Johnson stutter problem at high 2700 RPM

[crabby_bob]

My brother and I recently bought a boat that came with a 1988 120hp Johnson motor. We had our maiden voyage last Sunday and noticed something peculiar that we're hoping someone here can help us with.

The first few times we rev'd up the engine we were able to get on plane at around 3600RPM. However after going at a moderate speed (less than 2600 RPM) for a an hour or so we were no longer able to get the engine past 2600 RPM. It would kind of stumble and misfire.

We headed to shore where we removed the cowling and checked for broken wires, lose connections, disconnected vacuum lines and things like that. We put the cowling back on and the boat ran great again. We figured we had "fixed" the problem but a little later, after going at a moderate speed for a while the problem came back. We pulled over again and just let the boat sit for about 10 minutes and the boat ran great again.

We were pulling the boat out of the water and while my brother was gone to get the trailer I tested the motor by getting it on and off plane a few times. It worked fine. Then a large boat came by kicking up a big wake which caused our boat to rock side to side at a pretty sharp angle while idling. After that I couldn't get motor back up above 2600 RPM. We're thinking this might be a carburetor float problem and could use some advice on troubleshooting.

Thanks,
bob

 

[tashasdaddy]

Re: Johnson stutter problem at high 2700 RPM

start with the basics, compression and spark test. could have ethanol poisioning, which could damage the motor, due to oil not getting to all cylinders. do some reading here.

http://forums.iboats.com/showthread.php?t=299680

that is one great motor, should be turning 5500-5800 rpms at wot.

 

[crabby_bob]

Re: Johnson stutter problem at high 2700 RPM

Thanks for the link Tashasdaddy. I should have mentioned that the motor was overhauled by the previous owner and has 40 hours on it since the overhaul. We tried to get details on what was done but there's not much information available. We've been told the pistons and rings were replaced but he's not sure if the carburetors were rebuilt.

The problem is intermittent and once the boat is on plane the engine runs great. It only occurs occasionally after we've been cruising at less than 2600 for a while. If we turn the engine off and let it sit for about 10 minutes the problem goes away. But there have been times when we would troll for an hour or so and be able to get up on plane without any problems. Does that suggest a fuel problem? Oh, another thing, the motor will not idle when it's tilted up, even slightly. I seems to flood. The first time out I was trying to idle near the boat ramp while my brother parked the trailer and the engine died. I couldn't get it restarted and was drifting into shallow water. I threw the anchor out and when I tilted the motor up gas came out from under the cowling. I let it sit for a while and lowered the motor and it started.

I own the boat jointly with my brother and here's some information he sent me this morning.

"Did a little investigating this morning on the boat. I found that the fuel line coming from the tank(or atleast what I can see) has this type of hose http://www.goodyearep.com/ProductsDetail.aspx?id=17166

Then from the oil mixer to the engine it has this OMC ES 1763. I haven?t been able to find out much about it but there is a thread on iBoats if you do a search. I think overall the fuel lines are ok to handle ethanol.

I was going to do a little more searching to see if I can find a manual for the engine seeing as I can't figure out where the carb is.

Also I pulled the plugs and they look fine. They all look the same carbon wise. Not sure if we should do a compression test or not."

Thanks again,
bob

 

[ezeke]

Re: Johnson stutter problem at high 2700 RPM

Your engine is going into S.L.O.W. mode due to an overheat condition. You should not run the motor until you have repaired the problem, which may be due to a weak water pump, but is more likely caused by the thermostats.

Check the thermostats and seats. If you are still using the white thermostat housing with the elongated nose #0394409, you may want to replace the units with part #0434137, which provide better circulation and do not stick in the rubber seat.

Once S.L.O.W. is activated, the motor must be turned off AND the head temperature reduced to 162? F. before the powerpack will reset for normal running.

 

[ezeke]

Re: Johnson stutter problem at high 2700 RPM

The Factory Service Manual for your motor is Part #0507664. There is a used one on EBay at a good price - search item #310162440880.

 

[crabby_bob]

Re: Johnson stutter problem at high 2700 RPM

Thanks ezeke. Would the S.L.O.W mode be invoked even if we're getting steady stream of water out the check port? Is S.L.O.W. mode triggered electronically? We have what appears to be a broken wire coming out of what I think is one of the thermostats. We haven't tested the wire yet but it looks like it's been pinched and feels like the conductor is broken beneath the insulation.

Thanks for the pointer to the manual. We'll check it out.

 

[ezeke]

Re: Johnson stutter problem at high 2700 RPM

The tell tale lets you know that the water pump is functioning, but tells you nothing about the flow of water through the engine's cooling circuits; you would need a temperature gauge on each cylinder bank to do that.

As a rule of thumb, you should be able to keep your fingers on the cylinder heads for a three count whenever the engine is running.

