Looking at a 99 353, with HP500 EFIs, which years had the bad valve springs or was it carbed engines only.

So Mike, is it true that the carb motors that were still available 1st half of the '99 model year are uneffected? I'm shopping also ,and have had this valvespring issue in my head. Thanks, Randy

I wouldn't let the valve springs stop you from looking at any boat. Just prepare to change them over if need be.

If you call Mercury Racing and ask them about the valvetrains on the 500's, both carb and EFI they will tell you to replace springs at 200 hours.

Rod

I have a freind who just replaced valve springs in his 2000 500 EFI's due to breakage. Not sure of the hours but it does appear that this is a weak spot for the motor.

I've got late '95 vintage HP500's with a bit over 200 hours. They were running fine, but I figured better safe than sorry and winter is the time to do it.

I'm doing the first motor now. I wanted to check the head gaskets as well since I've been told that the stock Merc part was a bit weak. So far I've found the head gaskets leaking very slightly between cylinder 1 & 3, 3 & 5, and 4 & 6. I'm having the springs replaced and set-up by an expert this week. I'll let you know if I have any that are broken.

Then comes motor number two...

Good luck!

Jeff

jbraurer makes a good point. Have a leak down test done on the boat first. Valve springs are easy to replace.

The article in Powerboat said 99 and earlier HP500 carb motors had the valve spring issue. In late 99, Mercury changed to a different spring which resolved the issue. Powerboat article last Summer said there was a technical bulletin on it, but my Mercury guy in Northern Va couldn't find it.

I could have sworn it was too strong valve springs causing the roller lifters to break that was the problem, not valve springs breaking. I thought Scott would have jumped in by now, he went through all that problem.

TARHEEL YOUR RIGHT ON ALL COUNTS ,,,USE 929 COMP SPRINGS
THATS WHAT WE'VE USED AND THEY STAY IN THERE !!