I am planning to buy a Yamaha Crux from the showroom. I'd like to know how to compare the engines in each of the available bikes of the same model. Will they differ in any way? One mechanic I know said that the engine beat differs even between the same model bikes.
Check out the Yamaha web site for the models you are interested in. They should list technical specifications for each model. If you are concerned about performance differences focus on peak horsepower and peak torque at a specific rpm along with top speed.
Modern motors (and other aggregates) produced by reputable manufacturers (Yamaha be one of them) for established markets (Western Europe, Australia, North America etc.) are engineered with low tolerances, thus should have little variances in operation.
In emerging markets engines could be (or could have been not long ago) older, and poorer manufacturing quality. For instance, Indian Royal Enfields were (until recent updates) notorious to the point where no two bikes could exchange parts at times. This could hold true especially in the lower end of the spectrum where Crux is. Subsidiaries could be making cheap and older designs on outdated machinery. In a way, it could have been out of necessity, because more sophisticated machines require better maintenance and higher skilled mechanics than could be available, say, in the remote jungle of Nepal. Therefore, it could be hit and miss.
That said, current Indian-spec Crux looks up-to-date enough. As the matter of fact, I have a suspicion it has the same crankcase as a global 125 motor (used throughout the world from Latin America, to Asia, to Western Europe in YB/YBR models as well as in Indian YBR 125 and SS 125), but with different side covers. That puzzles me (why slap different cover on the same motor?) so I cannot be sure without properly comparing 110 and 125 engines.
Another thing to consider is that most of the countries Yamaha present in offer 125 motor, but not the 110. I have not seen any other Yamaha bike in any of the markets using the same engine (or 110 displacement for that matter). Also, bikez.com shows Crux with older engine — something to be mindful of. I cannot make educated assessment of quality or reliability of that motor as well, but I would take modern global power plant any day.
Considering all of the above, if Crux is much cheaper, given the engine is the same shown on the website, it still could be fine. As I said, it looks like modern quality-built motor. If you can confirm that it is, in fact, the same basic motor as worldwide 125 — all the better. In all, it should not vary much from bike to bike of the same model, but one should pay close attention to quality of assembly, and make sure the dealer did decent job on tune up (especially carburettor settings).