There have been several people who have reported this problem, and I had it when I first fitted the throttle bodies to my Plus 4 which has the same exhaust system as the Plus 4 SS.
Firstly there is a problem with the exhaust in that it is difficult to get the cat up to operating temperature as it is a long way from the engine. My car has now been fitted with exhaust wrap which has the effect of preserving the heat to the cat.
My car has also been fitted with a 400 cell cat. This has helped a little too but has reduced mid range torque
and has reduced the sound which I found a tad load
Finally careful tuning of the fuel map has now completely eliminated the problem.
Personally I would be very wary about changing the spark plugs. The plugs and gap specified are particular for the Mazda MX5 engine.
Below is my previous post on a related problem.
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Omex did not supply the engines, nor did they fit the cam change, and did not set the plug gaps (although they did specify them to be 0.9mm) but if the plugs have been changed (which they may have been in a three years old car) it is possible that they are not correct. Omex did not design the exhaust manifold and catalyst either...........
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Regarding the spark plugs; if anyone is using normal Duratec (Plus 4) spark plugs they will have serious issues as they are different lengths. The plugs fitted to Supersport were standard MX5 supplied by Mazda, I’m pretty sure Mazda know what plugs to fit!
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However there is software that allows the dealer and owner enough access to setup the idle and TPS etc, this software was supplied to Morgan on CD for each car and should have been distributed with each car. Omex have that software if it is required but if you did not receive the CD with the car I would try to get a copy from Morgan. Naturally the first port of call should be the dealer from whom you purchased the car.
Omex are aware that there have been a few instances of the Supersport having an issue with MOT emissions and obviously they would like to find a solution. They will contact MMC to see if they are intending to do anything. However any solution Omex comes up with (unless of course the error is due to them) will have to be paid for by the owner and it will be up to them to take that up with MMC. However the cars are now three years old and out of warranty (a warranty that the customer agreed to by buying the vehicle) so it is probably not unreasonable for them to have some cost involved.
As an aside I am told that with a 200 cell cat it is quite usual to hold the engine at a higher RPM for a period before testing the emissions performance. This has no detrimental effect on anything and sometimes has to be done with normal production cars.
In my opinion, and I stress that it is only my opinion, Omex would be prepared to help if Morgan agreed and if approached in the correct way. My feeling is that this would be best handled through a dealer, such as, but not necessarily, Williams. Omex are perhaps better set up to deal with technical and experienced mechanics than they are with owners. They do support owners but would expect a certain level of technical expertise, availability of tools, etc. etc.
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