84 Mercury 200 HP - Low RPM at Wide Open Throttle

wcsd106

Petty Officer 2nd Class
Joined
Mar 27, 2010
Messages
182
Hello All,

Last summer I rebuilt all three carburetors on my 1984 Mercury 200HP V6 outboard. After reinstalling the carbs and synchronizing the timing and carbs per the OEM Shop Manual for my engine, I noticed that my WOT RPM was low. A description of the problem is listed below:

  • Outboard starts and idles without much difficulty
  • Outboard will rev into the mid/high 5000 rpm range i.e. 5500 or so during the hole shot
  • Once on plane, the outboard seems to lack power and will only achieve around 4400-4800 rpm on plane

Here are the testing procedures I have done so far:

  • Checked for spark on all 6 cylinders (While Idling)
  • Checked and rechecked that the carburetor butterflies are opening completely at WOT
  • Checked spark plugs, wires, etc for cracks or damage
  • Verified WOT timing at 23.5 degrees (Measured at cranking speed)

A local marine mechanic told me today that he believes it is a fuel related issue and that possibly there was some miniscule piece of trash still in one of the carburetor passages when it was cleaned and rebuilt. He recommended that I try Starbrite Star Tron additive during my next tank of fuel.

Today I drained the old fuel and removed the water separating filter. I will be fueling up with new fuel tomorrow along with the Star Tron additive. A new water separating filter will be installed at the same time.

I am hoping the weather will allow for a test run tomorrow so that I can report any changes.

Any ideas as to what else it could be would be greatly appreciated. For instance, is it time to break out my DVA meter and check the stator and trigger again?

Thank You for any help you may be able to provide.
 

racerone

Supreme Mariner
Joined
Dec 28, 2013
Messages
36,033
Re: 84 Mercury 200 HP - Low RPM at Wide Open Throttle

You likely did a compression test on this motor , what values did you get ?
 

wcsd106

Petty Officer 2nd Class
Joined
Mar 27, 2010
Messages
182
Re: 84 Mercury 200 HP - Low RPM at Wide Open Throttle

Racer, I did. I am leaving for work at the moment, but will post the results in the morning. It was within spec.
 

wcsd106

Petty Officer 2nd Class
Joined
Mar 27, 2010
Messages
182
Re: 84 Mercury 200 HP - Low RPM at Wide Open Throttle

Racer,

Here are the results of the compression test.

Test Conditions:

Motor - Cold (Ambient Air Temp 75 Degrees F)
All Spark Plugs Removed
Throttle / shift lever in Neutral (Carb butterflies closed)
Ignition switch OFF
Remote Starter Switch connected to Starter Solenoid

Results:

Starboard Bank:
Cylinder 1: 130 psi
Cylinder 3: 130 psi
Cylinder 5: 127 psi

Port Bank:
Cylinder 2: 127 psi
Cylinder 4: 132 psi
Cylinder 6: 127 psi

Would it be beneficial to start the engine up while on the flushing muffs and allow it to reach operating temperature (start peeing) and then repeat the compression test? Also, I have read conflicting stories as to whether having it at full throttle or not (carb butterflies open or closed) will affect the test results. I can repeat the test under different conditions if needed.
 

Chris1956

Supreme Mariner
Joined
Mar 25, 2004
Messages
27,035
Re: 84 Mercury 200 HP - Low RPM at Wide Open Throttle

Are you trimming the motor for max RPM once on lane? Is the hull straight and true? A hooked hull can cause extra drag.
 

wcsd106

Petty Officer 2nd Class
Joined
Mar 27, 2010
Messages
182
Re: 84 Mercury 200 HP - Low RPM at Wide Open Throttle

I am starting with the engine trimmed down. As I accelerate and come up onto plane, I begin trimming the motor up. Once fully on plane, I begin to accelerate and have tried trimming the motor up and down in an attempt to get the RPMs up.. I know where my sweet spot USED to be, but it's not there anymore.

The hull is in good shape and appears to be straight. It's a fiberglass v hull bass boat.. I've looked all around it and can't find any problem areas.

It's been suggested that it may be time to take a trip to the lake and bring along the timing light to check that all 6 cylinders are firing at WOT. Weather permitting, that is now on the agenda for tomorrow. Will be fueling up with new gas/oil, some Star Tron, and a new water separating filter installed. I may also steal the tank from my john boat and bring it along with a different fuel line just to rule out those types of problems.
 

Chris1956

Supreme Mariner
Joined
Mar 25, 2004
Messages
27,035
Re: 84 Mercury 200 HP - Low RPM at Wide Open Throttle

Take a look at the spark plugs after running at high speed. If you have one that is real clean, like new, you may have a water leak into the cylinder, killing the power that cylinder would make.
 

wcsd106

Petty Officer 2nd Class
Joined
Mar 27, 2010
Messages
182
Re: 84 Mercury 200 HP - Low RPM at Wide Open Throttle

Unfortunately, the weather was not cooperating with me for a water test today. We are under a lake wind advisory and the winds are gusting around 30mph or so. While I would fish in that type of weather, I didn't feel comfortable making WOT test runs while the lake was white capping.

I did take some time today to test the fuel pump according to a procedure I saw here in a sticky at iboats.

Here are the results of testing the fuel pump:

First, I replaced the line running from the engine side of the primer bulb to the engine fuel connector with a section of clear vinyl tubing the same diameter and length. I squeezed the primer bulb until it hardened up and then fired up the engine while on the muffs. I was looking for air bubbles in the clear vinyl tubing while the engine was running. There were none.

Afterwards, I connected my combination gauge (Vacuum / Pressure) between the engine fuel connector and the primer bulb using a tee fitting and two short sections of vinyl tubing. While the engine was running and up to operating temperature, I squeezed the section of tubing between the tee fitting and the primer bulb to cut off the fuel supply. The gauge read approximately 3 inches or so of vacuum which, according to the procedure I was going by was on the low side of acceptable.

One thing I did notice was that when the tee fitting was installed, I saw an almost constant stream of air bubbles coming from the primer bulb to the tee fitting. This was not present before placing the tee fitting inline.

I then removed the tee fitting and fired up the engine one more time. I notice that if I squeeze the primer bulb while the engine is running I can get a stream of air bubbles, but those bubbles quickly dissipate.

When I did a visual inspection of the fuel pump, I was unable to locate any tears, pin holes, or rips in the pump diaphragms. The screen at the input of the pump had a very small amount of trash in it, so I cleaned it out and put everything back together.

It looks like Saturday morning is going to be my first chance to get out onto the lake and test the engine at WOT. I am wanting to get a list of all the on-water tests I should perform so that I can get them all taken care of while testing. I'm hoping that the problem will have mysteriously disappeared and can be attributed to a clogged water separating filter or a bad primer bulb (I just replaced the bulb this year because the old one had gotten too hard to squeeze)

So far, these are the tests I'm planning on conducting while on the water:

1. Over the course of 6 WOT runs, observe a timing light to make sure all 6 cylinders are firing while the throttle is wide open and the engine is running under load.
2. Disconnect the yellow wires connecting the stator to the rectifier and then perform another WOT test run to see if the problem disappears.
3. If the WOT RPM is still low, press and hold the enrichner switch while running at WOT


I would appreciate any other tests that may help to diagnose this problem.

Thanks
 
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