A Conundrum!

It may be a case of stating the obvious when suggesting that the mass of air ingested through the carburetor and into the engine at it`s most efficiently operating point has to, by definition, eventually leave via the exhaust pipe. This must be the situation despite the fact that some mixture can take several engine cycles to fully clear from the crankcase.
A good 125 Bantam engine might produce 8 bar BMEP at peak torque revs of 10,500rpm, peak at 11,000rpm and have a typical, maximum crankcase volume of 435cc ( 1.4 ratio).

Delivery ratio could be as high as 1.1 with the scavenging efficiency at .85; these statistics would imply that some 137cc of mixture passes through the transfer ports and into the cylinder each revolution!
137cc is a long way different to 435cc, so what has happened to the balance of that mixture. Is it a case then, that the breathing ability of a modest 8 bar engine is such that the full crank-case volume of 435cc could never be filled in the time allotted, and the prevailing pressure differential of the combined action of crank-case and exhaust pipe?

It is always the way that at the point of desired peak efficiency there is the least time available to achieve it……flat out!

Looking at all of this might offer the proposition that low BMEP engines give of their best with smaller case volumes? A conundrum indeed!

Trevor

Might it be useful to consider the amount of charge lost by blow-back through the carburettor? It is noticeable that piston ported two-strokes are much more prone to this than reed or disc equipped engines. I raced a DMW 250 with a Villiers "Starmaker" engine, and the back of the bike would be liberally coated with oily mixture at the end of each race. That was on the occasions that it actually made it to the end of the race, of course.

Frankly, when I look at the principles behind the working of the two-stroke engine, I'm amazed the things work at all.

Very useful to point that out John, I well remember blipping the throttle of one of my ill-advised 50mm stroke engines and seeing a plume of mixture puthering out of the carb. No idea if the same happened during racing but I wouldn`t be surprised if it did. It might just be that at higher revs the ram effect of the inlet gas column overcomes the rise in case pressure to curtail blowback. Inlet ram pressures of 1.2atm have been recorded in 125 cylinders so it cannot be discounted. However that may be a trifle optimistic in a piston port Bantam situation at 8bar BMEP, very useful though for helping to get reeds to lift more rapidly!

If the inlet duct dimensions and timing are somewhere near providing optimum resonance for engine operation within its power range then blowback such as you describe should be minimal. British two-stroke engines conceived in the 60s sadly do not fall into this category, and your experiences and my own witness confirm this!  You have my, very belated, commiserations?

However, just to digress I would offer one caveat to that blanket criticism, the Enfield GP5 engine designed by Herman Meier was a powerful and technically pioneering engine for it`s era. To say that the rest of the power unit of gearbox, clutch and ignition were poor is to be kind to that British industry. After enduring public criticism and very vocal rows with the Enfield management they sacked Herman! He is reported to have told a MCN reporter that the inside of that factory at management level was, “total insanity, I`m glad to be free of it”. No one could have put it better, one succinct observation summarizing an industry wide malaise!

Yamaha managed to minimize, but not effectively eliminate, as far as they could, the adverse effects of blow-back within their race engines with clever research and design work on their piston port inlets and carbs even with timings of 204* but were eventually force to concede that a reed valve solved a lot of these problems for them.
Yamaha were also very grateful to Herman and the multi transfer port concept he conceived and they and we all copy to this day.

Rotary disc valve equipped race engines are in fact are the worst offenders in this respect. Aprilia GP engines when running at the base of the power band below 10,000rpm would deliver such extreme and violent reverse flows that combustion deposits could be wiped from the carb bell-mouth and up inside the air box. Riders were cautioned not to go there, particularly under partially closed throttle conditions, and the engine management readout analysis would graphically reveal if they did!

Suffice to say that my own experience, and that of our rider, with my Bantam engine with a cylinder reed valve installation, is that I would never return to piston port induction control, except of course for a 175. The blow-back you refer to is tiny to the point of being inconsequential and once a few thousand revs are recorded there is none to speak of. With the reeds offering up to 275* of inlet opening beginning at just after BDC and the closing point determined by pressure difference alone, therefore not being delayed by the symmetry of piston port, there is all to gain in reducing potential blow back.

One last point worth airing again is the profile of the carb bell-mouth. Correctly designed, blow-back can be reduced and is well worth experimenting with. Again the 60s trend of a largely straight angled job is not helpful.

Trevor