posted 03-18-2011 06:28 AM ET (US)        

I took it to Mercury dealer and they put the fuel pressure gauge on that fuel regulator and it was fine. He said sometimes they get stuck and run rich. He played with the TPS but really nothing significant came of it. He tried to pull the plug wires off one at a time but shocked himself and decided to abandon that line of troubleshooting.

Here is my question: What is actually happening in the combustion cycle to make it vibrate so much and how is this engine designed in terms of its fuzzy logic to adjust itself for smooth operation?

My buddy thinks it is running uneven fuel/air mixtures between the cylinders and some are firing harder than on others which is making it shake. He thinks there are four diaphragm pumps that attach to runners on each side of the intake that could be leaking or the injector o-rings are leaking air. At high speed it isn’t mattering as much. At low speed its messing with the air/fuel ratio.

posted 03-18-2011 07:52 AM ET (US)            

Until then, I would recommend you do what most good mechanics would and that is check the basics to start with just to make sure it isn't something less complicated to diagnose than failed parts within the FI system. Often, it isn't. There is a tendency I've noticed for people to immediately jump to suspect FI problems on an FI equipped engine. This is a mistake. Most outboard FI systems have proven to be very reliable. Mercury's FI systems are no exception.

There are many things you can do and should do first that are free (if you are mechanically inclined) or cheap even if you have to hire someone:

If there is any downloadable data, do it (I honestly don't think so but don't know for sure with that particular engine). Ensure that you are feeding the engine good, water-free fuel. Give the engine a THOROUGH visual inspection in an attempt to find loose connections, broken wires, out of place links, disconnected hoses, etc. Take your time with your visual inspection! It's free. Put your hands on everything you can reach. While maintaing pressure via the primer bulb, continue your inspection and look for any leaks in the fuel system. Use a flashlight if necessary.

Check to see that your propeller shaft is true and that your propeller is true. Check to see that your engine is securely mounted to the boat transom. Try to twist, push, pull on the engine (engine not running of course) to see if you've got any obviously broken engine mounts. The engine should move only slightly, if at all.

Warm up the engine and then check engine cylinder compression and also spark quality. Carefully inspect the plugs as you pull them out to look for clues as to differences between cylinder running qualities. Tap plugs into the palm of your hand to check for evidence of water leaking into a combustion chamber.

posted 03-18-2011 09:22 AM ET (US)            

Boat vibration results from the complex environment of the engine, the elastic isolators in the engine mount, the propeller, and the boat being mechanically coupled together. You may have a combination of all of those elements that is causing the vibration to be amplified by a resonance. Try changing the propeller to one with a different number of blades. This should move the propeller vibration to a different frequency.

The engine speed range of 1,500 to 2,000-RPM for many boat-engine combinations is often a transition to planing from displacement. The engine works hard in this transition, and it may be laboring. It is best to give the engine plenty of throttle advance to get through this engine speed range as fast as possible. If you restrict the throttle advance and keep the engine in this speed range, the engine may run roughly and stumble due to the heavy load on it.

I don't know the technical explanation for effects of a heavy load on the engine's combustion cycle, but typical effects when an engine is trying to accelerate with a heavy load and lacks the torque to increase its speed are for the engine to stumble or miss, and for the exhaust to become sooty.

To reduce the load under acceleration for an engine, reduce the propeller pitch. If you feed like the propeller pitch is proper, try venting the propeller hub. If you have a Mercury propeller it may already have holes fabricated in the hub for venting; replace the plastic plugs with vented plugs.

The heavy load during acceleration onto plane will reveal any weakness in the engine, and the vibrations you are experiencing may be due to a problem in the engine. From your narrative, I would not characterize your present mechanic as particularly skillful.

posted 03-19-2011 06:07 AM ET (US)            

Just a point of clarification. Engine does not lack power coming up through the rpm. The vibration is there at those rpm regardless of how fast I advance the throttle. Albeit for a very brief period of time if one where to floor it. Boat leaps on plane with no issues. I realize this may not be the correct analogy for the design of this motor but its almost like it vibrates until the secondaries kick in.

Moreover, while its less than silky smooth at say 900 rpm, the real vibration that says I've got to get this fixed is between 1000 and when it planes or approximately 1800. Because the boat transitions from displacement to planing in the 1400 to 1800 range, I can't say I spend alot of time there to give the exact point where vibration stops. I'm also going to split hairs here and report that when you come off of plane to an approximately 1100 rpm throttle setting it seems to be somewhat vibration free for a second or two until everything catches up. But I admit, I have not concluded this under strict testing conditions. Its merely anecdotal.

My only thought on the shaft/prop balance issue is that I would suspect the vibration would get worse with speed and not stop all of sudden at about 1800 rpm. Wouldn't it be shaking like crazy at 5400 rpm?

posted 03-19-2011 08:16 AM ET (US)            

At this speed the engine is turning anywhere between 1,500 and 2,000 RPMs depending on traffic. There is a fairly heavy vibration in this range and I work the RPM around it as well.

