RPM's drop off in Mercrusier 120 (1972 vintage)

[islander46]

My Mercrusier in my 1972 (?) Reinell 18' speedboat with a Mercruiser 120 HP out drive has a mystery problem: Starts and runs great. After leaving the harbor, it runs great for 8 minutes or so at higher rpm's (under way) and then the rpm's begin dropping. They continue down until the boat is well off a plane (plowing through the water) and will keep running like this for a long time. After this happens, when the throttle is pulled back it runs very rough.

If it shut off for a long time (at least 30 minutes) it will start the cycle all over again. I have replaced the fuel filter, put in a temporary electric pump to a seperate fuel tank (to confirm I didn't have a fuel hose/pump/fuel delivery problem). I have replaced points and plugs, coils, plug wires, condenser, rotor and cap.

I have tried swapping props (to reduce the amount the engine has to push) to no avail. I should add here that we have lost track of the original prop, and have an aftermarket prop on. While I thought I was doomed with the prop change and losing the original, keep in mind that when the engine runs wellt the boat zips along just fine. It is after this specific amount of time when it acts up that the rpm's drop.

I have had this going on for two seasons now, and am very frustrated (as you might imagine after the list of trials and errors above!) I will appreciate any suggestions and will do my best to answer any questions. As Lake Superior is still frozen, I have a few weeks before it will be in the water!

Thanks in advance!

Bob T

 

[Don S]

Re: RPM's drop off in Mercrusier 120 (1972 vintage)

http://forums.iboats.com/showthread.php?t=167561

 

[islander46]

Re: RPM's drop off in Mercrusier 120 (1972 vintage)

Don- Thanks for taking the time to lend a hand. I saw this thread when looking through the earlier posts on this web site, and see you are a steady contributor.

While I did not test the fuel system with a vacuum pump, here is what I did do (which I felt eliminated the fuel issue): I bought an electric fuel pump at NAPA and ran a temporary gas line to a 5 gallon can of gas. This fuel pump bypassed the engine's fuel pump completely. I took the boat out with this temporary set-up and had the same problem: It slowed down after 8 or so minutes with a short clean gas line, seperate fuel tank, and the temporanry in-line electric pump.

Actually, I thought the fuel line itself might be delaminating, and that is what prompted me to bypass the boats fuel system completely.

Do you agree that this eliminates the fuel system as the cause of the problem?

Thanks again for your help!

Bob T

 

[Don S]

Re: RPM's drop off in Mercrusier 120 (1972 vintage)

You seem to be throwing a lot of parts at it without finding the prolem. Have you checked fuel pump pressure when the problem occurs? and while you have used an electric pump, give the gauge a try "Under Load" not a simulation. Yea, it might be a while before you can do that, but ??????
have you checked the fuel filters in the fuel pump and the carb?
When you have the problem, How is the spark on all the cylinders. How is the fuel flow to the carb, if you pump the throttle, does gas squirt into the intake from the accelerator pump?

 

[JustJason]

Re: RPM's drop off in Mercrusier 120 (1972 vintage)

theres a filter inside of the mechanical fuel pump... don't believe there is one in the carb.....
If the time is pretty consistant... in between when it slows down. I'd check the coil and the condenser.... run it around the lake untill it starts actin up... then recheck the coil and condenser...
it's an old boat... does it have a steel tank?

 

[fishrdan]

Re: RPM's drop off in Mercrusier 120 (1972 vintage)

Here's a thought, when the bogging problem occurs remove the spark arrestor off the carb, try to get the boat back up onto plane and slowly squirt a fine stream (a very small amount) of fuel into the carb. If the engine picks up it was starving for fuel and you need to look at the fuel system, if it stumbles more it was getting enough fuel and the problems are elsewhere,,, ignition.

Caution/Disclaimer: Only do this with 2 adults in the boat (NO KIDS), make sure no one gets close to the engine's moving parts or over the carb and DON'T use a container (IE:beer bottle or Big Gulp cup) that can spill the fuel. I wouldn't want to see someone get hung up in the engine, missing eye brows or burning the boat to the water line!!! USE AT YOUR OWN RISK

 

[JustJason]

Re: RPM's drop off in Mercrusier 120 (1972 vintage)

oh yeah after it starts actin up check the ballast resister to

 

[islander46]

Re: RPM's drop off in Mercrusier 120 (1972 vintage)

First I want to thank everyone for contributing.
I am not sure my comment regarding my temporary fuel system have been understood or noticed: The fuel system that is built into the boat was bypassed completely up to the carberator by using a fresh fuel tank (a five gallon gas can), an electric fuel pump with a line from the five gallon fuel can through the pump and into the carb. So I don't think it is the fuel system. I agree with you on the filter in the carb comment: I don't see where there could be one there.

