Ok chaps, before I do something more stupid than usual, I've just tried to start my '64 B40 for the first time.
I've changed just about everything, just to stop it being too simple.
I've fitted an 8.75:1 piston, ss90 valves, an Electrex world cdi ignition and a JRB carb. I opened up the inlet port to match the carb.
I set the ignition to 31.5 degrees btdc, after reading a post here that said the recommended 35 degrees was way off.
On the second kick, it kicked me back, but only gently. On the third it fired four times but generated huge clouds of smoke out of the air filter. After that I can get the odd spit through the carb, but nothing else.
I did check the valve timing several times (before fitting the barrel) but that doesn't necessarily mean it's right.
Any ideas?
Oops, I forgot to mention B25 cam and followers
With all the mods to the engine, who knows what the correct timing is!
I would suggest retarding the ignition a little and fire it up again. You may need to experiment with the timing until you get it right for your engine.
The inlet valve could be open longer and sooner if that makes sense. Igniting too soon.
Good call Adrian. Backed it off two degrees, which made no difference. Backed it off another five and it runs.
Huge clouds of smoke, insanely fast tickover and neither the ignition switch nor the decompressor (both of which I'd tested) would stop it. But that's all just fettling.
I looked at RR's book and it seems to say 5 degrees btdc for an ss90. That sounds hard to believe. It's currently at about 22, which is as far as I can get it without moving the rotor on the crankshaft. Clearly, that's the next step, along with fixing the decompressor and ignition switch.
It sounds glorious, though ;D
Looking much better now. Did a proper fit on the exhaust. Fitted a temporary kill switch (The new ignition switch has gone faulty) tweaked the decompressor and it's sounding almost civilised. A couple of times when I've put it on the side stand, the float has stuck down though. Not sure why that is.
What carb have you got?
When you say "stuck down" I would have thought the carb would constantly flood.
All the vibration of the engine running would free any stuck float I would have thought.
What is the condition of the float needle? If worn there is a possibility of it sticking or leaking.
The 5 degree figure is for timing when set on full retard.
Thanks Julian, that would make perfect sense. What I can't quite figure are the numbers from Electrex.
The blurb says that it provides 30 degrees of advance at 5000rpm. Given that it's now set at 26 deg (advanced) that suggests that fully retarded it's at 4 deg Atdc.
I read that as saying that with a fully advanced figure of anything less than 35deg I can't get the fully retarded figure right.
The carb is a brand new JRB, which is a Keihin with an Amal mount.
It doesn't seem to be a problem running (though I haven't left it running on the stand yet) as you say, the vibration should prevent it.
Once stopped and put on the stand it has indeed started peeing out of the overflow a couple of times.
It didn't do it after the last run, so maybe it's just settling in. I'm sure I'll have remove it again anyway as I sort the jetting out.
The fully advance setting is the important one, the retarded setting is directly connected to the advanced setting and cannot be set independent of the advanced setting.
The unit should be automaticaly retarded for starting.
26 sounds far too retarded.
Yes, it does to me too, but at 29 it would only spit smoke out of the carb.
Once I get the jetting somewhere near I'll start edging it forward again and see how it behaves.
If too far advanced I would expect it to kick back through the kickstarter. Spitting without kickback would suggest too weak mixture.
That's a fair point. I should have made it clear that it did both. However, retarding the ignition made it run.
I did some more checking via a Lucas document and RRs book, because I seemed to remember that the dizzy provides nowhere near 30 deg of advance.
According to RR and the figures on my (redundant) dizzy, which I've just stripped, it provides 15 degrees of advance.
Looking at RR figures a stock B40 is set at 15.5 deg (retarded) yielding a fully advanced figure of 30.5, roughly where I started.
But, a '64 SS90, which is what my engine effectively is (in terms of compression ratio, valves, valve timing and inlet port) is given a fully retarded figure of 5 deg, thus fully advanced will only be 20 deg btdc.
I can only assume that the increased compression ratio results in a much faster flame front.
It does sound very retarded, so I'd appreciate a check on my logic/numbers, if anyone is interested/sad/bored enough.
Even stranger, I've just realised the stock b40s and c15s are also shown at 15.5 deg in '62, 5 deg in '66 and 33.5 (advanced) in '67.
My head just imploded. I guess that 15 degrees of advance, marked on the dizzy is at the points, which equals 30 degrees at the crankshaft.
Does anybody have a conversion from crankshaft degrees to piston position for the B40?
That is correct.
Makers stopped using fully retarded figures to set timing because variations in manufacture and wear in the a/r unit caused inaccuracy at the more important fully advanced figure.
When using a degree disc to set timing it is crucial to accurately find TDC - using a piston stop the easiest way. Several degrees can be lost around TDC when using rodnthrough plug hole method.
Very true, but finding tdc isn't really my problem. Trying to determine a sensible timing figure is.
The very fact that there's a 10 deg difference between the 1962 and 1964 figures rings alarm bells for me.
If I take the '64 ss90 figure of 5 degrees as correct, then depending on whether the advance figure on the dizzy is at the points or the crankshaft I get either 20deg, which seems very retarded or 45 deg, which seems insanely advanced.
Add to that the fact that cdi produces a much faster and shorter spark than points (and an adjustment for modern fuel) and it all starts to look somewhat random.
With a dizzy, I'd just twist it to get the best running, but that's not really practical for moving the stator.
Oops, arithmetic failure...35 degrees.
In 1963 BSA changed the distributor from 15 degree advance to 10 degree advance but the fully advanced setting remained the same.
Hence the up to 1963 retard setting of 15.5 and 5.5 1963 on.
Thanks Julian, I didn't know that.
So is that also implying that the advance figure on the distributor is at the distributor, hence the ten degree change at the crank shaft?
Thats right.