Compression testing

[Greenfield]

The bike's a 1950 B33 plunger fitted with what I think is a 1952 engine ZB33 130**. The piston is flat topped as in Pete's photo, so in theory it should be the standard 6.8:1 low compression. There were no markings on the piston crown, so probably a replacement of unknown manufacture? The cylinder head is cast iron as it should be.
So, in theory, with timing and carburation adjusted correctly, it should work. I thorough road test is now due.

[Spaceman]

Sounds like a standard long conrod engine. As said, engines can sometime pink if they are coked up but this does not seem to be the case here. You can also get pinking if the mixture is too rich which effectively advances the ignition timing as it burns faster. If the mixture is normal as indicated by a light brown plug colour, advanced ignition timing can be the cause of pinking especially on a low compression engine and is worth double checking.

[Mike Farmer]

 

We always found using 2/3 star petrol in a higher compression engines, anything over about 8.5:1 could also cause it. Instantly cured by using 4/5 star. or 100 octane Low Lead Avgas. Note Low Lead as opposed to unleaded and I believe still no methanol. If you have a friend with an airplane or works at an airfield.

Slightly retarded can cause dieseling. IE compression firing before the spark.

Mike

[Pete C]

Sorry, should have specified that my engine is 1947 and so long rod.

The pistons are not interchangeable due to different compression heights.

[Spaceman]

As said, I'm not an expert on the B33 but understand the later short conrod was about 1/2" shorter than the earlier one. In order to keep the engine stroke geometry exactly the same, BSA simply moved the gudgeon pin down the side of the piston by the same amount so that, at TDC, the top of the piston was at the same height. Hence the reason the pistons are not interchangeable.

Although rather counter-intuitive, the length of the conrod makes no difference to the stroke length of the engine and, so changing it, does not affect the engine capacity - in other words, both versions of the B33 are 499 cc in capacity. What changes with a shorter rod, apart from the weight of the moving parts, is the maximum speed and acceleration of the piston at a given engine rpm and the degree of vacuum created which, somehow, must have been beneficial to the B33 engine.

[Steve.S]

Without wishing in any way to be pedantic, I wonder if it might be better to use Sir Harry Ricardo's wording. In his book on combustion, he points out that Pre-ignition and Detonation ( or Knock) are two distinct things. Pre-ignition is simply ignition before the timed spark and (mostly) inaudible. It is quite rare, and very difficult to identify.
Detonation, on the other hand, is audible and common. It is caused by the flame front travelling part way across the combustion chamber, whereupon the unburnt portion of the fuel/air mixture in front of it, spontaneously combusts, or explodes. This sets up a very high pressure wave in the  combustion chamber, the result being the "pinking" noise we hear, as the pressure waves strike the cylinder wall. This can destroy an engine.
Moving swiftly on, assuming your kickstart isn't prone to jamming, I would use a screw in compression tester, and using the exhaust valve lifter, kick the engine over,  release the valve lifter, and quickly (while the engine is still turning over) continue to kick repeatedly to keep the engine turning. I don't know if this would work, but it might be worth a try.
Personally, I would use a quite different approach to your Pinking problem. I would discard the compression plate and check the ignition timing using a degree disc, finding TDC with a rod jammed firmly against the cylinder half way down the bore and against the plug hole. Gently turn the engine until the piston touches the end of the rod and note the degree reading. Turn the engine backwards until the piston touches the rod and note the reading. When the readings are the same,  you will have TDC on the disc. If you don't use this method, you will almost certainly be a few degrees out. Be sure to set the timing on full advance.
Other thoughts are that since I imagine you have a manual advance lever, could you not retard the timing until it stops pinking?  If this does stop the pinking but the engine goes quite flat then we need to think further. I assume there are no carbon deposits in the combustion chamber and you are using the correct grade of spark plug? Maybe try a colder running plug and definitely use an octane booster.

[Spaceman]

I agree that a distinction should really be made between pre-ignition and detonation although detonation can follow pre-ignition and in both cases the engine can knock or pink. In this case, the CR is so low to start with, it's unlikely to be the cause of the problem in all likelihood. I assume the engine has effectively been de-coked now and so either an ignition timing issue or a fuel-air mixture problem would seem to be the likely cause of the problems.

However, having just re-read the previous posts on this issue with the engine going back to March this year, I think carburation and is probably still the main suspect?