No power under load

**Warmstrong** · 11-09-2019, 12:14 AM

I have a 1978 Mariner 175 serial number 5316460 that is not making power under load. It starts extremely well and idles great. When taken out of gear it will rev through the rpm range, but when placed under load, it won't go above 2000 rpms. I have checked to ensure the throttle cables are working properly and the throttle is going all the way to the stops with the carbs completely open. I recently purchased the motor to go on a Ranger boat that Ive been restoring. Prior to purchasing, I took it out on the sellers boat, and it ran great and was very strong. While it was being installed on my boat, the mechanic had to replace the wiring harness due to what he described as multiple jumped wires. Im trying to get an idea of where I should start my troubleshooting and would appreciate any suggestions.

**Mark Poole ModVP** · 11-09-2019, 12:32 AM

Did the mechanic replace the wiring harness to the control box/keyswitch or the wiring harness on the engine itself? If the engine wiring was replaced I would check to see if you have spark on all 6 cylinders. If you do then you could have a fuel restriction. Easiest way to test for that is to run the engine on a different tank and fuel hose. Did the engine sit up without being used for an extended period of time after you bought it? If so, could have gummed carb jets.

**nitro_rat** · 11-09-2019, 08:18 AM

could be bad high side on the stator too...

**Merc 2.5** · 11-09-2019, 07:07 PM

Could b sooo many things , start process of elimination, look at plugs to get idea, test spark, clean carbs, etc

**Motv18** · 11-09-2019, 09:44 PM

I’ll go with last thing fixed. It ran and someone fixed what wasn’t broke and now it doesn’t run.

The jumped wires either buypassed a broken broken part, or you have a Frankenstein and needed the jumps to wire it right.

Have ve fun with the birds nest.

**Warmstrong** · 11-09-2019, 11:06 PM

Thanks for the replies and suggestions. I’m going to start some trouble shooting tomorrow. What is the best method to check spark?

**Mark Poole ModVP** · 11-09-2019, 11:27 PM

Run engine on water hose. Either pull plug wire off one at a time with insulated pliers or use a timing light.

**Capt.Insane-o** · 11-10-2019, 02:50 AM

-.- one guy in this thread is on the right track.

**kellytheaker** · 11-10-2019, 08:08 AM

Easy now - we are all at times struggling to satisfy our wives - they can trickier to diagnose . I am not certain but I believe he is referring to the stator but not certain there are several decent posts . I would have thought a compression check would have been the starting point . But he is a knowledgeble guy and a strong member here and does help allot . So what thread exactly were you referring to sir I'm pretty sure we have all found the G spot - LOL

**Mark Poole ModVP** · 11-10-2019, 02:00 PM

The one who made the comment about doing their wife is probably a virgin.

**Merc 2.5** · 11-10-2019, 02:48 PM

Originally Posted by kellytheaker

Easy now - we are all at times struggling to satisfy our wives - they can trickier to diagnose . I am not certain but I believe he is referring to the stator but not certain there are several decent posts . I would have thought a compression check would have been the starting point . But he is a knowledgeble guy and a strong member here and does help allot . So what thread exactly were you referring to sir I'm pretty sure we have all found the G spot - LOL

Ya I thought was few gud suggestions to give starting points

**Warmstrong** · 11-10-2019, 02:56 PM

Ok so here's an update on what I've done so far today and some observations.

First off, I warmed up the motor for about 10-15 mins and then conducted a compression test. Test was conducted with all plugs out and throttle at wide open. Numbers are as follows: #1 75, #2 80, #3 75, #4 80, #5 75, #6 75. Im not sure what the specs are for a 78 2 stroke, and I expected them to be a little low due to the age, but I'm encouraged by the fact they're relatively even.

Second test I conducted was utilizing an in-line spark tester and checked each cylinder to ensure it was getting spark. Test sat.

Third thing I did was review the fuel system. The fuel line coming out of the fuel pump is 1/4" I.D. The fuel line on the boat side is 5/16" I.D. I did notice that while the motor was running the fuel boat was very soft. I would pump it up while the motor was running and it would get firm, and then get soft again. The fuel line had a date of 2012 on it, so I will be replacing that today.

The one other thing that I noticed that could be an issue is a ground wire that appears in fairly poor shape. In a separate post ill put a pic of it up, as Im not sure exactly what its purpose it or how it could be impacting things.

**Warmstrong** · 11-10-2019, 03:06 PM

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**Merc 2.5** · 11-10-2019, 03:13 PM

So at any point , have u got it to plane out and gain any rpm? Do u have a smaller prop around to try? Just to see if it let's motor rev up anymore

Wire on head is temp wire or temp alarm wire
On side of motor it's hard to tell. See if u can trace that one little farther

**Warmstrong** · 11-10-2019, 03:24 PM

I have only had it out once and it didn't plane out and wouldn't go over 2000 RPMs. It currently has a 25p 3 blade prop on it. From what I have been told from other folks that own the same boat I have, is that a 24p tempest plus or fury is about perfect for my boat. I don't currently have another prop to try, but I don't see why a 175 wouldn't turn a 25p more than 2000 rpms. I plan to change out the boat side fuel line as its a 2012 and probably should be replaced anyway. Im curious about that wire though. It's definitely in rough shape, and Im pretty sure its a ground wire. Not sure what it goes to though and Im leery of digging into it before I know what it goes to.