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1995 Mariner 50hp 3Cyl Engine cutting back to idle from full throttle

Martinm

New member
Hey,

I am new to the forum and seeking some help with my 50hp Mariner.
The issue: Took her out a few weeks ago and up on the plane cruising, then with the throttle fully forward the engine cut itself back to almost an idle just leaving us to limp back to the boat ramp.
Initially I thought it was a fuel delivery issue as the fuel hose was pretty bad with air leaks at most joins, I manufactured a new hose with automotive fuel hose thinking it would be ready for the next weekend!
Went out the following weekend... something happened and also with the throttle FWD it was surging and back to low revs all the way back. During fault diagnosis I can’t replicate the fault under load but it is miss firing at mid throttle.

Fault diagnosis carried out so far:
Compression test ✅
All fuel filters replaced ✅
Initial fuel pump diaphragm inspection ✅ (haven’t put them up to the light but they looked in great condition)
Spark plugs replaced ✅
HT leads replaced ✅
Resistance check of coils ✅
Isolate each coil while running by removing lead from spark plug ✅
Stator visual inspection- looked like new ✅
Stator resistance check ✅
Timer assembly visual inspection ✅
Timer assembly resistance check ✅
Timer Volts checked with Peak voltage adapter ✅ (did drop put that down to magneto pick up)
Power pack volts with Peak voltage adapter ✅- volts did drop across all cylinders close to misfire!

I am going to check the fuel pump diaphragms closer and vacuum line to the pump.

it looks to me to be the power pack but I don’t want to spend money on that in case it isn’t, in my head it still feels like a fuel issue when driving and it dropped back it felt like the engine was choking.

I’m not too sure what faults the power pack can cause on the engine, any help on this would be greatly appreciated thanks in advance.

Martin
 
You need to clean out the carbs. (That's the first thing you should have done.) With that deteriorating fuel hoses, lots of particles are plugging up the works.

Jeff
 
I have performed a peak voltage test on the stator today, it is good on the low rev side(blue). When I tested the high rev side (red) the volts were nearly double what’s recommended... It is obviously faulty but can’t think how that is going to give me the issues I have? Could the CDI be bad too? My next thought was head gasket or cracked head, but have been told they are “one piece” and unlikely to be the problem. Anyone have some ideas?
Cheers
 
Changed the stator and timer, new carburettors and used a donor CDI , no change still have a miss at mid revs. The only other things I was think were all 3 coils or the fuel pump? Or just scrap the engine?
 
Don't scrap it! You might have a bad reed valve. I ran into that on one of those I was working on. Pull the carbs and the plate behind them and take a look.

Jeff

PS: This is one of the three carb triples, right?
 
I have taken the plate off and can see the valves from the front side, how do I know if they are seating correctly? Do I have to take off the whole unit behind?
 
I have found on number 1 cylinder 3or4 valves with daylight coming through when a torch is held behind it. From first inspection with the ***** eye they look ok but when using light behind it there is definitely a tiny gap, how much deformity does it need to be bad?
 
I have changed the reed valves, it seemed to run better the first time I ran it. The second time I am getting the miss again :( what do you think is the next option going forward?
 
You should have the model with the big pack with quik-connectors...these can cause a miss also. Best to check spark with timing light when missing to see if it is a cylinder dropping or fuel problem.
 
I have checked the spark while running with an in-line tester, spark doesn’t drop off when it misses. I have re checked the compression and come out level on al 3 @115psi, I have taken out the spark plugs which are brand new during this fault diagnosis process and #3 cyl was quite badly fouled, #2 had a bit of build up and #1 cyl was clean.
 
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