Leaning/Fuel cut issues?

[Jacob]

Apologies if this question has been asked and answer, but I have done a search and can't seem to either find the thread covering this, or find the correct keywords to find more information.

What I'm running;

• MX5 NA6
• 1.6L engine with 300KM; bog stock everything.

Here's how it started;
My engine started to feel "sluggish". It wasn't noticeable at all and I had it checked over by my local mechanics to no avail, but I know my car and I could feel that there was a problem. I decided to do a compression test and from memory, all four came up to about 147psi with a variation of about 3-4psi.

After I did that, i drove the car around and it was fine until about a week later when it started idling at 300RPM - only hunting by a fraction of a MM.
The car will bog down on take-off (standard rolling off the clutch and taking off in first), and won't get power down even if i'm WOT at 5k RPM.
There isn't a "flat spot" because the "flat spot" covers the entire rev-range.

It "fixed" itself after some time. I decided to give my car a hit after it seemed to be running fine for a while and the car started bogging down after cracking 5k rpm. The car would then continue to run like sh*t for an undetermined amount of time afterwards.

I'm not too sure where to go. Happy to answer any questions that may help you think of a reason why this is happening and I appreciate any help you give

[Rolley]

Besides the idling at 3k the symptoms sound like a slipping clutch.

Does the engine struggle to build revs in neutral? If you were to put it in 4th and drop the clutch does the car stall or rev and stay still or take off very slowly.

I may have missed the point completely but I have come across clutches that feel fine cold or under normal load and then let go when hot or under extended load.

Luke.

[hks_kansei]

I would remove the spark plugs and see how they look, black and sooty means rich, white and dry means lean, wet means fouled (fuel means running extremely rich, or the ignition isnt working, oil means the engine needs looking at)

A normal well running MX5 will generally have the plugs come out a little black and sooty (not caked though)

[Jacob]

Hey guys,

Thanks for the replies!

With regards to the clutch; I have tested this out and tried taking off up a hill in first, second, third and fourth. First it took off fine, no slipping. Second I kept on the clutch a bit but if I hadn't, it would have died. Third and fourth died instantly.
I'll try dumping the clutch in fourth to see what happens.
My mate bought the exact same car as me and he has a slipping clutch. I can definitely feel that clutch slipping in his and mine feels nowhere near like his feels, so I've ruled out the clutch for now.

With regards to the spark plugs; they are about 20,000km old (i changed them 10 months ago), so I don't think they're the culprit. I pulled them out when I did the compression test and they didn't look like they were off. They're pregapped mazda sparkplugs for the NA6 - I got them from MX5 Mania - and they seem to be fine. They are - as you described - a tad dirty, but it's not caked on and the plugs don't appear to be heat-affected.

Another oddity is that the NA6 struggles to go above 110KM/H, and will definitely not go over 130KM/H no matter how much road, throttle or revs I give it. I feel like this could be related.

Two things that I feel could have at least contributed to having an issue;

• The charge light is constantly on when on reds. Goes away when engines on. I'm going to test the battery this weekend.
• The car has been running hot lately due to a burst rad pipe on the cold side. I'm hopefully picking up a replacement hose on Saturday but for the meantime I have been keeping it topped up with coolant/destilled water.
• Since I bought it, the car has had an oil leak around the sump. I have bought some liquid gasket to try fix the sump this weekend. The oil pressure gauge doesn't act sporadic and I always keep the oil topped up as well.
• The car has always had issues with power since I bought it. So other than the above, I don't know what history could be a contributing factor.

TL;DR
My previous theory was compression in the cylinders but they're holding strong.
I still need to check the fuel.
The sparks are fine.
The clutch feels fine and have had expected results from load tests.
Oil and coolant are a little shitty atm but they're being fixed.

[cookie]

Timing slipped a tooth perhaps. Foes it sound lumpy/odd? An na6 will happily get to 150 quickly enough, probably 200 if long enough straight.

[Mr Morlock]

High mileage and by the look of it a number of issues. Things wear out. You cannot diagnose multiple issues by writing on forums and its better to get a mechanic to take a look and get some reports.

[Jacob]

Cookie; love the username haha
It doesn't sound lumpy or odd. It actually revs out both under load and in neutral without skipping a beat. It's just like the engine has absolutely no power ... like 20kw... but it feels like it can do more.

Mr Morlock; I completely agree that this isn't something that you can diagnose on the forums. I was kind of hoping to snipe someone who may have experienced this in the past and gone "oh this! btw it's the X valve" or something along those lines. I agree that it could potentially be multiple issues, but I'm a little bit lost at what it could be.
I don't want to spend heaps of money on a shotgun-style solution (get a bunch of solutions and throw them at the car) and I've generally found that mechanics in my area are a bit clueless with non-conventional issues. I want to try get to MX5 Mania when I get paid and hopefully they can look over it

[johnnyreble]

My car was behaving like this right before the end sheared off the crank, the end of the crank actually twisted off so timing was out until it went completely, if this is the case then the crank is already beyond repair, and it's very difficult to check as the pulleys will still be tight.

BTW I hope i'm wrong it's a bitch to fit a new crank

[johnnyreble]

Also had an MG doing the same .. it turned out that the Cat had collapsed internally so the engine couldn't breathe.