Author Topic: Sort of copy of Daisy 747 (Read 3830 times - 2 votes)
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Rob M · « **Reply #20 on:** July 22, 2023, 08:16:35 PM »

23 lbs is super light . I also think the lever being on the left side is a good idea.

JPSAXNC · « **Reply #21 on:** July 24, 2023, 02:11:22 PM »

The 23 lbs. closing force was wrong, It's 27lbs. I tried increasing the preload on the pump handle the gun lost power, I'm pretty sure it's do to the pump linkage flexing and opening up the head space, the same thing happened on my ssp rifle. The power is back up now with a light preload. I did see a few higher velocity's up to 495 fps. I haven't seen any peewee pellets but a 45fps. gain from even a peewee seems like a lot for such a weak system? I have a couple more ideas to try.

Rob M · « **Reply #22 on:** July 24, 2023, 05:44:14 PM »

its a delicate balance , when I overdid the camover on the linkage , i lost power also. I contributed it to less overall stroke distance but your idea might be more acccurate ( or a combination of both factors) What weight pellets were hitting 495 ?

JPSAXNC · « **Reply #23 on:** July 24, 2023, 06:18:59 PM »

I was using the cpl's, the velocities were jumping around, lower than usual 440 fps. usual 450-455fps. and higher 480-485-490. The gun now is shooting in the 450's PS. The breech was made from a piece of CR1400 barrel.

Rob M · « **Reply #24 on:** July 24, 2023, 08:25:35 PM »

nice, thats actually perfect material for starting a breech

JPSAXNC · « **Reply #25 on:** July 25, 2023, 11:09:51 AM »

I had made a 1/4" thick felt wiper and saturated it with oil, I wonder if the oil could account for the faster than average shots. I don't smell oil burning, I thought maybe oil build up was reducing the volume of the firing chamber raising the final pressure. But from what I've seen in multi strokes oil build up in the valve reduces power. ?

JPSAXNC · « **Reply #26 on:** July 25, 2023, 12:24:54 PM »

I think the valve stem is shooting it self in. I didn't have a reamer for the valve stem hole and just drilled it, but the 7075 alum. doesn't like small holes I could feel a couple of hard spots so I think the hole is a little rough. Also I shortened the distance the poppet could open hoping to reduce turbulence, It's worked in other guns. Here's some five shot strings. 8.3 grain superdomes 485-490. 7 grain hobbies 520. 10.5 grain cps 397. 7 grain meisters 480. 7.9 grain cpls 502.

Rob M · « **Reply #27 on:** July 25, 2023, 06:18:24 PM »

That thing is cooking ! 520 with hobbies is faster than any ssp pistol I'm aware of. I wish I had built mine as 177 at the time. I've since lost interest as the shop too hot to mess with it ..

JPSAXNC · « **Reply #28 on:** July 26, 2023, 08:20:40 AM »

Comparing the velocity of the 7 grain miesters at 480 to the 7 grain hobbies at 520 really shows the difference in how velocity is effected by pellet fit.

JPSAXNC · « **Reply #29 on:** August 09, 2023, 09:47:20 AM »

Update and a Question, I'm trying different poppet diameters trying to stuff my valve to raise the pressure. What I'm seeing is that with a bigger poppet diameter the gun shoots faster. But I don't know if it shoots faster because the pressure is higher, or if the air flow is being restricted and acting like smaller porting pushing on the pellet longer. The poppet diameter is very large and restrictive compared to the id. of the firing chamber. I'll check to see if there's a measurable difference in closing force on the pump lever.

Rob M · « **Reply #30 on:** August 09, 2023, 11:14:00 PM »

thats an odd one fore sure , perhaps the smaller poppet was overtraveling blocking airflow

Offhand · « **Reply #31 on:** August 10, 2023, 01:25:59 AM »

Great work, James!

JPSAXNC · « **Reply #32 on:** August 11, 2023, 09:09:12 AM »

Thanks for the replies. The firing chamber is .250 deep with a .250 id., poppet diameters have varied between .192 to .221 poppet length is .200. The larger diameter poppets increase the final pressure. When I first assembled the gun the higher velocities I was seeing were caused by an excess of oil on the wiper being pushed in to the firing chamber causing some small degree of combustion. After I replaced the wiper pumping the gun to the same pressure and even a little higher I could not get the higher velocities. I checked out the crony the original velocities were real. Starting with the largest diameter poppet and reducing the diameter by .002 there was a 5 fps. loss of velocity, which keeps going down with each reduction in poppet diameter. Right now the gun shoots .460-.470 with 7.9 grain pellets.

JPSAXNC · « **Reply #33 on:** August 13, 2023, 12:23:34 PM »

I've made 4 valves for this gun with valve throats ranging from .125-.148 and transfer ports ranging from .098 - .125. The differences in velocity are about 10 fps. the smaller porting making the higher velocity. I've ( increased ) the final pressure with all 4 valves with no gain in velocity, which makes me think that to get more velocity I would need more swept volume. I don't think it would take much, if the tube was a 1/4"- 3/8" longer I think that would make a difference in velocity.

Rob M · « **Reply #34 on:** August 13, 2023, 01:50:30 PM »

I went through similar experimentation with my ssp , and figured out i was making statistically insignificant changes. Up a few , down a few. Ultimately , the swept volume was the limiting factor , and when i actually did the math ( not just estimating ) i was right where i should have been power wise . looking back id keep an ssp pistol 177, or simply make it an msp designed for 2 strokes. which i think is ideal actually .. Most ssps can be cocked a second time if the valve was designed to retain

JPSAXNC · « **Reply #35 on:** August 13, 2023, 02:50:19 PM »

Hi Rob, I'm about done with testing. I have front and rear sights for it.

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Sort of copy of Daisy 747