Monday, May 22, 2006

bikes -n-cars

Well I was finally home for an entire weekend. Felt good.

I thought I would do some catching up in the bike department and try to crank out a couple of paint jobs that have been waiting. I stripped both frames and amazingly found defects in both frames. Talk about a bummer, especially for the customer.
One of them was brand new, a custom build by a low-buck midwestern builder who shall remain nameless. He had overcooked the chainstay in the simplest of operations, installing the rear deraileur cable stop. Really dumb mistake, but he buckled the chainstay right at the stop and created an almost certain future failure point. So now I have a paint job and a chainstay replacement to do. Sigh.

The other frame was a tandem made by the biggest marketing company in the world of tandems, the big "S". I'm amazed at this one as they typically overbuild all their stuff by about a factor of 3, yet buried under that paint was a buckle in the captain seat-tube caused by a mistake in welding and a few years of riding.

It really sucks having to make those phone calls to people to tell them the frame that they innocently brought in for a repaint has a pretty major problem buried in it. It's a really rare occurrence, infact I've hardly ever had to do it before, but here I was having to make two of those calls in one weekend. yuk.

Since I wasn't going to be painting Sunday I decided to do some work on my other hobby, playing with my 1992 Miata. Granted I'm a die-hard cycle advocate, I still love to tinker with this car, it's the only car I've ever actually enjoyed working on. It's logically designed and well laid out, easy to service and tune. Usually I have it out in mid-April, but due to all my travels this year I hadn't even taken it out yet. It's always started right up with no problems after a long winter's nap, but this year it started up fine but just didn't want to idle at all. This vintage of Miata is known for it's persistent idle drop problem, but this was far more, it just kept cycling up and down never leveling out.

I've done quite a bit of engine modification to this car in my continual efforts to draw far more power out of it than Mazda felt it needed. In the process I've developed a pretty good understanding of this particular engine, but this idle business was getting the better of me. The car has a pretty primitive computer set-up that reads about 10 different basic error codes if there are abnormalities. I kept getting a code for a faulty air-flow meter, but the voltages coming out of the meter seemed fine. I broke down last week and bought another used airflow meter from the junk yard to plug it in and see if it got rid of the error code. So I tore out the old one, installed the new one and things got worse. The fuel pump wouldn't stop running and the idle got even more uneven. Turned out Mazda had made a change in the wiring of the AFM in 1991, so despite the part # being the same for 1990-1995, the actual part was not identical. Bummer.
I decided the problem was probably not the AFM as it really seemed to test out fine. I checked every vacuum line in there, but found no leaks, so I started checking the fuel system. I have a supercharger on the engine with quite an array of fueling enhancements that run off the boost level and the engine seemed to run better when I tricked the fuel system into thinking it was under boost. So I checked the fuel pressure and found that the factory-set Aux fuel pressure regulator I had in there was completely mis-set, the engine was getting 10 psi of gas at idle and about 60 psi under boost. It's supposed to be getting 37psi at idle and and extra 5 psi for each lb of boost. That means the car is running waaaaaay lean. bad, very bad. I pulled the plugs and sure enough they were really dry and getting white. That's bad, hopefully I caught it before any serious damage was done to the pistons. The compression seems ok, so I hope I'm alright there. A quick adjustment of the Aux fuel pressure reg and things started looking better. it would idle, but still not as solid as it should be.
Eventually after a few more hours of measuring volts and checking connections, I checked the simplest part on the engine, the PCV valve. Shoulda done that sooner, it was plugged and wasn't opening all the way. A quick spray of carb cleaner and it moved free, reinstalled it and idle was rock solid. Doh. Still has some drop at start-up, but about 95% of the problem went away. I think I can live with it where it is now until I can find time to play with this last little idle drop. I think I'll be installing an air/fuel ratio gauge as well to make sure I'm not running lean anymore. it'd be a real bummer to melt down a piston since this engine really does run great now!

Oh well, now I can focus on bikes again, and maybe the house a bit too, still have much to do outdoors in the yard, but that can wait for anther day to write about....


1 comment:

Jim G said...

Bob, would it be too much to ask if you could post a close-up photo (so as not to reveal any details of the frame/builder) of that cooked chainstay? I'm just really curious to see what that looks like... Thanks!