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m102404 · September 7th, 2012, 09:12

Going to have to assume your shimming is good...cause it's impossible to know via online/forum. Once you think it's set...then it's set and won't change except for one of the tests below.

Long and short of it is that one of two things are happening
1. When it's hot your motor is not providing enough force to crank your mechbox. Either because there's something less than optimal in your setup, your motor is dying/not scaled to your setup, etc...
2. Your battery cannot sustain the power required to force it's way through a cycle. Most any topped off battery can crank a couple of shots...but when after it's dumped a bit of power then you see it's behaviour.

When things are acting weird...it's always worth going back to basics:
1. In another known good gun/mechbox with a known good battery...try that motor.
2. With known good gun/motor...try that battery
- shoot both semi and short bursts
- if you're shooting at a rate of 2 shots/sec on semi there shouldn't be any drop off in trigger response for a long, long time. If that's good on semi...but when you put on a burst of FA and it dies off...then it's your battery.
* more often than not that will pin it down really quickly
* note...you risk breaking your mechbox shell if the front is unsupported...you can help that by putting the front of the shell against a solid table...or putting your finger over the nozzle to create a bit of an air cushion for the piston.
* see #3...you might want to do all this without putting the mech in the receiver since it's just simpler/less work if there are issues.
- if both work then the problem is likely with your switch/wiring/setup

3. Try the motor/battery/mechbox/grip outside of the receiver...weird/poor setups can bind in the receiver/grip and it's good to isolate those factors
- if it doesn't cycle well then...
...try a different grip (some are tight with some bushings and they'll bind the bevel gear bushings. The grip shouldn't be sqeezing the sides of the mechbox/bushings...a good fit is good, but it should slip on easily. The sides of the grip where they ride over the bushings can easily be relieved with a file/dremel.
...loosen up your shimming. Some mechboxes flex quite a bit and once it's under load it doesn't work as well as when there's no spring in it. I've run into some where I can rig it up with motor/batt/grip/no spring and run the gears....then pinch the sides of the shell hard enough to alter the "tune" of it. A good test to see if your stuff is binding under load is to just slightly back off the top mechbox screws (starting from the back). Not falling out loose, but just enough. On a lot of problem setups this will indicate that either the piston/shimming is binding under load...and or your bushings/gear axels are binding.
...consider reseating your bushings...or swapping them out. A cockeyed bushing will add a lot of drag to things...which just gets worse under load.
...check that your piston is completely free in the mechbox (take the sector gear and spring out). Screw up the mechbox. You should be able to freely push the piston all the way back by pushing a long allen key/rod/whatever down the nozzle. If it's binding either on the rails or the spring guide then swap parts (a non-bearing "plain" spring guide is often handy when troubleshooting)
...check that your piston can go fully over your spring/spring guide...place the spring guide on a solid table...put the spring over it...put the piston over that. Line it up and press down evenly. There shouldn't be any catching/binding/stuck-spots...it should go over nice and smooth. If it's hitting the bearings on the spring guide...swap the spring guide.
- set the mechbox up with everything but the spring guide, spring, piston, tappet plate and cut off lever. Setup the grip/motor/batt and run the mechbox...obviously it will not turn off without letting go of the trigger. Run it for a while...listening to changes in pitch or rubbing. Then take it apart and look for wear/scratches/worn spots. Areas to check for are the back side of the large flat gear rubbing the mechbox shell...or any of the gears hitting/rubbing the mechbox shell, reinforced sections, the post that the spring for the switch block hooks onto. It's also a good way to check if any of your gears are cockeyed enough to be rubbing.

4. You don't mention what spring you're using...if it's anything up to a M120 you should be able to simply test your setup with a known good large cell 8.4v/2000+mAh...9.6v/2000+...7.4v/2000+/20C (like an RC race pack). If it works with that but not with the battery you want to use (i.e. stock pipe/mini's that fit into a certain stock, etc...) then you know definitively that it's the battery.
- from there you can alter your setup for the battery you want...i.e. improve gear ratio, lower spring power, "plain" motor, etc...

That's just off the top of my head...would need more details of your setup.

