

The JDM (Japanese Domestic Market) FJ's were both power restricted and speed restricted as a condition of their sale in Japan. I have no experience with these models but I recall something about a restriction in the carb boots (smaller diameter) and a restrictive rev limiter in the tachometer.
Perhaps someone will chime in here.
I know the FJ guys over in the UK have come across this issue before. You may want to check in with them.
http://www.fjclub.co.uk/

Good for you on getting that FJ back on the road.   Cheers  Pat

Correct Pat...... yes my JDM '91 would not go past 180KPH as there is a circuit inside the back of the speedo that somehow cuts ignition, and yes the intake manifolds have a smaller diameter.

Cheers
Harvy

[aj52]

Read this article
https://forum.thinkbike.co.za/archive/index.php/t-21969.html

Info on the 4CC engine is at the bottom of the page.

Hannes

[FJ12IRL]

Thanks for the replies everyone but I didn't mention that my engine won't rev past 5k rpm even in neutral.
It sounds like a rev limiter or fuel starvation to me but I've replaced the fuel pipes and paid attention to their routing so there are no kinks and checked fuel flow to the pump and to the carbs and it's all good.
I can't see how the restriction in the speedometer can affect engine rpm if the bike is standing still..
Anyone any other ideas?

[Pat Conlon]

Hmmmm....

Maybe your fuel pump is not flowing enough fuel?
Try bypassing the fuel pump and run a gravity fed line with an overhead suspended fuel container to feed the carbs directly.

If it were an ignition interrupt you could hear a pop when the ignition cuts out then refires the plugs.
The pop would come from the flash off of the unburned fuel in the cylinders.

[T Legg]

what do the plugs look like? do they carbon up after it misfires? maybe one of the ignition coils is failing

[Pat Conlon]

Read this article
https://forum.thinkbike.co.za/archive/index.php/t-21969.html

Info on the 4CC engine is at the bottom of the page.

Hannes

Thanks Hannes for this link

Holy crap, did you guys see the jet sizes in the JDM 4cc model?  It's no wonder it won't rev past 5k
87.5 for a main jet? (normal 110) That's gotta be a misprint

WTF?


For the Japanese domestic market:
Engine type: 4CC
Carb type: BS36 (manifold diameter 32.5 mm)
Main jet: #87.5
Main air jet: #45
Pilot jet: #42.5
Pilot air jet: #135

Full-powered models:
Engine type: 3XW
Carb type: BS36 (manifold diameter 36 mm)
Main jet: #110
Main air jet: fixed
Pilot jet: #42.5
Pilot air jet: #155

[aj52]

Read this article
https://forum.thinkbike.co.za/archive/index.php/t-21969.html

Info on the 4CC engine is at the bottom of the page.

Hannes

Thanks Hannes for this link

Holy crap, did you guys see the jet sizes in the JDM 4cc model?  It's no wonder it won't rev past 5k
87.5 for a main jet? (normal 110) That's gotta be a misprint


WTF?


For the Japanese domestic market:
Engine type: 4CC
Carb type: BS36 (manifold diameter 32.5 mm)
Main jet: #87.5
Main air jet: #45
Pilot jet: #42.5
Pilot air jet: #135

Full-powered models:
Engine type: 3XW
Carb type: BS36 (manifold diameter 36 mm)
Main jet: #110
Main air jet: fixed
Pilot jet: #42.5
Pilot air jet: #155

Pat

While I am waiting for carb kits,I've put my ABS's carbs with standard exhaust system on the other FJ.
What I can confirm is that the jets on the intake boots side,is 135.
I wll open di carbs up when other carbs are sorted to put back on the bike.

What I also can confirm is that bike is nowhere near as lively with the ABS's carbs than with the other set and aftermarket exhaust.

Hannes

[FJ_Hooligan]

For US models with stock components;

The Main Air Jet is fixed and not replaceable.

The Air Pilot Jet is #155

Pilot (idle) Jets are either #37.5 or #40

Main Jets are either #110 or #112.5

[Pat Conlon]

Thanks David! I made the corrections on the pilot air jets.
I know the USA models had smaller pilot jets (37.5/40) than the European models (42.5).

What do you think of that main jet size on the JDM (87.5) Is that a typo?
I have a hard time believing that a FJ would run with a main jet that small.

[RPM - Robert]

I know the XJR1300 we worked on that came from Japan had a 95 main jet in it. Whether that was stock for those years or not I do not know. I do know the stock main jet needle on the XJR1300 has very little taper making it very lean. The slides also have a smaller hole next to the main jet needle compared to the US FJ.

[FJ12IRL]

Well I've torn down my carbs (yet again) only to discover that the main jets are in fact #875's ..
I rigged up a gravity fed fuel tank and tried it that way before I stripped the carbs and there was no improvement.
So could the small size of the main jets really cause this issue?
And if I replace them, what size should I go for?

[Pat Conlon]

To start with, I would stay with stock sizes: 112.5 main jets and 155 pilot air jets.  All available from RPM

Later on if you add an aftermarket header, Unipods and ignition advance, you could go to 115 or 117.5 mains and shim the needles 1/2 step.

[FJ12IRL]

Thanks Pat, I've ordered the jets and I'll keep you posted on how she goes when I've fitted them.