Gen 8 with 13:1 gears

So I have a gen 8 m4. I’ve upgraded to red chi motor, metal gear plunger, alloy piston and piston head, 7mm barrel. I’ve just put in 13:1 SHS gears and a 1.3mm spring. The issue is I have very low fps. The only thing that I can think is happening is the tappet is engaging to early and losing air. Any help would be appreciated.

Tappet timing would be the likely candidate

Low guido does a thing around tappet timing from memory

I’d start by walking through a cycle by hand. At the very least you’ll be able to identify where you’ll be cutting if you need to.

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Did you have a win with this?

Haven’t had chance to do anything yet, should get to it on the weekend. Will post results when I can.

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SHS gears will not fit properly in a Gen8 box, the centre rod of the spur gear is too high. See if you can find a photo of the cheap cast gears with the brass sleeve around the spur centre and compare it with the SHS ones, you will either have to use a different spur gear or grind the SHS one down.

i’ve seen that pickle a few times now.

there are 250$ mini lathes that would make a good investment for these sorts of issues.

i know there are a couple of people here who have one or access to one

Well I think I’ve had a win, doing 22rps @ 285fps. I ended up having a file down the shoulder of one of the gears (was half a mil longer then the others). Plus I shaved 1mm off the tappet where the gear hits it, plus 2mm off the bottom of it. I’ve taken a total of 7 rings off of the tappet spring. Pretty happy with the outcome

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