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Tan, MerCruiser Certified Technician
Category: Boat
Satisfied Customers: 10601
Experience:  40 yrs. experience with complete engine repairs both inboard/outboard and MerCruiser Certified Technician.
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Have '79 evinrude 150hp outboard. Runs great on hose but in

Customer Question

Have '79 evinrude 150hp outboard. Runs great on hose but in water, in gear will barely run 5-8 mph.... A couple of times during the day it would run good and boat would plane but that would only last a minute or two then back to running like crap.. Starts right up every time.. At a loss for what to do or check... It has 3 carbs....
Submitted: 5 months ago.
Category: Boat
Expert:  Tan replied 5 months ago.
Hello Russ. My name is***** are a few things we need to check before we can know what is wrong with your engine. We need to start with a spark and compression test. The compression must be 80+ psi on all cylinders with no more than 10% difference between cylinders. Then we must check spark with the use of a spark gap tester. The spark must jump a 7/16" gap on the spark gap tester. If these check good then we have a fuel related issue. Here are some tests you can perform. When the engine is acting up squeeze the primer bulb. Does it run better? If no change choke the engine when it is acting up. Does it run better.If you have no change in the engines performance during these tests we will need to run the engine and use a timing light to verify spark at all rpms on all cylinders. Do all of these checks and post back with findings.
Customer: replied 5 months ago.
Did the fuel related measures on the lake... Bigs more when choke turned on, no difference when squeeze bulb... Advance bar under flywheel moves correctly... Plugs are new NGKs... During lake run , barely noticeable surges intermittently.... Getting gas to fuel filters, didn't go farther with fuel than that... Tank pressurizes.. The gas may be mixed with some year old gas, not sure.. Gas/ oil is mixed 50:1....
Expert:  Tan replied 5 months ago.
Have you done the spark and compression tests?Surging is either spark or fuel you have eliminated the fuel leaving spark. We need to use the timing light test to find which cylinder is misfiring. Check each cylinder with the timing light. You want to see a steady flash of the light with no dead spots. Leave the light on each cylinder till you feel the engine surge. If the light stays steady while surging then mo\ve to the next cylinder.
Customer: replied 5 months ago.
It will take me a couple of days to do compression test.. I don't have access to boat right now. Next time I have boat in water I will pull plug wires off one at a time to check cylinders/ spark plugs.. I will pull plugs and confirm the gap is correct first
Expert:  Tan replied 5 months ago.
When I was referring to spark gap I didn't mean spark plug gap. They make a spark gap tester that tests the spark gap of the ignition when it fires. This give us the quality of the spark. The gap must jump a 7/16" gap to have enough power to fire the fuel. Here is the gap tester. Post back with findings when you get the chance to do the checks.

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