Intermittant low power 120HP

I need some help on this one. I have an 88 120 HP with VRO Evinrude that over the past several years has had an intermittant low power problem. In the past it was always something that came and went for 15-30 seconds but recently it ocurred and the problem stayed for several days but I was in a remote location and did not have access to any diag equipment to sort out the problem. The problem began recently while under power at 4200 RPM the engine bogged out to 3000 rpm and wouldn't go any faster. No alarms were going off and the engine had only been running for 5 min or so. The primer bulb was not soft and pumping it had no effect. I also noted that if I pushed the key in at a regular rythm I could get the RPM up to almost 4000 RPM. It ran for about 1/2 hour like this and then it suddenly took off like a switch had been thrown-This is also the way it had acted in the past although prev it had only bogged for 15-30 sec. Then after running fine for 15 min it bogged out to 3000 RPM again. The next day I tried it again and it was still bogging. It would idle fine and run fine at low speed but still wouldn't run over 3000 RPM. I pulled all the plugs and they all looked fine. Checked water separator filter and fuel filter under the cowl-ok. Checked primer bulb check valve-ok. I tried running on my aux tank and it was still the same. I put on another fuel pump from another evinrude and it was still the same. I checked the pulse hose to #4 cyl-ok. Swapped backfire valve from another engine-same. I noted that the gas filter under the cowl had only a little fuel in it with either fuel supply and with either fuel pump and if I pumped the primer bulb (even though it was already firm) I could fill the filter but the level would go down again. It was as though the fuel pump was not sucking enough but I did not have a vac gauge to check it. I finally left the second pump on and ran on my aux fuel tank with mixed fuel as I did not know for sure that the oil side of the fuel pump was working because this was a scavenged pump. On the way home it was bogging for about 15 min and suddenly took off again and worked fine for the rest of the trip. I note that now the fuel filter is full and the engine is not bogging so now is is going to be really difficult to diag. Sorry for the long post but I'm sure someone has had a similar experience and that is probably the only way to find this problem . I am an automotive mechanic so I understand mechanics but I am no 2-stroke expert.

Re: Intermittant low power 120HP

Welcome to iboats.

I would start by checking spark and compression. Get out a timing light and install while underway to see if you are dropping a cylinder.

If spark and compression check out, look to fuel delivery.

I would also ensure that the motor is not going into SLOW mode, restricting it to 2500 RPMS. Use a temperature probe or infrared thermometer and verify that the engine is not overheating. Disconnect the TAN temp wire from the pack and retest. If the engine now performs properly, replace the temp switch. Also make sure that the TAN temp switch wire is not located next to a spark plug wire.

Re: Intermittant low power 120HP

Thanks for answering my problem-however the problem is not there now and it only happens occasionally. I plan on doing a compression test however this problem comes and goes but I also wonder if marginal compression on #4 can effect the fuel pump operation as this is the cyl that has the fuel pump pulse hose on it??? The engine was running at a steady 3000 RPM and anything I read has 2500 RPM as a limit for slow mode. Are there any carb or fuel feed line problems on these engines that can be intermittant?

Re: Intermittant low power 120HP

If your motor recovered when the key was pushed in (activating the primer), this would seem to indicate a problem with one of the carbs. You said that swapping the fuel pump had no effect, and the primer bulb never really went soft. I see air in fuel filters quite often, it doesn't always indicate a fuel delivery problem. I think what probably happened, was a chunk of varnish broke off from the inside of one of the float bowls, partially blocking one of the main jets. It eventually sucked it through, and the motor performed properly afterwards, or it may be something too large to pass through the jet and just floats around in the float bowl occasionally blocking one of the jets.
The main jets on this model can be removed without disassembling the carbs, remove the intake silencer cover, and the main jets are in the bottom of the float bowl behind the bowl drain plug. I have also seen inlet needles stick, preventing fuel from flowing into the float bowl, the primer bulb will feel hard but no fuel may be going into the carb.

Hope this helps, Jimmy

Re: Intermittant low power 120HP

Thanks Jimmy,
I wish I had thought of that when the problem was occurring as then I could have pulled the float bowl drains and see how much fuel came out of each one! I may have to attempt to duplicate the problem again then drain the float bowls when it happens. Has anyone else experienced this problem? One
of my fellow techs found a thread online from another web site that had a similar problem and the needle and seat was sticking closed intermittantly caused by the new fuel formulations(ethanol). Points to ponder!!

Re: Intermittant low power 120HP

Also does anyone know the function of the gray wire to the VRO pump. The tan wire I understand is the alarm wire, The black wire is ground but what does the gray wire do?. My schematic doesn't specify what it does and I can't find any text that describes these circuts. This is a 3 wire harness.

Re: Intermittant low power 120HP

The gray wire is tach signal from the voltage regulator.

Re: Intermittant low power 120HP

OK-Here is an update. I replaced all the fuel lines from the tank and checked all connections for a leak by applying vaccuum to system. It holds vac to 10 inches. Checked fuel pump vac and it never goes over 2 in vac but tested pump vac by plugging a hose and it pulls 6 in vac. installed fuel pressure guage and took the boat on the water. First 1/2 hour it ran perfect but then started bogging. Checked fuel pressure gauge and pressure was between 3 and 5 PSI. I immediately shut off the engine and pulled all the float bowl drains-they all drained fuel and all were an equal amount. With it bogging I pulled off each primary coil wire and top cyl on port side made no difference. Checked spark and there was lots of spark. Swapped spark plug with lower cyl and the miss stayed on the top cyl. Ran it again and confirmed that the revs go up when you press the choke primer. The problem must be in the carb for the top port cyl!!!! I will keep you posted. Any ideas of what to look for when I take this carb apart???