Preignition

[Greenfield]

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.

[Bess]

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...

[idie]

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.

[Greenfield]

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?

[Pete C]

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.

[Greenfield]

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?

[idie]

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.

[Greenfield]

Cheers Idie

[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.

[berniej]

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

[idie]

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.

[berniej]

As you like - was just trying to help but have seen enough now.

Bye all.

[Greenfield]

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.

[Pete C]

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 

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

[Mike Farmer]

 

I would suggest that you get some REDEX (or equivilant additive) and add to your fuel before doing anything mechanical.

Mike