Help diagnosing bucking & stuttering  By  rcq92130 (Read 7,348 times)

It's common for electrically-related problems (i.e. - a weak spark) to present themselves "suddenly". I'd try a new plug, check all connectors/kickstand switch/kill switch/ground points (engine especially), swap CDI's if you have a spare, etc...

If it was running acceptably before - and you know the carb didn't suck a plug of nasty goo or something into the float bowl, and diaphragms are OK - you're just wasting time chasing your tail with mixture settings/jet sizes/idle mix screw adjustments/etc. Follow the most logical explanations, first - all of those things didn't just suddenly change.

Bob

130-140 psi is OK... not great, but OK. And anyways, compression a bit on the low side won't produce the symptoms you describe (bucking/missing at WOT).

Again - try a new, known good plug (and cap - or even coil, if you have one) - and check all of the associated ignition wiring, connectors and grounds. That will at least rule those things out, without costing any $$.

At idle, the strobe should illuminate the "F" mark on the flywheel when aligned with the pointer (looking thru the rubber plug-covered hole in the fan shroud). That mark is before (when turning in a clockwise rotation) the "T" mark. Even so, ignition timing itself being a bit off, too, won't cause those symptoms - less power or overheating/detonation rattle, yes - but not bucking/missing.

Although - if it's a 12V timing light and you can rig it up to watch it while riding the scoot, it very well may visibly show the stutter while it's happening, if it's a failing spark.

A weak spark, caused from, say, a dirty primary coil connection, insufficient ground connection or bad plug or plug cap - can actually be "snuffed out" by the increased combustion pressures that come from opening the throttle further. So it may run fine at lower throttle openings, but "snuff out" and misfire at big throttle openings.

Maybe not what's wrong, but just some things to chew on.

Bob

Here is today's (non) progress:

1.  Tried a different CDI.  No change
2.  Tried another spark plug.  No change
3.  Tried an electric fuel pump.  No change
4.  Tried a 105 main.  No change (previously tried 115 - back in now - & a 120)
5.  Attempted to get a strobe timing check, but the timing light wouldn't fire.  Checked it on my Goldwing and it worked OK.  Maybe the voltage in the scooter isn't high enough of the inductive pickup ...
6.  Checked vacuum with the electric pump, not the vacuum fuel pump --- still at 10 in mg
7.  Tried the 28 pilot jet.  No change.
8.  Put a gun in my mouth.  No change.

Recap:  this started after installing a BBK.  Drove 20 miles with everything fine -- great, really -- and then suddenly heard a stumble that, within a mile, became so bad I didn't know if i could get back home.  Pulled the head and found i had bent the intake valve during installation (stupid mistake; didn't relieve the tensioner & the rocker arm pushed against the side of the valve as I torqued down the head nuts.  Got replacement SS valves, installed, scooter still stumbles.

Wondering:  I did NOT use lapping compound to seat the replacement valve, thinking with only 20 miles on the head it would be OK.  Maybe the valve isn't seating fully?

Maybe should put the old OEM 150cc cylinder head on and see if anything changes.  but would it fit on top of a 60mm piston and jug?  Or would the valves hit the piston?

TOTALLY grasping at straws at this point ..........

.

I just bought a new NGK cap and mine actually dosent have the screw in end . But as I read your problem , it does sound like a coil problem , however is it just a coincidence that the coil problem started exactly the same time as the bent valve " possibly " or is this still a valve problem . How bad was the bent valve ? Did this affect your spark plug causing a bad spark ? " you tried a new one so that can't be it " but I would check both ends of the plug wire to make sure they are screwed into the boot and coil .

Yes - bad ignition misfire or way too rich (which you've already mostly ruled out).

Intake tube or air filter collapsing (or other major intake restriction) under load, is another thing that comes to mind in looking at those pics... how are they? All clear, stiff-walled, well supported, etc? Didn't leave a rag in there or something, while it was apart (known to happen... don't ask!)?

Bob

Are you trying to start it without the air filter on? Believe it or not, you need some restriction going into the carb in order to create the proper vacuum. Sometimes a uni filter just doesn't work on a particular setup, requiring the stock airbox to be put back on (though that is usually solved by proper jetting and tuning). While this may not be the root problem, it could be causing additional issues which are muddying the waters.