Valve Replacement

I replaced the valve in my 98C yesterday, I had problems getting over 240 FPS with HPA, and I was surprised to find a dead bug in my power tube. I have absolutely no idea how it managed to get in there and it looked like it was in there for quite some time.

Did you just switch to HPA?  It could be that your tank is low pressure (450 psi out of the regulator) vs. high pressure (850 psi output).  Tippmanns need HP.

Also, have you tried replacing the drive spring?  Velocity can decrease as your gun cycles more and the spring wears.

Two branches here, is it mechanical, or is it an air issue. Do you have a standard air setup? Nothing crazy like 3 expansion chambers in a row or an inline regulator or anything? If the air is fine...Either your mainspring is old and weak like folks are saying, or theres friction being generated somewhere.

Personally, I would check:

Is the gun cleaned and lubricated? And no, the "drops of oil in the ASA" trick is not lubricated. Take it apart and do it right.

Is the sliding surface areas inside the reciever halves rough? Run your finger along the areas where the front and rear bolt and linkage arm slide. Is it rough? If so, get some fine-grade steel wool (#00 grade) and polish the areas up. Dont go overboard, just get it smooth.

Are the reciever half bolts overtightened? It usually doesnt matter, but Ive seen some guns that had roughed up internal areas (see above) that if you overtightened the half bolts, the thing would barely work.

Gimpy_Butzke wrote: I unscrewed the FVA so much it was peeking out of my marker.

This will not help.  Normally the FVA only increases velocity during about the first four turns out from all the way in.

Try installing the air tank, then turning it back out about a quarter turn.  If you get better results it means the pin in the ASA that opens the tank valve is blocking air flow.

Also try a CO2 and another air tank on your marker.  In warm weather CO2 delivers at significantly more pressure than most compressed air tanks so if the marker only works better on CO2 you will know that for some reason your marker requires more pressure to operate.  If it works better on both the CO2 and the other air tank, then you will know you have an air tank problem.

You mentioned that you are using a Flatline Barrel.  Velocity problems will result if the Flatline barrel is not fully seated in the marker as air loss can occur around the end of the barrel.  You should check this as well.