My Mercury 7.5hp will not rev up properly ?

Question

Hi my mercury 7.5 blue band I think it is an 80's model and when I am in the water on occasion's it gives a quick burst then goes back to just quicker than idle, does any one know how I can fix the problem is it spark or fuel related thank you.

Answer 1

Quick check- pull one plug wire and see if it runs consistently (of course slower).  Try the other next. Runs the same on either, probably fuel. Runs well on one but not the other, probably spark. Check for water in your fuel, a vacuum in your tank or a bad or leaky fuel hose (after market hoses are notorious- try replacing with a known good one) as easy to try fixes before getting complicated. By switching coils, wires and plugs individually cylinder to cylinder you can usually isolate the bad component. Also some Mercs have a easy to replace reed valve seal that wants attention every few years...

Answer 2

It sounds like you have a spark problem.You need first see if it is an electronic ignition or it has points in it.I suspect it is points because you say that on occasion it will rev up.If it is points you will need to replace them.There is also a small plastic piece that the points fit through.Make sure it is not cracked because this will cause a loss of spark.The point setting is .020.To check to see witch cylinder is not working,remove the spark plugs to find the one that is wet.I hope this will help and good luck.

Answer 3

That's the question- is it spark or fuel--and here is how you find out; get 2 timing lights. Hook one up to each cylinder. Tie down the triggers and put them a$$ down in a battery box and go for a ride. If one drops out when the problem repeats - it will be in that circuit. Most likely the coil. But now for the dangerous part. There is a bundle of wires coming out from under the flywheel.  Push them from side to side and see if it makes any difference. If it has no effect- switch the coil wires (not to the spark plug) and see if the trouble follows the wires. If not the spark- then you have a vacuum leak some where. Look on the right side of the engine and find the tiny tubes which move excess fuel from the bottom to the top cylinder. If none of this is the problem you may have a bad lower seal. They tend to dry up the bottom cylinder when conditions are right. Check your bottom spark plug for a real dry look.

Ps -The ig problem will leave a wet look and the vac leak or bottom seal will leave the dry look.