Hello, and welcome to Justanswer. My name is XXXXX XXXXX I will be helping you today. If you need clarification on something please post back here.
There are about 15 diffent things that can cause lack of full throttle RPMS, instead of giving you all 15, we need to do some basic checks to get us pointed the right direction.
That being said, we need to do some preliminary troubleshooting to figure out which system is causing the condition. When testing you should do it in the order I am writing it here, jumping around can have misleading conclusions.
What we need to is engine compression and spark, and you will need these tools to accomplish this.
1. Compression. Your results should be 100psi or better, all cylinders should be within 10-15% of eachother. No cylinder can be below 100psi
2. Spark. These are the testers we use in the industry.
But you can also get cheap spark checkers at most good autoparts stores.
You need to hook the spark tool up and set the gap to 7/16ths of an inch. The spark should easily jump that gap, be hot and blue in color, and be consistant with engine rpms. Weak looking yellow spark will not cut it. Items such as cap, rotor, points and plugs are once a year items, so if you have weak spark, you should start with replacing those items, spark plug wires are good for 3 to 5 years.
Also, we should check the quality of the fuel itself. Either pull a sample of it and put it in a clear jar overnight. In the morning, look for any kind of dirt/debris/water in the fuel, or any kind of seperation line.
Or, Get a small outboard tank (like a 3 gallon) and fill it up with known new fresh gas and try it.
Lastly, if compression and spark check out, we need to look at the fuel system. First we look at the boat side fuel system and test it. Here is how;
Boat side fuel system test - click here
If the problem does not appear to be in the boat side after testing, then the problem is going to be in the carbs. Carb jobs are very common on marine engines. They make up 80+% of all my work. It takes a piece of dirt the size of a piece of sand in there to gum up the works.
You will also want to have a quick look at the prop to make sure it is not all dinged up, as that will effect rpms.
After you perform these tests we will know what direction we need to go in.
The last bit of advice I have is that if you wish to work on this motor yourself, you really should get a copy of the factory service manual. That is the most important tool you will need.
Post back here with questions or test results.
Good luck my friend.
This did not address my problem.
The engines rev to any rpm, the rpm's are not stable at anything but idle. they fluctuate up and down.
I'm sorry if I miss understood your question. You are saying that you can make full RPMS under a load all of the time then?
i do believe so. i had the boat running today with the starboard engine running steady at approx 3200 and the port engine rpms were unsteady bouncing between 3500 and 3000. the sync meter would catch it when the fluctuating rpms would be 3500.
after i docked i revved the engine to 3000 with no load and the rpms would about the same, just a little less magnitude (about +-100 rpm fluctuation)
First and foremost, and I tell you this so you do not damage anything, reving an engine that is not under load is not good for the engine and can damage it, 3000 rpms is not all that bad, but it is still bad practice.
Also, reving an engine that is not under load does not tell you or me anything, as any engine will run to 5k+ rpms in nuetral firing on 1/2 its cylinders.
Now back to the first answer I gave you, just about anything in there will help you troubleshoot the problem. Troubleshooting is just as much ruling things out as it is finding the actual problem.
100 rpms fluctuation I would not worry about to much, as that will happen.
But under load a 500rpm fluctuation is something that does need to be explored further.
The first thing would be to verify that the other engine is dropping rpms, as this could simply be a case of a bad tach. Can you hear/feel the engine drop rpms? Does the boat push to port as the port engine drops rpms?
Is the problem repeatable, and is it always between/at the 3000-3500 rpms mark? If I went out on your boat could you duplicate the problem for me?
I do have more items for you to check, but I am trying to get a feel for what is going on first. Obviously this is harder for me to diagnose over the internet, so please bear with me here.
yes i can hear and feel the rpm changes. at first it looked like a bad tach, but it appears that the tach is accurate. The boat does not pull to port. i get on plane and the starboard engine runs dead on at ANY rpm say 3300, while the port "surges" between 3000 and 3500. it seems like the power is there, but the engine cannot regulate its rpm's. the same would happen if i throttled back to 2000 rpm. the port would bounce between 1800 and 2200 at a frequency of about 1 cycle per second.
i assume that the problem will still be there next time I use the boat.
it worked fine yesterday.
the rpm's do not fluctuate at all when idling when in gear or neutral.
i was fishing with the port engine idling for 4 hours before I got on plane and noticed this problem. as I said the idle is smooth.
Alright then, we can confirm the variation in rpms.
The reason why the engine rpms are fluctuating, is because something is happening to the air fuel ratio of the engine.
The engine needs a very specific ratio of fuel to air for proper combustion, if you add fuel or air, or decrease fuel or air to that ratio, the rpms drop.
If the engine has either more fuel, or less air than it is supposed to it is called rich.
Vice versa, if the engine has less fuel or more air than it is supposed to, than it is called lean.
First thing is the fuel itself. If each engine runs on its own tank, take a fuel sample from the problem engines tank and look for water. Look for debris in that engines fuel water sep filter. Run that engine on a portable tank to see if the problem goes away. If the engines share a common tank you can skip this step.
A compression test should still be done to rule it in or out. If you have a leaking cylinder, and that leak is through the intake valve, it will push extra air into the manifold and the other 7 cylinders will be lean.
If the compression test passes. Next would be to spray down with oil the exterior of the intake manifold and the exterior of the carb body, including the carb base gasket and the rod that goes through the carb that attaches to the throttle plate. If the rpms smooth out when you hit a certain area, then you have found an air leak. All air must enter the engine through the throat of the the carb, if air enters any other way then the engine runs lean. WD40 works fine for this.
If everything passes so far, then the problem has a 99% chance of being nothing more than a dirty carb. Carbs, injection systems, and fuel systems in general make up 80% of my work, it is very common.
Let me know how you make out, and as always, post back here with any questions.