Uncontrollable revving

[wrekkah]

Other than a stuck throttle or air leak, what could cause uncontrollable revving to maximum rpm with the scooter on the stand?

I was tuning the carb on a newly rebuilt engine (I did the work) and while turning the air mixture screw in, the engine just took off revving uncontrolably to maximum rpm. This occurred when the air mixture screw was at approximately 1 turn out.

I plan on trying to repeat the process tonight but wanted some input first.
Any thoughts would be appreciated.


Specs: Deanspeed 190, BGM 30mm PWK carb (jetting 130/45/JJH 3rd clip), PM tuning pipe, Indian stator and flywheel, MB loom, Ducati CDI and reg, 19 deg BTDC, 1.5mm squish.

Thanks

[GP Kevo]

Did you force the air mixture screw in and run it through it's seat?

[wrekkah]

Don't think so but will check. However, as I was turning the A/M screw in, the revs changed from slow to high, then uncontrolably high so if the seat was blown I would expect A/M screw position to have no effect on the revs, correct?
Thanks

[byron]

blocked idle circuit

[scoothappy]

Hey
Sounds like it's dieseling. it's caused by a overly lean carb mixture by which the heat in the cylinder ignites the following fuel pulse. this causes the motor to rev uncontrollably. :flaming:

[wrekkah]

Thanks for the comments

I've disassembled the carb and blew through pilot circuit - seems clear
Increased to highest taper jet I have (HJK) - no help
Raised needle, lowered clip - no help
I would think a 50 pilot should be big enough - I will order and try a larger pilot.
Checked A/M screw seat - seemed fine, no visible damage
Engine still revs uncontrolably as I screw in the A/M screw to around 1.0-1.5 turns out. At 2.0 turns out the engine wants to stall.
The engine is certainly acting lean (searching for idle, won't come off rev smoothly).

Tried a smaller PWK 28mm carb, with similar settings and engine exhibited same tendencies.

I'm leaning towards an air leak somewhere. Any other thoughts?

[dapper]

Was just going to say air leak

[wrekkah]

Well I found the problem. The base of the MB intake manifold was a little too wide and it was hung up on the lip of the cylinder creating an air leak.
A few minutes with a dremel took a care of it. Engine is all sorted now.
Must remember to get my eyes checked soon!
Thanks

[byron]

I found the problem.

cool. always good to learn of the reason