Has anyone else experienced this? I have a 2004 496 Mag which only has the adjuster screw on the alternator to adjust the tension on the serpentine belt. I put new belts on this season and had them pretty tight. After about 20 hours

they loosened up a bit and they both jumped over a groove or two to the inside on the upper idler pulley above the raw water pump. It seems like that pulley does not have a high outer ridge at all. An the only thing keeping it in the grooves are the grooves themselves. When I look at pictures of a new idler pulley online, they seem to have a higher outside edge. Has anyone else ran into this? I'm nervous that if I don't check the tension of the belt every time I take the boat out, it might skip a few grooves and start rubbing on the hose clamps that hold the heat exchanger on! I'm also contemplating converting to the auto-tensioner system on the new 496 mags. I heard that the power steering pump bracket will have to be modified to do that. Does anyone have any info on that?

I would loosen the belt and check pullies to see if they wobble any. Also check sea water pump for correct alignment and no missing mounting bolts. BTW did u use OEM belt or from auto parts store. and part number of one used?

Since it only has the issue after the belt starts to get loose, then it would appear to be the adjuster. Loosen the jam nut and back off the tension and remove the belt. Then turn the adjuster bolt back where it would normally be after adjusting an see if there is any play it the threaded block and the bolt inside. Only way for it to loosen is for the jam nut to loosen.

Looking at the drawing I see that the nut shows on the other side of the block but if its jammed tight it still should not loosen

Check for play in the idler pulley's and if bracket fasteners bracket bolts are tight

The adjustment on the alternator is not moving. When I said loosen up, I meant the belts seems to be stretching. I used Gates belts from Napa to replace the belts. The Mercury belts were too short and would not fit over the pulleys. I had the idler pulley bracket off at the beginning of the season and I made sure they were tight. I don't think that is the issue. I will double check that as well as idler pulley play. I'm not sure what else to look for.

NAPA part number 25-061120.

That's definitely not my favorite tensioner system, but it does work. I wouldn't worry about it drifting aft and hitting the hose clamps for the HE. Mine never did that. The belts can stretch a little as they break in, and with the heat. You may just need to go back and do a re-check after a few hours of use. I open the hatch before every weekend outing anyhow, and that was always one of my checks. I had a temporary belt jumping (off) issue once, and it was tension related.

The reason I don't like that tensioner is I had the bolt seize in mine, and had to replace it after snapping it off. That sucked as it cost me a weekend way back in the day. If you feel froggy enough to make the changeover to the newer style, I say go for it. Once I got past my issue (in first season or two of ownership), I never had a problem for the last 10 years that I owned my 496HOs.

I put close to 15 hours on the belts after initially replacing them this season. Then on my next trip on the boat, I checked the motors like I do every trip and both motors the belts had jumped a groove or two on the idler to the aft. I would say this was my fault because the one port belt was so loose that I could move it back over the grooves to the correct position without loosening the adjuster on the alternator. The belt had been rubbing against the hose clamp for the HE. I adjusted them at that time to what I thought was pretty tight. And then just recently over the weekend when I checked before starting out, I noticed that the stbd motor had skipped one groove to the aft. And I thought they were pretty tight. I ended up loosening it to move it back and tightened it again. I tightened both sides at that time. I had no problems with about 5 hours of running on Saturday, but I'm nervous that I'm not going to catch it next time.

Belts don't loosen up that fast without something else going on, or junk belt
Both motors doing the same thing is a issue, and I'm not a napa fan because they don't have quality stuff anymore.

The belt is listed as 112x6 rib

Agree, use a straight edge

Like you said, it's happening to both motors - so I feel like the belts are stretching too much. I did contact Gates and they do supply Napa with their belts. This is their lowest grade belt. They offer the same belt in the RPM series (highest grade belt) K061120RPM which is a lot more money - $100. I might try a set of those belts. I will try a straight edge, but I don't see how both motors could be off.

rock has K061120RPM for $55