1976 Chevrolet 250 I6 question

[mjsmith1223]

Got a question for all you experts on the classics.

 

My boat is powered by a 1976 Chevrolet 250 I6.  It's a Mercruiser setup tuned for 165 hp.  About halfway through the boating season (July-ish) the motor will start to tap after we run it hard for 10-20 minutes.  When I back down on the throttle and let it idle along for 20-30 minutes the tapping goes away.  The motor has done this for the past 2 seasons.  April-June, no tapping, July-October, tapping after running hard.  Any ideas what may be causing this?

 

Mike

[76BBSub]

I'm no I-6 expert, but it sounds like your lifters are collapsing, although I have no idea why it only does it at certain times of the year.  Maybe when it gets hotter in the summer the oil is thinner and the lifters don't pump-up properly?  Have you checked the lifters?  You might try a little thicker grade of oil, or run some Marvel Mystery Oil on your next oil change, it helps clean up the sludge that can build up in your valve train.

Jeff

[mjsmith1223]

Thanks for the thoughts.  Since I'm fixin' to change the oil and park it for the winter I'll give that Marvel Mystery oil a shot and see what happens next season.  It might be that I'll have to start changing the oil half way through the season.   Might need new lifters too.  On that motor it doesn't look like too big of a deal to change them out.  There's a couple of access panels on the side of the block that come off revealing the lifters and push rods.  Maybe I'll do that next spring.

 

Mike

[76BBSub]

If you do put the Marvel in it before you park it, try to run it until it reaches normal operating temp before parking it for good.  This will allow the Marvel to circulate into the engine and if you have some crud built up it will be working all winter.  It works best when hot, during operation, but it won't hurt to let it sit there all winter doing it's stuff...

You are probably right about the lifters, and they should be relatively cheap and easy to replace.

Jeff