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Replaced Belts on Carbureted Engine

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When I was getting ready to park my 1979 Pontiac Catalina last night after driving home, I heard a loud snap and metallic ding, then the battery light came on. I knew that old alternator belt I tightened a few weeks ago snapped. I had a brand new spare in my front seat in case it broke. I replaced it and the power steering belt this morning since that one is cracking and needed to come off anyways. 

 

I noticed the engine on base idle was running slower and sounding like it was struggling a bit with the new belts on, so I adjusted the base idle screw inwards to smooth it out. Is this normal behavior after replacing belts on a carbureted engine, or do I possibly have a belt too tight?

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Maybe the alternator is binding. Did alternator spin feely with the belt off?

It seemed fine. I spun it pretty easily with my finger. It was brand new around 4 years ago and has sat most of its life. The old belt was put on incorrectly by the shop that serviced it before I bought it. The belt had a twist in it on the crank pulley. That's why I suspected it might fail after being corrected and tightened. It would squeal with engine speed changes.

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Might have been getting some slip on the old belts though it's hard to imagine it being enough difference in drag to affect the idle. However as I've said before those old barges are always running open loop so do not adapt automatically to changes.

There are tension gauges you can get to check on the belt tension, but back in the day the seat-of-the-pants method we used was that you should be able to deflect the belt with thumb pressure by about 1/2" on its longest run.

I expected it to run a tad bit slower, but not like it was. I just loosened the alternator about a bolt head's width and started it back up. It didn't sound labored anymore, and the belt doesn't seem to be loose. It was sounding like it did when I had the idle screw almost totally out and it would want to stall with the air cleaner on.

A metallic rattling sound I used to hear is gone now that I replaced both of those belts. Would you keep the power steering belt as a spare or just toss it?

I usually keep old belts as spares.

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