gpz flatspot

[maxpower125]

howdy
i have a gpz 500, it has a full system on from motad, but i think maybe whoever fitted it didnt change carb settings, as its muscualr lowdown, but the midrange is non existant, then at 7000rpm picks up really hard all the way to the 11000 redline, and mayb more is this likely?

[stinkwheel]

Absolutely normal. That is what they do.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[iCraig]

Sounds like a normal GPZ to me too.

[maxpower125]

well its still rides real nice so this mans happy,

[Whosthedaddy]

What causes a flat spot, my ZXR seemed to do the same thing at 6k, wind hard past it and feck me my arms were pulled out of their sockets.
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Current : MSX 125 Past : CBR 900RR Monkeybike : c50 LAC : ZXR750 H2 : FZR600 : ZX7R P3 : YW100 : TRX850: Trophy 900 T309 : GSXR 600 L0: Monkeybike : XJ6S Whosthedaddy

[iCraig]

The GPZ on the flat spot is just the way it was originally tuned, so that it had low down grunt and power from the mid top end to the redline.

Flats spots can also be caused by bad fueling set ups when people put different exhausts on bikes and cause it to run lean/rich etc. Basically anything that changes the fuelling can cause a flat spot.

[Whosthedaddy]

[iCraig]

No, when a race can is fitted the carbs must be properly set up for the can, using a dynojet kit or other jetting kit and time on a dyno making sure its set up right is usually required.

[0ddball]

It depends what you mean by flatspot.

If it is a true flatspot then it seems to be a common problem that lots of them develop but they certainly were not like that from new. Mine was fine for the first 4000 miles but has now developed a noticable one at 5-6Krpm.

But even a perfect running one feels slightly flat in the midrange due to the way the engine makes it's power.

Only you can decide whether it's a genuine flatspot (a good service should sort it) or just the character of the engine.

[Nath]

I also would be interested to hear an explanation of flatspots, and why they exist in the rev range rather than being related to throttle opening. Is it due to the overlap between slow running and mainjets, ie too much fuel being drawn in through the main jet whilst the engine speed is still slow enough to allow the pilot jet to supply fuel?
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Hard livin', hard luck.

[stinkwheel]

Lots of factors. Basically it is because the fuel:air ratio or simply the volume of air that can be drawn through the carb isn't exactly right for what the engine needs at a particular speed.

There are a lot of different tricks that are used to get past this. One of them is the airbox design, as far as I am aware, this is so the carbs get laminar flow of air going into them. Laminar flow means the amount of air being sucked in is as high as possible. Take the airbox off a modern Japanese bike and it runs like shite.

On the GPZ there are also little additional jets to give a 'boost' of fuel in the midrange. You can see them round the edge of the carb air inlet. These kick-in at certain carb openings to smooth the power out. Mine got blocked a while ago and the bike would only run at either low revs or flat-out, totally dead from about 5k to 8k.

The carb/airbox on a GPZ500 are set up to give maximum power at high revs, the sacrifice for this is a dead midrange. If you put the carbs and airbox from a KLE500 on a GPZ engine, it pulls well all through the midrange but struggles to rev to the redline (I know because I did this last weekend).
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.