Huge flat spot from 5k-6k rpm ('84 FJ600).

Hello all,

I've recently got a FJ600 1984 model with 75000KMs.
It wasn't running so I cleaned the carbs everything, did the valve clearances, etc.etc. and it fired right up. (i've done this many times on many bikes).

Anyway so it runs fairly decent, I've already ridden about 300kms and it's pretty ride-able.
However there is a huge flat spot starting around 5000 until 6500rpm. It feels like it's a 250cc and has run out power. It's still accelerating but noticeably slower than from 4k-5k rpm. The engine also sounds a bit more rough. After 6.5k rpm it rockets to the redline.
Also if I mainting 5k RPM in 2nd gear and go WOT, it stumbles before it goes.

So the bike is fully stock except it has a full 4-into-1 marving open exhaust. It has stock air filter and airbox, stock jets (102.5). I put the pilot screws at 2.5 turns out, and float height is 3mm below carb body mating service.
Idle mixture seems pretty spot on, it doesn't start without choke, throttle response is good and rpm drop fast when I close the throttle.

The diaphragm has no tears and the slide moves up without effort.
The needles are not worn at all and the needle jet isn't oval.
I've got adjustable needles, I already tried 1 down (+1 rich) but this didn't remove the flat spot and only introduced very bad throttle response at 1/8-1/4 throttle.
I've blown out the airfilter with a compressor.
I cleaned the spark plugs.

Could this be exhaust related?
What else could it be? Why is +1 rich on the clip too much already with a fully open exhaust and stock mains? (this is totally different from most bikes I've had).
I think I will try to remove the snorkel (if possible) and see how it behaves...

Thanks

 

most likely it is

doesn't matter why, at those rpm's your engine wants more air or less fuel. you gave it more fuel and you proved that yourself.

tried that and it makes a little difference but in the right direction, try it.
i think it's the open pipe that's the problem.
maybe try something like this "lollypop baffle"
https://www.google.com/search?q=lol...X&ved=0ahUKEwiFksyO_sHOAhXLGR4KHe70D9IQsAQIGw
or restrict the exhaust somehow and see what happens

 

QUOTE="Polock, post: 534566, member: 782"]most likely it is

doesn't matter why, at those rpm's your engine wants more air or less fuel. you gave it more fuel and you proved that yourself.

tried that and it makes a little difference but in the right direction, try it.
i think it's the open pipe that's the problem.
maybe try something like this "lollypop baffle"
https://www.google.com/search?q=lol...X&ved=0ahUKEwiFksyO_sHOAhXLGR4KHe70D9IQsAQIGw
or restrict the exhaust somehow and see what happens[/QUOTE]

Yeah, the weird thing is I would expect a 4-into-1 fully open marving exhaust to be more flowing than the stock exhaust (= allows more air, leaner). I can't see why it would be the other way around.
On most bikes I can easily (partly) remove the airbox cover to test, but on this bike the cover is used to hold the air filter.

One more thing is that yes the bike ran worse at 1/8 1/4 throttle when I gave it more fuel with the needles. However this may still be unrelated to the flat spot at 5k-6k rpm (which happens after the 1/8-1/4 problem with richer needle). Maybe I need bigger main jets instead rather than adjusting the needles (because the needle is already way up at 5k rpm).

It's just that I've never seen an exhaust give such a negative effect regardless of jetting.

 

Have you tried an 'at speed' plug chop. i.e rn it up to 5k on the open road and kill the motor. Coast to the side and check the plugs. Also you tried +1 rich, I'd try +1 lean on the needle (did it start off in teh centre notch by the way?)

J

 

I haven't tried that because it's a pain, would need to go 5000rpm, kill it and push it home.
edit: I just realized I could check the outside plug on easily.

Anyway even with this method I've never really believed much in spark plug readings on 4-strokes.

I haven't tried +1 lean, I thought about doing it but like I said I can't fathom why it would require even leaner setting with a open exhaust than the setting used in the factory! (which is usually already on the lean side), unless the needles etc. showed obvious sign of wear.
Yes it was in the middle when I opened it.

