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tuning with the chronograph

July 15 2017 at 11:23 AM
john  (Login john2468)

I want to tune my Crosman 1720T and 1701P for fps with the chrono. Can you give me some simple directions or direct me to a web site with easy to follow directions ?
Thank you.

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Frank in Fairfield
(Login MRodDude)


July 15 2017, 11:54 AM 

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Scot Laughlin
(Login classicalgas)

Stroke adjustment is your friend..that's where you find efficiency

July 15 2017, 3:38 PM 

That adjustment wouldn't be there if it weren't useful.

Think of the valve as metering energy, not flowing gas. You want the least dwell(valve open time)that will give you the velocity you want.

I'd start with the spring adjuster at 75% in, the stroke at about 50%,back it out 1/4 turn at a time until you see max velocity.You can run the spring adjuster in a little more if you insist. That's about all you are going to get unless you open up the TP. Now adjust the stroke in(forward) until velocity starts to drop... that will be close to your most efficient setting for that velocity. I wouldn't run the spring preload at 100%.

I wont repeat Gubb's notes on the velocity curve, efficiency, and tradeoff of power vs shot count...he's dead on.

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(Login dan_house)

dont say "dwell".....

July 17 2017, 3:11 PM 

one of the words air gunners cannot speak...... happy.gif

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(Login shambozzie)

John, simple directions for tuning 1720/1701p.

July 15 2017, 12:21 PM 

First if you are looking for power gain inlarging the tp is the way.you have two adjustments. 1.Hammer spring and 2.is the stroke adj. Back the stroke adj out to zero.
You won't be using it anymore. Hint blue lock tight. You have approximately 7 turns from zero with the hammer spring adj. Start at 4 turns. Adjust according to fill pressure and
Fps. The hammer spring is your final and only adj from now on.just tuned my 1701p. 1.bstaley mod.3 #113 orings. 2.replaced stock tp .043 with 1720 tp .067.3.stroke =0. 4.Hammer spring =5. Jsb 8.4 @ 670 average for 30 shots. Nice cloverleaf @ 35 yds.

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(Login gubb33ps)

Re: tuning with the chronograph

July 15 2017, 2:48 PM 

I'd look at it in 2 parts.

Get the striker hit to the valve balaned so that the velocity at max pressure turns out to be about 4% under whatever the fasest shot turns out to be.

Lets look at that this way. If you started off shooting at 820fps...where would you call a halt? Perhaps by 788-787fps (which is 4% under max.)...so you'd count only the shots from 820-788 as "good shots".

But if it were to start at 770fps, climb to an eventual max of 801fps, and then decline back down to 770, could count the shots going "up", the shots at the leavel area, and the shots coming down and stay inside the same 4% range.

Now if that is not fast/powerul enough for you, would work at the air passages...valve work, transfer port size, transfer passage sixe/smoothness....which would bring the energy level up and still keep you near the optimum shot count (some shots increasing, some level, some decreasing in speed, but all within 4%).

If you demande 3% or evwen 2%, can manage to get there,,,take a lot more fiddling around and adjusting, but it can be cone over a short pressure range.,

Bad news is this: it's a dispropotional increase. Might get 80 shots at 5 foot pounds (400foot pounds total for a fill) but more like 33 shots at 10 foot pounds...or 18 shots at 15 foot pounds...or even 10 shots at 18 foot pounds.

So set a gaol...X amount of energy vs Y amont of shots, and work towards that. Open-ended goals (like "all I can get") are a work-trap (can work for decades improveion one thing or the other for some super-minor amount of difference).

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