The comments posted going to the ignition interest me; particularly the ballast resister one. How does one test a ballast resister? By measuring resistance with an ohm-meter? If the ballast resister was bad, would it behave like this (malfunctioning after the engine gets nice and warm)?

capJason suggested the coil and condenser be checked. I have replaced each of these once already. Would you recheck them if the symptoms remained the same after replacement?

It has been suggested to me that there are two "weights" in the distributer, under the mounting plate foe the points... some kind of timing advance device or something like that, and they may be rusty and not moving freely. Does this sound familiar to anyone?

Are their any other distributor issues that you guys have run across that would cause these symptoms?

Once again, I thank you for your time a consideration here guys. This has been perplexing me for quite a while now. Hopefully I can solve this with your help and advice.

-Bob T

 

[Caveman Charlie]

Re: RPM's drop off in Mercrusier 120 (1972 vintage)

There are weights in the distributor. It's called the mechanical advance. There east to check just remove the cap and rotor and you should see them. There may be a plastic plate covering them up. But, there was none on my boat. I don't see how these could be part of your problem though.

Did you give it a good general tune up. Plugs, wires, points,and cap?

Are you sure the points are adjusted correctly? A friend says a problem similar to your can occur if the points are bad and this makes them over heat. Mis adjusted or burned points could be the problem.

Has anyone been doing wiring repairs on the engine? I ask because there is suppose to be a resistance wire inline to the points. If someone removed it the points will be getting full 12 volts while running and this will burn them.The correct wire itself should have printing on it but, it may be hard to read.

Are you sure your little electric fuel pump could pump enough gas to keep up with the motor at full throttle?

If you run it on your muffs does the engine seem to smoke bad? I ask because I wonder if it's running really rich?

Remember, I'm not a mechanic just another boat owner like you!! I'm just pulling ideas out of my head. (some might say I'm pulling them out of somewhere else lol)

 

[Bt Doctur]

Re: RPM's drop off in Mercrusier 120 (1972 vintage)

what about the filter IN the carb body?was the carb rebuilt recently.
float needle swelling/sticking from methanol fuel?

 

[Maclin]

Re: RPM's drop off in Mercrusier 120 (1972 vintage)

When a coil is supposed to have a ballast resistor or resistor wire but is actually getting a full 12 volts it can run good for a bit then get hot and act like that especially in a points system. Be double sure the coil ballast wiring and components are correct. Also be sure the plus and minus wiring is correct on the coil. When this is wrong they can still work but not support all rpm's under load.

Once the symptom takes over can you tell if the carb has enough fuel in the bowl(s)? If the fuel level is ok in the carb (not way low or too high and spilling over into the engine) then it sounds more like an electrical component is getting hot or overworked.

I suppose there could be an internal problem with the carb that could act like that, but that is just a wild guess.

There can be a small inline type of filter in the carburetor fuel fitting itself. These are small and usually copper colored and feel like they are made of metal. Usually when these clog they stop all fuel flow but sometimes they can clog gradually under pressure and once the pressure goes down the junk moves around or something, I am not quite sure what the dynamics are that can cause this symptom. I have a diagram attached, note that it shows a little bit different type of element than I was describing but the configuration is typical.



Typical carb fitting filter layout....
http://img.villagephotos.com/p/2006-12/1234103/carbfilterdiagram.gif

 

[Maclin]

Re: RPM's drop off in Mercrusier 120 (1972 vintage)

Hey Bt Doctur,

We are thinking the same thing, your post was not there when I started....It took me a few minutes to get all of mine typed in and get the pic....!

 

[fishrdan]

Re: RPM's drop off in Mercrusier 120 (1972 vintage)

Reason for spraying additional fuel into the carb is to diagnose the problem. Even though you have a separate tank, hose and fuel pump, not everything in the fuel system has been eliminated. Maclin mentioned the carb overflowing, spraying in additional fuel will make the engine run worse if that's happening...

