Paul Hopewell kindly gave permission to reproduce his notes on curing a backfire
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I’ve reopened this topic because of a problem I had with an annoying backfire that seemed to have appeared when I fitted a new stainless steel exhaust system. And I’ve seen more queries [on the Deauville Owners' Website] regarding this subject. This backfire is relating to carburettor equipped models and the backfire in this case is the type of backfire that only occurs while using the engine as a brake, decelerating or closing the throttle to slow down, often known as over-run (not a back fire through the carbs which would indicate a timing error or burnt inlet valve for instance).
The cause of the backfire in this case would be because of a problem with air getting into the exhaust system through a leaking seal or gasket. Air leaking into the exhaust system can often be remedied by a simple re-torque of the bolts along the entire exhaust or replace the gaskets.
Sometimes an air bypass valve is
introduced into many machines to reduce the ever demanding lower cO2 level requirements.
When the throttle is closed to slow the bike down the closed throttle still
allows a small amount of fuel through the carburettor, this fuel may get burned
in the combustion chamber or it may pass straight through to the exhaust and
mix with air getting into the manifold end of the exhaust system. This fuel
air mixture can be ignited by a glowing carbon ember in the exhaust system or
via a spark / flame from the opening exhaust valve resulting in a loud pop or
bang.
I read many of your notes on this overrun backfire problem and decided to investigate my machine a little more thoroughly. Armed with various tips about the PAIR valve and the loose downpipes that could cause the problem I completed a thorough inspection of the intake and exhaust systems. Both carburettor sockets and both down pipes were secure and all settings appeared to be in tolerance, but I did manage to nip the front exhaust pipe flange a little more. I also investigated the PAIR valve and found no diaphragm leaks and a vacuum of 290mm/Hg caused an effective air shut off.
However, quite by accident, I discovered that the air transfer hoses from the PAIR valve to both of the reed valve covers where not properly secure on the reed valve covers. On further inspection I found a white deposit inside the end of both tubes indicating that the rubber pipes could have been loose and leaking air. A temporary fix was to cut 15mm off the end of both pipes and refit them tightly on to the reed covers securing them with more suitable clips. Success, no more Backfire just a light and gentle burble on the overrun. I can't be bothered to strip the bike down again just to take a photo. Just so you know, I'm not a bike mechanic. However, I'll try and describe the air valve and how to get at it and the two pipe ends on the reed valves. This description is of my Deauville and I generally use the Haynes manual as my maintenance reference, I assume most of you have a copy of the same book.
The PAIR valve is a black mainly plastic bodied three-way valve it looks, in part, like an old-fashioned car fuel pump. Out of which four pipes protrude, one is a thin rubber vacuum hose connected to the rear cylinder intake port on the left hand side. The three larger rubber pipes, about 18mm diameter, trot off to the bottom left rear of the air-filter box we’ll call this pipe ‘A’. Another goes to the rear cylinder head rocker cover, call this ‘B’ and the last one goes off to the top of the front cylinder head rocker cover ‘C’. This valve is under the fuel tank beside and to the left of the carburettors. To get at the valve the seat, side covers and tank need to be removed, I suggest you use the Haynes manual to guide you through any disassembly and subsequent reassembly. After removing all the above paraphernalia, the PAIR valve will now be exposed. It’s below and to the left hand side of the air filter box.
Remove the air filter and box there is a picture of the PAIR valve in chapter 4.15, illustration 14.b. Haynes call it the ‘secondary air system control valve’. The next thing you need to do is test the PAIR valve function so you need to get something to use as a temporary fuel tank, I use the fuel tank off my petrol mower and plug it into the fuel line. When ready, get the engine running and warmed up, but beware, the carbs are open to the atmosphere and if anything can fall in ‘sods law’ says it will. Now, while the engine is running establish the pipe on top of the PAIR valve that connects to the air filter, pipe ‘A’. Put your thumb over the hole you should feel suction, the suction will increase as engine revs increase. Take your thumb off the pipe and listen to the sucking sound coming from it, while doing so rev the engine to above 6000rpm and snap the throttle shut you should hear the valve click in with a hollow ‘pop’ and the suction sound will stop until the revs drop to below 2000rpm, then the sucking sound will restart.
Do the same with your thumb over the pipe again and you will feel the vacuum drop when the valve shuts and reactivate when the valve reopens. If this is the case the PAIR valve would in my opinion be perfectly OK. Before going any further re-torque the exhaust pipes.
Now turn your attention to the reed valves on top of the rocker boxes chapter
4.15 (in the Haynes manual) illustrations 14.1a, 14.3a. Illustration 14.3a shows
the connection to pipe ‘B’ on the rear rocker cover, the connection
to pipe ‘C’ is a similar set up. Where the rubber pipe fits on the
aluminium covers is where I believe the source of my air leaks were. Follow
the pipe refitting instructions mentioned earlier and after you’ve carefully
re-fitted everything you should have sorted your backfire problem providing
the carbs are properly balanced and within tolerance and that the spark timing
is OK, the exhaust pipes are well torqued and if you’re running on fresh
fuel "
- thanks, Paul - a well written up report. .