Timing belt issues

  1. My model of MX-5 is:mk1 eunos
  2. I’m based near: isle of man
  3. I’m looking for technical help or recommendations on: __timing problem
    Hi all, having had the head off for blown gasket, I have fitted a new timing belt but I cannot get the inlet cam to line up with the timing mark on the back plate the exhaust cam is spot onto the crank pulley mark, the inlet cam is either half a tooth out up or half a tooth out low, I know I am on compression stroke as both cams first lobe points outwards, I am now wondering if the cam belt has a slight error it was bought from a reputable supplier, I have tried several ways of fitting the belt on but always the same problem, I have even loosed the back plate to see if that alters but not really, I have now stripped the top half of the engine down three times, any thought guys, thanks.

Extremely unlikely to be the belt, or the teeth wouldn’t fit the pulleys.
Some thoughts.
Did you line up the marks before disassembly, and were they correct?
Are the ex/inlet pulleys marked in/ex, if so are they mounted on the correct cam ?
Is the engine a known quantity, many have parts changed over the years so at some point an earlier/later slightly different Yer Dion of something may have been fitted, resulting in the marks not lining up.
Sounds to me like it’s out with the dial gauge and vernier wheel to check the actual valve timing.

It’s a fiddly belt to do as you have to move the cams whilst trying to put the belt on and the marks are tricky to see.
This is a good video, I think you should have 19 teeth between the marks at 12 o clock when the timing is correct.

Hi, I didn’t line up the marks before stripping as the engine was running sweet but noticed the tell tail white steamy exhaust, I needed to put on new water pump, hoses, cam cover was leaking etc etc, so stripped down I kept all parts as they came off I’m sure I kept the cam pulley’s with the cam it came off, all cam clamps were kept in order for both sides, I’m quite meticulous with things like this as I’m an ex aerospace engineer, I have watched several videos on this and I am sure I have assembled correctly, there are 19 teeth between cams, I have done this about 10 times now different ways using cable ties I get the same each time half a tooth out on the inlet side, exhaust spot on with crank on its mark.

Hopefully someone who has changed the belt on lots of these engines will chime in - it may even be normal for these, although I must admit it doesn’t seem right.
If the crank and exhaust cam line up, it can only be the length of the belt between the pulleys that affects the inlet timing, and if the teeth on the belt were at incorrect spacing, they wouldn’t engage properly in the pulley teeth…
Thinking about it, have you tensioned the belt correctly (so it’s pulled hard down into the cam pulleys) before final check of timing?

I suspect you’re over thinking it a little bit. It can’t be half a tooth out, it’s either a tooth out or correct. The marks are a little bit vague. Setting it one way will look better than the other, go with the best of the 2 options.

Yes cam belt feels tight watched a very informative video and he showed to turn the engine over by hands few times then release the sprung pulley onto the belt and tighten

Looking direct onto the impressed marks on the back plate against the cam marks it is half a tooth out on the inlet, I even tried locking the cams in correct position using spanners on the cams and clamping them then tried fitting the belt, the belt would not fit it actually showed half a tooth out trying to stretch it over impossible, as you say maybe I am being to critical as the back plate is only a pressed plate, I did loosen the bolts on it to see if there is movement but only slight, I am hoping someone else has had the same situation and tells me the plate is slightly inaccurate.

Yes, that’s what I’m getting at, the pressings are a bit vague. I usually rest a screwdriver along the groove in the tooth to see where it is in relation to the pressing. Then try it on the groove either side and it’ll be obvious that it’s in the best of the 3 positions. The belt on the pulley will either be correct or a tooth out either way.

My mk2 1.8 non vvt doesnt line up perfectly with the marks (I even took photos etc before changing it so know it wasnt something I did!) Its very close but there isnt any way to get it exact.

Ah thanks for your input can you remember if it was slightly high or low and was it inlet cam.

The exhaust cam was slightly more clockwise than the inlet, by maybe half 1/4 tooth

Thanks for that, I have settled on the inlet cam being half a tooth slightly high refitted everything engine started first crack but idle engine shakes , running on four cylinders but got an injector leaking badly all over my newly painted manifold so hoping the leak is causing the shake, home mechanics, great fun eh.