Aug 3, 2011 update to the Mazda MX-5 Miata project car

Crank Pulleys Can Indeed Fail

Setting timing on a busted pulley is a tough job.
The old pulley had seen more than 250,000 miles, but has needed replacing at least 60,000 miles ago. It was clearly not working quite right; it had rubbed through the plastic lower timing cover.
With a proper, precise timing mark, I could set the ignition advance with confidence at 14 degrees BTDC. This was shot with the engine running, lit up with the timing light.

Setting the ignition timing on my ‘91 Miata has always been a strange experience. The first time, I couldn’t see the mark when I had the timing light reading off of spark plug #1. But it was dead-on when I read off of cylinder #2. The next time I tried it, I couldn’t get the mark to line up at all. My ever resourceful and clever dad suggested that we make our own timing mark by dropping a rod down spark plug hole #1 and finding top dead center (TDC), and marking that on the pulley with a silver permanent marker. Click the first picture below and look closely: You’ll be able to see the silver marks.

The next time I checked the timing, the mark had moved again. Not willing to trust our less-than-precise method for finding TDC, I set the timing to what looked like 8 degrees before TDC (BTDC) to be on the safe side. The OE-recommended setting is 10 degrees BTDC, but most enthusiasts set it to 14 for a bit more power.

Adding to the strangeness, my accessory belt would stretch and require regular re-tightenings. What’s more, the pulley was rubbing on the plastic lower timing cover. The crank pulley is a 2-piece affair, sandwiching a hunk of rubber between the inner and outer parts to damp vibrations. After 20 years, the rubber must not have been holding up very well.

Yesterday, I got my brand-new crank pulley (and timing cover) in the mail from Mazda Motorsports. It’s a stock piece. Through Mazda’s “Discount Parts for lolol rac3r d00dz” program (I’m sure that’s its official name), it cost me in the neighborhood of $80. A used pulley from most Miata parts specialists sells for about $50, and its rubber isn’t necessarily in better shape than mine. A new one’s worth the extra $30.

I considered an underdrive pulley, but the only option for my V-belt-equipped car was the Corksport unit. I wasn’t ready to spend nearly double the OE Mazda price for it. Later cars with multi-rib belts, however, have very cheap options available on ebay. People on teh intarwebz will cry bloody murder if you put one of those on (citing their lack of vibration damping), but I’m not confident they’re such a bad thing. These engines are pretty robust; I think they can handle slightly less damping.

My co-workers, Joe and Tom, saw me getting my tools ready in the parking lot after work and were, of course, intrigued. Joe offered his driveway for a workspace, so I followed him there. It was merely a mile from the office—not far enough to get the engine too hot to work on—and I got to park next to an incredibly awesome AMC Eagle, a car I’ve always had an unjustifiable affinity for.

I got the job done with the meager tool set I keep in my car, moral support from Joe and Tom, and some excellent snacks that Tom provided. The foods he discovers are both unbelievable and delicious. He has been crowned the King of Snacks.

Afterward, I went home and checked my ignition timing and idle speed. My dad’s improvised TDC-finding method worked well: it was set precisely to 8 degrees BTDC. I advanced it to the go-faster 14 degrees, adjusted my idle accordingly, and have fallen in love with my car again. The difference in power and smoothness is immediately noticeable. It’s nice to have my go-go fast car back.

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