FJ1100 Bad flat spot from 2000-2800 RPM

[tscheel]

I owned the bike for 20 years and kept it in a usable state. Now I need new ideas - the flat spot has been developing slowly over the years, but now it have become unbearable. Uphill I can sit with full trottle at 2000 RPM - it never goes above. A gearchange to 3000 RPM and it takes off like an FJ should. Choker or throttle positioin changes nothing.

I have cleaned the carbs (no access to untrasound cleaner, but I did my best with carb cleaner), replaced all brass and o-rings in the carbs and adjusted float heights - it starts a lot better now, but the flat spot remains. Then the diaphragms were replaced as they had some tears after 39 years - but no difference.

Valve adjustment are not perfect but OK, plugs are equally grey. Fuel economy ca. 15-16 km/l with relaxed touring.

Can bad compression/worn cylinders cause this? Ignition box leak? Ignition timing? What am I missing?

[Motofun]

Is the bike still (mostly) stock?  Messing with intakes and exhaust and not compensating with jetting can cause a weird torque curve though yours sounds really off.  Still sounds like a bad jetting or perhaps a serious in-leak of air through bad boots?  Once enough fuel is dumped into the mix to compensate for the extra air it sorts itself out?

[tscheel]

Not stock, but the same setup for 19 years. New jets are same size as the old, needle in the same spot, and the problem is exactly the same after the cleaning and replacement of parts. It have just developed slowly, so the suspects are compression, and air leaks. But maybe I'm missing something obvious.

Original airbox, 1100 engine and ignition with 1200 cylinders, std. jets for 1100, Remus 4in1.

[Pat Conlon]

Assuming your needle jets and pilot jets are clean.
If you are running full throttle at 2k rpm while going uphill and not accelerating, something is holding your engine back and I suspect it's lack of fuel.

1) Check the opening at the bottom of the needle jets, aka emulsion tubes (where the jet needle passes through) these holes can become elongated causing weird rich fueling gremlins. These needle jets wear out and should be replaced. The vibrations of the jet needles rattling against the sides of these openings causes this wear on the soft brass needle jet.
Here's some further reading on the subject:
https://store.moto-lab.com/articles/emulsion-tube-inspection-guidelines
http://www.factorypro.com/tech/needle_jet_wear.html
[edit] I re-read your post, if you replaced "all brass" I would read that to mean your emulsion tubes are also new....if so, disregard the above...^^^

2) Check your CV vacuum circuit for blockages. Possibly under low vacuum the slides are not raising thus causing the bog, yet once you downshift and raise the rpms the stronger vacuum is able to raise the slides.

3) I assume the slides move freely in the bores of the carb body?

4) With the air box off, with your finger, reach in and lift the slides....let them drop....are they all falling at a constant rate or is one or two falling faster than the others? They should all fall at the same rate. The ones that fall fast are the ones that have a vacuum leak.

Report back, we will get to the bottom of the problem.
Once upon a time your FJ ran right, rest assured,  it will do so again.

Cheers.  Pat

[tscheel]

Assuming your needle jets and pilot jets are clean.
If you are running full throttle at 2k rpm while going uphill and not accelerating, something is holding your engine back and I suspect it's lack of fuel.

1) Check the opening at the bottom of the needle jets, aka emulsion tubes (where the jet needle passes through) these holes can become elongated causing weird rich fueling gremlins. These needle jets wear out and should be replaced. The vibrations of the jet needles rattling against the sides of these openings causes this wear on the soft brass needle jet.
Here's some further reading on the subject:
https://store.moto-lab.com/articles/emulsion-tube-inspection-guidelines
http://www.factorypro.com/tech/needle_jet_wear.html
[edit] I re-read your post, if you replaced "all brass" I would read that to mean your emulsion tubes are also new....if so, disregard the above...^^^

Carbs are in a good state - and yes, emulsion tubes, needles, jets are all new. Cold start got a lot better, but the dead spot didn't move.

2) Check your CV vacuum circuit for blockages. Possibly under low vacuum the slides are not raising thus causing the bog, yet once you downshift and raise the rpms the stronger vacuum is able to raise the slides.

3) I assume the slides move freely in the bores of the carb body?

4) With the air box off, with your finger, reach in and lift the slides....let them drop....are they all falling at a constant rate or is one or two falling faster than the others? They should all fall at the same rate. The ones that fall fast are the ones that have a vacuum leak.

Report back, we will get to the bottom of the problem.
Once upon a time your FJ ran right, rest assured,  it will do so again.

Cheers.  Pat

I suspected the diaphragms and changed all four - exactly the same behavior after. But yes, it does sound like a vacuum leak. Next are test of rubber boots with propane and blocking off the vacuum to the ignition box.

[Pat Conlon]

Do a suck test on the vacuum line going to the ignition box. If you can pull air thru the vacuum line it means that the internal vacuum diaphragm is leaking. This vacuum line is run from the #2 cylinder intake manifold so if there is a leak your #2 plug should be a different color than the other 3 plugs.
You can disconnect the vacuum advance line and cap it off with no degradation in performance.
https://fjowners.com/index.php?topic=11690.0

I am interested in what you find...

Cheers. Pat

[tscheel]

You can disconnect the vacuum advance line and cap it off with no degradation in performance.
https://fjowners.com/index.php?topic=11690.0

I am interested in what you find...

Easy to try - next time I'm with the bike. Thanks, I'll report back.

[tscheel]

Did the vacuum test - and yes, I could draw air through the ignition box. But blanking the line did not change anything.

Now I have the bike at home after a 1900 km's ride from Italy - it did between 12 and 15 km/liter. Not fantastic, but not crazy high. Rides fine above 2800rpm, not lacking power.

Now it is time for compression test, valve check, carb balancing and leak test of the carb boots.

[fj1289]

Reading through all this - three things come to mind.
1. Replace o-rings for the intake manifolds - in case a sight vacuum leak - though I'd assume it would also affect the idle?
2. Same with the vacuum caps - make sure they aren't dry cracked.
3. Then - although a change in jetting "shouldn't" be needed - I'd try shimming the needles a little bit.  Who knows, maybe gas formulation has changed enough to lean out an off- idle transition that was already a hair lean, while the rest of the jetting might have been a hair rich and are still happy?

This all assumes the carbs are properly synched first! 

[tscheel]

Reading through all this - three things come to mind.
1. Replace o-rings for the intake manifolds - in case a sight vacuum leak - though I'd assume it would also affect the idle?
2. Same with the vacuum caps - make sure they aren't dry cracked.
3. Then - although a change in jetting "shouldn't" be needed - I'd try shimming the needles a little bit.  Who knows, maybe gas formulation has changed enough to lean out an off- idle transition that was already a hair lean, while the rest of the jetting might have been a hair rich and are still happy?

This all assumes the carbs are properly synched first! 

Good point about the transition - it would make sense! Intake manifolds are very old, maybe original - so I'm looking for replacements. But I want good quality, and it's difficult to tell the bad from the good. But first a proper service.

[fj1289]

http://www.rpmracingca.com/proddetail.asp?prod=36Y-13586-00-00%2C36Y-13596-00-00

These will definitely be quality.  Understand shipping and/or customs can be difficult some places?