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· Registered
12 Posts
Discussion Starter · #1 ·

Bike is a 1996 750 SRAD, 55000 km.

Earlier this year it slowly began to run poor (one or two cylinders were clearly not giving everything, confirmed by plug color on cylinder 4).
After I lifted the tank I saw that some sparks were jumping from the cables to the engine and one carb was leaking.

I did :
  • 4 spark plugs (NGK CR9E)
  • 4 spark plug caps (NGK SD05FM)
  • 4 ignition cables (coils to caps)
  • 4 rebuild kits (bowl gaskets and float valves with float heights adjustment)
  • 1 suspicious carb membrane (on cylinder 4)
  • both ignition coils (new but not OEM)
  • removal of carb lift control system solenoid, tubing, clogged connections to carbs caps (connected to cylinder 4 <- I think this was the main reason for my misfires)
  • valves clearance (all OK, already done 10k earlier)

Misfires on cylinder 4 now solved: bike now runs OK for commuting, smooth, no vtwin noise anymore, all plugs same color.

But new problem : when I begin to run it hard (high rev, big throttle), it cuts for about one or two seconds
: gauges go back to zero, fuel light and temperature light come up, engine cuts. It seems to me that headlight remains. After the cut it usually does an exhaust backfire.
Runs OK when not pushing, does not happen at high rev and light throttle or full throttle at low revs.

This seems pretty much electrical...
It never did it before, bike is stock (including exhaust and air filter).

I checked:
  • stator resistance and voltage (55V at 5000rpm)
  • battery charging voltage is OK. Note: the bike was converted to a mosfet rectifier some years ago after burning the stator plug (do not try to run on a dead battery thinking that you can start your bike pushing it everyday...). Not "direct to battery mod" done on DC side, only AC side is direct from stator.
  • changed rectifier to an original Shindengen FH012AA (before that, it was a FH020AA knockoff)
  • fuel pump runs ok, voltages comes and flow rate is fine.

I plan to try:
  • another ECU
  • another battery (it seems OK with 12.80V even after some days of rest, but in the past I already had a battery tested OK that did not run fine).
  • putting back old ignition coils
  • riding with a voltmeter on after ignition 12v to see if something happens there
  • making direct connection between mosfet rectifier and battery with a dedicated fuse (rectifier DC side)

Any idea that you might have? I begin to get tired. o_O

One thing that surprises me: on idle with lights on, stator current is ~10A (AC) on each of the three wires, measured with a clamp. Isn't it quite high?
DC current on rectifier output at idle is ~5A with no load, 10A with headlight, 15A with hi beam: seems OK

· Registered
12 Posts
Discussion Starter · #3 ·
Actually I measured both voltage and current:
  • unplug 3 phases from stator, check voltage between all pairs (at least 55V AC at 5000rpm per service manual): I had something like 60V at 4000 rpm if I remember well.
  • plug everything back of rectifier, check current intensity using a clamp on the wires: ~10 amps AC on AC side on each phase, ~5 amps on DC side
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