The Infamous "Flat Spot"

1980 XJ650 Maxim
I would like to inquire if anyone has successfully reduced or eliminated the 5.5K to@ 7K RPM flat spot felt while accelerating. Mine has that well described and documented symptom. All mechanical, fuel ( carbs exhaust stock) and ignition systems are unmodified and seem solid. Plug running color is excellent. She runs like a scalded dog above 7K to redline + and accelerates good from 2.5K until you reach that 5.5K threshold. The flat spot is REAL obvious when you roll on throttle in 5th at @4.7K which is about 60MPH. Then, when you get to around 7K, its like she hitches up another team of horses.
Any tips would be greatly appreciated.
Many thanks all.

 

The only time I have ever heard refrence to a flat spot is when the stock airbox is ditched in favor of pods. If your machine is in tune it should pull strong below 5.5K and pull like a mommy-effer above that, right-up-to and above redline.

Are you running a paper air filter?

Is the clutch old or new?

 

My XJ750 runs strong throughout the power band. She accelerates strong even below 4.7K when in fifth gear. Sounds like something isn't tuned right.

 

As described above my xj550 actually takes off like a banshee @ 5500 rpm and continues through redline.

 

With the carbs off, reach in and push gently up on the vacuum slide piston it should go up smoothly and slowly come back down.
If one were to hang a little going up, that cylinder would be lean until it came free.
Could cause what yours is doing

 

Thanks for the feedback. Humm... sticky/slow slides. Haven't synced up the carbs in 5k anyway. Think i'll pop the hats off the carbs too, and more closely examine those 36 year old diaphragms. Just might be some liquid electrical tape in the forecast, or a second job..$$$...

 

There's a place that has new ones, I'll get the link after work.
Not a whole lot of $, but your slide might just need cleaned up.

 

I have no flat spot on any of mine......

 

Some of that flat spot is from that K&N filter. It makes the fuel mix a bit lean until you get into the upper RPM range. A fine tradeoff if you're racing around a circle all day, but not if you're riding on the street.
If you've gone back to using the paper filter then Polock's suggestion is definitely the next thing to look at.

 

Where do you set your coffee?

 

Oooh K&N filter, loose that and get a stock one. For sure

 

And when I said some, I meant all (unless a sticky slide is also contributing).

 

Many thanks in advance for the link.

 

Ok. Put the OEM paper filter back, and the flat spot is still there, but much less obvious now - however - the extra pop at the top is gone. Makes sense - less max air flow now. I'm on a learning curve here. I cut my teeth in the 70's on motorcycle multi-carb racks of the "Slide" variety, not C/V. They had a whole different set of operating principles, problems, and remedies. #1 problem on slide carbs was BOG. Nail the throttle at idle and the bike cut off, period. You had to have 1.2K RPM or more on them (on my 79GS850G anyway) to wick the throttle. For a short time some bikes with slide carbs came equipped with accelerator pumps. Solved the problem, then Mr. EPA buried a hatchet in the forehead of that solution. Always give and take ain't it?
So hears what I'm thinking on the XJ - Stock air box, K&N, Y-20 needles, and if perforations are found in the diaphragms repair or replace. Give that set up a go, and after 1 tank at steady cruise speed, read the brand new plugs installed . If they load up, drop down 1 on the main jet. If they are snow white, back up and punt. Got to learn these rules on C/V's. All thoughts and tips much appreciated.

 

Mine only did that in the early miles of getting acquainted with it. The trick was to know how fast the inbuilt response time would be and not open the throttle faster or wider than the engine could handle it. That was fast enough, anyway. It wasn't a tarmac blisterer but was satisfyingly quick, even with all the porkiness of it.

 

http://jbmindustries.com/
there ya go

 

Got that right. She was fat girl, but a fun one and had a mighty smooth comfortable ride. I wound up beating the BOG odds on those carbs after endless trial and error. Solution - Overjet the idle mixture circut +4 I think it was, shimmed the jet needles a little, and kept the mains the same. Stock air box. Now, you couldn't let it idle for long obviously, but on semi-long cruises (200+ miles) the plugs were fairly clean. MPG went to hell though, and you did have to clear her throat once in awhile. But it held almost 6 gallons of gas at about 75 cents a gallon then.
Many thanks for the reply.

 

YDM! Thanks! an affordable solution if needed! I paid less for the whole bike than the price of 2 new slide assemblies.

