Poor Planing Merc 50L

"I have a 20 ft Celebrity with

"I have a 20 ft Celebrity with a MerCruiser 5.0L engine. The boat planes HORRIBLY! The engine dosn't seem to "kick down" or "down shift" (ignition advance??) appropriately to generate the power necessary to get the boat up on plane. Once it finally does get up on plane, all performs as expected.

The 5.0L is outfitted with a Quadrajet and Thunderbolt ignition. Prop is pitched correctly as well (18).

I do see there are 2 vacuum nipples at the base of the carb. One of which has nothing attached.

Any thoughts?"

"Down shift??? Are you trimmed

"Down shift??? Are you trimmed correctly? and check carb secondary and vacuum ports, may as well check timeing too."

Close off the vacuum port. Che

Close off the vacuum port. Check for extra weight in the back of the boat.(false floor may have water trapped under it)Make sure the drive is all the way down. Check engine compression. Are the secondaries on the carb opening and lifting the needles for the rear jets? What is the top rpm when it does get going? Should be about 4600. The advance in the distributor could be frozen. Check for a loose / worn timing chain. (late valve timing = power loss).There is no down shifting on a mercruiser. If the power is falling off as in slow to recover in a turn you could also have a blockage in the exhuast system. The flaps in the exhuast Y pipe may have broken off and are lodged in the exhuast port where it exits into the outdrive. Closing most of the exhuast port. The shift rod holds these flaps in the port so they will not be able to get out. There are many things that can cause your problem. These are just a few that come to mind.
Hope you find the cause.

"Thanks for the replies.

So

"Thanks for the replies.

Sorry to confuse people with the automotive jargin of "downshifting" - was only using as a relative description.

I believe the trim is correct when starting out = trim all the way down

I can see the secondaries open as expected. Not sure if needles are lifting in rear jets.

Once the boat gets up on plane, it performs well. Will reach 4600 rpm's and run nicely.

Not familiar with this distributor - what do I look for under the cap regarding possibility of a frozen advance (this sounds like the right direction to look into)

This is a fairly new engine with 100 "gentle" hours on it. I am not thinking the timing chain or engine compression as being an issue, but will check it out.

The exaust port suggestion may also be a good lead. Any way to test the functioning of the port valves?"

Sorry no test for exaust port.

Sorry no test for exaust port. The only way to check for this is to slide the drive off. No big deal to do that. If these flaps are loose they will be in view where the shift rod drops down in the housing. If only one is there the other one is not far behind it. Try and find it. I've had several that could not be found but sure enough would show up on the first or second time out. These are kinda hard to remove without taking the shift rod out. This is not easy at times so I use a cordless sawsall to cut the flaps down enough to remove them. Hold the flap with vise grips while cutting it. Good luck.

some questions
Is the boat ne

some questions
Is the boat new to you?
Is the choke pull off dashpot in place and hooked up?
I have not seen Marine carbs with vacuum ports. If it is a car carb it is not calibrated for marine use and is also unsafe (fire).
If it is a marine carb the power piston in the carb could be stuck thus not allowing enough fuel to enter during low vacuum acceleration. Plug all vacuum ports and try again.

"This boat is new to me.
I be

"This boat is new to me.
I believe the choke pull off is working properly, but will double check.

The carb has (2) vacuum ports for hoses on the carb base toward the front side of the engine. One port is connected to a hose....not sure where this hose leads to.

How do I check the ignition advance? Is it vacuum, mechanical, electronic?

Will check carbs power piston."

"Just had my Mercruiser 228 &#

"Just had my Mercruiser 228 (305 V8 4bbl) with Roch QuadraJet serviced. Mechanic did all kinds of things including carb rebuild., filters and ignition tuneup parts. He finally dropped in another distributor and the boat now powers onto plane without hesitation. The old distributor mechanical advance inside the distributor was not advancing the spark at higher RPMs causing power problems. So if the ignition timing is not changed (spark advanced) as the RPMs increase by the mechanical advance in the bottom of the distributor base you will loose power from the fuel-air mixture combusting too late."

Any way to actually see if the

Any way to actually see if the mechanical advance is functioning properly? What was the cost of a new distributor? Thank you for the feedback!