Engine won't rev out

[Paul_from_Thornbury]

Got LI150 engine converted to MB RT195, with Clubman exhaust and 24mm PWK.
Had jetting done by Paul Baker at SRP race tec at the beginning of summer.
Had carb off yesterday to try and change flip up choke to cable choke and after discussion with Paul via text a minor change to needle setting.
However now the engine won't rev out, gets to about 1/2 revs and then just hunts, pulsing up and down in revs. Sometimes it will also rev away to very high rpm even with throttle closed off and have to put hand over carb to shut it down as turning off ignition doesn't stop it.
Only time I have had this before is with a split carb rubber, but it's ok on this occasion.
Returned all to settings that Paul had set and back to flip choke but it's still the same.
The only other change since the set up at SRP is a change in stator plate and CDI after breakdown.

[CHRIS in MARGATE]

Is the cable nipple still held in firmly in the slide?



Also check has the circlip come off the needle?

Has the pin come out of the float ?

[Paul_from_Thornbury]

Yes slide moves fine, no circlip has not come off and float bowl is fine.
Spoken to Paul Baker and he suspects the stator although green/white and red/white resistance readings seem normal at 532 and 110 ohms but these may be breaking down under high demand when revs are high. I have changed plug lead and CDI, so will put new LT coil on my currently unserviceable spare stator.

[coaster]

The stator won't cause the engine to over rev, that sounds more like an air leak, I'd have another look if I were you, make sure the rubber is fully seated in the grove if it has one. Also don't over tighten the Jubilee clips, just tight enough to hold the carb from twisting, over tightening can cause distortion.

[Paul_from_Thornbury]

Checked that, they are not over tight. Top of engine below fuel tank us a bit wet, but I'm convinced that is fuel tank spillage. Have dried off to ensure that there are no gasket leaks, but when I rebuilt all was torqued correctly with new gaskets, I'm sure there would be a noticeable performance deficit if it was a base gasket, inlet gasket or magneto gasket issue.

[coaster]

Well if it was running ok before you worked on the carb and now it isn't it must be related to what you have with the carb.

[Paul_from_Thornbury]

http://youtu.be/2tCVuP9XzI0

Attached is a youtube link which is a sound recording of the engine being initially held at 1/2 throttle and the second at full throttle, the engine won't rev out and has the 'pulsing' of the engine hesitating.

I have ordered a new stator today, I have tried to BGM stators, but want a new one to baseline from.

[Eden]

pickup on the stator fucked by the sound of that!

could be cdi
could be ht lead
could be ht cap
could even be ignition switch dropping out at a certain vibration.


but my money would be on the pickup

[CHRIS in MARGATE]

I am just re-reading and didnt notice first time that you wrote ignition switch would not kill engine and it revs up. Classic sign of pre-ignition where excessively weak mixture caused piston crown to heat up and detonate fuel without a spark.
I might be wrong but it happened after carb was taken off, so I am still going with that.
Hope you get it sorted.

[sunrisemac]

For the engine to rev up with the throttle closed it would need to be getting plenty of fuel from somewhere.

[rossclark]

For the engine to rev up with the throttle closed it would need to be getting plenty of fuel from somewhere.

Classic air leak symptom, racing on a closed throttle, lean running it gets so hot that the fuel ignites without a spark

[Paul_from_Thornbury]

New stator fixed problem, so looks like I have two BGMs with dud pickups. When I spoke to them they said it was a rare failure and the chances of having two with failed pickups was very slim.
The over revving I'm guessing was coming from the engine taking great gobs of fuel/air mixture that it wasn't igniting due to the pickup failure

[Adam_Winstone]

I wonder if this was a pickup failure or LT coil failure? likewise, with BGM's good reputation, I'd have thought 2 x component failure was unlikely. I wonder whether the height from underside of stator plate to pickup triggers was the same on all 3 stators or whether the height of trigger differs and alignment of stator to flywheel could be a factor?

[sunrisemac]

For the engine to rev up with the throttle closed it would need to be getting plenty of fuel from somewhere.

Classic air leak symptom, racing on a closed throttle, lean running it gets so hot that the fuel ignites without a spark

I do know a bit about engines I build race cars for a living, fuel won't ignite if there's none there, the engine can run on very lean with the throttle closed but is unlikely to rev up for long and run faster without more fuel.
Glad to see the problems been sorted, it seem that Eden knows his stuff he was spot on

[CHRIS in MARGATE]

Pleased for you. Well done.

[Eden]

Glad to be of service


ive seen many bgm stators failing just lately. Its not rare at all.

Adam as im sure you know, when the low tension coil fails its because the wire brakes or the protective covering comes off the the wire in the windings somewhere which results in no spark at all. Pickups breaking down can give many different symtems but they always run well at a certain rpm like the one in this case.
Ive had pickups do exactly what this one was doing. Ive even had one that ran fine upto 6200 rpm then just not rev any higher than that. In each case rather than changing the stator i have just replaced the pickup to rectify the problem.

I only ever use indian stators with the pickup and low tension coil replaced with genuine vespa px components. They aint that easy to get hold of now a days though