View Full Version : Starting issue since engine refurb and cosmetic...

05-06-2012, 09:47 PM
Ever since we did a small refurb / cosmetic upgrade to the ole 84 S&S 351w, we had a problem with getting the engine to crank without giving the motor one shot of gas. We had everything set stock - mixture, idle, timing. Of course we have some aftermarket parts including GT-40 heads and a Edelbrock Performer intake. However, we figured that stock settings should still apply for sheer craning purposes. In either event, we have compensated for our problem for the last year and a half by having the mixture richer by several turns. Stock I believe is about one and a half turns out on each side. We had it compensated by having both mixture screws richened to about 4 and a half turns out. Also we had the idle turned up a bit. In either event, this set-up got old real quick. The shifts forward and backward were a little too rough and the bottom line is that it just didn't seem right.

This weekend we finally figured out and solved the problem almost thinking a good bit about our issue. After much consideration we realized or at least thought we had a vacuum leak of some kind. We checked everything and could not find the leak. The last thing we were going to do was install new intake gaskets. Thank goodness before we did that we noticed where the "vacuum leak" was. You may have seen a photo of our engine posted here before after the refurb. If you have not, see this photo...


Before I state where the vacuum leak is, can you see it? One of the cosmetic pieces we installed when we refurbed the motor was the cobra valve covers. Now can you see it?

On the right side, we have a Moroso breather that installs over a long neck oil fill tube. On the left side, however, the long neck tube will not fit and the same breather will not fit either. On the old set-up, we had a stcok set of valve covers and a stock intake. With the stock intake, you can use a spacer plate under the carb with a vacuum hose that runs too your PCV valve that installs in the stock valve covers on the left side.

When we installed the new covers and intake, we could not find a gromet that would fit a standard PCV valve. So we simply installed a breather with a breather hookup nipple. We also had to get rid of the spacer plate to allow the engine cover to fully close since the Edelbrock intake has a higher rise. I did not think about it at the time, but the breather I installed instead of a PCV valve allows air to freely flow without any resistance unlike a PCV valve which has a check ball set-up inside to prevent too much vacuum from escaping while at low RPM. The breather that I installed is actually designed to plug up to the air cleaner above the carb, NOT to a carb vacuum source itself. Well stupid me in trying to make everything look clean hooked he breather directly to a carb vaccum source on the base plate itself.

Well thank God we finally realized that this was the issue. After pulling the breather line from the carb and plugging that vaccum source, we set the carb and idle to stock and the boat fired up immediately and set right down to a perfect idle. Smoothest forward and revers she has ever had and slowest idle speed ever too. Run stronger and better than ever as well. The problem was obvious. We had to richen the mixture a significant degree to compensate for all of the air the carb was sucking from the valve cover itself. Once the carb stopped sucking so much air, the mixture was perfect at stock settings. Love it when you find out what you engine problems are and fix them. Great day!

05-06-2012, 10:16 PM
Good find, iirc pcv system has to be a closed loop type on a boat so you also need get rid of the open breather and get a closed type to hook up to the flame arrester. I believe the instructions that came with your valve covers went over the closed loop pcv system, That way will work correctly as its still pulling fresh air into the engine its just not legal.

I had a similar time finding grommets and breathers to work when I replaced my covers with ford racing ones, ultimately mr. gasket had correct grommets and breathers.

Here in an 83 ss 351w pcm engine pic you can see how the pcv is hooked up...