Hi all,
Thanks again for all the previous replies. Progress has been slow, owing to not having had a nice dry garage to work in for a while.
I bought a new Amal carb and new, longer, “original” silencer from Hitchcocks, cleaned the head, and lapped the valves. Also had one of the stripped engine stud threads repaired.
Old setup
Amal Mark I concentric, 107 Needle, 220 Main, 3.5 slide, pancake air filter (instead of the stock air box). Had a shorter silencer on it. Ran, but it had an unstable idle so would cut out waiting at traffic lights if not holding on the throttle a bit. Mechanic said bigger cut-out on slide would fix idle (though I don’t understand that since since I would have thought that the air flow at idle would be restricted by the long edge of the completely descended slide and so have no effect at that stage of throttle opening). Also suspected it was running rich.
New setup
New Amal Mark I concentric, identical to the one that was on the bike before - a 930 (900 series, 30 mm), with a pilot bushing (not an interchangeable pilot jet) and the newer/better float and float needle. Currently with a 105 Needle, 200 Main, 3.5 slide, same pancake air filter.
Current sitch
Initially we couldn’t get engine to turn over more than a few beats because of what seemed to us to be a fuelling problem. I say that because kicking over engine and holding finger over spark plug hole we noticed no wetness and no petrol smell. Triple checked that the pilot circuit was clear, it is. We’d get a very short run after flooding the float bowl by using the tickler or by syringing some petrol directly into the cylinder via the spark plug hole, but once that fuel is used up, nothing.
But fuel flow from petcock seems fine, and there’s no inline filter. We shortened the fuel pipe, so there was no U (to rule out the possibility of air lock).
We struggled for ages to even get it to idle. The breakthrough came when we closed the pilot air screw completely. Now it starts, and idles steady as a rock. We started it with the choke on, but releasing the choke made no difference to the idle running. Now the problem is that the engine sputters/chokes when we open the throttle past about a quarter (regardless of whether quickly or slowly).
Spark plug is clean and correctly gapped, compression seems good, timing set, points gap checked, good bright spark, battery charged, pushrods correctly set. Last time I checked the spring-loaded advance gizmo it moved freely and I cleaned and regreased it. So I’m happy in that the problem seems at least to be limited to how the carb is set up, and/or air flow into the carb (admittedly, perhaps naively!).
Other observations for what they’re worth:
- Pressing down the tickler on the new carb doesn’t have the fuel coming out the float bowl gasket near as quickly as doing so on the old carb did.
- Is it usual that the part of the pilot fuel circuit has no O-ring or bushing where it passes up from float bowl to carb proper? (i.e. where it passes through the float bowl gasket)
- I have a COlourtune kit I'd love to put to work, but unfortuantely wrong spark plug hole size becasue i'd bought it for another bike. not having much joy finding an adapter or right type.
Does this make sense to anyone? I do love these bikes! Excellent learning curve
