Venturi screw

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Billyboy
Posts: 11
Joined: Tue May 31, 2016 6:56 pm

Venturi screw

Tue Aug 20, 2019 9:09 pm

Iv recently. Bought an s 410
When tested on the chrono at 150 bar it is 10.30 ftlb with screw tight in.
With screw removed it is at 11.20 ftlb
Is this standard set up with this variation or is there something not quite right....

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tonyc
Posts: 376
Joined: Mon May 29, 2017 10:55 am

Re: Venturi screw

Tue Aug 20, 2019 10:20 pm

You haven't said whether carbine or classic, what calibre and what pellets you used. For .177 classic using JSB-manufactured 8.44gr, 11.2ft lb is where you want to be IMO as the max power will be around 11.6-11.7ft lb with 10.34gr.

If you have removed the venturi screw completely and sealed the hole (for example with a 6mm cap screw) you will have added a little extra "dead" volume of the transfer port. Max power will be with the venturi wound back so the transfer port is just unrestricted but no dead space below the screw. The difference may be small (I've not measured it). Then the ideal set up is with the power limited by the valve spring pre-load, rather than the venturi. It takes some fiddling to get it right - too much time/hassle/cost for the manufacturer to set each rifle this way which is probably why they use the venturi and AT.

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Jammy13
Posts: 43
Joined: Tue Jul 02, 2019 5:00 pm
Location: Northumberland

Re: Venturi screw

Tue Aug 20, 2019 10:34 pm

Hi Billyboy,
with an s410 working correctly you should get 14+ft/lbs with the venturi screw right out.

If the gun hasn't had a service recently the first thing I would try is polish the hammer rail. I use Autosol metal polish and wipe with kitchen roll till it is clean. I make sure no polish is left on the rail. Don't put any lube on it. I also clean the bushes in the hammer. If you think the rifle has fired many shots replace the hammer spring. Julian in spares can supply that and anything else you need.

Because you will have taken the gun apart make sure the transfer port 0 ring is fitted correctly, maybe replace it. Replacing the breech seal 0 ring will also be a good idea. They don't cost much and a replacement will give peace of mind.

Give the barrel a clean.

Follow Tonyc's advice also.

Keith

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Archer50
Posts: 200
Joined: Mon Jun 20, 2016 5:06 pm
Location: Perth

Re: Venturi screw

Thu Aug 22, 2019 2:48 pm

Giving the rail a clean and checking the o rings is always a good idea, but if the power is consistent, and there is no air blasting around, the 'problem', if that's what you call it, is most likely to be the tension on the firing valve spring being a bit high, ie the pot extension being a bit too short.

Personally, even before I regged them, I adjusted my S series guns so none of them would do more than 12 ft.lb with the venturi screw right out. In theory, this reduces the shot count slightly, but I can't say I ever noticed it.

I always try to get as near as possible to 11.2 ft.lb on all my PCP's anyway, as I have found it gives the best consistency and precision, so if your MV is consistent, I wouldn't bother adjusting anything.

Good shooting

Alan

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Billyboy
Posts: 11
Joined: Tue May 31, 2016 6:56 pm

Re: Venturi screw

Thu Aug 22, 2019 3:49 pm

Many thanks for the reply’s.. it’s a410 f carbine in .22
I’ll have a good play with it and as long it’s shooting good I’ll leave it alone..
If not I’ll get it serviced
Cheers

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