I'm just getting my 1950 B33 plunger back on the road after probably several years out of service. I bought it just over a year ago in poor condition, although it would start and engage 1st gear. It now goes through the box and having overhauled the speedo, its now up and ready to go. It now starts near enough first kick (full choke, no throttle and ignition about 1/2 advanced), and once warmed up runs fine on the stand. However, on the road when the ignition is advanced beyond full retard, there's an unpleasant rattle from the engine which disappears as the ignition is retarded. My initial thoughts are preignition. Its got an amal 289 with standard jets but I've raised the needle to 4 to richen the mixture. If it is preignition, my next thoughts are that a build up of carbon in the head could be the issue. To rule this out I'll need to remove the head. Is this possible with the engine in situ?
Any thoughts would be welcome.
Hi,
Before you strip the engine, have you tried Shell V power fuel. It might stop the pre ignition and during use helps clean the Carb and engine.
Best wishes...
Before you rip the engine to bits, have you checked the ignition timing. 3/8" btdc. It sounds as if if it is over advanced.
Thanks Bess, yes that was the first move to drain the fuel and put a gallon of good stuff in - put it didn't make any difference. I've had a chat with Burlen and everything about the carb looks as it should do apart from the throttle needle, which I can't tell whether its correct without having the right one alongside. I might just renew it anyway.
Idie, I've got the timing roughly about right but what's the best way to check timing accurately?
use this guide from the WM20 site. http://wdbsa.nl/ignition_timing.htm
I set my ignition at 7/16" advance on M21, B31 and B33. An electronic indicator for the opening of the points is much easier than the fag paper method.
It's a pain to get the timing 100% right but worth it.
You can remove the cylinder head with the engine in situ and this would allow you to accurately measure the piston position for timing purposes as well as give it a decoke.
Thanks Pete
A job for tomorrow.
I've just checked the points and they were perhaps a tad wide. I'm wondering how advanced it would make it if the points were say 14thou instead of 10 thou. Also, when the lever is used to advance the ignition, what would the difference in ignition timing between fully retarded and fully advanced. Is this info available somewhere?
The points should be set at 12 thou. If wider it will advance the ignition I set my B33 and M33 at 3/8 btdc. 7/16 I believe is the earlier setting, I doubt if it makes a lot of difference on our old clunkers.
Cheers Idie
If you are setting the timing and haven't got buzzer for when the points open. Do as I do use a 1 1/2 thou feeler gauge. At least you won't rip it like a fag paper.
I would always suggest setting the points gap first and then timing at full advance at 7/16" BTDC (remember earlier mags are tight to advance and later ones slack).
Although these engines are pretty tolerant of ignition timing it's more critical with mags as they only generate maximum flux (spark juice) at one particular point in the cycle. So, when the timing is too far advanced - or retarded - there will be a reduction in spark energy as the points open.
I start my M21s with a little bit of advance, adjust for a nice idle and then, once warmed up, advance with engine revs listening to the engine to make sure that I'm not over advanced.
As mentioned, pre-ignition may just be the result of too much advance for the condiitons.
Cheers,
Bernie
Greenfield has said that his motorcycle is a 1950 B33 which is a 500cc OHV engine. The correct ignition timing for this machine is 3/8 inch BTDC, not 7/8 inch which is for a 500cc sidevalve engine.
As you like - was just trying to help but have seen enough now.
Bye all.
I had another look at the instruction manual last night (1946-1955). It states 3/8" btdc (fully advanced) points open 0.002". I hadn't noticed the 2 thou gap before and for many years have used the window out of envelopes to set points on bikes as its about 1 thou. To be honest, we probably need a bit of give and take with these old bikes as long as things are roughly in the right area.
I used a 1.5 thou feeler this morning on the B33 and the points open at near enough tdc when fully retarded and 5/8" btdc fully advanced. I therefore conclude that if I advanced the ignition lever, ignition would be too advanced, but if I adjust the timing to 3/8", it would spark atdc when the lever is retarded ie slack cable on mine.
Again, any thoughts chaps.
Quote from: idie on 30 Mar, 2022, 19:40
Greenfield has said that his motorcycle is a 1950 B33 which is a 500cc OHV engine. The correct ignition timing for this machine is 3/8 inch BTDC, not 7/8 inch which is for a 500cc sidevalve engine.