Your motor has temperature switches in the cylinder heads which are basically grounding devices that activate the warning devices. The port cylinder temperature switch has a second wire (white with black stripe) which runs to the powerpack. The second wire controls QuikStart in your powerpack.

 

[crabby_bob]

Re: Johnson stutter problem at high 2700 RPM

Thanks again. We have a temperature gauge on the dash and I don't recall if it has degrees or not but would it be a useful indication of whether or not the engine is too hot? I remember looking at it while the motor was stuttering and the needle was at about the halfway point, maybe a little hotter.

 

[ezeke]

Re: Johnson stutter problem at high 2700 RPM

Temperature gauges get their signals from separate variable resistor sensors in the cylinder heads; you would have to have one for each cylinder bank to let you know if there were a problem in that bank of cylinders.

Most operators use the temperature gauge as a leading indicator, because the temperature switch warnings usually come later, after damage may already have occurred.

Your thermostat system is overly complicated although good when working properly, incorporating a water control valve. The system was not used in the 90 degree loopers after the early 1992 models and you may want to consider upgrading.

 

[crabby_bob]

Re: Johnson stutter problem at high 2700 RPM

Thanks for the information. I think we need to get a manual, this thing is complicated.

 

[jonesg]

Re: Johnson stutter problem at high 2700 RPM

If you don't have any luck on ebay get your factory manual at this website.

www.outboardbooks.com

 

[imported]

Re: Johnson stutter problem at high 2700 RPM

Sorry for interupting but i have a question.

If it were indeed slow mode that he was experiencing why would it not go into slow mode when he was at WOT? would not the same issue casue it to do so?

Just trying to learn a thing or to so thanks for the info.

Imported......

 

[ezeke]

Re: Johnson stutter problem at high 2700 RPM

The 1988 V4 Loopers have a complicated water control system that basically changes the flow of water as the engine RPM increases. The motors typically run hotter at lower RPM than at WOT.

I use the same motor in the 140 version and have had the same problems.

Usually switching to the newer style thermostats cures the problem, otherwise it is the water control valve, or the small holes in the thermostat cover gaskets are blocked.

Anyway, the only way to check thermostats is to take the covers off and check. This is a useless exercise in guessing with no feedback.

 

[Silvertip]

Re: Johnson stutter problem at high 2700 RPM

When you take the cover off the motor the carburetors are staring at you. It's just that the pretty plastic air silencer is covering them. Thermostats are not electrical devices. They are temperature sensitive devices that control the water flow within the engine. Stuck shut, a thermostat causes overheating at slow speeds. Overheating triggers the overheat sensor which signals the ignition module to activate the SLOW circuit which limits engine rpm to about 2700 RPM so you "hopefully" don't damage it. There is also an overheat warning horn that should sound when you initially turn the key. If that is not happening, then the horn or something in the circuit related to it is also not working which is why the horn did not sound when the overheat occurred. NO! Water coming from the tell tale tells you the water pump is working. It has no bearing whatsoever on overheat conditions except for lack of water. Anyway -- keep doing what you are doing and you will be in for another rebuild.

 

[imported]

Re: Johnson stutter problem at high 2700 RPM

When you so you "hopefully" don't damage it.

How would one determine if they damaged the engine? what would one look for and where?


Thanks for the great information guys. I'm learning a lot.

 

[crabby_bob]

Re: Johnson stutter problem at high 2700 RPM

Thanks for the help. At this point I suspect the overtemp alarm isn't working due to the pinched wired and the engine is overheating, as you said. I know this has been a bit frustrating without feedback but we don't have any experience in trouble shooting marine engines so it's been very helpful to get this much information before "tearing into" the motor. We also haven't had time to do any actual work on the motor but plan to get to it this weekend.

I just bough a manual for the motor online. It probably won't be here by the weekend but we should be able to poke around the motor a little without it. Thanks again.

 

[crabby_bob]

Re: Johnson stutter problem at high 2700 RPM

My brother and I followed up on some of your suggestions this weekend. The manual hadn't arrived yet so we focused on things we could get at easily.

The first thing we did was check the warning buzzer, and it didn't work. Someone said it should make a noise while starting the motor so we attached a set of muffs and ran the motor briefly. It didn't make any noise so we went looking for it.

We found a melted buzzer under the center console. There was a tan wire connected to it and we traced it back to the hermetically sealed power pack. We disconnected the buzzer from the tan wire and connected it directly between 12V and ground. It didn't make any noise so we went looking for a replacement. The only buzzer we could find was a 12V piezo buzzer at Radio shack. We connected it up still don't hear anything when we start the motor.

Does the warning circuitry require a specific type of buzzer? The one we took out was marked "MX 10V 16" Any idea what that means? Should the 12V piezo buzzer be acceptable?

