I’m not familiar with this particular model but all of the similar appliances I have worked on usually have a cover that wraps around on three sides. After removing all of the cover screws, slide the cover towards the rear of the oven for about half an inch. The cover should then lift away from the main body. Hope this works for you.
Check to make sure you don’t have a leak on the intake manifold or the manifold side of the carb’. you can check this by dribbling a little oil along the joint to see if it gets sucked in. If this is the case, then the engine will be sucking in air without the petrol vapour to ignite and so causing a ‘flat spot’.
Further to Mayer's very well informed suggestions, may I add a suggestion of my own? I am not familiar with this type of machine in the UK, but if it has a motor that employs brushes, I should check that the brushes are not worn down, do not have broken or weak springs, and are not sticking in the guides. A new set of brushes can remedy a lot of obscure faults for relatively little cost.
The pressure switch indicated by Mayer can be tested by disconnecting the tube from the tub and gently blowing into it. As the pressure builds a faint click should be heard from the switch unit ,indicating that the switch contacts have been activated. Another faint click will be heard only when you release the pressure. If the switch clicks before you release the pressure, then the diaphragm inside the unit is faulty. Another cause could be a faulty solenoid coil on one of the inlet valves. There are usually two, (hot and cold). Isolate the power from the machine and disconnect the two wires from one of the solenoids. With a multimeter set on a low OHM's range, place the prods one on each terminal of the solenoid. If a reading is obtained, the coil is good. If no reading shows, then the coil is open circuit and a new solenoid valve is required. Hope this is helpful.