Air leak and vibration?

[Danbretta]

Hi Chaps,
With the good weather forecast as always on cue my scooter decides to start running like shite. Went for a ride last sunday all was good until the way home when the tick over started to fluctuate. Also a vibration and deeper tone for a section as the revs dropped. I'm guessing this is pointing to an air leak. I'm also expecting it to be the exhaust manifold again. Will run a test over the weekend to confirm where the leak is.

I put a new plug in before the ride (pics attached). Seems to be oily on one side more than the other, what does this indicate?

I have been running the engine on shell V power with a 4%mix of Maxima formula K2 100% ester based synthetic. I had put a new piston and rings in last time I stripped the engine hence the 4% mix, the engine is standard nothing tuned, is it ok to carry on with the 4% or should I be running a 3% mix now?

I'll report back after the leak test but if anyone has an idea what the vibration and tone would point to I'll look into it.

TiA
Dan

[missing lynx]

What engine set up have you got?

[Warkton Tornado No.1]

As soon as you add more oil to the petrol, you weaken the petrol/air mix ratio.

If that were not bad enough, there is no point in thinking additional oil will somehow help the bedding in of the engine.The opposite is true. Running in is a process of deliberate wear for components to match each other better, which should be done without labouring the engine or staying @ low revs where the fan is not cooling adequately.

The other common misconception is that rolling off the throttle will somehow be good for a two stroke, thereby starving it of oil.....

[Danbretta]

What engine set up have you got?

All standard as it came out the factory, Li150s, 18mm carb, all jets to original spec, I'll have to dig out the details but nothing has been upgraded.

[Storkfoot]

You are wise to suspect an air leak as they are a main reason for two strokes to start running poorly. If the exhaust manifold is leaking, you can usually see a black oily gunge escaping from the connection with the exhaust. Also, look for a discharge from the cylinder head and base. Even though you can’t see all the cylinder head join with the barrel, you can usually see some evidence of a leak around.

I would do this before I bother with setting up an air leak test, personally.

If I was running a standard engine with a good synthetic and semi synthetic oil, again personally, I would be putting in something between 2 and 3%.

Good luck.

[Warkton Tornado No.1]

What engine set up have you got?

All standard as it came out the factory, Li150s, 18mm carb, all jets to original spec, I'll have to dig out the details but nothing has been upgraded.

Apart from my aforementioned (which applies regardless of tune):

The head gasket (if fitted) will almost certainly be weeping.

i'd be surprised if the points ignition is faulty, though the plug cap should be un-suppressed with the "old, unreliable points" that so many swap out for reasons of fault elimination. .....You did say:

What engine set up have you got?

All standard as it came out the factory, Li150s, 18mm carb, all jets to original spec, I'll have to dig out the details but nothing has been upgraded.

Sorry if I appear cynical (I am ) but I'd suggest a strip down of the top end inevitable, but can be so informative. Prior to doing so, I would also strongly suggest you procure a good selection of alloy, re-useable base gaskets with the intention of head gasket elimination. Whilst you are about it, a solid copper exhaust gasket will not leak. The problem will be finding somebody that knows what one is! Scooter Restorations list them, but out of stock. Such a solid copper gasket, fitted with too long nuts (run an M7 tap through M6 extentension nuts from Toolstation etc) lockwired together are bulletproof.

Prior to doing anything though, measure the squish with some solder. The chance are that you will want to improve that to the optimum in the interests of heat shredding from piston crown to head.

Your choice of fuel is excellent BTW. The alternatives are BP Ultra or Tesco Super Unleaded. The choice of oil is very controversial @ times, but I would have thought you can drop to 2%. (I get away with 2% Exol on a tuned Rapido 71 x 61 but have noted other comments concerning big end failures on reed valve engines. Perhaps my big end bearing is somehow a very good quality...)

Apart from playing Devil's Advocate, I do hope to have helped. Building a good engine is not really a 'Black Art' but more a case of being methodical.

[Danbretta]

Thanks Storkfoot, I'll take this onboard and have an inspection prior to the leak test.
Also noted on the fuel mix, 2-3% I guess the 4% mix was what was suggested back in the day

[Danbretta]

Thanks for the advice Warkton,
As you say, I think a strip down is on the cards, I'll have a look over it first before I strip it down, the squish is something I haven't checked yet, what should I be looking at for the optimum gap?
Looks like stickys book is coming out again over the weekend.
Cheers guys much appreciated and always..

[Warkton Tornado No.1]

Thanks for the advice Warkton,
As you say, I think a strip down is on the cards, I'll have a look over it first before I strip it down, the squish is something I haven't checked yet, what should I be looking at for the optimum gap?
Looks like stickys book is coming out again over the weekend.
Cheers guys much appreciated and always..

If the crankshaft & all bottom end bearings are known to be good, then 1.0 mm is the practical safe optimum & what to aim for. Saying that, I always centralise the head to the bore via dowelling @ the very least.

[Danbretta]

Managed to get time with the scooter yesterday, tested the squish and its 0.9 to 1mm. This is using the solder method.
So far the leak test is showing a big leak at the exhaust manifold. The copper gasket is showing signs where it's been blowing.
I was using longer studs and doubling up the nuts, I might put shorter studs back in and look at the k nuts, although not cheap if they do the job I'm happy.
Any advice or tips on securing a good secure fit here or is it a common failure. It wouldnt be so bad if I could get a socket up to nip them tight once in a while but the bend on the manifold makes it impossible for one nut.
I've been using copper sealant, anyone have a better product they've had success with?