 

I have this EXACT problem with my FJ600. It has stock airbox but a fully open 4-1 marving exhaust system.
I've only just gotten this bike and fixed her up. Carbs are clean. I think the needles must be in the stock position..and it's too lean. Gonna try to go richer. Or doesn't it have grooves on the needle?

 

Does it have the stock air filter (paper element)?

 

you need some back pressure in that exhaust, it will be quieter and the bog will go away

 

Yes, it looks like this

Really? I've heard about idle bog without back pressure but never mid-range bog.
I don't have a baffle and looking at this exhaust it's not made for one.

My bandit 400 also has an open exhaust and has stronger midrange than ever. But I can change the needles on my bandit. From videos it looks like the FJ600 only has 1 groove. Anyone have experience with shims to increase the needle height?

It kinda feels like the needles don't wanna go up... from 5k-7k rpm hardly wanna rise and it sounds very rough. Then it goes.
No tears in the rubber though..

 

look at it this way, with stock air filter and pipes there is no bog so with your pipes the back pressure is changed and the bog started.
my 750 came with a Mac 4/1 pipe and stock air filter, and a bog at 5K. i changed the air filter to a Uni foam type and the bog got worse. after all sorts of jet changes and needle shims the bog was still there. finally got a Supertrapp muffler and with the best combination of jets, tuned the Supertrapp and the bog went away.
you have to balance the in and out to work together.
you can put washers under the needles but that only changes when they take effect and that's only half the solution

 

Thanks. Jetting is so confusing. I've had quite many carb bikes now, but the symptoms of lean/rich are never consistent...
I rode again on the FJ600 today.. when I go in 2nd gear at 5000rpm and open the throttle fully, it stutters for 1-2 secs and then goes. I had this same symptom at 7000 rpm on my GPZ 900 when it was too rich on the main circuit. How can the FJ600 be too rich with open exhaust and stock airfilter. The needles weren't worn at all.

But maybe stuck'ish diaphragms cause the same thing?? who knows.

 

there's a old school way to tell rich/lean. get to the point where the hesitation is happening and very quickly close the throttle to about half of whatever it is.
if the power surges, that means there was too much gas for the amount of air. you quickly reduced the air and just for a moment the mixture got better(that was the surge).
sometimes it's very noticeable, sometimes not. it's worth trying while you're out for a test ride.
if nothing happens your good or rich

 

Yeah I've tried to test but with CV carbs it's very hard, because at a constant throttle opening the vacuum slide can be opened more or less depending on RPM.
If I go to the 5k rpm in 2nd gear (from where normally I would stumble when giving it WOT), and then upshift to 3rd or 4th gear (maintaining same throttle opening) it pulls great with no hesitations.
Why would you say that you are rich when nothing happens? If you are rich and you 'quickly reduce the air' you would go even more rich..

 

true

that would change rpm, vacuum and slider position. so now you're under completely different conditions and can't tell what condition everything is in when the stumble happens.

because nothing will happen, if it's lean and you chop the throttle it gets richer and surges (better mixture), if it's already rich it gets richer and you don't feel anything

 

Sorry for stealing this thread, but it appears to be dead anyway and the problem seems to be the same.
So I opened the carbs again today and noticed mine does have the adjustable needles as expected (different in Europe).
I set them all to +1 rich and tried it out. Well it didn't like it and was way too rich. Would hesitate on 1/8 - 1/4 throttle and stutter on partial throttle.
I put it back to stock and it's much better again.

It puzzles me how it can be way too rich with only +1 on the needle and a full 4-1 open exhaust system and stock airbox. All my other bikes loved +1 on the needle with open exhaust.
I'm thinking it might be worn needles etc because of 75000km, but the needles look perfect and the needle jet doesn't look oval at all.

Any other ideas? Could the exhaust simple be crap for this bike at this certain RPM?
It pulls great at 3k 4k rpm, then huge flat spot at 5k-6k and then 6k-9 it goes.

I rechecked the diaphragm, no tears, and the slide goes up without effort. (although the rubber isn't as flexible has a bit of wrinkles, but I've seen this on other bikes before with no problem)

I'm thinking about removing the snorkel on top of the airbox to see if it's somehow still too lean...

 

Please start your own thread so people are clear about what's happening with YOUR motorcycle. If you already have a thread for you bike, please put your question in there.