Seneca Sumatra 2500 500 cc .22 loss of accuracy

[ShakySarge]

I was very happy with this gun when it arrived, and already own two more in .25. However, the last time I shot it I noticed some serious accuracy issues with the JSB 33.95 gr .22s. This is surprising as it was a laser accurate shooter from the time I got it until now. I have cleaned the barrel and still no improvements to how it was. Some of the pellets are even spiraling that I can see when the sun is behind me. I suspect that there may be an issue with the poppet but it also doesn't lose any air.

I am certainly open to suggestions before I bite the bullet and pay someone to tune it as I have never had to work on any of the others I own and this is an entirely new set up from discos, Mrods, and Bulldogs.

Just to reiterate, please don't comment if all you have to say is to send it to someone.

[steveoh]

I was very happy with this gun when it arrived, and already own two more in .25. However, the last time I shot it I noticed some serious accuracy issues with the JSB 33.95 gr .22s. This is surprising as it was a laser accurate shooter from the time I got it until now. I have cleaned the barrel and still no improvements to how it was. Some of the pellets are even spiraling that I can see when the sun is behind me. I suspect that there may be an issue with the poppet but it also doesn't lose any air.

I am certainly open to suggestions before I bite the bullet and pay someone to tune it as I have never had to work on any of the others I own and this is an entirely new set up from discos, Mrods, and Bulldogs.

Just to reiterate, please don't comment if all you have to say is to send it to someone.

My .25 Sumatra Carbine was accurate until one day the plastic bit on the valve stem broke in two. Replacing that fixed the rifle.

[ShakySarge]

From what I have read on here that seems to be a big accuracy issue if it goes. I suspect it may need to be replaced. That would indeed lead to severe inefficiency with air flow and too much would cause the spiraling.

Thanks for the advice.

[ShakySarge]

Valve stem is good. On to the next thing.

[Dairyboy]

New tin of pellets?? I've had that before on a different gun but never know could be the case here...

[Ribbonstone]

  Can just ignore this as I don't have a mondern made Sumatra....mine is old and evidently a good bit different internally (WISELY the folks who have a modern version were not going to let me taking it apart just for a look-see).

On the other hand, been in use for about 12-14 years, so if something can go wrong, this one pretty well has had it happen.

Folks don't LIKE taking Sumatras apart. Aren't any real tricks, they just don't come apart the way you expect them to and are kind of overly complex ...so many onwers just send them off rather than dealing with it themselves.

The trick on the valve stem is that you really won't know if it has cracked unless you take it apart and visually look at it. Will mostly likely crack the rebated section at the rear that guides the retrun spring.   Will often NOT leak with that section cracked off, will hold air as well as it every did becasue the sealing section still seals, but the velocity will be both a drastic see-saw type graph.
Design is actually quite good at hiding bolt probe type air leaks, and the transfer port would be sheilded by the covering reciever shell. Doers take longer to diagnose those possible problems that it does on other PCP's (good news is that is is harder to get a face full of leaking air).

Which is where I'd start as it's holding air. Chronograph readings can actually help pin-point a problem, not just measure the power.


Not clear (at least to me) if you were always shooting those 34gr. JSB's or if the accuracy problem came about when you started shooting them....of it it is a new tin of JSB's (which the above post mentions).

[Lion]

Has the side of the valve stem that comes in the valve a 'round' scape, just like on the picture? Or has it a dent?
(The picure is not of the valve stem of a Sumatra, but for the single shot Sam Yangs. But the shape is the same)
On the front of the barrel is an o-ring. Have you put already a new one on it?

[ShakySarge]

  Can just ignore this as I don't have a mondern made Sumatra....mine is old and evidently a good bit different internally (WISELY the folks who have a modern version were not going to let me taking it apart just for a look-see).

On the other hand, been in use for about 12-14 years, so if something can go wrong, this one pretty well has had it happen.

Folks don't LIKE taking Sumatras apart. Aren't any real tricks, they just don't come apart the way you expect them to and are kind of overly complex ...so many onwers just send them off rather than dealing with it themselves.

The trick on the valve stem is that you really won't know if it has cracked unless you take it apart and visually look at it. Will mostly likely crack the rebated section at the rear that guides the retrun spring.   Will often NOT leak with that section cracked off, will hold air as well as it every did becasue the sealing section still seals, but the velocity will be both a drastic see-saw type graph.
Design is actually quite good at hiding bolt probe type air leaks, and the transfer port would be sheilded by the covering reciever shell. Doers take longer to diagnose those possible problems that it does on other PCP's (good news is that is is harder to get a face full of leaking air).

