91 GSXR 750 flat spot

[Regan28]

My Gixxer 750 slingshot has a flat spot around 5.5k in its rev range (feels like the bike is stalling but never cuts out), The plugs have been changed and it has a K&N fitted (not by me) on standard carbs which I was told were dynojetted level 1 when I bought it.

I was hoping for any ideas or experiances people have had with K&N filters or if people have experianced this before ad solved it.

My thoughts are:

- K&N allowing to much air to flow and hence causing a problem with the air/fuel ratio and then a flat spot at this specific range.

- Possible deteriorating coil(s).

- Or some sort of problem with the carb although I striped and cleaned them previously and it made no difference.

Any ideas would be greatly apprieciated as its quickly becoming one of thos pain in the backside problems


Thanks in advace

[The Artist]

Sounds fuel related.

If you want to get it sorted properly then take it to a dyno and get them to sort it out.

Is it fine after the flat spot?

[Regan28]

Yeah fine after, well I'm considering going to a standard air filter, maybe a bit more restrictive give the proper A/F ratio

Also, trying to do it on the cheap at the moment as the bike ain't worth tonnes, so not looking to go to a dyno if possible.

cheers

[sickpup]

There are two different size rubber rings with the K&N and you need the small one fitted.

What exhaust is on it?

[Regan28]

It has a micron system on it, this could also cause these problems, but riding it feels more like a fueling problem, but I'm not 100% sure.

Do you know that diameter of the small rubber ring for the K&N?

[The Artist]

exhaust and air filter affect fuelling. e.g. micron exhaust as opposed to stock will be more free flowing allowing more fuel/air through the engine which is why bikes need to be set up properly when installing any mods like that.

[tone]

Its a fuel problem, carbs are incorrectly set for the changes in induction & exhaust

You need to rip the carbs off & check what size main jets are in there + if they are dynojet jets & if you also have the dynojet needles, also what the clip position is on the needle
DJ & stock parts dont play nice together if you mix them !

What plug colour do you have ? what are the float heights set at ? what are the mixture screws set at ?

You say you cleaned the carbs how far did you go ? pilots, mains, emulsion tubes all out & spotlessly clean ? did you sync the carbs afterwards ?

sorry for all the questions mate, i'm trying to help but need all that info to have a hope of getting it anywhere near right
tone
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Of all the things ive lost , i miss my mind the most

[Coxyzxr]

Unlikely soley a main jet issue, seems to be the cross over point from needle to mains.

Likely needle tapering too early or too rich a needle. If it's dynojetted it will be adjustable. Try lowering it down 1 notch, should help quite a bit.
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**WAS FITTED "WRONG WAY ROUND" !** CANT ACCOUNT FOR THE STUPID !

[Regan28]

Cheers, had teh carbs out again and stripped, its on standard needles and jets, not dynojetted, jet needles are non adjustable, and I cleaned the emulsion tubes out properly.

I'm sure its a A/F ratio problem, I recon mainly due to the K&N rather than the exhaust. Granted I haven't put it all back together yet to test it but I might try restricting the intake into the K&N to reduce the air flow slightly, maybe get it closer to standard air flow in.

Plugs are normal. As you said I also recon its something to do with the switch over between jets. Gives me a headache

Thanks for the help so far good to get second and third opinions on things.

[Regan28]

Should also mention my bike is a Californian import and hence has a smaller (36mm) venturi instead of the standard (38mm) due to emisions laws over there. It also means the mixture screws are set and blocked off so you can't adjust them

Might just buy a set of 38mm carbs for it or evan the 40mm of the 1100

[Coxyzxr]

[Regan28]

Ok cool, so any ideas what would cause a rich mixture, because as far as I'm aware they look like the original jets etc when I stripped it. Numbers were the same as stated in haynes etc

thanks

[tone]

I agree it does sound like a mixture problem but would expect the crossover from pilots to mains to be at lower revs ( maybe 4K)

you say the mixture screws are blanked off as its an import model, mostly this takes the form of a thin brass plug that can be removed & the screws are just the same as any other bike underneath, it could be worth checking if this is the case the usual way of removing these is to drill a small hole in the plug (very carefully so as not to damage the mixture screw) then screw in a self tapper that you can pull on to remove the blank

disclaimer .......... if you are a ham fisted sod & mess up your carbs because you do NOT understand the term "very carefully" the author of the above takes no responsibility
____________________
Of all the things ive lost , i miss my mind the most

[sickpup]

First get a set of 38mm carbs, fit them and remove the cali crap. Then fit either a dynojet kit or a factory pro race kit.

I have a new factory pro kit here somewhere but it won't be cheap. It will however be cheaper than the £169.34 Dynojet want.

[Blau Zedong]

[Regan28]

Thanks for the help everyone, just an update, took all the californian exhaust gas recirculation stuff off etc and its running fine, no hesitation.

May look into getting a set of 38mm theres a set on ebay that were used on a 750 and have individual filters on each carb.

However my smart head says that now its fixed leave it alone

Once again thanks for all the input