Carter Tri-Car fires on hard pump stoke - "slam fire"?

[caylegeorge]

Hoping someone might be able to help -

I have a Tri-car I've owned since new in 1991, recently gave it a rebuild as it had been dead for years.

Everything seems to be working perfect except one thing -

When you pull/slam the pump handle back really hard, the gun will burp or lightly fire. To reiterate - It is firing right when the pump is back all the way only on a very hard stroke.

It feels like when the bolt hits the hammer hard on the back stroke to lock up with the hammer, that force is causing the power tube to bump and fire lightly?

What can I do to tune this out? I don't have access to a chrono, could it only be because the velocity is way too high or low? Or should I be tuning the springs one way or another?

Thank you for your help

[russc]

This is pretty normal for most Nelsons...they rely on the valve pushing back against the hammer to cock. It happens when your pressure is too low, when your mainspring is too hard, and/or when your valve spring is too weak.

[caylegeorge]

Gotcha, thank you for the reply.

I am using HPA and the tank I am using is outputting ~840psi. Given that, sounds like I could increase tension on the valve spring?

Would shim-ing the valve spring with a few washers be acceptable?


Follow-up - I imagine if I increase the tension of the valve spring, then I probably have to adjust the velocity/bolt to achieve the same fps - if I do that, will I be back to square one?

maybe best to start with some new springs as they are almost 30 years old? Anyone recommend a place to buy a set?

Thanks

[Lt. head-shot]

One of the things that got lost during the hack was all the tuning threads... wish I could give you advice but I'm no expert.

FYI - A "Slam" fire is when you are able to pump the gun and it fires just as the pump handle is returning back to full forward. Carter's (specifically Buzzards/Comps) can be known to do this as they have very tight tolerances around the body snub, trigger, bolt, etc.

[caylegeorge]

Yeah its a shame all the threads are gone.

- Yeah looks like I got the terminology wrong, its firing on pump stroke back - russc's comments seem to like up perfectly.

[desertT1]

Hi Cayle,

Is this happening no matter what the tank PSI is? If the tank pressure is less than your reg output pressure I imagine it would happen more easily, but if you can do it no matter what the tank pressure is it might be more easily resolved with changes within the marker.

[russc]

If pressures are fine like your say, I'd say the first thing to try is a different mainspring. If the spring's compressed height is too long, it'll bind up and act like a rigid piston on the valve at the end of the cocking stroke.

[caylegeorge]

Great Ideas - I'll look into the main spring, thank you

Desert - Yeah, tank is at 2k psi and output is ~840. It acts the same with a 650 psi out tank, but is a little worse.

I did recently do a full rebuild, and I did have to replace the cup-seal with an appropriate sized one, though it is certainly not a carter original as afaik those are NLA.