The maintenance manual arrived Tuesday and it has diagnostic procedures for testing the temperature sensors. The manual was printed 1987 but seems to match our motor which we were told is a 1988.

It looks like there are 2 different types of sensors. 1st there's the temperature sensor that connects to the gauge on the console. I believe that one works OK but there's only 1 sensor installed. Both heads have a spot where a sensor can be installed should I install a second one? It looks like the gauge was wired separately from the rest of the boat. It's not part of any bundle and goes directly to the sensor on the block. Is that normal?

There are two other sensors connected in series to the power pack. I believe these trigger S.L.O.W. mode. Is that correct? The manual has a test procedure for these but we haven't done it yet. According to the manual we should remove them from the head and drop them into some motor oil. An ohm meter is used to verify that the contacts close and open according to the temperature of the oil.

From looking at the wiring diagram I can see the isolation diode someone mentioned but I can't find it in the wiring harness. The tan wire from the buzzer seems to go into the power pack. Is that where the diode is? According to the wiring diagram the anode side of the diode allows for additional connections but we haven't traced them out yet and I don't recall seeing them.

We also took the thermostats off and didn't seen anything that looked like it would be restricting the water flow.

We found a cut in one of the fuel lines. The line goes from one pair of carburetors to something we think is a primer pump. When we choke the motor gas flows out the split in the hose. We plan to fix that by reconnecting the hose with a fuel line connector from an auto parts store.

We believe we're making progress but would appreciate it if someone can let us know if 12v piezo buzzer is acceptable and if the isolation diode is in the power pack. Also my brother wants to change the impeller. Are there any tricks to it that we should know about?

Thanks,
bob and Ray

 

[ezeke]

Re: Johnson stutter problem at high 2700 RPM

The 1988 warning horn did not sound at startup; the test circuit was a slot into which you could insert an extra key. On the side mounted control, raise the warmup lever slightly and you can see the slot, just above the ignition key. Any 12 volt horn will work, but it is important to use one with low amperage.

The tan wire for the overheat alarm is wired through the main harness to the temperature switches in the cylinder heads and indirectly to the powerpack.

To test the warning horn circuit, disconnect both of the quick disconnect leads from the temperature switch and ground the engine end of the tan wire to the engine block with the ignition switch in the run position.

Your motor is a 1988 model which, like a 1988 car, came on the market with the manuals in 1987.

Be careful in your testing. It is best to use an ohmmeter whenever possible and to have the battery disconnected. Any stray voltage at all reaching the powerpack or rectifier can instantly destroy them.

The engine came with temperature switches in each cylinder head that sounded a constant alarm.

The temperature gauge was an option that operates from a variable resistor that is screwed into the cyinder head. You would need two gauges and sensors or two sensors and a switch in order to read each cylinder head.

 

[ezeke]

Re: Johnson stutter problem at high 2700 RPM

In preparation for servicing the impeller, it is easiest to remove the port side lower cowl first. This is accomplished by loosening the four screws from the starboard side of the lower cowl and pulling the two clips from under the engine.

The shift rod is disconnected by removing the screw that is facing you from the port side, under the carburetors.

There are seven bolts to remove to release the lower unit: two on each side, one in front of the trim tab, one holding the trim tab and one partially hidden above the trim tab.

Mark the position of the trim tab before removing it so that you can replace it exactly as it was.


Try not to turn the shift rod, but if you do, turn to the last pages of the V4 instructions in the gearcase section of your service manual.

 

[crabby_bob]

Re: Johnson stutter problem at high 2700 RPM

In preparation for servicing the impeller, it is easiest to remove the port side lower cowl first. This is accomplished by loosening the four screws from the starboard side of the lower cowl and pulling the two clips from under the engine.

Try not to turn the shift rod, but if you do, turn to the last pages of the V4 instructions in the gearcase section of your service manual.

Thanks for the tips Ezeke. Removing the lower cowl was an excellent suggestion. We started by disconnecting the shift rod which was a pain to get at with the cowling on. It wasn't until we tried to put the gearcase back on that we realized the cowling needed to come off. We were careful not to turn the shift rod but found we had to rotate it 90 degrees to the starboard side to get it back up to through the hole on the bottom of the motor mount. We rotated it back to connect it to the shift linkage. I don't believe we changed the height of the rod.

The only other problems we had were aligning the water pickup tube and keeping the exhaust boot in place between the midsection and the gear case. We found that tilting the motor a little made aligning the water pickup easier and we used masking tape to hole the boot in the midsection while we worked the gear case close enough to install the 4 screws that hold it to the midsection.

By the way, I missed the part of your posts where you said the white and black wire are for the QuickStart feature. Sorry about that and thanks for the information.