Johnson boat motor bogging down

[Kenny T 55]

I have a 89 Johnson 70 horse 2 cycle motor that has always run very strong. It has started bogging down after about 30 minutes of running at about half throttle. I've replaced the spark plugs put all 3 new coil packs on it but still does it. When it bog's down you can idle a few minutes and it will run fine for a few more minutes then does it again. I'VE put new fuel pump new fuel line and pump ball and new clear filter and keeping plenty of fuel . Only thing I know left is the power pack. Need professional advice before I go buying a new power pack and it not take care of the problem either . Any Advice ?

 

[racerone]

Throwing parts at is the wrong approach.------Do a compression test first.----Have you looked at the water pump impeller ?------And does the tell tale hose come off the side of the block or the top ?

 

[oldboat1]

^^agree. Compression, measure spark…. Suggestion is that it may be overheating — most obvious solution is the water pump (need impeller replaced). Less obvious is an airlock in the water passage on some models, involving relocation of the tell tale elbow to resolve.

When the motor acts up, touch the top of the head with the heel of your hand. If you can hold your hand there for a few seconds, temp is probably OK. If too hot to touch, assume it’s running hot. In any case, replace the impeller if it’s been more than a couple of years. That’s a part you should replace as part of regular maintenance.

If you have an infrared temp gun, surface temp on the top of the head should not exceed 160F.

 

[Kenny T 55]

Thanks for input , checked compression and it's 125 per cylinder. I put palm of my hand on top when acting up and not hot at all and it pee's strong. I noticed this go around when i ran it through it sounded like running on 2 cylinders from the start with not near the power as usual then it picked up and took off strong again for about 10 ten minutes then bogged down sounding like 2 cylinders again.. any more advice before I have to break down and put it in the shop. Thank you for your time.

 

[Kenny T 55]

One other question ? How do I measure the spark on the plugs. Thanks

 

[hardwater fisherman]

You need to make the spark jump an open air gap.....You can also make a tester with basic materials you have...

 

[tramsdell]

Service carburetor