My cylinder head and other close friends

[Snails]

Well I reassembled it all, but forgot to do the leak test whilst the engine cowling was off. However I'm glad to say my plug colour is so now chocolaty brown Cadbury's want it to feature in their next advertising campaign

I guess I will never know the actual root cause as I have tidied up and changed so many things, new piston rings, lapped head and barrel, new fuel tap, welseal to all gasket faces but if I had to put money on it would be the exhaust down pipe to barrel seal as this is now much better, thanks to the welseal and I also used fireputty on the downpipe to exhaust joint and it so much quieter!

And now that I think about it this is what it used to sound like when I first built it. I guess you don't notice it creeping up over such a long time.

[Snails]

Arrrrrrgggggggggggggggggghhhhhhhhhhhhhh my air leak is back

I'm sure it was ok for a bit - so I am working on trying to find it again - stripping the engine again - as it comes and goes I'm assuming it something working loose rather than a cracked head or similar.

Wish i'd remembered to leak test it last time before I reassembled.

Anyway one thing I did notice is that its leaking a little fuel around the carb bango. Not that musch but you can see two stroke residue on the flywheel cowling. Could this be the cause? Would it actually add enough air to the fuel mix to cause problems?

If it is any suggestions for stopping this? Its a difficult to access position with the engine mounts getting in the way so i have tended to fit the bango and fuel line to the carb before fitting the carb to the inlet and I think that motion helps loosen it. As I'm also a little wary of over tightening the bolt and stripping the thread, would adding an additional washer stop that from happening?

I've not so long ago replaced the washers for bango and bolt.

[coaster]

Couple of points:

The fuel leak will have NO effect on the mixture unless it is leaking so much that it's causing fuel starvation....that would mean it was p1551ing out all over the place, not a tiny seepage.

If you do a leak down test, you will need a bicycle inner tube (mountain bike works) a lambretta one is way too big. You can buy a suitable inner tube from Asda for about £3.

Exhaust flanges are often too thin and folded copper/fibre exhaust gaskets squash down and allow the flange to bend at each end causing them to leak in the middle. It's very possible your carefully sealed exhaust joint has gone again.

[Snails]

Thanks. Would I not see blue fumes coming out from around the down pipe if it was leaking?

I'm using an MB gasket - the use once and throw away type if I remember rightly.

http://www.lambrettaspares.com/spares/e ... b0078.html

My down pipe has always struck me as being a bit odd as its not possible to remove the engine cowling with the down pipe in place but my friend says that he can on his. I find that quite annoying so I am tempted to seek a replacement although it will probably warp too?

[olliewtf]

unless exhaust manifold was really loose, no you would not see any fumes, but the slightest gap will be enough to draw air. did you lap your exhaust manifold? these are always to thin and can guarantee being bent everytime you tighten it down.
Also, bare in mind your head nuts may have come loose since last rebuild, i torqued mine up properly when i first built my engine and after 50 miles the head was squeaking. retightened with new washers and no problems. i know its frustrating, we've all been there, but try not to assume that if you had a leak before, rebuilt top end, and soon after you have a leak again, its the same place causing you problems again. It could be any of the usual suspects as theyve all been disturbed.
out of interest, this time you say its an air leak, before it was just a white plug, what symptoms other than plug colour are you experiencing?
are you experiencing the scooter revving itself to bits or just sitting above normal tickover? or is the head squeeking when you kick it over?

the reason i ask is that a white plug can sometimes be symptom of over advanced ignition timing..... I think. Im pretty sure. 99%.

[Snails]

unless exhaust manifold was really loose, no you would not see any fumes, but the slightest gap will be enough to draw air. did you lap your exhaust manifold? these are always to thin and can guarantee being bent everytime you tighten it down. - yes I did, it does turn up at its extremes (where the stud holes are) quite badly but the area around the gasket face appears flat enough
Also, bare in mind your head nuts may have come loose since last rebuild, i torqued mine up properly when i first built my engine and after 50 miles the head was squeaking. retightened with new washers and no problems. I'll check again although they were definately ok when I stripped it last time and that was after 8 years and 8k miles. i know its frustrating, we've all been there, but try not to assume that if you had a leak before, rebuilt top end, and soon after you have a leak again, its the same place causing you problems again. It could be any of the usual suspects as theyve all been disturbed. yeah good point, bleeding scooters do test your patience dont they, especially when its so hot and you want to be riding it not taking it to pieces!
out of interest, this time you say its an air leak, before it was just a white plug, what symptoms other than plug colour are you experiencing?
are you experiencing the scooter revving itself to bits or just sitting above normal tickover? or is the head squeeking when you kick it over?

Yes, not revving to bit, its takes a long time to return to tickover. What do you mean by head squeaking?

the reason i ask is that a white plug can sometimes be symptom of over advanced ignition timing..... I think. Im pretty sure. 99%.

Cant hurt to test that too. I will try to borrow my mates strobe

[Snails]

Anyway I go to carry on the strip down process and I notice that my carb clamp is untightened. Now I may of loosened this the other night as it was at the point of carb removal that I stopped but I dont remember doing so - in fact I'm 75% sure I didnt. So am I barking up another wrong tree or could this be a cause?

Also how many miles would I need to travel for the plug colour to give a indication of being true? I've just cleaned the plug which may have been a mistake for this.

From my distant memory of doing plug chops its not that far is it?

The speed of tick over return does not seem much different but it was pretty hard to tell anyway - it had to be pointed out to me for me to notice in the first place

[rosscla]

For plugs chops you need a nice long bypass with a lay by run for about a mile at each throttle setting.