Back pressure?

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Maestro
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Joined: Sat Jul 12, 2014 7:27 pm

Back pressure?

Postby Maestro » Fri May 26, 2017 7:58 am

Recently my BigM seems very finicky about the exhaust outlets being submerged.

Under typical conditions they sit 2-3 inches above the water.

When we back it down the boat ramp, she won't start if the outlets are under water. Then, a few days ago we had some extra people on board which caused the exhaust to be slightly submerged. Boat ran fine at speed but was fussy at idle speed. Running sort of rough and with delayed throttle response.

Did a test run last night with just me and the admiral aboard and she ran fine.

My carb and distributor both need a little work so I'm sure that's a factor, but what role could mufflers and back pressure be playing here? Mufflers are original and baffles could be having issues.
Maestro
1970 28' Express
Single Chrysler 440
Mississippi River

Fastjeff
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Posts: 867
Joined: Sat Jul 12, 2014 5:06 am

Re: Back pressure?

Postby Fastjeff » Fri May 26, 2017 5:53 pm

Weird. You might try this: With the outlets high and dry, and the motor warmed up and running, have someone block the opening with a rag in the opening and see how the motor reacts.

If the idle goes away within a few seconds, you're not imagining it. It not, look for something else (like carbs not as level with extra weight back there. Mine weren't level with the boat sitting "level"; they were tilted backwards.)

Jeff
"We live at the bottom of an ocean of air, not at the top." General Marvage Slatington