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may not be the cause of your problems but they are the first things I look for when I have a problem.
You can feel free to email me with your questions but this page pretty much encompasses all my
knowledge and advice I give anyone. The best resource for help is the ongoing thread on Reef
Central. When you post on RC please don’t just say “my mod doesn’t work please help” because no
one can use that information to give any real advice. Tell them exactly how you built it, what things you
have already tried and post some pictures of your mod assembled and unassembled. People are
always more likely to put in more effort to help you if you give them the information they need to help
you and show that you are also giving this some effort.
my housing doesn’t sit flush on the MJ…
You need to take off the rubber O ring of the MJ or it will never fit. Because this is such a common mistake I will say it
again , you need to take off that O ring : )
In my opinion these are the top three reasons why a mod might not work
1) you didn't take off the o ring: )
2) you bent the stainless steel rod when putting it together
3) the housing isn't fully seated on the MJ and it is causing the magnet to rub.
In my opinion the only reason a MJ mod is not going to work is because there is excess friction or drag and the
motor isn’t powerful enough to generate the torque required to start the prop , keep it going or both. This can be
caused by a number of things including but not limited too.
• The Prop is too large for the MJ you are using and the motor cant generate the torque required.
• The magnet is rubbing on the side of the housing.
• The impeller build doesn’t spin freely on the shaft
• The Prop is hitting the stopper
• There are not enough inlet cuts .
It is almost always one of these things that is stopping the unit from working properly. Before I address each one of
these individually id like to give a two simple tips that normally work for me.
First, simply turn the housing a ¼ turn and try the unit again. Repeat this until you are back where you started. To
be honest I don’t know why this would make any difference at all but it has made a world of difference in so many
mods that this is always the first thing I do if one doesn’t work. Second I try jump starting them by bringing them to
the surface of the water and then putting them back down into the water. The Mj might not have enough torque to
get the prop moving from a dead stop but it has enough to keep it going once it is already moving water. Once it is
running let it run for a week or so before you turn it off and try again . By letting it run you are letting everything ware
into place and reducing the friction/drag and there by reducing the amount of torque required to start it so it may
start without “jump starting” it. If after a week or so of running this way and you find that you still need to jump start
the unit every time you turn it on or off, you need to either look for one of the bulleted problems above or take the
lazy mans route and just accept the fact that you will need to jump start them after you do a water change.
The Prop is too large for the MJ you are using and the motor cant generate the torque required.
My recommendation for the 900 is the Octura 1435 and for the 1200 the Dumas 3004. The 900 can run the 2035
and generate more flow but the results seem to be inconsistent and the 1200 can run the larger Octura props and
theoretically should produce more flow but again the results are not as consistent. The reason being is these props
have a higher pitch and require more torque to get the prop moving. I personally will give up a bit of extra flow for the
consistent results and reliability. I do use the 2035s sometimes but if this is your first mod id stick with what works
best so your mod works and you don’t get discouraged by the project. Keep in mind that all MJ’s are not created
equal and just because you have a new one it does not mean that it will out perform a old one. If you build one
with one of the larger props and it doesn't work I would sand some of the prop off, works for almost
everyone !!! It should also be noted that not all MJ's and magnets are created equal so you may just
have a weak MJ or magnet , again id just shave/sand a bit of the prop off but make sure you try the other
solutions first so you know that this really is your problem.
The magnet is rubbing on the side of the housing.
If the magnet is rubbing on the side of the housing this is obviously going to cause excess friction. Good way to tell is
to assemble the unit and flick the prop with your finger, it should jiggle pretty freely as it bounces between the
polarities of the magnet. If you flick it with your finger and you don’t know what I’m talking about when I say jiggle
then it is not doing it and this is your problem. You can also use a flash light and just look down the barrel of the
unit. Keep in mind that it can just barley be touching and still cause excess drag/friction. First thing is to make sure
the housing is fully seated on the MJ and then turn the housing ¼ turns and see if that helps, If that doesn't work
look elsewhere for reasons why the magnet may be rubbing. The most common reason is because the housing isn't
fully seated on the MJ . The shroud "must" be seated flush with the maxijet.