 

My bike was very 'fluffy' (actually unrideable) mid-range as the carbs (bought 2nd hand) had been set with the needle up and the plugs confirmed this. Lowering (+1 lean ) made a big difference. My bike also has a 4:1 without much baffling too..

https://lh3.googleusercontent.com/c...herFTtLT5FWMVkFdqzT2dfbOeCQ=w1280-h1024-rw-no

What have you got to lose except about 30 mins spannering..?

 

You're right I have nothing to lose.

It's just that I always want to understand, want to know why. So you went from +1 rich to +1 lean? Going even leaner than factory with a more flee flowing exhaust is illogical to me. I don't know any aftermarket exhaust that restricts flow compared to stock exhaust. I could understand if you put a 250cc exhaust system on, but the one I have seems to be for the XJ600: http://www.wemoto.com/parts/picture/mv-y_65_nc/.

Maybe your carbs (or mine) do have worn carbs and allow more fuel then stock.
Anyway I will try this week and see what happens!

 

You should. The plugs get read slightly differently than on 2-strokes, but the science is sound.

 

"What have you got to lose" he said.
"Only 30 min spannering" he said.

Well I'll tell you what you got to lose.
A god damn last-carb needle c-clip that catapults off your fingers at mach 3, disappearing into your over-full garage with rubbish-stone-impossible-to-find-anything-floor.

...

Guess I'll also tell you what you got to win .

I found the c-clip after about 30min searching with my flashlight in the dark... such luck. It could've been lost forever but I got lucky.
Anyway. The results are obvious I have to say. It runs much much better. It's still not perfect but about 75% of the flat spot is gone. Also when I WOT it at 5k rpm like I did before, there's no stumble!

I'm really confused why it wants to be even leaner than stock. I re-checked the needles and to be honest they look new. Maybe they have been replaced with non-stock needles??
On the other hand, I'm glad it was rich because these symptoms were exactly the same as my GPZ 900 (like I said before, I got confused because logically I thought it had to be lean but the symptoms were like I knew from rich).

In all honesty, it feels like it wants even more air. Very weird.

 

I feel your pain, some days it is like the gods are angry at me and they test my limits.

 

Ah yes, the old circlips in the dirt caper. Did the same with a bicycle ball bearing just the other day - never did find it! Glad the carbs are back in tune. I was convinced my stumbling was fuel starvation so I went +1 rich and it was just hopeless, I then went +1 lean and bingo! My situation was slightly different though as I wasn't staring from stock settings (2nd hand carb rack..) Anyway great result

 

Yeah! Now here's hoping there's someone here who can tell me the WHY . This is what the needle looks like. Though I'm sure even the stock ones here look different than US ones. All my bikes with mikuni carbs had the adjustable needles.
Also made sure it's installed correctly with the black spacer pin in the right place etc.
Here it was in the +1 rich position obviously.

It did quite well if this has been in for 30 year and 75000km?!

 

You know what curiosity did to the cat right ? Maybe it's something to to do with riding sub-sea level - more atmospheric pressure etc.. BTW you're not native Dutch, are you? Even by their standards your English is too good (mind you, you said 'flashlight' and an Englishman would have said 'torch'..)

 

I'm actually a bit above sea level, +-40m according to the internet.... so that shouldn't be it. It also varies very little over here.. -5-50m for most of the country. My Bandit 400 (also mikuni CV) is very happy with +1 rich and even bigger mains with open exhaust (though stock pipes/headers). Had a couple of other carbed bikes and never had to put the needles leaner.

Anyway yes I'm natively Dutch so thanks for the compliment. My 'high-school' was 50/50 Dutch/English, so that might explain. Apart from that most influence in the Netherlands comes from American TV-shows/movies/games, so that would explain my use of flashlight . I would use torch for : " pre-historic-fire-on-a-stick", if you know what I mean... and flashlight for today's version.

 

Well that is curious, but I can't help further with the reasoning.. Where in NL are you? I'm over to Eindhoven
several times a year to visit the parent company I work for.