10 replies to this topic

[Andy L]

I have a 1993 Miata I'm building for SM and I just finished rebuilding the engine. I have done a few engines but no dual overhead cams. The cams ARE in there correct positions though. I feel like an idiot posting this on here but I'm out of ideas. The engine won't fire, won't even hit at all. I've gone over timing, fuel pressure, electronics (a little) CAS and compression. What's even more confusing is that when trying to start it, it's pulling from the exhaust and exhaling through the intake. I checked the rotation and it IS turning clockwise when looking at it from the front.

Any ideas would be GREATLY appreciated as I am stumped.
Thanks!

[Tom Hampton]

Since your crankshaft isn't going the wrong way, its got to be cam timing. I don't know another way for the exhaust valves to open on a downstroke, and intakes on the up.

I just went through this exact problem with a lawnmower engine I rebuilt. In the end, my crank keyway had busted and everything was 180 degrees off. I was glad I hadn't been an idiot and put it together wrong, but I still had a broken crank key.

So, either your crank or cam pully keyway is busted and the pulley has rotated, or your timing belt isn't on right.

It might help to take the valve and timing covers off, and slowly rotate the engine through the entire suck, squeeze, bang, blow cycle. Watch the valves as the engine goes through each phase and see what happens. Place a long dowel in the plug hole so you can see the piston move up and down. Somewhere as the engine goes around the circle, it should be obvious what's happening.

You don't really have to take the timing cover off....but, odds are whatever you have to fix is going to require taking it off.

[FTodaro]

It sounds like you have the cams 180 degrees off Looking at the front of the motor the left cam (intake) the I has to be at 12:00 the cam wheels are interchangeable so you have to make sure that the timing marks for intake and exhaust are at 12:00 and then you need to get the crank at 12;00 (TDC) Have you verified those timing marks? You will have to pull the valve cover and the timing belt covers to confirm. there is no other explanation I can come up with.

[dmathias]

There is a small arrowhead cast into the front of the oil pump, just above the crank pulley. The arrowhead must be lined-up with a small notch in the crank pulley. If not you are 180* off. You can shine a Maglight, etc., down into the timing belt cover and visually see if the marks line-up.

[Dave Lewis]

When you have verified what Denny said above, look at the cams, the intake #1 lobes should be pointing directly to the left and the exhaust #1 to the right.

[dmathias]

And the small alignment pins on the front of the cams should be at 12 o'clock.

[Sphinx]

suck, squeeze, bang, blow

There you go, the 4-cycle engine reduced to a fantasy.

Not one peanut gallery comment?! Are you kidding me?!

[Tom Hampton]

I can only lob the meatball. If no one chooses to hit it out of the park....

[Funracer]

[Andy L]

Thanks for the info! I'll try this stuff and let you know what I come up with. Since I have been playing with timing for a few days I'll try slowly rotating the engine with the valve cover off to see what's going on. It's gotta be 180 off.

Thanks for the "peanut gallery comment" by the way Sphinx. No topic is complete without one.

[Andy L]

And the small alignment pins on the front of the cams should be at 12 o'clock.

This was the problem! I was around 60 degrees off on both cams, the gears were placed on the alignment pins incorrectly. (DUR!)
It actually tries to start now now that it's sucking, squeezing and blowing in the proper order. Now if I can just get it to bang I'll be in business! I think I just need to tweek the cam angle sensor and it should run. Thanks for all the help guys!