S300 pot and Venturi screw factory settings?

[Skid]

Hi all, new member here although I've had my S300 from new and never even thought about joining until now

Not sure if anyone else is still using this antique but the gun is an original S300 .22 classic, non-FAC 12 ft/lbs single shot version which I bought about 20 years ago.....annoyingly just a couple of months before the 310 came out and has until recently had hardly any use.....as in probably a dozen air fills in all that time!!!
It has suddenly become wildly inaccurate and wouldn't zero better than a 2" group at 25 yards so first thing I thought was that the cheap Tasco Golden Antler sight had broken internally (I know I know but it was all I could afford at the time ) and the parallax was particularly bad too.
Bought a new Hawke 3-9x40 AO which is really nice but it made no difference. Borrowed a chrono and found it was way down at around 390 fps but quite consistent. There were no leaks but I decided to give the gun a service.....figured it needed one after 20 years
Anyway, everything looked really good, no rust in cylinder, hammer guide clean and not sticking but I polished it all the same, no real wear anywhere although I didn't strip the firing valve at that time, got the new modified barrel seal direct from AA and fitted it (small brass washer that the o ring fits into, which is the one they used before sleeving the end of the barrel with a brass fitting on later guns) and fitted a new hammer spring.
Put it all back together thinking it would have to be better.........and speed dropped by another 100 fps!!!!! Tried adjusting the Venturi screw but it made zero difference whether fully in or out. Still no leaks anywhere either.

Really puzzled now so stripped again, including pot and valve this time. I measured distance before unscrewing the pot which was 20mm from flat end of hex drive of the pot to where it screwed in. There were no signs of it having moved and locking screw was tight. The valve is the old style white one and doesn't seem to have any wear but I'm going to buy a new valve and spring anyway.

The question is, before I get that far, being a numb nuts I didn't measure the Venturi screw before I moved it so does anyone know the default distance or number of turns in/out that it should be for factory settings? Likewise for the pot measurement?
I know I'll have to fine tune things but need a solid starting point to avoid too much faffing about. I have contacted AA by webmail but not had a reply yet so was hoping one of the experts here would be able to help.
Apologies for writing War and Peace on my first post

[Skid]

It looks like the new pot will be the same design now as the 400 series pot from the parts lists I've seen online. They obviously updated it. Which means I can set it to 57/58mm from what I've read on here unless you know different.
I still need to know what the air metering screw should be set at though. Any ideas please?

[TenMetrePeter]

The venturi screw can be wound out til it is flush with the outside . At this point if everything else is OK you should have gone over 12ft lb. then turn l it in til it goes to 11.0. There is no recommended turns out like you have with the pot

If you have low power with the screw wound out to max you have another problem with the hammer and valve setup. New hammer spring or valve spring maybe?

Oh by the way make sure there isn’t another venturi screw under the grub screw you are turning. Some owners added a second locking grub screw.

[Skid]

Thanks for the info TenMetrePeter. I was hoping for an easy solution with a simple base setting but that's too much to hope for when engineering is involved
I fitted a new hammer spring when I changed all the seals so suspect the firing valve and/or its spring. For the sake of about £25 I'm going to order a new spring, valve and pot on the assumption that new stuff should be more improved, what with design changes too. Mine has the original old style short pot with the hex drive and a grub screw which acts as a meter for the air going into the pot.
From the parts lists I've seen that style has been updated to a longer one. I've also noticed that the hammer spring has a spacer and centralising washer on newer guns so wondering if that also causes issues.

I don't think the Venturi screw was a problem as the locking screw (single item) was tight against it. I just need to get the gun to a point where it's even capable of being powerful enough to reach the limit
The last couple of shots I fired before stripping it didn't even register 200 fps and one pellet didn't make it out the barrel!

[Skid]

For anyone in a similar situation, I rang Air Arms and spoke to a really helpful tech guy. Good news is he gave me a start point for the Venturi screw.....one third covering the hole. The bad news is he's just as stumped as me why the muzzle velocity had dropped so much so suddenly

The interesting thing is that he probably built my gun as he's been there for 23 years! All the suggested fixes he mentioned I've already tried, barring changing the valve, spring and pot. Even then it would normally only gain tens of fps, not the hundreds that Ive lost somewhere, main benefit being better regulated airflow through the newer style pot. Anyway, I'm ordering them to see what happens because there's nothing else to try.

[Blackbaronfish]

Does the rifle cock and fire as smooth as it should.
Did you check to see if the seal in between the block cylinder was in place.
Have you checked the barrel transfer hole lines up.

BBF

[Skid]

Hi BBF. Thanks for the tips but yes, gun cocks as smooth as before, no undue effort needed. The seal between the barrel/block is still good and in place, I made a point of checking it each time it was stripped and reassembled but I've ordered another new one for when I replace the valve and pot anyway..
Barrel port is also good. I took it out and checked it again to make sure it lined up when the grub screws were centralised.
Hopefully the new parts will fix things but it really is baffling
It's not like it's a complex mechanism.

[TenMetrePeter]

Two more obvious ones not mentioned. Is the hammer rail dry? You said you polished it nig it must have no oil or grease.
Second, what pressure are you filling to and is the gauge reliable?

[Skid]

To quote a favourite phrase from an old apprentice master of mine many years ago......The hammer rail is as dry as a witches tit.
I did initially try with a minuscule dab of oil although it made no difference but after speaking with AA a couple of weeks back they also said to keep it dry because longer term it tended to slow the action slightly when it gummed up so I dried it off.

Max fill is 190 but usually slightly less as I currently only have a 200 bar charging cylinder so it empties quickly. Pressure gauge seems pretty accurate though and checked against the one on my brand new HW100 last week they both read he same. I'll triple check it against my new 300 bar rig when it turns up hopefully this weekend.

Keep the tips coming guys....might be something really stupid I've missed!

[Skid]

Well that was easy. Fitted the new pot, valve, valve spring and another new mainspring just to be safe. I also really polished the hammer guide rod so it was like a mirror. First shot was well into FAC territory using my standard Bisley LRG, so screwed the Venturi screw in a touch to bring it back to a consistent 600 fps.....and I mean really consistent. The new design pot was giving me only 5 or 6 fps variation so I'm well chuffed

Trouble is....the OCD engineer in me wants to know which part cured it but I couldn't be bothered to mess about doing it bit by bit. Impatience won the day

I didn't really need to buy another gun but hankered after the HW100 for a while so probably would have got one anyway. Got a good deal too so had to get it didn't I. Impatient again.
So now I just have to explain to the missus exactly why I NEED two air rifles, well ok....3 if you include the old BSA spring gun. Not sure I'm going to win this one