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[TimG]

Hi all

I wondered if someone could help me with this...for the last few weeks my Classic has occasionally been blowing back through the carb, usually on startup, but now more with increasing frequency while coming to rest at a traffic light or similar. This is beginning to get very scary, as now and again (and it's starting to happen more often) there is a noise like a Howitzer and the carb is blown right off the inlet stub. I have done the circular clamps up as tight as I can, but still it happens. I have improved this behaviour by routing the throttle cable correctly to ensure that the slide returns fully, but it still happens. Apart from anything else, hazardous early morning startups mean I am expecting a fatwa from my neighbours sometime soon. Also, since replacing the points the idle has become very slow (I didn't touch the carb itself at all when the points were done), and I have an idea the two problems may be connected, but can't immediately think how. Any ideas please?

Cheers

TimG

[Dennis C]

Hi Tim.------ My guess is that you have altered the ignition timing when replacing the points, check and reset the timing and the problem should be solved.

[Les H]

Hi Tim. The difference in tickover speed has been caused by you fitting the new points with a different points gap than it had with the old set. It would be most likely the old points had a large gap compared to your ones set with an accurate (Or inaccurate!) smaller points gap. The bigger the gap the more advanced the ignition is, and with a smaller gap, the more retarded the ignition timing becomes. At tickover the the revs can change quite a bit with just a small change in ignition timing and this explains what has happened. Check you points gap again and if correct simply set the the carb for a faster tickover. The spitting back in the carb will be caused by the pilot jet becoming slightly blocked, most likely, but just a chance that the inlet tappet clearance is tight and slightly holding the valve off it's seat...remedies are obvious.

[Les H]

CORRECTION: It would be most likely the old points had a large gap compared to your NEW ones set with an accurate (Or inaccurate!) smaller points gap.

[Bullet thrasher]

Me too. I have Boyer ignition and if there's any voltage drop mine will spit the carb off. Trouble is it still does it from time to time. Sometimes I'll just start moving the kickstart to ease it past tdc and it'll just start. Other times it'll cough and spit the carb off. It's annoying and I have to have a screwdriver to hand always. The definite cure is a carb that bolts on rather than clips. Be careful you don't puncture your manifold. Over tightening won't help the force is enought to blow my whole airbox back off it's one securing bolt. My bike runs great at the moment but still does this sometimes. Wish I had a simple fix.

[another Allan]

Be very careful if this 'spitting back' through the carb is due to over-advanced ignition timing (caused by inaccurately set points timing, or low voltage to a Boyer.)
That 'spitting back' is the engine trying to run backwards (because the spark occurs too early.) The real damage that can be done is to the oil pump spindle and worm.
Mine did this some months ago when I gave it a half-hearted kick. It 'spat back' with a bang. I tried again and it started fine. Then it started making a terrible racket. I switched it off and unscrewed the return pump cover and turned the engine over slowly - the pump did not rotate.
I stripped the lot down and found that the pump spindle had lost the teeth over half its circumference, the worm had lost about 5mm from the end, the exhaust cam spindle was bent, the exhaust camwheel lost a tooth and the inlet camwheel half a tooth, and the cam follower feet were scored. There were fragments of steel liberally spread around the inside of the timing case.
So, not a cheap repair! Our host's technical notes mentions having stripped a pump spindle on the first startup of a rebuilt engine; it's nice to know it's not just me!
So, don't just tighten your carb, check the ignition timing, and don't set the idle speed too low.

[TimG]

Thanks all. Dennis C: I was most careful not to disturb the timing plate AT ALL when replacing the points, so I think that Les H may have nailed it. It may well be inaccurate points gap, as I find it almost impossible to see where the max opening point is on the points cam. Will retry. Will also look at the inlet valve clearance (was dead on when I looked at it a couple of months ago, so didn't bother adjusting it, but will double check). Another Allan - I think if I had overadvanced ignition to that extent I would have all sorts of starting and running problems, but even so will check to see that nothing has slipped...my money is on the points. Thanks once again.

Cheers

TimG

[Dennis C]

Hi Tim.---- Perhaps I oversimplified the reply, by having the incorrect points gap the timing is incorrect as well, too wide and the ignition is advanced and too small and the ignition is retarded. ----- must try to give a more complete answer in future.

[Dennis C]

Just to add to my Previous reply regarding points gap, ---- it would be wise to check the timing as well, --- if the bike has previously been timed with the points gap incorrect then correcting the points gap now will not necessarily give you the correct timing.

[TimG]

Hi Dennis C - the timing has not been altered (by me anyway - my local bike dealer may have done it as part of a service) since it left the factory, so I'm tending to treat timing as a "given", particularly as all this started to happen after I replaced the points. Opinion tends to vary as to how sensitive to points gap the timing is, from "anything between 12 and 15 thou will do" to "it has to be more or less exact, otherwise the timing can be significantly affected". My problem - as I outlined to Les H - is that I find it almost impossible to see at what point on the points cam I have to adjust the gap, and getting that wrong, combined with an inaccurate points gap, could well be causing my problems. Have you got any tips for finding the correct point on the cam? It does seem that Classics tend to be a bit more sensitive to correct gap AND opening point (and therefore, effective gap) than I had been imagining...

Cheers

TimG