Different FPS using semi vs auto/burst
 

Setting the stage, CYMA P90 stock spring with just over 1.5 coils cut. Original stock FPS is 400 and ROF at 9.5.

Changed (some say downgraded) to Lonex 6.03mm tightbore barrel (247mm length), Lonex Bearing Spring Guide. Absolutely everything else is stock.

On semi, the FPS is a stable ~260. On full auto it's ~385 with ROF at 10.5. On 2 or 3 round burst the FPS clocks at ~385.

The question is, what's happening (or not) in semi that has such a drastically low FPS? I thought that was a normal expected velocity drop for 1.5 coils cut but this thread debunks that (http://airsoftcanada.com/showpost.ph...9&postcount=10). In that case, the issue is with the semi mode itself. Where should I begin to troubleshoot this?

EDIT: current CYMA stock hop up set to 50% of dial for the sake of testing above.

POTENTIAL ISSUE FOUND: It's double feeding in semi.

Hop up. Unless you have flat/r-hop you expect a significant fps reduction due to hop. Firing full auto tends to reduce that reduction, my guess, due to the heating up of the rubber. Basically this is analogous to what happens with fps reducers in full auto.

So if I understand (and I probably don't), if I dial the hop to zero then I should be getting close to 385 as I'm doing in full auto. With that, the hop may be doing zip (no pun intended) at the current setting in full auto? I'll test that theory, thanks for giving something to go by and try.

Hop might be on too much, could be a bad airseal thats better in full auto.
Its definitely not anything to do with the spring, no way in hell you dropped 140fps by cutting just 1.5 coils.
Dont turn the hop OFF since you'll get feeding issues, but turn it to low and try.
And then inspect your compression parts for leaks.

Thank you dmitril and TC, you've given me lots of food for thought. For starters, I should do another quick air seal check again via the -- loading hole (proper nomenclature is?).

I'm going to say that you should try it with your hop on all the way.

OR, double check your anti reversal latch. You could be consistently double feeding in semi, which is caused by anti reversal latch failure.

Woo, that's something exotic for me. I only remember that thing was a pain in the ass to hold on to when reassembling the v6 gearbox! Luckily I can test the double feeding by simply shooting and seeing what ends up at the other end. Thanks for that tidbit though!

Well mate you've got it. It's double feeding in semi. Now you say it's likely an anti reversal latch failure (or bonehead reinstall), is there any video or other reference you have on proper install? I'm having a hard time finding a relevant v6 video that's also clear.

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Things below here are my research so I can come back to it:

You probably have the spring in there incorrectly. Just make sure the spring is pushing the latch toward the catches on the bevel gear. Make sure the spring has a hook on one side for the latch and is straight on the other to push against the mechbox.

I'll check that after doing the simpler things since it takes 4 hands to reassemble this bad boy, and I have a proper spring/sorbo on the way so I'll need to take apart then.

In the meantime, more searching and I'll reinstall/change the OE barrel and hopup assembly...

Lets put it this way. If it wasn't double feeding before you cut the spring, and then it started double feeding after you cut the spring, is it logical to assume its a gearbox issue or is it logical to assume it somehow became a hop up issue?

If your gun has an extra whine after each shot in semi, it is an anti reversal latch issue.

Given that I also mucked with the barrel and hop up, it's plausible. Having said that, I just ruled that out by a complete re install twice using the OEM bits :) I'll park this thread until the spring arrives. Thanks!