This one is a real mystery so I thought I'd toss it up here in case someone else has come across this problem.
I've reassembled the Speedmaster which was lacking parts (see my earlier inquiry) and have it up and running again. It's a long story, but amazingly, after missing a bundle of parts and being terribly neglected it's ticking away happily and the chronograph works a treat- but there is one BIG problem. The watch runs about two seconds per minute fast. That adds up pretty quickly. The timegrapher trace isn't too clean but it is consistent and it shows the watch running much better than that (note the image only shows about fifteen seconds of testing on the timegrapher- rest assured, I've had it on there a lot longer than that and have pretty much the same results in all positions)
So why does the timegrapher show the watch running less than a minute off per day but the real world results are so much different? Well there are a few possible problem areas. The escapement may be the problem but the pallet and balance (complete) are NOS replacements from Omega. The escape wheel may be an issue- perhaps it's "slipping" and the pallet isn't locking consitantly. I haven't seen this on the timegrapher though nor have I seen it while watching the escapement under the microscope.
Still, there are some obvious issues in the movement. Whoever disassembled it before I got it made a real mess of things. For example the Delrin wheel on the coupling clutch was vandalized terribly by a wayward screwdriver. It took quite a while to get the gear teeth back in order (the "before" picture is below). I secured a NOS replacement just in case the movement didn't accept the repaired coupling clutch.
Despite this damage, I don't think the coupling clutch is the problem. The fourth wheel is positioned between the coupling clutch and the escape wheel. It's regulated by the escape wheel and the running seconds hand is attached to the fourth wheel. When I time how long it takes for the running seconds hand to make a full revolution using the stopwatch built into my iPhone, I come up with 58.02 seconds (give or take a few tenths).
I'm thinking the problem lies with the fourth wheel. The Lemania 5012 chronograph movement (Tissot 2920) runs at a slower 21600 BPM. The replacement of the fourth wheel in this movement (a Lemania 5100, 28800 BPM derivative) with one manufactured for the 5012 movement would result in the watch running fast. It's questionable whether the gear teeth would mesh properly though. Right now it's just a guess. I may need to tear the movement back down to check the gear ratios for each of the wheels in the train to confirm they're the right wheels for the movement but before I do that I was hoping someone might have solved a mystery such as this before and have some good advice to offer! Thanks in advance for the comments as they are always helpful!
Here's a few pics of the watch as it came together- just because posts with pictures are always more interesting.
Before service begins...
Why you should periodically replace your spring bars-
Omega gaskets self destruct (turn into tar) if they aren't replaced for forty years...
This movement is plastic fantastic but I love it. Other than not keeping time at all, it's the easiest chronograph movement I've worked on.
Look at all those scratches- previous watchmaker's tool of choice- screwdriver or machete?
It is a pretty beast though.