I think you mean 7/16 not 7/8 :o
I use the 7/16 BTDC setting on a 1952 M21, 1947 B33 and 1954 B31 and haven't had any pinking issues. I can't see it making much difference (3/8 or 7/16) on the OHV engines but I do try to get it spot on for the sidevalve as that runs hot anyway.
If you are finding 5/8 at full advance, that is to much!
cheers
:) :) :) :)
I would suggest that you get some REDEX (or equivilant additive) and add to your fuel before doing anything mechanical.
Mike 8) 8) 8)
Definitely 7/16 inch not 7/8 inch my mistake.
Taking one step at a time, by the look of things, I need to retard ignition by about 1/4". By my reckoning the magdyno pinion needs to be rotated on its taper. I have removed the magdyno pinion nut and I need an extractor to ease the pinion free. Can someone confirm that the extractor is 61-1903 before I buy? The inside thread looks about 5/8".
Once the pinion is free I suspect the fun will start getting everything into position and tightening the pinion nut without disturbing the set up. Happy days!
It's a 5/8" 20 tpi thread, the same as the the cylinder head bolts (in the crankcase). If you have a spare/scrap cylinder head bolt just tap a thread for a 7/16" BSF or whit bolt in it and you've got a mag pinion puller - see photo.
:) :) :) :)
Re tightening without disturbing positioning.
I set it in position then press hard to stabilise it. Then get a suitably sized socket put over the spindle against the sprocket and give a "vigorous" tap with a hammer. This will set it on the taper. Then with care it wont move as you tighten things up.
Mike 8) 8) 8)
Fortunately a local B31 owner heard my cry for help and kindly lent me a puller.
Once the timing had been set, I started it up and it was awful. Although the carburation was pretty close before altering the timing, perhaps slightly on the weak side, it was now way out and running very weak, so much so that the exhaust blued in just a few minutes. I rechecked the timing and found it slightly retarded, so its now been set correctly. It still runs weak. The pilot screw is fully in and the engine will not pick up from idle on full retard; it spits out of the air intake and cuts out. The positive thing is a short trial run with the advance set around middle, I've now got rid of the suspected preignition.
Having stripped the carb and removed the jet block to make sure that everything was thoroughly clean, I would now recommend to myself that I fit a slightly bigger pilot jet. But that's not possible with a 289. I'm fairly sure that air isn't leaking at the manifold and the throttle slide was renewed not long ago.
Apart from the ignition timing, I think a 289 is too large for a B21, standard would be 276 1 inch bore or a 376 monobloc.
Trying to diagnose problems when the initial setup is non standard makes it more difficult. :(
He has a B33 500cc isn't a b21 a pre-war 250cc
Yes its a B33. Everything about the carb is as to specs apart from the fact I've raised the throttle needle a notch. I've just taken the carb off yet again and probed every orifice with fuse wire to make sure everything's clear. Having started up and warmed it up, the pilot circuit really needs to be a bit richer. I hadn't appreciated how closely linked the timing and carburation were. Running weak will also contribute to preignition.
What has crossed my mind is how much air still enters the idling mix when the airscrew is fully in, and if the screw is not seating properly, could additional air get through?
Normally when sorting out ignition timing and carburation problems on a bike, the last thing to worry about is the tickover and the pilot mixture. In general, the pilot circuit will have no effect on how the bike runs after the throttle has been opened slightly. Clearly, in this case, there are more fundamental problems with the way the engine is running and it might well be a case of having to go back to basics including, possibly, a top-end overhaul, a check of the valve timing and a thorough check of the magneto and the ignition timing. As said, worrying about the pilot mixture would be bottom of my list!
Thanks Spaceman. That's what I'm thinking. I'll take it out for a decent run and see how it goes. Yesterday on my one mile circuit it seemed to pull ok at a range of revs. So as long as I can rely on it not to stop and I can reliably start it, I'll live with it for the time being. Anything major can wait for 'a rainy day'.
At least the clutch, gearbox, brakes, speedo etc are all performing now.
Quote from: Pete C on 11 Apr, 2022, 14:31
Apart from the ignition timing, I think a 289 is too large for a B21, standard would be 276 1 inch bore or a 376 monobloc.
Trying to diagnose problems when the initial setup is non standard makes it more difficult. :(
Sorry I got confused by a previous post and thought it was a B31 (my B21 was a typo! ::))