Couple of questions, did you use marine parts specific to your 120 Mercruiser or generic auto parts? Was the coil for a 4 cyl engine? Did you adjust the points/dwell with a dwell meter or just feeler gauge it? Set the timing with a timing light? Gap the plugs? Just want to make sure you didn't introduce a problem while trying to repair a problem. Hate to admit it,,, but I've done it before.

You mentioned it will run fine for 8 minutes, bog, and then after 1/2 hour of being shut off (cooling period) it will run fine again for awhile. This does sound like the ignition system,,, but I have seen stranger things. Here's a few other trouble shooting steps, sorry, on the water again since this is the only place you can duplicate the problem.
1- hook up a volt meter to the coils + post and ground, then monitor the coil voltage while cruising around, it should be 9-10V (from what I remember), but not battery voltage 12-13V. Probably best to make some long leads so you can put the volt meter on the dash of the boat. If you see a voltage drop when the problem occurs you know it's an electrical problem.
2- Right after the engine starts bogging connect a jumper wire with alligator clips between the coils + terminal and 12V to bypass the boats wiring. (CAUTION/DISCLAIMER, THE KEY IGNITION SWITCH WILL NOT SHUT OFF THE ENGINE, YOU WILL HAVE TO PULL THE JUMPER WIRE since it's powering the ignition.) Does the engine run fine now? Yes = you have a problem with power to the ignition, loose/corroded wiring, ballast resistor, ignition switch. Remove 12V jumper wire after testing so it does not over heat the coil.
3- If everything looks fine but the engine still bogs, take a can of "canned air" and shoot it on the coil for a minute or 2 to cool it down rapidly, then see if the boat runs fine. Even though you replaced the coil doesn't mean the new coil is good.
4- (I don't know if this is going to help your problem, but I'll throw it out anyway) When the engine is running rough connect a timing light to the coil wire and look for a steady strobe, or firing of the coil. Next, connect the timing light to each spark plug wire one at a time to see that each plug is firing steady. Is the coil missing? Is one of the plugs missing?

Yes, check the distributor weights to make sure they move freely and that the springs aren't rusty and working properly. On my 73 140HP the weights and springs were rusty and limiting travel. The distributor bushings were also worn out in the housing causing erratic timing/dwell so I replaced the distributor. Though as Caveman said, I don't see how this could be your problem since it should run bad all the time.

 

[islander46]

Re: RPM's drop off in Mercrusier 120 (1972 vintage)

Once again I thank everyone for their time and expertise.

While I felt like I had eliminated the fuel system as the culprit, I see there are other considerations I had not thought of; particularly in the carb. I will be looking for the filter in the carb, which I thought I had checked for already, but I'll give it another shot. As far as checking whether or not it is getting under or overfilled, I am not sure how I would go about this... do I open the carb once before leaving the dock to loosen the screws/gasket, reassemble, and when it acts up take the top off the carb to see if the bowl is real full or empty?

Regarding the ignition questions from fishrdan: Over the years there have been a number of explorations by various mechanics, including myself. So the questions about whether or not marine parts were used is a good one: I don't know. Also I don't know if the coil was specific for the 120 4 cylander engine. I will double check this.

The last time we adjusted the points we used a feeler gauge. I don't know about the timing light, and we did gap the plugs.

Am I correct in understanding that resister wiring will have some marking on it to identify it?

Thanks for the details on what readings to watch for when checking the coil. These details are what I need now!

Maclin, you mention the coil + and - need to be on the correct terminals. Thanks for mentioning this, as they may very well be reversed.

So right now it is a full raging blizzard with 6 inches of fresh snow outside. While the late season snowfalls melt quickly, it is a bit discouraging. It's nice to think about my speedboat and getting out on the water, hopefully sooner then later!

Thanks for everyone's help, and when it warms up a bit I will be digging into the ignition system and the carb as suggested. Any other thoughts, please post!

-Bob T

 

[trm]

Re: RPM's drop off in Mercrusier 120 (1972 vintage)

BobT,

My 120 was doing the exact same thing your's is at the end of last season, so I am VERY interested in how you solve your problem. Please be sure to come back with your solution.

Thanks,

swabbie

 

[captk]

Re: RPM's drop off in Mercrusier 120 (1972 vintage)

About 100 years ago a whizzend old boat wrench gave me this sage advise." Son when every thing on a pt, fired marine engine looks for all the world like a fuel problem, GO TO IGNITION. ok so maybe it was only 40 or so years ago.

 

[pktaske]

Re: RPM's drop off in Mercrusier 120 (1972 vintage)

Did this problem ever clear up?