YZF750 running issues

I seem to have a high rev fueling issue on the YZF750.

All through the gears up to approx 90mph the engine runs perfect. If I twist the throttle into the powerband (starts at around 9k), the bike stutters and won't pull smoothly. As soon as I ease off the throttle normal progress is resumed. The carbs are cleaner than my brand new front carpet. The fuel tank is cleaner than a cup just out of the dishwasher and the fuel in the tank is fresher than a freshly dropped cow pat. The bike before it was laid up (for some 8 months) ran perfectly all the way through the gears and rev range. (The subsequent driving ban can contest to that). I have a K&N filter which is clean. The engine idles just dandy at a steady 1250 rpm without choke.

I have a suspect alternator/regulator which I think is over charging (replacement on the way) but can't see this as this issue. The carbs were given an overhaul and it was found the main jets were of different sizes and depths but these were placed back in exactly the same positions as previously and assume the previous owner must have had it set up on a dyno for it to be like this. Remember the bike runs perfect at lower revs all the way through to the powerband.

I thought possibly the exup valve but then it would be running badly at much earlier revs if it were that. I would say it seems as though it is being starved of fuel.

(Whilst writing that I just thought I have not checked the fuel filter, so will do this this evening).

What do you think might be the cause of my troubles (other than the fuel filter)?

Originally Posted by Stig_Of_The_Dump

The engine idles just dandy at a steady 12500 rpm without choke.

[gruntygiggles]

+1 to Plowsie......12500rpm on mine will have me doing a ton in 3rd gear.....sounds pretty high to be idling

Being a numpty, the only things I would think of would be jets and valves in the carbs, air intake and fuel intake...and making sure filters are clean.

Okay so I got a little excited with the 0's.

[gruntygiggles]

Originally Posted by Stig_Of_The_Dump

Okay so I got a little excited with the 0's.

Oh....so that's what caused the ban then....lol

[TSM]

i have a similar issue with my 7R, if I give you WOT with a high loading on the engine it boggs down and stopps accelerating, close the throttle a little and it starts to accelerate to the redline, if the engine load is not high when you WOT then it pulls hard to the redline, apparently it all points to a very very rich mixture.
Can you still reach the redline?

Originally Posted by TSM

i have a similar issue with my 7R, if I give you WOT with a high loading on the engine it boggs down and stopps accelerating, close the throttle a little and it starts to accelerate to the redline, if the engine load is not high when you WOT then it pulls hard to the redline, apparently it all points to a very very rich mixture.
Can you still reach the redline?

Have had as above on the 400 - stupid size main jets choked it when winding it on. If it used to run fine it obviously aint this though (ie not magically morphing jets).

Never underestimate the daftness of previous owners.

Otherwise dunno.

I know nothing.



Edit: so too much fuel or not enough then....

If it was me (knowing nothing) I might also check the fuel tank breather. & If I had new electric bits on the way I'd probably wait before peering at anything else.

[yorkie_chris]

Sounds lean, try tape off an inch or so of the filter and see what it changes.

Ricardo says running rich and Chris suggests running lean.

Doing the good old nose down the end can test, I'm quite sure it isn't running lean. But then it doesn't smell like its running rich either. I shall dig out the plugs and see what they say. Failing that I am guessing it's carb issues which probably means buying a new set of jets for it.

[yorkie_chris]

With my suggestion if it is rich it will get worse