GPZ500s - loss of power/acceleration at max throttle

[robbiestich]

This seems an obvious question, but with not a particularly obvious answer... about 300 miles ago, my 1993 GPZ 500s suddenly lost power going at 70mph on a motorway in top gear, approx. 6000revs. I automatically asumed i had run out of fuel and whacked the reserve on, but also noticed that it wasn't complete power loss.

Over the next few months i have done lots of work on the bike and recognised the following pattern:

1. Very good running at low revs
2. approximately 60% power on accelerating (full throttle, regardless of gear), made worse if carrying extra weight (like a pillion), and much more noticebaly in gears 4, 5, 6
2. At full throttle, engine is 'jerky' as if it is being starved of fuel, and when I bring the throttle back to about 70% throttle it works fine, through obviously with less power...this results in very poor acceleration above about 60mph.
3. Otherwise bike is working well with regular servicing and in good condition.

Work completed:

1. Went straight to the fuel and examined and cleaned the tank.
2. took the fuel tap of and thouroughly cleaned out
3. checked all pipes to carbs - both air vacuum and fuel
4. next checked air box and filter etc, everything fine
5. carb overhaul (despite completing this 2 years ago, about 10'000 miles ago) - no noticebaly blockages anywhere (and i mean everywhere!!) - cleaned through and blown out
6. checked spark plugs - firing well - replaced with new anyway, obviously this should clear the HT leads
7. Ignition timing is non-adjustable anyway though couldn't do the check
8. Valve clearances adjusted though nearly within tolerance anyway
9. carbs balanced, though almost within tolerance anyway

Any ideas? - i'd be very grateful. cheers, Rob - Cardiff, Wales

[TUBZ69]

ive had exactly the same problem yesterday, picked up a dirt cheap gpz 500, mot +tax bit rough round the edges, but starts 1st time, as i need a cheap runaround whilst i fix the zx6r, anyways its started out fine all up the rev range, would happly sit at 90 an still pull.
I got halfway home and sure enough it would get to 70 but then u try to open the throttle and it just vibes alot an doesnt wana go that much faster unless ur goin down a hill, as for goin up a hill, the power just drops. weird, i think it mite be the wheel or headbearings creating to much vibration, so much so that the bike wont go any faster. i knew the bearings were on the way out, an im thinkin that they must have pretty much given up the ghost at that point, for example id let go of the bars an let the bike slow down and a massive tank slapper will develop wich to me seems like wheel or headbearing knackered,either that or the knackered front tyre... new ones goin on 2mora!!
____________________
those who risk nothing, become nothing, achieve nothing....
c90 rvf 400, vfr 400, gpz 500, Gsxr 750, R1, Zx6r....

[TUBZ69]

anyone got any other ideas??
____________________
those who risk nothing, become nothing, achieve nothing....
c90 rvf 400, vfr 400, gpz 500, Gsxr 750, R1, Zx6r....

[poontang boy]

hi, it sounds like its the head gasket, check the oil if it is creamy and white the head gasket has gone alowing coolant in to the oil gallerys, also check fore compression.
good luck , james.

[TUBZ69]

[thegubner]

[robbiestich]

Hi, ok i doubt it is the head gasket, the oil seems absolutely fine and certainly isn't whitish at all, also the bike is running 100% fine except when throttle is on hard (as described above). I guess it could be compression, but would that then point to the pistons? The bike seems in pretty good condition except for this problem, I have had it for 3 years now.

Again i really doubt that my problem is headbearings.

I'd also note that it will still pull to well over a ton - i reckon it would do 110 or 115, whereas it has done 125 in the past - just you have to hold the throttle less open.

Another note is that the fuel economy seems the same.

Any thoughts? - cheers so far guys, just need more food for thought