1982 Mercruiser 470 3.7L - difference in air pressure with inboard cover on & off

[Blueghost924]

Hi - I have a 1982 Mercruiser 470 (the ol faithful 4 banger) with the 2 bbl carb. I've finished work on the exhaust, and started it up and ran it yesterday in the boat on the trailer at my house (not in the water). The sterndrive is not on. While I've been making maintenance repairs and replacing old parts, I've always started it up with the inboard cover off (to watch the engine). Yesterday, I had the engine purring at 1200 RPM and decided to put the inboard engine cover shell on while it was running. Immediately, the engine RPM dipped down a little bit and I could detect a difference in the way it was running. I believe I had to bump up the throttle a little to bring it back up to 1200 RPM. The only thing I could think is putting the hard top engine cover back on causes a change in air pressure going into the 2bbl carb. Does this make sense? Any tips, advice, etc... ? Again, this is all at the house on the boat trailer, not on the water.

 

[stonyloam]

Not a difference in air pressure, but a difference in air density. The air inside the housing would be warmer than outside air, therefore less dense. Could make a difference. Make sure you are getting plenty of fresh air in there. You could try running the blower when at low speed.

 

[JustJason]

If the engine definitely runs differently with the cover on vs the cover off chances are you have an exhaust leak and the engine is ingesting exhaust gas.

 

[dozerII]

Are you sure the engine cover is not interfering with the throttle cable in some way?

 

[Blueghost924]

I've verified the throttle cable is ok - and I don't seen any leaks from the exhaust manifold or exhaust elbow. I think I know what the issue is - the exhaust hard pipe that fits inside rubber sleeve before the shutter is off slightly. Water and exhaust have been spraying slightly off to the side and into the bilge. I've been ignoring while I've been making sure other areas are tight while running the engine. Let me lift the rear of the engine up and get that hard exhaust pipe down tight in the rubber sleeve before the shutter and then see if that makes a difference!

 

[Blueghost924]

It was definitely getting the hard exhaust pipe down solid into the rubber sleeve and making it air and water tight. Ran it with the cover on and off, and it runs the same now.

 

[GA_Boater]

Makes sense, Blue. Thanks for the update.