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Ask Dominick Your Own Question
Dominick, Marine Mechanic
Category: Boat
Satisfied Customers: 1009
Experience:  25 Years Experience as Mechanic, 20 Yrs Boat Yard Owner/Mechanic, Factory Certified
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I just bought a 1980 boat with a 100 horse Johnson and

Customer Question

Hello I just bought a 1980 boat with a 100 horse Johnson and I took it out yesterday and started rite up but when I headed out I gave the boat full throttle and it only went a at 6-7 mp. I took the cover off and it shows its getting all the throttle but no speed u think u could help
Submitted: 2 years ago.
Category: Boat
Expert:  Jason replied 2 years ago.
Hello my name is ***** ***** am I speaking with?
Is the engine getting full rpm (about 5k rpms) but just no speed? Or is there no rpm and no speed?
Customer: replied 2 years ago.
Well the rpm gauge doesn't work but by listening to it yesterday it didn't sound like it had full rpm. I tightened the throttle friction today and cleaned out the plugs sounds like it's getting more rpm's but can't really tell
Expert:  Jason replied 2 years ago.
Sooo without a tach there is no way of knowing how high the engine is getting. What do you think, is it getting 1/2 rpm, 3/4 rpm?
Customer: replied 2 years ago.
1/2 rpm I can buy a rpm gauge if I need to today but don't really know how to wire it up
Expert:  Jason replied 2 years ago.
Chances are this engine probably has more than 1 thing wrong with it. Do you have another propeller you can try to see if you get more speed out of it?
Customer: replied 2 years ago.
No I don't have another propeller. The guy said he just paid 800 dollars worth off motor work I think he said all of the wiring all electrical work. I can try to wire up a rpm gauge and take it out
Expert:  Tan replied 2 years ago.
Hello. My name is ***** ***** do a spark and compression test? This is where I would start. If you are loosing spark on one or more cylinders this could be your loss of power. Your spark should jump a 7/16" gap on a spark gap tester. The compression must be 90+ psi with no more than 15% difference. Is this something you can do?
Customer: replied 2 years ago.
I don't have a tester I guess I'll just have to take it to a mechanic
Customer: replied 2 years ago.
Buy thanks tan for the advice.
Expert:  Tan replied 2 years ago.
Ok chances are you have an ignition or carburetion issue. Can you do a couple of tests that don't require any special tools? With engine running pull each spark plug wire off does the engine drop rpms? Use a pair of insulated pliers for this test. When you are running the boat push in the choke does it pick up rpms? Squeeze the primer bulb does it pick up rpms?
Customer: replied 2 years ago.
K I'll try that when I get it to the water. Hope it's not a lot of work and money to fix it
Expert:  Tan replied 2 years ago.
Ok that all depends on what we find. If you find a cylinder with no spark swap one of the firing ignition coils with the non firing cylinder. Does it fire now? Post back with findings.
Customer: replied 2 years ago.
But the way the motor sounds and acts it might be the carb
Customer: replied 2 years ago.
K I will post back might be a little while before I can test it out
Expert:  Tan replied 2 years ago.
It's very possible the choke when it is acting up will tell us if it is a carb issue. Squeezing the primer bulb will tell us if you have a fuel restriction or fuel pump issue. So when you are running the engine just briefly push the choke if the engine picks up rpms then we have a carburetor issue.
Customer: replied 2 years ago.
Kewl IL try it out thanks a lot
Expert:  Tan replied 2 years ago.
Ok keep me posted.

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