Forum

[wilf]

My bullet has started to spit back through the carb again, mainly when started from cold. Having stripped and cleaned the carb several times, to no avail, I turned my attention to the sparks dept. I found the auto advance not fully returning, so removed the points back-plate to take a closer look. With the back-plate out of the way I found the mechanism working as it should. With everything back together I found that if turned the cam clockwise and let go, once again the auto adv. wouldn't fully return until I opened the points with a screwdriver. Is it that the springs in the auto adv. have weakened and are no longer able to overcome the tension of the points spring? The bike has covered 15,000 kms.

[Leon Novello]

wilf: If the weights return with the back plate removed while you worked it by hand, they must be binding on either the back plate or the condenser. Look behind the condenser, it can get worn through by the weights rubbing on it. You will need to bend its bracket forward, away from the weights. The condenser could be changed if you want to whilst you are in there.

[Thack]

I wouldn't have thought ignition timing would be likely to cause spit backs. They happen when the previous charge is still burning when the inlet valve opens, and this requires an extremely slow rate of combustion. As far as I know, the only way that happens is when the mixture is far too weak (I'm sure that's why it's more common during a cold start). Of course, I might be wrong about this - let's see what our resident experts say.

[wilf]

Leon, I had noticed the wear on the back of the condenser,and have bent the bracket slightly. Still the springs don't fully return (only by a whisker) unless I open the points with a screwdriver.
Thack, I was given to believe that incorrect timing could cause erratic slow running and spitting back. As I'm having difficulty starting from cold (sometimes first kick, sometimes flooding) and the only area of doubt carb-wise is a very slack pilot air screw which seems to have limited effect on setting slow running, I thought the problem might be ignition related.
When she does start first kick I have the devil of a job keeping the old girl running. She just keeps f**ting away through the carb until warmed up. She runs fine after that, except for a slight hesitation when opening up the throttle from fully closed after descending a hill, which started about 100 miles ago.
(The hesitation, not the hill!)

[Leon Novello]

wilf: All I can suggest is to check for an air leak, possibly a split in the rubber manifold between the carburettor and cylinder head, you might have to look very closely to find it. Symptoms are usually as you discribe and pinging under any sort of light load. A few light drops of oil on the moving parts of the weights might help them move more freely. Sorry, that is all I can suggest. Others will be able to help you more I`m sure

[simon]

Check your compression too it could be a badly seating valve.

[PeteF]

Mr H has some better quality UK springs for only a few quid.
If you in doubt about them why not change them then it's one less possible cause of your problem.
As with all jobs like this - do ONE thing at a time and test.

[wilf]

Morning gents,manifold rubber is OK,recently renewed. Just changed head gasket to cure oil leak from pushrod tube.compression good /ex. valve seating OK.performance generally excellent. Slightly concerned about pilot air screw being slack. Changing ADV springs looks fiddly. Can it be done without further dismantling?

[Leon Novello]

The springs can be changed if you can remove the points cam without the special puller. Spread a large white cloth beneath the whole assembly to catch a spring if it goes ----- TWANG! Long-nose pliers would be handy.
This will throw the timing out of course, so mark the plate before you start.

[Thack]

Wilf, I'm still of the view that it is running too weak at small throttle openings. In particular, I'm worried about this "slack" air screw. Do you mean it is so far out that it is loose? You didn't say what carb you have, but if it's like an Amal Mk1, unscrewing the air screw weakens the mixture. Thus a so-far-out-it's-loose air screw may well be the problem. If I were you, I'd "find" the correct setting again, as follows. Get the engine warmed up. Wind up the throttle stop screw until it will hold a fairly fast tickover. Starting with the air screw well out, screw it in until the engine speed peaks (or starts to slow down, in which case unscrew it 1/4 turn). Unwind the throttle stop screw until the engine is running more slowly, then readjust the air screw for maximum tickover speed. Keep doing this in stages: unwind the throttle stop, then tweak the air screw for maximum speed, until you've arrived at the right tickover speed and, finally, optimised the air screw for the last time. Now, I actually like to run the tickover just a bit rich, so you might want to consider screwing in the air screw perhaps 1/8 turn from the "correct" position and readjust the tickover speed to compensate. Would you consider doing this and then reporting back?