Cyma 74U double feed or no hop on semi. Fires like a train on auto.

[Groot]

Hello,

 

I have a weird one. I am going to guess to start its either the hop rubber or the nozzle but before i order both and take it apart, let me get your group opinion.

 

It is running a 11.1 with a mosfet and running at 1.1J. Pretty sure it was short stroked and the hop is a maple leaf with omega nub. 
I have checked the nub placement and everything externally looks in place and as it should. 
Rate of fire is nothing extreme.

When i fire it on full auto. It will go through the full mag and all the bbs will fly straight and true to the end.

When i put it on semi. I get either a double feed, a few flying fine and then the 3rd / 4th having no hop.

Put it back to full auto on the same mag and no issues. 

Tried on two different ammo weights and same issue.

What you saying?

Cheers

[Cannonfodder]

If you're getting double feeding on semi auto then my first thought is the battery. Have you tried a lower voltage?

 

[concretesnail]

The double feed on semi sounds like the cycle is over running. See if it does it on a 2s pack. 

What model mosfet is fitted?

[Sewdhull]

Certainly sounds like overspin if you are getting 2 shots with one trigger pull, which is what I am thinking you are describing, instead of 2 bbs per cycle.

 

If you rely on mechanical timing to kill the motor power, and you spin everything faster ( 3S battery ) is keeps going longer until it can compress the spring twice with one trigger pull. Even if it doesn't do it every time the stopping position can creep around until 2 shots happen.

 

Battery is an easy change, Posh Mosfet is a bit more expensive but popular.

 

You can try a 2S Lipo (7.4v) or a 3S Life (9.9v)

 

The other thing that can happen is that the cut of lever can wear at the sector gear end which ends up delaying the trigger reset, resulting in the motor being energised for longer than intended. If your gun is relatively new though this isn't likely.

If the gun is short stroked and teeth taken from the release end you alter the cut off timing, effectively removing the load from the gearbox, for example a tooth early, but still energising the motor (off load) which will cut off at the normal time having continued to spin the gear box.

 

The more conservative the build the less likely battery, wear and mods will affect things, but when things are done to decrease the cycle time, that when stuff that normally doesn't matter starts to matter.

 

But battery for sure is a good starting point. 2S to 3S battery is a 50% increase in offload speed, under load...well it depends on how much load and how much current can be supplied.

Edited October 15, 2023 by Sewdhull
more thoughts

[Groot]

Hello,

Just tried it on some 7.4V 25C vs the 11.1 20C i was using. 

It is better but does still give me the odd double feed. 

The mosfet is a Gate PicoAAB micro 3rd gen Active Break

Cheers

 

Edited October 15, 2023 by Groot