Tuning TX200 Problem

I’m concerned that the harsh slammy shot cycle may be due to piston bounce. I’ve been advised that this can be corrected by going with a SMALLER (sounds counterintuitive) transfer port. So is this something that can be done by me or do I need a tuner? Uj

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Try moving weights inside.. Are you using a steel tophat? If so try a plastic one. If you added any shims to the spring move them to the bottom not in front by the tophat. One thing a tuner might try is shortening the spring so weights and or shims (washers) can be moved ahead or behind the spring. No need to alter the transfer port unless someone has already done that..

Now are you sure the piston is bouncing or...is the piston slamming home at the end of the stroke? This causes even more harshness in the shot cycle...without actually feeling the shot cycle, I can only guess what is going on. 

I’m concerned that the harsh slammy shot cycle may be due to piston bounce. I’ve been advised that this can be corrected by going with a SMALLER (sounds counterintuitive) transfer port. So is this something that can be done by me or do I need a tuner? Uj

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If you are running a stock-stroke TX at 12fpe it's highly unlikely that it's NOT bouncing. 

And a smaller port would make bounce WORSE, just as you suspected. Now if its slamming, a smaller port will help with that, but only if the gun is sealing well in other areas. 

How does the piston seal fit? Is it tight enough to have some resistance all the way down the compression tube? Does it happen to feel snug at first, then lose resistance right at the front of the tube? That can certainly make one slammy. 

Also, the TX compression tubes are screwed together with a thin oring to seal. They CAN leak here sometimes, and cause poor power or slammy firing behavior. Not common but it has happened before.

And how is the pellet fit and weight? Do they seat with some resistance, or basically almost fall out if you tipped the rifle up? A little resistance at least is good. Heavy or overly tight pellets will make bounce more pronounced with a long stroke/low preload setup like you are running now.

Again, first thing IMO is to put two fresh breech seals in it and see if it feels any different at all. 

But before you do, try this: Cock the gun, and close the lever back just as if you were ready to fire it. Now take your thumb and try to move the silver compression chamber fore and aft. Does it move? If so, that's an issue with the lockup. Wear or just poor tolerances on the cocking shoe, worn or poor tolerances on the cocking arm, or just tolerance stackup overall. It can cause anything from consistency issues, to low power output, to funny noises or nothing at all in some cases. First and easiest step is to pop in some fresh harder breech seals (they are just cheap orings, don't spend a fortune ordering specialty "seals" here) and see if it changes anything. 

Lots of things to check, and it may not be anything "wrong" at all if it's accurate and makes good power. I've had some with your same setup, and they all felt and sounded weird to me, for what that's worth.

I’m concerned that the harsh slammy shot cycle may be due to piston bounce. I’ve been advised that this can be corrected by going with a SMALLER (sounds counterintuitive) transfer port. So is this something that can be done by me or do I need a tuner? Uj

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Out of curiosity how do you make the transfer port smaller? Weld it up and re drill? I was under the impression you could make them bigger but not smaller.

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That's one way to do it, but probably the hardest way. I usually bore the port out much bigger, and then use a press fit bronze sleeve that's pressed in with the receiver/compression tube heated. Then redrill the port to your desired size. It's always worked for me. 

Some guys drill and thread the port and use drilled out set screws to change port size. I don't like to do this because it's ugly, more work, and the length of the port (which is as important as size) is altered due to the length of the setscrew used.