The impeller build doesn’t spin freely on the shaft
When you take it apart the impeller should spin freely on the shaft . when I say freely I mean it should glide. With
one spin it should go around 20 times and the only drag should be the prop in the air. If you have drag it can be
from a few things . First the shaft can be bent or you got some glue on it, its not all that hard to bend the shaft when
you are putting it together. Second, you are using the old impeller build and attaching two pieces of shaft together
with a piece of soft airline, there will always be some excess friction this way , take some effort to make sure you glue
this thing together as strait as possible to reduce friction. Tons of people have used the old style impeller build with
success so if done right it will be ok but there will always be some excess friction. Thirdly you used the new style
impeller build and the hole in the magnet was so tight that it squeezed the tube a bit . my solution for both of the
impeller builds is to take a piece of .098 carbon fiber rod, stick it in the end of my drill and then stick it through the
impeller and turn it on . the small ridges on the CB rod will ware away some of the tube and make the impeller glide
more freely on the shaft. Do this in short bursts so you don’t heat up and melt the tube.
The Prop is hitting the stopper
Obviously if the prop is hitting the stopper it isn’t going to work
There are not enough inlet cuts .
If you don’t make sure to cut enough inlet slits or holes the prop is going to have to work too hard to stuck the water
in through the back and it will cause the unit not to function properly.
If you try all these things and it still doesn’t work
I have no idea what may have gone wrong or been overlooked. Ask on Reef Central, make sure to tell them what
you have tried already and take some pictures. If no one there can help, take it apart and rebuild. The second one
can only be better than the last after all you have learned : )
Few notes on units that function well but produce excess noise.
I’m not really sure why some of them are noisier than others but I can give my own personal guesses. Again the first
thing you want to try is turning the housing in ¼” turns.
My first guess is the tube you are using is too large for the shaft you are using and the noise you are hearing is the
vibration between the tube and shaft. The rigid airline had a extremely inconstant inside diameter so bring a piece
of the rod you want to use to the store and find a piece of airline that fits it well.
I personally have had better luck producing quieter mods with the new style impeller build so if yours makes too
much noise for you may want to try this.
Keep in mind that water dampens the noise quickly so try to keep the unit as far away from the sides of the tank as
possible and this should help.
It is also generally thought that a 900 will not make as much noise as a 1200. I'm not so sure this is the truth
because I have built 1200's so quiet that you can't hear they are on unless you press your ear to the glass. This
should be noted anyways because so many people believe this to be the truth.
Again if these don’t work, ask on RC with as much info as you can give and pics , If they can’t help , rebuild : )
Seems i missed one: If your MJ shoots water out the inlet slits and not the front opening then you forgot
to put the stopper in .
Because the maxi jet runs on alternating current (AC) power it can spin both left or right and will do so randomly
when you first plug it in. if it starts going the right way then perfect but half the time it will start going the wrong
way and end up sucking water in the front and blowing it out the fins on the side. This is where the stopper
comes in. the stopper is simply a piece of acrylic rod shoved through the side of the housing. I drill a 3/16” hole
about a 1/8” below the top of the housing (just under the cap) and glue a ½”-1" piece of 3/16” acrylic rod. What
happens when the pump starts the wrong way is it pushes the whole impeller up towards the cap when this
happens the prop will slam into the stopper (acrylic bar) and bounce back the other way and start running in the
right direction. This is extremely easy to implement and works very well. Some people have found with the
octura props that you don’t need a stopper because the ends of the prop extend up and actually slam into the
centering cap itself. I personally always use the stopper bar because it works so well , costs almost nothing and
takes about 20 seconds to install . Mother always said “if it an’t broke don’t fix it” : )