Which is where I'd start as it's holding air. Chronograph readings can actually help pin-point a problem, not just measure the power.


Not clear (at least to me) if you were always shooting those 34gr. JSB's or if the accuracy problem came about when you started shooting them....of it it is a new tin of JSB's (which the above post mentions).

I will re-check the valve poppet once the new one arrives on Saturday. I may have overlooked it if it is below the brass retainer. I have always shot the 34gr out of it which is why it is puzzling me. Once the new poppet arrives, I will report back after installation.

I have NOT replaced the front oring on the barrel and will go ahead and swap it out as well while it is apart.

[ShakySarge]

Has the side of the valve stem that comes in the valve a 'round' scape, just like on the picture? Or has it a dent?
(The picure is not of the valve stem of a Sumatra, but for the single shot Sam Yangs. But the shape is the same)
On the front of the barrel is an o-ring. Have you put already a new one on it?

It has a round scape. It may be cracked or split below the brass ring and I just didn't notice it.

[Ribbonstone]

I'd go simple first. Chronograph readings and a change of pellets.

Just a little illustration.]

At one point, my Sumatra was shooting EunJins great.

Then they changed the EunJins….lucky it was an easy to see visible change so I pretty much knew it was the pellets. But the changes could be more dimensional than easily visible.   Without any other choice, had to adapt (retune) the rifle to work with the new pellets (as that was all that was out there)



One glance...even Mr. MAgoo would tell them apart (OK...old reference...not PC...made fun of nearly blind folks):





There are two official 34gr. JSB's, maked on the tins.  But I think there were MORE varations befor they settled on the "new improved"version.  Every order made before the offical tin-stamping was a little different than the last. Seems to have settled dwon with the current versions (but I've so many of the old versions that I've not used them up, so I've not gotten enough of the new versions to tell for sure).

[ShakySarge]

Going to run the Chrono tomorrow and see what it is shooting at again. The Beasts were both the newest batch, both shot like lasers until all of a sudden, the POI started changing up to 10 inches at 50 yards. Have some other heavy pellets, including the Senecas on the way and they should arrive this Saturday along with the new valve stem. Gonna go ahead and do a full shoot through the chrono with each pellet and then if nothing changes with accuracy go ahead and put the new poppet in along with all new seals for good measure.

[ShakySarge]

New valve stem is installed along with all new o-rings. No leaks, so that is a plus. Did some PB range time all day yesterday so no time to run the chronograph yet but did shoot a couple groups with the Seneca RP hunting 28.5gr pellets and the issue just may be that it has changed it's taste as mentioned above.

Spent today out at a local park with the Boy Scouts cleaning up the place and am beat for today. If the weather is still decent tomorrow gonna run some strings and groups and see what else is going on.

[notshootingbunnies]

I'm sorry to hear about the loss in accuracy in your .22 Sumatra.  Did you have a chance to try it out? If so, how is it shooting now?

[ShakySarge]

I'm sorry to hear about the loss in accuracy in your .22 Sumatra.  Did you have a chance to try it out? If so, how is it shooting now?

Switched pellets to the seneca hunting pellet at 28.5gr and it is doing fine now, back to excellent accuracy.

[Lastdog]

My 25 never liked JSBs does like Seneca, baracuda and Benjamin dome pellets.

[zippotter]

hey every one i was reading your post because i have a sumatra 2500 .22 coming and its used. right now all i know is the gun has been rebuilt but the power wheel is broke and set on high power or so im told. how hard is it to replace the power wheel gear? and what should i look for being that this is a used gun? thanks

[Nomadic Pirate]

hey every one i was reading your post because i have a sumatra 2500 .22 coming and its used. right now all i know is the gun has been rebuilt but the power wheel is broke and set on high power or so im told. how hard is it to replace the power wheel gear? and what should i look for being that this is a used gun? thanks

I would re-route it to stop at Will Piatt shop before it gets to your house,....well worth it.

[Habanero69er]

hey every one i was reading your post because i have a sumatra 2500 .22 coming and its used. right now all i know is the gun has been rebuilt but the power wheel is broke and set on high power or so im told. how hard is it to replace the power wheel gear? and what should i look for being that this is a used gun? thanks

I would re-route it to stop at Will Piatt shop before it gets to your house,....well worth it.

+1

[ShakySarge]

If ya can get the parts it is fairly easy. If not, although I have never used Will Piatt, I have seen enough of his work to recommend him.