1979 Johnson 70 hp bogging past 5500 rpm.

[J.avant]

I've got a problem with my 79 70 hp Johnson bogging after 5500 rpm. The first and second time I took the boat out it ran fine all the way to 6000 rpm. Now it sounds like it is running out of gas about 56 5700 rpm. But when I let off the throttle and rpms drop back to 5500 it runs fine.

It could be just about anything. Could someone point me in the direction of what to check first? Maybe a cause?

Could it be electrical? The carbs are supposed to of been rebuilt.

Any help would be appreciated.

John

 

[Mntom]

Re: 1979 Johnson 70 hp bogging past 5500 rpm.

I would start by checking the fuel system. Look at the filter, check fuel lines for small cracks, check for soft fuel lines that may collapse, and possibly a tank vent.

 

[mikesea]

Re: 1979 Johnson 70 hp bogging past 5500 rpm.

also check your sinc and linc,particularly the throttle butterflies are opening evenly

 

[jtexas]

Re: 1979 Johnson 70 hp bogging past 5500 rpm.

Electrical seems unlikely to me.

I had a hi-speed misfire on mine once, kinda like you described, only at full throttle...back down just a bit and no problem. It was immediately following a carb rebuild -- turns out I had installed one of the floats upside down, so that carb wasn't getting enough gas to keep up at full throttle. Implication here I think might be a float or two sticking shut.

BTW I think you're a little underpropped. The max recommended RPM for that motor is 5000. I run mine at 5200-5300. I wouldn't run it at 6000.

 

[J.avant]

Re: 1979 Johnson 70 hp bogging past 5500 rpm.

Thanks for the input. I will start with the fuel system. I installed a fuel/water seperator. Maybe somethings not sealed properly or came loose.

I was on the prop forum and I was told that these engines don't like to be lugged. They also said that 5800 to 6000 with Todays fuel would be good for this engine.

I'm running a 13 1/4 X 17. Old SST-1.

 

[J.avant]

Re: 1979 Johnson 70 hp bogging past 5500 rpm.

Checked all fittings on the fuel/water seperator and no leaks.

I took the boat out again and this time when at full throttle I squeezed the primer bulb and did not make a difference still fell off and when I let off throttle ran fine.

My next step will be to check the float hight to make sure it's good. Does anyone know what the hight is, in inches, on the float? I have a manual just for my model engine but all it has is the part number of a special tool.

Thanks

 

[jtexas]

Re: 1979 Johnson 70 hp bogging past 5500 rpm.

With the carb held upside down, the float should hang parallel with the carb body. I been using that method for years with never a problem.

Few weeks ago someone reported on the forum, a problem with the plastic hinge pins that come in the kits. Seems the float arm etched a groove into the plastic, which caused the float to stick. If you still have the brass pins, use them.

mikesea had a good point about the butterflies, be sure they're all still tight and parallel, and not going beyond horizontal at WOT.

Re-checked my manual, the max recommended RPM is 4500-5500...guess I misremembered somehow...in light of that, 6000 sounds just about right. sorry about the confusion.

 

[J.avant]

Re: 1979 Johnson 70 hp bogging past 5500 rpm.

Thanks jtexas...I will be taking the carbs apart to check the float level.

This model 70EL79, does it have the larger lower unit? The gear case has a 12:29 ratio. So if I'm at 5500 RPM does 35 to 36 MPH sound right to you,with a 13 1/4 X 17 SS prop?

Can a power pack going bad have the same results? I want to make sure I cover all the bases.

Thanks,John

 

[jtexas]

Re: 1979 Johnson 70 hp bogging past 5500 rpm.

according to my manual, only the short-shaft 75hp has 15:28 lower unit, the rest are 12:29

5500 RPM, 17" prop, 36 MPH, 12:29 gear ratio means only 2% slip. Sounds too good to be true. 15:28 is 32% slip, too poor to be true.

I get about 35mph with a 19" prop at 5200 RPM. probably could be better.

Spark test (not a "spark plug" test). get an inline spark tester (couple bucks at autozone), set the gap for 7/16", clip it to the engine block, attach the plug leads (one at a time) and look for a strong blue spark while cranking. If it's firing in the hole under heat & compression, it'll jump a half-inch gap in the open air without a spark plug.

 

[J.avant]

Re: 1979 Johnson 70 hp bogging past 5500 rpm.

Thanks, Just had the prop reworked because it was bent. Mike hoods prop shop when I took my prop in for repair told me it needed to have more cup put in it.

I'll try the spark test too.

John