LCGB Forums

Out on the scooter friday and it wouldnt tick over unless the choke was on even after warmed up. I've stripped the carb and cleaned it but before putting it back in I have done a leak test and found the welds at the mouth of the exhaust box are leaking both sides as shown in the image where the soot marks are. Would this affect the tick over or should I be looking elsewhere. This is a new tutto lambretta exhaust that was recommended to me as being one of the best original type exhausts. Just want some advice before I weld them up. Thanks in advance.

Not sure if the exhaust would effect tick over, but if the exhaust is new you should be asking for a replacement or money back as that is a manufacture fault

I purchased it direct from Italy as I couldn't find one in stock in the UK. The cost and communication to post back would probably be more hassle than tacking a couple of welds on there but I get what you are saying...

An exhaust as per OEM will be contributing to back pressure, so leakage should be more obvious, as you have discovered.

Any leak is not good, but just those depicted are unlikely to cause significant weakness.

I would pursue the problem. You say that a full leak down check has been made, but the two you found easily might be masking others, not so obvious.

I would remove the 'U' pipe in any case to visually check for leakage & test for leaks again.

The exhaust may be a problem but i'd be more inclined to check the carb'. The choke is merely adding more fuel so a blocked or pilot jet may be the cause or you may require a slightly larger jet.

Warkton Tornado No.1 wrote:An exhaust as per OEM will be contributing to back pressure, so leakage should be more obvious, as you have discovered.

Any leak is not good, but just those depicted are unlikely to cause significant weakness.

I would pursue the problem. You say that a full leak down check has been made, but the two you found easily might be masking others, not so obvious.

I would remove the 'U' pipe in any case to visually check for leakage & test for leaks again.

Fair point, I'll dig deeper and see if any other leaks show up. I've taken the u pipe off to remove the exhaust and get the cowl off to get better access and view to what's going on. The face of the exhaust manifold may be very slightly warped. It might be slightly pulled up at the stud hole ends so I'll have a look at this and see if this was a cause too.

What's the vest way of mating these surfaces, is it worth a skim of copper exhaust past along with the copper gasket or just use the copper gasket by itself...?

MickYork wrote:The exhaust may be a problem but i'd be more inclined to check the carb'. The choke is merely adding more fuel so a blocked or pilot jet may be the cause or you may require a slightly larger jet.

I'll take a look once I confirm no air leaks...hopefully one or the other solves the issue.

Cover the gasket face of the u-bend with with pen or paint and flat it off with a file or on a sheet of sandpaper on a flat surface.

Copper silicon sealant and a decent solid gasket - annealed copper - should ensure that joint. You should have the u-bend on if you have a fixed flange when you do a leak down test.

With that joint sealed look for leaks around the sparkplug, head, inlet manifold, cylinder base, mag flange. Spray soapy water and watch for bubbles. It will loose some pressure over time but of it holds for 20-30 mins I'd call that OK.

Thanks Ross, I'll have a proper go at it again later when I get home.
I was running with the mixture screw 1 and 1/2 turns out. This seemed to be ok but j came across this yesterday, does this still carry weight with today's fuels. From this original chart my 150S should be set at 1/2 then out. Is this ok or would it be too lean now?

rossclark wrote:Cover the gasket face of the u-bend with with pen or paint and flat it off with a file or on a sheet of sandpaper on a flat surface.

Copper silicon sealant and a decent solid gasket - annealed copper - should ensure that joint. You should have the u-bend on if you have a fixed flange when you do a leak down test.

With that joint sealed look for leaks around the sparkplug, head, inlet manifold, cylinder base, mag flange. Spray soapy water and watch for bubbles. It will loose some pressure over time but of it holds for 20-30 mins I'd call that OK.

Good that somebody else uses solid copper gaskets. They are the only sort that I would use, though I only know of Scooter Restorations that sold them.

They serve a dual purpose in allowing easier port matching, as well as maintaining the seal.

If port matching is not a concern (not sure why it wouldn't be ) or if the manifold & exhaust port are already matched & flat, then the gasket may be omitted with a good silicone. Although nowadays I use the Loctite copper coloured stuff, basic silicone sealant that smells like vinegar never failed for me....

I tried a copper solid gasket today and there is a leak at the back. The face of the manifold is flat as I sorted that today, maybe its the face of the cylinder or the copper gasket needs flatting off. I'll use the copper gasket sealer and look for more leaks.

Danbretta wrote:I tried a copper solid gasket today and there is a leak at the back. The face of the manifold is flat as I sorted that today, maybe its the face of the cylinder or the copper gasket needs flatting off. I'll use the copper gasket sealer and look for more leaks.

Sometimes the 'U' pipe manifolds may not allow complete seating. The solid copper gaskets can also have been punched out of line so that they need dressing top & bottom so they don't foul on the cylinder. In fact, the last ones I bought were really on the p1ss! Solid copper gaskets are made from sheet material, so the thickness (usually 10 SWG) will be very consistent & unlikely to be the cause of leakage. Unless you have burrs, of course....

If the cylinder is OEM cast iron, it may need a clean up. Best done with studs out & a file.

The cut out on the copper gasket is pretty good, I think maybe it would benefit from a quick file over each side to get flatter. Failing that it looks like I'll have to take the studs out and go to work on the cylinder side.