Middleby 350 problem

I have a shop I help out a bit here and there with computer stuff, and they have a stack of middleby 350 ovens that have been converted to 360 conveyors some time back. Anywho, the top oven in the stack has developed a couple problems recently. First, the speed controller went out and has been replaced. Secondly, The oven just cannot maintain temp like it used to. The day i was there, it took an hour and 15 minutes to go from 100 degrees to 500 degrees, and once they put a pizza in, it dropped to 475 and just couldnt bring it back to 500. For those of you familiar with these ovens, any ideas? I suggested he have someone come in and change out the gas valve and set the pressure right.

P.S. The inducer fan had been replaced a few weeks ago.

First thing I’d do is swap the temp controllers and see if the problem moves to the other oven. If it does, then you’ve saved yourself a lot of time testing out all of the other parts that might be failing.

i worked at a place years ago that had a similar problem with the temp. The oven took forever to heat up and cooked fine with one pizza at a time but would lose temp as soon as a secon pizza was loaded in it. The problem turned out to be that one of the blower motors was out. The 360’s have two blower motors so check to see that both are working.

Change the fans on the back of them oven. The small black ones that cool off the controller. Those small fans are easy to choke with dust and grease, and are a cheap repair of things like the controller are going out.

If I had to guess, I would say that 75% of the stores I have worked with had dirty fans. Just pop your head behind the oven and look for the wire cover, if it is covered in grease and dust, replace them even if they look fine.

Hi Stebby1:

The foregoing posts covered the subject very well;

With the cooling fans placed in the rear and most ovens backed up to a wall they are easily over looked.

They can get clogged up fast and should be cleaned monthly.

If the suggestions offered do not resolve the problem the last resort is to call in a service compamy.

George Mills

I would first check the main motors as paul7979 suggested. What you are describing is sometimes caused by a main motor spinning too slow, not working, or wired incorrectly (making the fan run the wrong direction). I would also verify that your hood is working correctly. I have seen many PS300 series ovens start to burn up parts because the design of the ovens put the top oven machinery compartment up high near the hood and sometimes even in the hood depending on your hood height. If your exhaust fan stops working or slows down it allows the controls to get very hot, overheat, and prematurely wear out from the prolonged heat. Replacing only the failed parts without identifying what may be causing the failure (such as bad hood system or fan) usually leads to repetitive failure of the same parts over and over.

If you do find that it is a blower motor not working (or sticking or running slow) - don’t panic.

Go to Lowe’s or Home Depot and pick up a $8 can of Electric Motor Cleaner. Put the little red tube on the sprayer stick it in the end of the motor where you see the windings. Spray the crap out of it, all around - anywhere you can find an opening to spray into. If they don’t have electric motor cleaner, use electronic contact cleaner instead - works almost as well.

I’ve “fixed” 3 or 4 supposedly “bad” blower motors by doing this.

The oven is out in the middle of a room, so its not backed against the wall. The cooling fans on the back looked brand new and were blowing plenty of air. As for the blower motors, the thought had crossed my mind, however, the service manual I have states that they have centrifigal switches in them to prevent the burner from firing unless both were running. Something to check i guess. Also, this oven has the thermocouples in the front of the oven and not in the back wall. They appear to be in the burner area. This makes me think that if I had a blower down, that compartment would read hotter than actual oven temp, but i could be wrong.

I’ll have him check the blowers and see if they’re running. Whats the easiest way to get to them to make sure they dont have a wad of foil or something in them?

He’s called a couple service companies. One of them called him backed and asked him how to spell middleby, so they were ruled out. The other is where we’re getting the parts from, but he’s too far away to just pop in.

If an oven is not coming to temperature fast enough or the oven is not maintain temperature when you place product in it. It can be a number of reasons.

>Blockage-Top oven especially
Do you use tin foil or aluminum pans? I have seen a number of top ovens suck these into the squirrel cage(s) and then do not allow the oven to disperse heat properly or quickly.
Bottom oven, never since the conveyor works as a filter.
>Solenoid valve
The standard solenoid valve(On/Off) is rated at 1.5M cycles, that is about a year and one half of operation. When a valve goes bad; it tears or doesn’t open or close properly, thus you are not getting the full gas flow to the burner to heat up or maintain temperature.

It is NOT

>Blower motor - In-Op
Unless you have bypassed the centrifugal switch on the motor(s), the safety is if the motor is not turning or running the circuit will not complete and allow the ignition sequence to activate(No Heat Generation).
>Blower motor - Running slow
The motor is designed to run at a certain speed. If the motor slows down or is inhibited by bad bearing(s). The motor will slow down and then the windings will overheat/melt and then cause the motor to stop/burn up, unless the high temperature relay kicks in first. The process is then repeated(motor cools down, restarts, ignition re-lights, motor overheats, and then shut-down again of the motor and the Ignition process)
>Temperature Controller
The temperature controller is a switch that functions by receiving a signal from the thermocouple. If the set-point is 500F and the temperature in the oven is below 500F the switch allows current to flow to the valve. If the temperature is above, it shuts off the current to the valve. By stating the oven is heating but is not fast means the temperature controller is doing its job and properly.

>>>>Good point on keeping the axial fans clean of debris. Most of the problems that occur in the oven is a direct relation to the axial fans. The axial fans cool the main blower motors and allow air circulation to the electrical components in the cavities.

Good luck!


They had an “oven whisperer” come in today to look things over after the other oven started acting up, and it turns out that the gas pressure feeding the ovens was low. Now I guess between getting that corrected and some other minor adjustments here and there, both ovens are running as good as new again.