mx5cartalk.com

Welcome & congrats on the purchase.

If the plugs hadn't been changed in quite a while then likely neither have the plug leads, so definitely replace them. Sometimes plug leads can test ok, but break down under load. One of the NB8B onward owners might be able to suggest a good supplier for the NB8B onward specific plug leads. But MX5 Parts UK are popular for a number of things: http://www.mx5parts.co.uk/leads-plugs-c-53_287_219.html

As a general rule you can't test plug leads with a multimeter unless they're seriously broken. Their resistance/performance at the 2 or 3 volts supplied by a multimeter is entirely different from what they might do when carrying 20,000V from a coil. Either replace them or have an auto electrician test them on a proper piece of test equipment.

Is your check engine light coming on at all? and have you read your engine fault codes?

I thought of valve timing problems as soon as you said chugging . Seeing that it's a VVT engine I started searching for something that upsets the VVT actuators and found that 2002 models did have problems, often associated with poor oil or oil change history. It usually throws a P0012 fault code. I don't have direct experience of this problem, it's just a shot in the dark Try googling "VVT stuck site:miata.net" and you'll see what I'm talking about.

OBD2 readers are handy if you like playing with that sort of thing. ELM327 ones are cheap as chips on ebay etc. Somehow I doubt they're going to tell you much this time though.

I'm new to Mx5's so not sure what plug you should be running but are the plugs you have chosen too "cold". Cold plugs will do what you have described. Do the IX 11's mean 1.1mm gap or are they the same classification as the NGK 11's which are way too cold for even a race car. Make sure you gap the plugs as per manufacturers recommendation. Something as simply as plug gap could cause you issues whereby you replace everything and get to the plugs last costing you lots of time money and a lot of head scratching before you sort out the problem.

Regards - John

They are the correct plugs.

Other things you can try:

coil packs
cam angle sensor
battery
O2 sensors
Air filter
Clean the throttle body