Mag side air leak?

[phil23fair]

I've got a Harry Barlow tuned Rapido 250 which is normally spot on - starts first time, pulls well, is quick and never given any problem. Until now that is.
A few of us were out for a ride a couple of weeks back I was giving my scooter a good thrash for a couple of miles or so, the last bit of which was downhill. I was full throttle until the last bit of the downhill where I shut off and started braking for a sharp bend. Opened the throttle up once I was through the bend and went up the hill on the other side no problem. I then pulled into a layby to let the others catch up and that's when my problem started. The engine was revving its nuts off and wouldn't drop back to tickover.I suspected an air leak. I wasn't far from home so nursed the scooter the last few miles which wasn't a problem with the engine under load.
Since then I have stripped the entire top end - base, head, inlet and exhaust gaskets are all new. The carb rubber is not split and is fitted correctly and tight. I run with an open mouthed carb, so it's not a filter problem. I am not burning oil so I don't think the drive side oil seal has gone, but I still have the same problem, once the engine has done 2 or 3 miles the off load, high revving starts.
What likelihood is it that I have a mag side air leak being as there are two oilseals? Is there any test I can do to prove it one way or the other?
Thanks
Phil

[rosscla]

viewtopic.php?f=12&t=26427

[phil23fair]

Thanks, I've PM'd Darrell.

[22/2]

Carb.

My experience of air leaks is erratic idling not "revving the nuts off".
The slide will not be open much to sound as if it is revving, do you have a tachometer ? What does that say?
Is the cable catching any where, can you disconnect the cable and ensure the slide is fully down on the stop?
What cap is on the carb, is this unwinding? Had an experience years ago when the slide on an Amal turned because the key on the carb body fell out, that was scary, couldn't shut off.

Also mixture can give similar symptoms to air leak at idle, could the screw have turned/fallen out, again though wouldn't have thought it would be high revs.

Good luck

Rich

[coaster]

....My experience of air leaks is erratic idling not "revving the nuts off".

I was thinking the same, it would need to be a SERIOUS leak for the engine to rev out. I'm fairly sure I have a mag side air leak myself as there is oil seeping into the stator area. It's been like it for a thousand miles or so and runs very well, just waiting for a winter strip down

Could it be a badly worn slide in the carb? does turning the choke on help/make any difference?

[phil23fair]

Could it be a badly worn slide in the carb? does turning the choke on help/make any difference?

I did try turning on the choke after it happened and it made hardly any difference. Normally the engine would have started spluttering and trying to stall. This is what made me think a gasket or carb rubber had gone causing an air leak. The cartb slide is slightly worn, but surely that would be a problem that got progressively worse. This was instant. going OK before we started the journey home, revving like mad 15 -20 miles later.
Could shutting off the throttle and going on to overrun after running flat out cause the crankcase compression to rise and blow a mag flange gasket or oil seal?

[Ricko]

Not saying you have this but, a mate was suffering similar problems and was at his wits end one night yarning over a pint I asked what had he changed just before the problem arose? He told me he had changed his throttle cable, so we took off the headset in the carpark and the cable end was just long enough to catch on the casting web now and again, more so if the throttle had been held full on then let snapped shut........ could have been nasty had the end not been bent back with a pair of pliers that night !!! Worth checking.
Rick.

[goldeneye]

I had this , and it was a headbuster. it was fine for 5 miles or so then would rev its tits off, even with ignition off! had to stall it with the clutch. a couple of re-builds later...... it turned out it was the mag side transfer in the casing had a hair line crack in it where it had been ported, was fine when cold-ish but when it heated up, it opened up . of course when rebuilding it, it was cold , so invisible to the eye. replaced everything into a new casing and all was fine (200 casings where only 70 quid at the time)

[phil23fair]

Thanks for the reply Goldeneye. I've ordered one of Darrell Taylors leak kits now, so i'll wait until that comes and pressure test the casing before I do anything else. If there's a crack that should find it but I'm hoping it's only a mag seal or gasket. I've got another project on the go and don't really need to be spending out on two scooters at the moment.