engine and drivetrain Engine vibration on a '76...

[dang]

I've been driving my '76 2002 around "getting the bugs out" before I put it up for sale.  It runs decent, considering it has all the '76 smog crap on it, but it has a vibration when revving the engine.  It vibrates worse as the revs go up, to the point where the rear view mirror is affected.  Last night I took off the belt for the fan to see if that's the culprit since it has the biggest diameter and easiest to be out of balance.  The vibration is gone without the belt on so its probably the fan.  How do you balance a fan?  I've seen balancing clips on blades before and could use something like that, but how do you figure out the balance?  You'd have to know the exact center and place it on a point.  Has anyone done this before?

 

Dan

[Guest dq48]

Probably not the fan,but,the water pump,crank pulley or alternator.........Or compressor if it has a/c.

[dang]

1 minute ago, dq48 said:

Probably not the fan,but,the water pump,crank pulley or alternator.........Or compressor if it has a/c.

 

I can put the belt back on without the fan to test it.  All the belts look smooth while its running, but I'm not sure that's a good gauge.

[avoirdupois]

22 minutes ago, dang said:

I've been driving my '76 2002 around "getting the bugs out" before I put it up for sale.  It runs decent, considering it has all the '76 smog crap on it, but it has a vibration when revving the engine.  It vibrates worse as the revs go up, to the point where the rear view mirror is affected.  Last night I took off the belt for the fan to see if that's the culprit since it has the biggest diameter and easiest to be out of balance.  The vibration is gone without the belt on so its probably the fan.  How do you balance a fan?  I've seen balancing clips on blades before and could use something like that, but how do you figure out the balance?  You'd have to know the exact center and place it on a point.  Has anyone done this before?

 

Dan

 

Eliminating vibration by accessory belt removal clearly points to an out of balance accessory.  Does the belt drive turn anything else, e.g., an alternator?

 

Before condemning the fan, how about the pulley?  I seem to recall a pulley (or two) that had a tack welded balance weight in its interior.

 

Traditional red five-blade '02 cooling fans, can be balanced several ways.  Using a bubble balance it is probably much easier to remove material from the outside edge of any extra heavy blade.  I have seen many examples of the fans that have had close encounters with foreign objects and seem to handle this quite well.  Spinning the fan on a lathe is another means of dynamically checking balance.  From personal experience, fan blades can be dramatically shortened.  However, a matched fan shroud loses much of its effectiveness with a smaller fan, although it tends to protect digits.

 

 

 

Hard to tell - balance weights? 

 

[kbmb02]

Check the condition of the bushings where the alternator support bracket mounts to the lower timing cover (and the condition / tightness of the bracket itself). -KB

[dang]

31 minutes ago, kbmb02 said:

Check the condition of the bushings where the alternator support bracket mounts to the lower timing cover (and the condition / tightness of the bracket itself). -KB

 

The alternator bushing is suspect.  Everything seemed tight when I started looking at belts and things but once I loosened the alternator to take the belt off the bushing side felt loose.  The classic worn bushing.  I'll start there...

[Mike Self]

I'd suspect something other than the fan--Several years ago I drove my '73 to Vintage and back (just under 1k miles) with a 5 blade fan that had no blades left that were at their full length. I know--just didn't have time to replace it before leaving--and I took a spare with me.  One blade was broken off at the hub, two were about half length, and the longest blade had a large chunk missing.  

 

Car ran fine, no vibration, and didn't even overheat in the WV mountains.  That fan, AKA Stumpy, gained a clock and became the 2015 hard luck award at Mid America.  It's now the "proud" possession of Andrew Wilson...

 

No way was it even close to being in balance, yet no vibration.  Those fans only weigh a few ounces vs the engine's weight, so I'd suspect something else that runs off that belt.  And BTW, did you examine the belt itself?  My wife's Buick came from the factory with an engine speed-related thump that it took the dealer nearly a year to find--turned out to be a defective belt with a lump in it.  Every time the lump passed over a pulley it thumped.

 

cheers

mike