Some of my customers have asked if I put my finished carburetor rebuilds on an engine and test them that way before shipping them out. There are extremely few rebuilders who do that that I am aware of. They naturally market that, and I would too. Sounds impressive. But that does not mean all the other rebuilders don't do good work, or at least those of the others who are pros, that is, have the experience and knowledge and correct ethic, and I'll add, passion. Then why do the few who do it, do it? It's a reasonable question. To answer it, let's begin by considering what we are talking about here.
There are only three things running the carbs on a test engine will do for you. One, it will ensure the rebuilder put all the pieces back in the right places. Two, it will catch leaks. And three, it will allow for a running sync. Every rebuilder worth his paint gun cleaning brushes ensures the last two items as part of their QC (quality control), and I am far from an exception. I am one also who tests for leaks and bench synchronizes the carbs as part of the last steps. So that leaves the parts placement, which is kind of silly, really. They go only in certain places so putting them in the wrong places is impossible. As for leaving them out, well, that is something only a beginning mechanic and one who is not concientious, would do. Fortunately, this doesn't even describe the worst rebuilder.
So at the end of the day, running the carbs has no practical benefit but rather a marketing one. If that weren't so, I would be doing it. This is my livelihood, and more important, my passion. Plus, and here is something to think about: running the carbs on an engine actually results in two negatives. One is that they get synced to an engine, and that engine is not your engine. This means two things. First, how proper is the tune on that test engine? Two, if on the other hand it is your engine that has problems, how will you know it unless the rebuilder did an exact bench sync against which to compare what your engine wants? Make sense? The other problem is gasoline. Once you put gasoline into carbs, the three-week clock is ticking. I have no intention of shipping carbs to customers with fuel residue in them. Not my idea of good customer service. Not my idea of good rebuilding technique. I don't even use gasoline when I leak test. Bad idea.
Bottom line, though some may use running on an engine as a marketing tool, and it sounds good, the presumed advantages are pretty vaporous, even non-existent, and actually counter to best practice when you think about it.
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