1975 Johnson 9.9 runs half speed.

[Gjmcp]

I am not very good with mechanics. I have installed 2 new spark plugs and two new coils. We also bought a new tank and hose. First trip out motor ran very well at full speed. I idled down and I trolled for 1/4 mile with no problem. Went to head to another spot and the motor would only run at half speed again. Is there anything easy I could try or what should I have a small engine mechanic look for?

 

[racerone]

Check for spark.----Sounds like it is running on one cylinder.----Gas and oil mixed at 50:1 ??-----Mat just need a new sparkplug again or points cleaned.----Time for you to check a few things.

 

[F_R]

Yep, that's what those motors did when brand new. A real lemon, we couldn't keep some of them running one weekend. Try the latest recommended spark plugs, I think it's UL-81C, not 100% sure.

 

[OptsyEagle]

Try some new plugs and see if she works again. If so, never troll with that motor again. Even if the plugs don't fix it, never troll with that motor again. Yes, I know you used brand new spark plugs but if they are Champions, 1/4 mile of trolling should be able to foul at least one of the spark plugs. As FR has stated that motor has issues with spark. It is very prone to dirty points and fouling spark plugs. For the latter, throw away any Champion spark plugs you have and replace them with NGK B7HS plugs. For some reason the NGK plugs resist fouling on that motor a little better then the Champions do. I have no idea why, I just know they do.

If it is not the plugs then check spark and if one cylinder is out it is probably a dirty set of points.

I should add, that my advice above is specific to the 1974-1976 9.9/15Hp models. Any motor after 1976 will troll all day with no complaint and Champions plugs work best in them. Unfortuneatly not your motor.

 

[racerone]

I tuned one up for someone and had it idling in gear for more than an hr----No hick-up at all.----There are issues with them as we know.----One is the exhaust relief holes in the tuner plugging.------All needs to be inspected.----Very cost prohibitive at $100+ / hr and then best to look for a good newer model.

 

[Gjmcp]

Thank you all for your responses, greatly appreciated. I will check for spark and