Very Compact V4 Piston Engine Design WIP

These are my two babies. I've spent quite some time getting this small v4 piston engine to the scale it is at now, and most recently I altered the design to reduce its size by a few blocks on the bottom of the engine (that version is the smaller of the two engines, to seperate them, break the yellow block). Although, the slightly smaller engine doesn't work well now and I'm not sure why. It fluctuates in RPM quite a lot and stops working all together eventually. On the other hand, the slightly larger engine works well (enough) when it gets spinning, but it is unreliable because it looses power over time. I think this is because the timing goes off due to belt slippage over time, but I'm not so sure. Either way, belt slippage or not, I think that these engines have great potential, and the smaller one moreso than the larger, but there is something wrong with them that I'm hoping someone can fix. I put the larger engine in a car once and it worked very well, but, like I said, I had to mess around with the w key for a while until the engine wpuld run at it's full power, all for it to suddenly change direction out of nowhere. Please fix them Thank you, - GC