[Warkton Tornado No.1]

Managed to get time with the scooter yesterday, tested the squish and its 0.9 to 1mm. This is using the solder method.
So far the leak test is showing a big leak at the exhaust manifold. The copper gasket is showing signs where it's been blowing.
I was using longer studs and doubling up the nuts, I might put shorter studs back in and look at the k nuts, although not cheap if they do the job I'm happy.
Any advice or tips on securing a good secure fit here or is it a common failure. It wouldnt be so bad if I could get a socket up to nip them tight once in a while but the bend on the manifold makes it impossible for one nut.
I've been using copper sealant, anyone have a better product they've had success with?

You've had my best advice.

Whilst you are about it, a solid copper exhaust gasket will not leak. The problem will be finding somebody that knows what one is! Scooter Restorations list them, but out of stock. Such a solid copper gasket, fitted with too long nuts (run an M7 tap through M6 extentension nuts from Toolstation etc) lockwired together are bulletproof.

in your latest response, you mention yours being copper. It won't be, other than folded copper 'tin' to retain the contents which may well be asbestos. Scooter Restorations would not list the "solid copper gaskets" if there were not a need & any well established dealers that doesn't know what they are, or will try to sell you the cr@p standard type should be ashamed.

As for acheiving better purchase on the nuts, or making 'spannering' easier the long M6 nuts I mentioned are, of course, only 10 mm AF. As I reiterate now, run an M7 tap through & cross drill one end for lock-wire & job done. Silicone sealant (the vinegar smelling one) will do as the additional sealant. It doesn't need to be 'high temperature' as silica already has a high melting point. Ever tried melting sand with a blowlamp?

[Storkfoot]

Make sure that none of the exhaust mounting points are putting the exhaust manifold joint under stress. If that connection is being continually pulled as one of the other mounting points is perhaps in the wrong place, that will often result in a leak.

[Danbretta]

It's a solid copper gasket, its rectangular rather than shaped to the profile of the exhaust manifold face. I binned the cheaper tin wrapped ones you mention, I was never impressed with them.

I'll look at getting those M6 long nuts you mention and tapping them out.

I'll take a bit more care when fitting the exhaust. As suggested It may well be pulling on the fitting causing it to blow.

[coaster]

....I'll take a bit more care when fitting the exhaust. As suggested It may well be pulling on the fitting causing it to blow.

Its also worth checking that the exhaust flange is flet as they're usualy not especially if they have been used with the squashy tin ones at any point. 5 minute with a good file will be time well spent. As for nuts, I've been using extended stainless ones which are drilled and come with a large 'R' clip that you put through the best positioned holes. Far easier than lock wire and have been good for me a few top ends for many thousands of miles

[Warkton Tornado No.1]

It's a solid copper gasket, its rectangular rather than shaped to the profile of the exhaust manifold face. I binned the cheaper tin wrapped ones you mention, I was never impressed with them.

I'll look at getting those M6 long nuts you mention and tapping them out.
re of thetting the exhaust. As suggested It may well be pulling on the fitting causing it to blow.

i stand corrected about the gasket not being of the solid copper type, though the thicker, the better. BTW, copper can be annealed by heating then quenching, which gives gaskets extra life.

....I'll take a bit more care when fitting the exhaust. As suggested It may well be pulling on the fitting causing it to blow.

Its also worth checking that the exhaust flange is flet as they're usualy not especially if they have been used with the squashy tin ones at any point. 5 minute with a good file will be time well spent. As for nuts, I've been using extended stainless ones which are drilled and come with a large 'R' clip that you put through the best positioned holes. Far easier than lock wire and have been good for me a few top ends for many thousands of miles

Not aware of the stainless fasteners with 'R' clips, but are they only 10 mm AF?

[Danbretta]

The exhaust flange and gasket were both put on the linisher, all faces were good and flat, do you have a link for the nuts and r clips you mention, are your studs drilled for these too?

[coaster]

This a link to the R clip extended nuts but I was fogetting I have stub fittings on my exhausts. That said, they can be shortened a bit but only so far as the ones I have arent threaded all the way through. I have them unmollested on a Franspeed exhasust flange but had to shorten one slightly for the BGM on my other scoot.. You dont drill the stud, the R clib is largr enough to contact the manifold/stub to stop the nut comming undone.https://www.ebay.co.uk/itm/153462401081 ... 1MQAvD_BwE

[Danbretta]

Ah I see know how they work, makes sense now I see the image. Thanks for posting the link.

[coaster]

Ah I see know how they work, makes sense now I see the image. Thanks for posting the link.

The only problem with them is that they too long for some applications, cutting a bit off the end seems obvious but the thread only goes for 20mm which in my case resulted in me chopping ALL the thread off the bit beyond the thread is machine too wide to just tap it through. Great for stubs though.

[Warkton Tornado No.1]

Ha!

Just for 'balance' (as in " you pays your money & makes your choice") when it comes to ease of use & cost:

https://www.toolstation.com/connector-nut/p29740

So, £1.39 for ten off.
M7 tap (most of us will already possess) but, say, about four quid...
Lockwire or some stripped copper wire.

Change from twelve pounds to do five engines.

Hmmmm. I think i'll 'stick'