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jeff-register.
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March 7, 2016 at 9:58 pm #3785
Have a 1956 71/2hp evinrude ran last may and it ran good. Tried to run it today and it acts like its only running on 1 cyl. I checked the plugs both have spark while it is running toke 1 wire off at a time and they both made a little difference. So i took a comp test and they both only have 55 to 60 psi any suggestions what could of happened.Motor was kept in garage all the time just dont understand what happened
March 7, 2016 at 10:38 pm #33080Could be one of several things. How much oil do you run in the gas for your motor? That one should be 16/1. Running at 50/1 will wear out the motor rapidly resulting in low compression. Stuck rings from carbon build up can also be a problem, or maybe after lots of hours, it is time for a rebuild. You may want to try a different compression gauge to verify your readings. With compression that low it should’t idle very well at all. Hope this helps..
March 7, 2016 at 11:00 pm #33081You can try laying the motor down, plugs facing up and level. Shoot some OMC power tune or Sea Foam deep creep into the cylinders and reinstall the plugs. Let sit overnight and repeat. If the rings are stuck, this might loosen them up somewhat and provide a better compression reading. I would not expect a huge increase. Like Jerry said, it sounds like and may be time for a rebuild. Don’t let some slick talking parts salesman or neighbor talk you into anything less then the 16/1 oil mixture Jerry indicated. Todays oils are good…but they are not THAT good to alter from that mixture.
March 7, 2016 at 11:10 pm #33082One other thing to look at. Pull the flywheel and have a look at the coils. Look for any cracks or deterioration in the outer shell. Sometimes a bad or on it’s way out coil will spark at low RPM but cut out at higher RPM. While your in there check and clean/replace points and replace condensers.
March 7, 2016 at 11:21 pm #33084If you are lucky you may have a blown head gasket. Since both are way down in compression that may be the case.
Jerry gave you some sound advice on oil ratios. Do not deviate from that.
Also time to check the water pump impeller and also possibly the exhaust cover gasket.
I believe for that engine the powerhead will have to come off to change the water pump impeller or complete pump anyway. That will give you easy access for the gasket inspections.
Rule of thumb on when it is time to replace the water pump housing is that if you can catch a fingernail in a groove inside either the pump housing or the lower plate…..replace the complete pump. Lots cheaper than a powerhead with parts that are almost made of "unobtainium" these days.March 7, 2016 at 11:49 pm #33086^^^ Good point. If you are lucky, the head gasket is blown between cylinders, or the head slightly low between cylinders, causing equal, low compression in both.
Worst case scenario, like many 7.5hps before it, it was run too lean, and is scrap or in need of rebuild.
If it were mine, I would pull the head, the bypass covers, and the exhaust covers. Inspect head gasket and head. Inspect the rings through the now open covers. See if the rings will bounce, when you press on them with a screw driver. Check the sides of pistons for damage. Check the bore for damage.
March 8, 2016 at 12:00 am #33087Just want to make sure I am understanding what you posted. The engine ran fine last may, which was the last time you used it…Now, it seems to only run on one cylinder….But, pulling either plug wire off makes little difference in how the engine runs….correct? Well, kind of hard to be sure it is indeed only running on one cylinder based on what you are saying. Usually, if running on just one cylinder, one plug wire removed will make little difference, but the engine will stall if the other wire is pulled….
Are you running the engine on a boat in the water, or in a barrel? Are you pulling the plug wires when the engine is in neutral, or in gear under load?
Why is it that you feel the engine is only running on one cylinder?
These engines have relatively low compression readings, usually higher than 55PSI though, more like 70-80PSI. BUT, compression readings will vary depending on gage used due to the small combustion chambers. Have you tried a different gage? True, the head gasket could be blown between the cylinders causing low readings in both cylinders, but the engine ran OK when you used it last year. Doesn’t seem likely that the head gasket failed while sitting in the garage. True, the cylinders/rings may have gotten rusted while sitting, so it sure couldn’t hurt to soak the cylinders with engine tuner/sea foam.
I’m just hearing some conflicting symptoms, and don’t want you to rip this thing apart needlessly. So, I would direct you back to basics. Try a different compression gage. Are you using fresh fuel/oil mixed at 16:1? Was fuel left in the carb that could have gummed up? Have you tried new spark plugs? You need to do a spark test as well, the spark should be able to jump a 1/4" gap while cranking with the recoil.
Like Casey says, are you sure it is cooling properly? The engine would be low on power if overheating. The only other basic advice I would give is to draing/check/refill the gearcase fluid. Water in the gearcase can cause it to bind, which might lead to a lack of power, poor running….March 8, 2016 at 12:41 am #33088yes i am running in a barrel .I pulled one plug wire at a time while running made very little difference on either cyl. It wont start when the lower wire is unhooked it will start when it is hooked and the top one is unplugged. I have that wrong it is the other way around i put a new pump in it thats why i tried to run it in a barrel It has always had 16/1 gas my grandpa bought it new. I always drain the carb when done it only gets used once are twice a year so i always drain it. With it being my grandpas im kinda anal about this motor i have 4 or5 other motors so im used to these older motors.Thanks for the tips i think i will pull the head and see whats up
March 8, 2016 at 2:12 am #33097By any chance, is it hard to pull the rope with the plugs out? Your shock absorber in the lower unit may be expanded or broken, making it bind against the bearing housing below the water pump. Disturbing it while changing the pump may be a contributing factor.
March 8, 2016 at 4:38 am #33102 -
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