This boat and motor combination like others I have been on are similar in that they share the distinct Mercury V6 vibration in this range. Mine rattles everything from the outboard to the pulpit at certain RPM's.

I have own this boat for four years and was a passenger on the boat for two years prior and it hasn't changed. The engine is very smooth and responsive otherwise.

I should also mention that this Mercury 225 received the best of care including several visits from a Mercury technician on several occasions while in the hands of the previous owner.

The trim motor on the Mercury 225 EFI is nothing short of a small engine starter motor. It will have a significant current draw while operating the hydraulic tilt and trim pump. This current draw is much greater trimming the outboard out (away from the boat) and compared to trimming the outboard in. This current draw will place a load on the engine alternator and reduce the RPM by 50 to 100 depending on the running speed at that time.

As for the trim tab pump, that pump motor can be compared to a automotive heater blower motor. It will draw much less current and is not nearly as noticeable when operating in either direction.

posted 03-19-2011 02:33 PM ET (US)            

How it works;

Except for DFI, all other systems still ultimately deliver fuel to the cylinder via vacuum.

Your crank case creates vacuum as the piston rises in the cylinder. This vacuum acts upon the reed valves, which allow air/fuel in, but now create a vacuum up stream of the valves which sucks air through the only available opening, the carb throat. Here is where the problem occurs.

At idle, the carb butterfly is open just a crack. There is an air bleeder (hole) on the butterfly but just ahead of and just below the butterfly leading edge is a series of holes that create a passage called the pilot (Picture a hwy overpass with an off-ramp/on-ramp where there is a road block on the overpass and traffic gets by via the off/on-ramp. With the butterfly closed, vacuum draws air in via the pilot system exit hole, which varies in size based on the position of the pilot screw (mixture). Open the mixture screw and there is more vacuum in the bypass, close the screw and there is less vacuum, since the screw basically acts as a valve. In the pilot bypass, there is a passage that goes down (vertical) into the carb bowl. In the bowl, the passage ends with the pilot jet and vacuum inside the pilot system siphons gas through the pilot jet, up the passage (against gravity) and into the pilot bypass. On the upstream side of the bypass, there is a series of holes (air inlet) that allow air in. As the throttle is advanced, the butterfly edge sequentially passes over these holes, they transition from being upstream to downstream of the leading edge and thus once the edge has passed over all the holes, there is no longer a pressure difference between the two sides of the pilot bypass and it essentially cuts out. At this point the butterfly is open enough that the air is sucking gas in via the main nozzle.

Based on the above, you can see that the "mixture" does not control the amount of gas relative to air, but the amount of ail/gas mix relative to air. Secondly, as the inlet holes are closed off one at a time by the advancing butterfly edge, but vacuum remains constant via the mixture screw opening, this forces additional gas via the pilot jet to compensate for the diminished amount of available air. At this point, the mixture going into the bypass has become relatively rich, which is good for proper acceleration. But, if you "cruise" for any period of time longer than a few seconds at this position, the mixture is too rich for smooth operation. Unfortunately, in the case of most carbed outboards, this is in this 1200-1800 rpm range, and unfortunately, this is also the range at which most motors operate in the dreaded "5 MPH Zone".

The pilot system was designed to allow for an engine to have the ability to achieve a smooth idle speed at which the engine could operate as well as allow a separate system to take over at higher speed/load requirements. If you had an automobile carb form the 50's/60's (pre EPA) this is basically how they were designed.

This system is designed so that the standard operating procedure is to accelerate through the transitional range rather than operate in it. As is, much engineering and testing has gone into the design of these pilot systems to make them as smooth as possible, the remaining flaws are just the nature of the beast.

I have been able to get rid of most of my vibration by adjusting the pilot screws to run rather lean at idle since this is the most forgiving speed range, but I constantly monitor my spark plugs for color and deposits.

With an EFI system, the amount of fuel/mix is regulated by the ECM so it is impossible to manually adjust.

posted 03-20-2011 08:24 AM ET (US)            

I don't know the precise details of the failure of the engine mounts on L H G's engines, but I suspect that the elastic or rubber engine mounting components probably needed replacement. The engine continues to be operable even with the deteriorated engine mounts, and, due to the significant expense, I believe L H G was just living with the problem. The last time I spoke to him about this problem he had not had the required repairs performed on the engines.

posted 06-11-2011 12:22 PM ET (US)            

After launching the boat and allowing it to warm up, the difference was dramatic. I can not say [the engine running characteristic with regard to vibration] is perfect, as there is some vibration above 1,000-RPM and below 1,800-RPM, but not so much [vibration] that I can not attribute [the cause as being natural for] to a 13-year-old two-cycle engine.

posted 06-11-2011 03:06 PM ET (US)            

Were the springs which you used to alter the poppet valve a normal service part available from Mercury Marine? Perhaps you could mention the source of the springs you used to change the characteristics of the poppet valves.