SF3 exhaust sealing

M

macavity99

Full member
Location
Bedfordshire, UK
Just completed 500 miles after rebore and valve guide replacement. The exhaust ports are not sealing properly: oil dribble from LH port and some smoke from RH silencer. (Non standard exhaust system with no balance pipe between left and right.)

I've added an extra gasket but it hasn't helped.

Will try high-temp silicone next, the stuff for sealing domestic flues. Is this a good idea?

Can the finned header retaining rings be flattened on abrasive paper?

Cheers, Steve in UK
 
You can of course flatten retainers on sand paper. They do warp but I'd check elsewhere first.
Dismantle complete exhaust and check methodicaly and meticulously. Make sure you haven't inverted left and right exhaust tubes. ( not sure it can be done but check).
Where does the exhaust come from?
Who did valves and guides, are you sure they're competent?
Paul
 
You are talking SF? Regardless, smoking exhaust is not related to exhaust sealing. It can be either oil coming down inside of valve guide or between the guide and the head, or your rings are not sealing properly on that side and smoking up, both these things have just been worked on.
How do your spark plugs look? Poor oil sealing rings, or valve guide leak on the inlet side will show on the plugs.
Oil dribbling from the exhaust port can also be the exhaust guides as mentioned above.
Remove the headers and look for oil leaking down from the exhaust guide area (it should be dry carbon), a bit harder to see on the inlet side but a torch should help.
How did you run it in? If you were too careful it may have glazed the bores (need re-honing), or you have synthetic oil (not generally good for running in). How bad was the bike before the work? if there was tons of oil consumption you could be just burning out the oil from in the exhaust from before on the side that smokes.
You can well use silicone on the exhaust gaskets to keep it inside but this is hiding the issue. If you can?t hear exhaust leaking then the gaskets are doing their job, they are not supposed to be oil seals.
 
Steve,

You'll have a difficult time sealing the exhaust, no matter what type of seals or goo you use.

The balance tubes, either type, are very effective stays and prevent the entire system wobbling about.  With the header clamped only to the head and the silencers hanging from a rubber bush over a yard away, the system can sway about and will slowly but surely wear away any type of header seal you might fit.  Quite certain your (header) nuts will be constantly loose... :D

Fitting a sturdy stay where the silencer/balance pipe was fitted effectively avoids any loosening issues.

piet

 
On 5000/8000 batch SFCs they hang loose too, but there are always spots where they rest against rub the engine which possibly helps. Never really a problem with these Laverdas.

Marnix
 
Piet

Interesting point about the exhaust header bracing. I hadn't thought of that but it makes sense.

Reconditioning (rebore and k-line liners) was done by www.vintage-engines.net, Bedford UK. Competence? There were

plenty of engines there waiting for work, motorbike and car, so I guess they are competent.

Searching the forum I've found reports of oil leaks caused by poor header sealing, fixed by carefully installing

with new gaskets.

The RH side smoke is more worrying. Embarrassed me yesterday: after waiting at traffic lights for a few minutes the

bike made a LOT of smoke when I  moved off. Makes me wonder if I broke a piston ring when I reinstalled cylinder

barrel. Can't see why the LH exhaust valve guide liner is okay but RH side is allowing oil past: both were done by

the same shop.

Cheers, Steve C
 
You never know who has done the actual work, regardless of "reputation", as many of us have found through bitter experience. Removing and fitting valve guides can easily score the alloy in the head if not done very carefully with the correct technique and drifts. Unlike many bikes Laverdas don?t have an o-ring to stop oil leaking down outside the guide, so it has to fit well. I really hope you are not the recipient of a poor job but it is worth at least looking in ports and checking plugs.
A leak-down test will tell you if you broke an oil ring.
A new rebuild should definitely not smoke as you describe.
One thing to check is that your breather hose is open to the head and the one-way ball-valve does its job. Try blowing and sucking on the hose end or at least feel with your hand that it "breathes". A restricted breather can cause crankcase pressure to find other ways out, past rings,guides etc.
On my SF2s I have had rubber mounted and fast mounted mufflers without a centre brace and have had issues with broken studs exhaust and muffler brackets, never with header gasket leaks (or oil dribbling past the gaskets). My plan is to fit a rubber mount to the frame for the rear of the header pipe and rubber mount the mufflers.
Lee
 
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