Metrallas in reverse
Each of us has experienced an episode when the engine of our Bultaco Metralla has run backwards, propelling the bike in the reverse direction to that expected. But finding a common explanation of how this is possible is difficult as the circumstances were
“I’ll say up front that I’m not a mechanic and certainly not an expert on the subject of internal combustion engines of any persuasion 2 or 4 stroke. As we all know the ignition process is caused by the points opening which initiates the spark which in turn ignites the fuel mixture causing it to expand and the piston to descend thus rotating the crankshaft. In the case of an engine with a kickstarter, the mechanism of the kick-starter is such that when downward pressure is applied to the lever the crankshaft is rotated in the correct direction, the inertia created by the weight of the flywheel spinning keeps the crank rotating in the correct direction. During this kick cycle the sparkplug fires igniting the mix and as the flywheel has the rotational inertia carries the piston beyond top dead centre as the igniting mixture expands causing the piston to move downwards, maintaining the rotation in the correct direction.
“As the kick-starter is on the left of the bike, my normal starting procedure is to put the bike on the centre-stand and from standing to the left of the bike, operate the kick-start lever by hand until I feel the compression build up. Then return the lever to the high point, place my right foot on the lever pedal and depress it rapidly. On this occasion I was being a bit of a smart arse splitting lanes to get to the head of the traffic so that I could leave them all in that beautiful two stroke haze as I took off. So, when the engine stopped, rather than dismount I turned the lever pedal out with my left foot and just continued to kick it from there and we know what happened next.
“I have a couple of theories on the reason my bike started and ran backwards: It basically began with me not following the normal procedure so the piston could have been positioned anywhere in the cycle when I commenced my kick. Now I’m going into the realms of pure guess work. Theory One I will refer to as the Compression Rebound Theory. “My kick commenced with the piston being somewhere after TDC. As the piston started on the compression stroke the inertia was slowing down due to friction etc. The force from the rising compression was too much for the inertia of the flywheel to overcome so the piston was forced back by the compression thus making the crankshaft go in the opposite direction. I think this was unlikely to be what happened as I don’t think the compression force would have been sufficient to rotate the crankshaft round to the spot where the points would open causing that initial spark that started the reverse running.
“My other theory: Again, my kick commenced with the piston being somewhere after TDC but it managed to get to the firing point which is ever slightly before TDC. When the points opened causing the spark the crank was for all intents and purposes stopped just before TDC, so not having the inertia to carry the piston past TDC the expanding mixture forced the piston down the path of least resistance causing it to start backwards. That initial firing was sufficient to carry it to the next firing point and so it continued. While I don’t understand the full implications that the porting has on this I believe that the porting arrangement on the Bultaco is such that it will make it easier for the engine to continue running backwards.”
Next from Rob Carmichael, who in OBA 92 attributed the explanation of how his Metralla’s engine could run backwards to Bert Flood saying that it was due to “the overlap of the intake and the exhaust ports”.
“A quick trawl of chat forums with Google reveals that incorrect ignition timing is the most common explanation of why two stroke engines can run backwards (though there was a lack of agreement as to whether it was through being too advanced or too retarded), yet Bert Flood, the person described by John Somerville as being in his day the best Bultaco tuner in the world said it was due to the overlapping ports. In this respect it should be noted that Bert Flood’s comment was made in relation to a Bultaco engine fitted with a racing head, barrel and piston. “So the tuning of porting is important, but the overlapping of intake and exhaust ports is not the complete answer because as Dave has explained, reverse running involves chance variables such as the exact location of the piston in relation to TDC when combustion occurs.
“One chat forum contributor had an interesting way of putting it: “assuming it’s a two stroke… the only thing reversed is the rotation of the engine. If you think through what the engine is doing it makes very little difference to the crankshaft which way it rotates and no difference to the piston… the ignition timing will be off but everything else, reeds, carb… will be functioning normally.” (trialscentral.com) – This insight might explain why Bert chose overlapping ports and not ignition timing as the distinctive mechanism, as it is the overlapping of the ports that enables the two stroke cycle to function irrespective of the direction of crankshaft spin, albeit much less efficiently in one direction than the other.”
Finally, Bultaco expert John Somerville has a comment about his unique experience of having a Bultaco engine suddenly run backwards without use of the kick-starter:
“It happened to me once while the bike was stationary with the engine running and idling at very low idle speed, when the engine suddenly started to sound “funny”, and when I released the clutch it went backwards. Luckily no-one was behind me! If the engine is already running it happens at low revs, or if when attempting to kick-start, by doing too ‘wimpy’ a kick, and maybe if the timing is too far advanced. The dynamics of the situation would be this: the piston is coming up towards TDC, is travelling relatively slowly and when the ignition fires BTDC there is not enough rotational force to push the piston past TDC once the spark has fired (BTDC), so because the intake and exhaust ports overlap, it reverses in the direction of least resistance – and so runs backwards!”
“Fundamentally I agree with Dave’s explanation of this phenomenon and, because it only seems to happen to single cylinder two strokes, the porting of which probably has some role to play, also with Bert Flood.”