Scooterotica

Hi, hoping for some help here.

So, I have a 1959 Series 2 Lammy, stage 4 175 conversion, 12 volt electronic ignition, standard Dellorto SH1/2 carb with standard jets.

It's been running OK, starting OK, idling OK.

Out for a run yesterday, bimble through town OK, then out on the open road running at 45-50mph on about ¾ throttle. All is good.

After about 4-5 miles, it starts showing signs of loosing power; ie, slowing down, engine note reducing, not responding to the throttle.

Having had the piston nip up in the distant past, I think perhaps it could be doing it again, so I close the throttle and pull the clutch in. The engine immediately goes to full revs with the throttle still closed. The only way to bring the revs down is to release the clutch. I can eventually get the revs back to idle with the throttle closed.

Playing with the throttle for a while seems to bring things back to normal so I carry on. I should of turned for home.

A few miles further on things become worse and eventually come to a stop.

Thinking dirt in the fuel, I strip the jets out, give them a good blow, all looks good and put them back. It now won't start and I end up getting recovery to get back home.

Does anyone have any idea what's going on?? I mean how do you get full revs with a closed throttle? The throttle cable and slide operates normally, no signs of sticking open.

Thanks.

air leak......possibly cylinder head (loss of power)

Before you touch anything i'd try a leak test to check if there is a leak....and where.

I would say air leak also. Plus if it's soft seized previously I would Deffo strip the top to see what damage has been caused.

Good luck with it.

Cgt75b.

Had something similar to that on one of my GPs, full revs with the throttle closed every now and then, after a lot of carb stripping ended up being the fuel tap restricting fuel flow intermittently

Before you touch anything i'd try a leak test to check if there is a leak....and where.

Thanks. How would I do that? Although I rebuilt it years ago, my mechanical knowledge/ability is very limited.

Plus if it's soft seized previously I would Deffo strip the top to see what damage has been caused.

Thanks. That was a long time ago, before the 175 conversion.

ended up being the fuel tap restricting fuel flow intermittently

Thanks. How would/could that cause high revs with the throttle closed?

There a good thread on here somewhere about how to do a leak down test.

The high revving can be caused by a lean mixture - it's drawing air through the leak and lean mix leads to overheating and preignition - it can get so hot that it can continue to run even with the ignition turned off, which can be quite scary

So where would this leak likely to be?

Head gasket, base gasket, inlet manifold, exhaust manifold + other

I holed a piston that was caused by a crack in the casing on a stud hole....

Long and short seal the inlet and the exhaust port - some use a length of suitably sized bicycle tube clamped over the inlet and the exhaust manifold - then pressurise the system to about 5psi. The piston should be near the bottom of the stroke. It should hold this pressure. If it doesn't, start to look for leaks by spraying a dilute solution of washing up liquid around the joints that are prone to leaks -

Cylinder head
Cylinder base
Spark plug
Inlet manifold
Exhaust manifold
Mag flange

A leak should be revealed by bubbles forming.

Leaks can be caused by failing gaskets, cracks in casings, failed oil seals on the flywheel side etc.