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Tuning FX Impact ll Power Plenum - 720, Need Help / Tuning Problems

Hi Guys,

I have ruined my 30 Cal Impact after I installed the Power Plenum 720. A bit of history on my gun, I bought it second-hand from a guy who had Ernest Rowe (yes the FX Master) upgrade the gun to Impact II and add his power plenum. He also did other improvements, installed a valve seat that allows more air, and installed a havier hammer and hammer spring.

The gun worked great, had lots of power, and was very accurate when I bought it. But it was a bit loud, especially after I increased the port size on the barrel and the probe that pushes the slug in the barrel. I upgraded to 720 Power Plenum, and I think that was a mistake. If I use the new valve spring C42 that came with 720 Kit, and the new valve stop, the gun will have no power and the slug will get stuck in the barrel. I use the stronger hammer spring that came with 720 Plenum kit and the heavy hammer from Earnest which is heavier than the hammer from the 720 Kit. I also use two washers inside the hammer with the sleeve that adds more weight and help compress the hammer spring further.

After the slug got stuck in the barrel I removed the small valve spring from the valve stop that came with 720 Kit, and I installed the old valve stop with the rubber ball (so there is no valve spring at all in the gun) and it works better, the pressure closes the valve without the aid of the valve spring. With the regulator set at 140 BAR, hammer wheel max, and valve seat close to line 4, I get around 950 FPS with NSA slugs 49.5 gr 0.30 CAL. So the speed is decent but NO accuracy. I get 2"-3" groups at 50 yards...I think the heavier spring compressed to the max and the heavy hammer cause a bit of recoil and that may throw off my shots...I really don't understand why I lost all the accuracy...

I watched Keith's Impact tuning (with Whiskey 68) and a few others but nothing gives me a clue on how to fix my gun. I really appreciate any tips from this group. 
I would of left it. I would first addres barrel whip before tuning for high power with the 30 cal. Well speaking for me. Good luck. I'm getting 2 to 3 inch groups at 100 with 5 10 mph wind. Don't forget to clean your barrel . I'm go to up my reg 170 . Because the hammer hits to hard. Stock m3 spring work better for me. 
Airgunmann do you have Markll upgraded with 720 power plenum? The Power Plenum 720 Kit came with its new valve tube. I assume it's the high flow valve tube you mentioned. Also I had no barrel whip problem before the upgrade. I was getting 940 FPS with 160 BAR reg press. And I had 1" groups at 100 yards. Also, most people say the barrel is self-cleaning and doesn't need to be cleaned. 
No I'm still waiting on my kit so you were already able to lower your reg pressure some is that the lowest you can go and still achieve that speed I would also put the valve spring back in with the new M3 valve adjuster and open it up past the 4 the line a turn and 1/2 and try that you may be able to drop more on the reg pressure yet to 
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Detornic, was your Impact a ppmkii or a earlier mkii? If it didn’t have the 50cc plenum, it would have a valve return spring inside the 35cc plenum and would use the rubber ball as a valve stop. If it had been converted properly to ppmkii or if it came from the factory as a ppmkii. There is no rubber ball and the valve return spring as been moved to replace the ball into adjuster area. 
There could a couple of different problems. It sounds like you’re gun as multiple valve springs. And an incredibly bad tune. I’d start by standardizing your gun, what I mean using all the 720 kit or going back to where the gun started life at. The point of larger plenum and valve of 720 kit is to allow you to drive the larger slugs and pellets with a lower regulator set point, thus allowing lower hammer weight and spring tension (hammer and valve return). With a mix and match of parts it would be real easy to get the results your getting. 
with a little more information about the gun and what your plans are we might be able to help you get back on track and have a great shooting gun. 
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Mrshosted, thanks for your reply. There is no valve spring in my gun right now as I described in my post. The gun was an Impact mark1 upgraded by the previous owner or more likely ER to Mark2 and it looks like the upgrade was done correctly. ER installed his high-power plenum and the gun was very powerful and very accurate before I upgraded to 720.

I was hoping to achieve the result you mentioned, driving havier slugs with lower pressure, that is why I upgraded to 720 and enlarged the barrel port and probe port but it looks like I'm not the only one having a problem with this 720 Kit. Other people had the same issue with the small valve spring and replaced it with a shorter and weaker spring...

I started tuning with a low press of 120 BAR when the slug got stuck in the barrel. Then I began to up the spring tension and pressure to achieve the 950 FPS I was looking for and I ended up with 140 BAR; It seems like that is still not enough according to other posts on this site, when using the 720 plenum and the slug kit, the pressure must be increased to 150 BAR so I'll try that next...
So you are talking a 50 gr slug then how long is your barrel sounds like your 10 fps over your original tune and at 140 bar your original tune was 160 bar am I correct people were also having trouble making extra power using the slug kit in the M3 they were adding a ball to the valve control I believe and there was al ready one in place if I remember correctly like you stated I'd give 150 a try and see what happens 940 must of been in a node for accuracy and now at 950 your not