Loose Belt

Flytrade

Flytrade

Active Member
Feb 20, 2018
302
Bradenton, FL
Boat Info
2006 320 Sundancer
Engines
Twin 6.2L Mercruiser
I started my Mag 350 engine only to hear a terrible screeching sound coming from the engine compartment. Upon inspection I found the engine is running, but the belt is not moving. It appears to be a little loose. I only had the engine running for a few seconds, but now I'm concerned about the belt. Should I just have it tightened or should the belt be replaced because of the rubbing on the pulley?
 
I would spin each accessory and pulley by hand if possible. That just happened to me(belt not spinning, screeching noise), and the idler pulley was frozen, and most likely need a new water pump and alternator (mechanic is diagnosing today).
 
Just had my alternator freeze up. Heard the chirp and then the engine started. Saw water flow from exhaust and thought it just slipped on startup. Then I got the burning rubber smell. :( By the time I shut down it melted my belt and idler pulley.

-Kevin
 
I've had that happen. I tightened the belt and ran it (approximately 4 hours) until the Amazon fairy in Brown delivered the replacement. That belt is now in my emergency spares drawer. As others mentioned, spin the accessories while you have the belt off, to insure nothing is seized.
 
bobeast said:
I've had that happen. I tightened the belt and ran it (approximately 4 hours) until the Amazon fairy in Brown delivered the replacement. That belt is now in my emergency spares drawer. As others mentioned, spin the accessories while you have the belt off, to insure nothing is seized.
Click to expand...
How do you tighten a belt? Hope this isn't a stupid question...
 
on my 350 MPI the idler pulley on the upper starboard side of the engine has a nut with a concentric stud. Put a wrench around the nut and loosen it. You'll see that the pulley can now be moved back and forth. replace the belt and put your wrench back on that nut. The concentric stud has flat spots where you can put a socket wrench on it. Use the socket wrench to leverage the pulley against the belt and with your other hand tighten the nut.

Once tight "twang" the belt on the opposite engine like a guitar string and listen to the tone. "twang" the belt on the engine you just replaced the belt on. If the tone is lower, then the replaced belt is looser than that on the opposite engine. if the "twang" is higher, it is tighter. If the tone is the same, your two belts are tightened to the same degree and should wear evenly. Of course this assumes the belt on your reference engine is properly tentioned to begin with.

In the future you can periodically check for belt stretch by performing the twang test.
 
Last edited:
5/16" wrench for the tension adjustment, 5/8" for the lock nut.

(Hope I saved you a trip or 2 looking for the correct sizes)
 
bobeast said:
on my 350 MPI the idler pulley on the upper starboard side of the engine has a nut with a concentric stud. Put a wrench around the nut and loosen it. You'll see that the pulley can now be moved back and forth. replace the belt and put your wrench back on that nut. The concentric stud has flat spots where you can put a socket wrench on it. Use the socket wrench to leverage the pulley against the belt and with your other hand tighten the nut.

Once tight "twang" the belt on the opposite engine like a guitar string and listen to the tone. "twang" the belt on the engine you just replaced the belt on. If the tone is lower, then the replaced belt is looser than that on the opposite engine. if the "twang" is higher, it is tighter. If the tone is the same, your two belts are tightened to the same degree and should wear evenly. Of course this assumes the belt on your reference engine is properly tentioned to begin with.

In the future you can periodically check for belt stretch by performing the twang test.
Click to expand...
He only has 1 engine to twang on
 
Todd320 said:
I would spin each accessory and pulley by hand if possible. That just happened to me(belt not spinning, screeching noise), and the idler pulley was frozen, and most likely need a new water pump and alternator (mechanic is diagnosing today).
Click to expand...

Same thing happened to my daughters car a few months ago. I went to move it out of the driveway a few weeks after she went back to school.
Started it up and it was squealing. I checked all the pulleys and found the alternator had frozen.
I ordered a new delco alternator and gates serpentine belt for her car from Rockauto
My advice to the OP is to replace the belt. Belts are cheap. You can probably get a new one for under $20.00 these days if you look for it online. See if you can get a number off of it.
 
Last edited:
'My advice to the OP is to replace the belt. Belts are cheap. You can probably get a new one for under $20.00 these days if you look for it online. See if you can get a number off of it.'

that's good advise....the belt has been damaged from slipping on the various pulleys.....most likely the inner surface of the belt has been 'glazed' by the friction....i would replace the belt and keep the current one as an emergency spare.....

cliff
 
Take the old belt to an auto parts store and they can match up with a same size auto belt. No sense in paying through the nose for an OEM Mercury belt. Auto belt will serve you just fine.
 
I like that idea about going to an auto parts store for the belt. However, I got lucky (one of the few times with a boat) and my mechanic just tightened the belt, as described above, and everything if fine. He said the belt did not receive any damage, so this was a quick and easy fix. Thanks for all the advise above.
 
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