September 7th, 2012, 09:12	#13
m102404 Tys Trader rating Join Date: Apr 2006 Location: Toronto	Going to have to assume your shimming is good...cause it's impossible to know via online/forum. Once you think it's set...then it's set and won't change except for one of the tests below. Long and short of it is that one of two things are happening 1. When it's hot your motor is not providing enough force to crank your mechbox. Either because there's something less than optimal in your setup, your motor is dying/not scaled to your setup, etc... 2. Your battery cannot sustain the power required to force it's way through a cycle. Most any topped off battery can crank a couple of shots...but when after it's dumped a bit of power then you see it's behaviour. When things are acting weird...it's always worth going back to basics: 1. In another known good gun/mechbox with a known good battery...try that motor. 2. With known good gun/motor...try that battery - shoot both semi and short bursts - if you're shooting at a rate of 2 shots/sec on semi there shouldn't be any drop off in trigger response for a long, long time. If that's good on semi...but when you put on a burst of FA and it dies off...then it's your battery. * more often than not that will pin it down really quickly * note...you risk breaking your mechbox shell if the front is unsupported...you can help that by putting the front of the shell against a solid table...or putting your finger over the nozzle to create a bit of an air cushion for the piston. * see #3...you might want to do all this without putting the mech in the receiver since it's just simpler/less work if there are issues. - if both work then the problem is likely with your switch/wiring/setup 3. Try the motor/battery/mechbox/grip outside of the receiver...weird/poor setups can bind in the receiver/grip and it's good to isolate those factors - if it doesn't cycle well then... ...try a different grip (some are tight with some bushings and they'll bind the bevel gear bushings. The grip shouldn't be sqeezing the sides of the mechbox/bushings...a good fit is good, but it should slip on easily. The sides of the grip where they ride over the bushings can easily be relieved with a file/dremel. ...loosen up your shimming. Some mechboxes flex quite a bit and once it's under load it doesn't work as well as when there's no spring in it. I've run into some where I can rig it up with motor/batt/grip/no spring and run the gears....then pinch the sides of the shell hard enough to alter the "tune" of it. A good test to see if your stuff is binding under load is to just slightly back off the top mechbox screws (starting from the back). Not falling out loose, but just enough. On a lot of problem setups this will indicate that either the piston/shimming is binding under load...and or your bushings/gear axels are binding. ...consider reseating your bushings...or swapping them out. A cockeyed bushing will add a lot of drag to things...which just gets worse under load. ...check that your piston is completely free in the mechbox (take the sector gear and spring out). Screw up the mechbox. You should be able to freely push the piston all the way back by pushing a long allen key/rod/whatever down the nozzle. If it's binding either on the rails or the spring guide then swap parts (a non-bearing "plain" spring guide is often handy when troubleshooting) ...check that your piston can go fully over your spring/spring guide...place the spring guide on a solid table...put the spring over it...put the piston over that. Line it up and press down evenly. There shouldn't be any catching/binding/stuck-spots...it should go over nice and smooth. If it's hitting the bearings on the spring guide...swap the spring guide. - set the mechbox up with everything but the spring guide, spring, piston, tappet plate and cut off lever. Setup the grip/motor/batt and run the mechbox...obviously it will not turn off without letting go of the trigger. Run it for a while...listening to changes in pitch or rubbing. Then take it apart and look for wear/scratches/worn spots. Areas to check for are the back side of the large flat gear rubbing the mechbox shell...or any of the gears hitting/rubbing the mechbox shell, reinforced sections, the post that the spring for the switch block hooks onto. It's also a good way to check if any of your gears are cockeyed enough to be rubbing. 4. You don't mention what spring you're using...if it's anything up to a M120 you should be able to simply test your setup with a known good large cell 8.4v/2000+mAh...9.6v/2000+...7.4v/2000+/20C (like an RC race pack). If it works with that but not with the battery you want to use (i.e. stock pipe/mini's that fit into a certain stock, etc...) then you know definitively that it's the battery. - from there you can alter your setup for the battery you want...i.e. improve gear ratio, lower spring power, "plain" motor, etc... That's just off the top of my head...would need more details of your setup.