Regulated Mrod inconsistent velocities

[Doug-T]

My buddy and I both decided to put HUMA regulators in our .22 Mrods. We put his in set at 125 BAR with a bunch of low pressure mods (nylon 40g hammer, captured hammer spring, enlarged 0.155" air path (valve port/transfer port/barrel port), Hill light valve spring). Everything was fine with it shooting JSB 18.1g pellets averaging 845fps and a pretty tight fps variance. 

He wanted a bit more velocity, around 875fps. So, we drilled out the valve port to 0.170" as the only change and reassembled everything the same as it was before. Now he has wild velocity swings. 810, 865, 845, 820, 855, 850, 825, etc. No consistency like there was before. After drilling the valve port we cleaned it with brake cleaner before reassembling. 

My Mrod doesn't do this and has a pretty tight fps spread. Tonight I swapped our two regulators and he's still getting wild velocity swings. A 20 shot string resulted in a low of 830fps and a high of 872fps with a majority being in the 855-868fps range. Swapping in a known working regulator isolates the problem down to his Mrod.

Any ideas what is causing this wild swing?

Thanks.

 

[T3PRanch]

Sounds like the transfer port might not be sealed evenly at one or both ends. Can you feel an air burst along the junction near the pressure tube and breech block when firing the gun?

 

[Rj]

How come just the valve exhaust and not all the way through? You basically just made a choke point. Maybe that and the combination of light valve spring are giving you the high ES's. Hopefully Motorhead catches this one and can shed some light.

 

[Dairyboy]

Did you change anything with the hammer spring? Find the max fps for what it is now at that reg pressure and then back it down 15-20fps then try again.

 

[Motorhead]

If all you did was increase transfer size ... will agree on there being a leak now in the transfer path. Or he blew out the probe o-ring in barrels leade ?



FWIW ... unless port in barrel is also made larger to match transfer path size valve body up, it remains the tight spot and gains will be minimal.

 

[Doug-T]

I ordered a Hill oversize transfer port but it does the inconsistent velocity thing with it too. I'll eventually oval the barrel port to remove the last restriction point but that shouldn't make velocities irregular, just consistently down.

I'll check the bolt probe o-ring.

 

[Doug-T]

I just removed the bolt probe o-ring and it looks great.

I also ovalled the barrel port, radiused the inside edge and installed the Hill larger transfer port. (I bored the stock port to 0.155"). Now the air has a larger channel the whole way (0.170" valve port, 0.180"? transfer port, 0.155"x0.175" barrel port). I was using the stock transfer port that I bored to 0.155". We'll see what this does. I still don't think it will help the velocity inconsistency...….

If this doesn't work the next move will be to disassemble EVERYTHING, clean it and reassemble.

 

[Hubertus]

are you using grease to lube the hammer ?

 

[John "spysir" Blount]

Hub had a nice thought, do run your hammer dry.

I'll pick bent of binding valve stem for $1.00 as the #1 issue.

#2 out of round stock tube where hammer runs.

Please do follow u and lets us know what it is/was. Great to have a spare reg to eliminate that, saved some hours there.



John

 

[Dairyboy]

Seeing you have a nylon hammer are you tightening it too much? I've had that before where I tightened it too much and was "sticky" in the tube. Slightly loosening it made it glide like butter again

 

[Doug-T]

The nylon hammer slides smoothly with a tiny bit of slop inside the tube. I use graphite powder to lube it.

I don't know how the valve stem could have got bent as I'm very careful handling things. I'll check though.

More chrono time tonight after work...….

 

[Motorhead]

The nylon hammer slides smoothly with a tiny bit of slop inside the tube. I use graphite powder to lube it.

I don't know how the valve stem could have got bent as I'm very careful handling things. I'll check though.

More chrono time tonight after work...….

I was the one who invented and did the R&D of the MDS NYLON light weight hammers ... RUN IT DRY and no graphite powder or any lube whats so ever !

Don't over tighten the cocking lug screw either or hammer will no longer be round.



Scott S

 

[707taildragger]

Also confirm the tp isnt leaking as T3PRanch suggested. 

 

[Doug-T]

I don't feel any air leakage between the tube & breech. I've felt that other times on mine and know what it feels like.

Good News!

I just got done removing all of the graphite lube from the tube & hammer and tested. It picked up 33fps more fps on average, up to 891 average. The shot string was much better: 903, 896, 880, 894, 894, 886, 899, 889, 890, 881. 23fps spread. I'm tired of working on this so I think we'll leave it alone.

 

[Doug-T]

Thanks for all the tips & info!