1978 70 HP Johnson Running Problem

[karl3419]

I have a 1978 70 HP Johnson that will not run above 1200 rpm. When I increase the throttle further the engine dies. Compression is good, fuel pressure is good, high speed jets cleaned, power pack changed. Checked with timing light and timing OK but #2 cylinder not firing. Swapped coils to see if problem is a bad coil and then all three cylinders fired as normal. Put it in the water and problem reoccurred. Had best mechanic in area check engine and it appeared that moving the throttle would cause plugs to stop firing so he replaced the timer assembly. Put boat in water and engine ran fine for 20 minutes but then the low rpm problem reappeared. Checked with timing light and all cylinders are firing but 1200 rpm is still maximum and engine dies on more throttle. The only thing that appears abnormal is when thhe engine is in the water a lot of bubbles come up around the lower unit. A friend said that the exhaust housing has probably come lose.

I have two questionns, First, could the exhaust housing cause a breathing problem for the engine? Second, is there any way that the throttle movement could effect the ignition? Any suggestions would be appreciated.

 

[keykev]

Re: 1978 70 HP Johnson Running Problem

bubbles are simply the exhaust gases rising to surface, throttle movement advances timing as carbs open, assuming all throttle linkage is properly adjusted it sounds like a fuel problem to me. did you check the screen under large screw on fuel pump? if so , have someone drive while you manualy choke the motor half way or more upon acceleration and see if that gets it going. you can also remove drain plugs at bottom of carbs to verify fuel in each bowl. hope this helps....

 

[karl3419]

Re: 1978 70 HP Johnson Running Problem

Thanks for the reply. Initially I thought it was a fuel problem. That is why I cleaned the high speed jets, lots of fuel came out so the carbs are getting fuel. Also I changed the inline fuel filter, put in new fuel line and squeeze bulb, checked the screen on fuel pump and measured the fuel pump pressure at idle and high rpm. Squeezing the fuel line bulb or applying choke does not solve the problem.

If I manually open the throttle plates on the carb the engine will go to high rpm (in neutral). All linkages look ok. The next thing I plan to do is to put the boat into the water at the dock, put a timing light on #1 lead and make sure I have spark in neutral and then shift into forward. I will advance the throttle and see if ignition continues as the engine dies or see if the ignition stops first. The mechanic who installed the new timer is supposed to stop by tomorrow.

 

[emdsapmgr]

Re: 1978 70 HP Johnson Running Problem

It's always possible that when the throttle arm is advanced and the timer base is dragged forward, that the timer base wiring may be contacting ground during it's movement. If the timer base is new, that's probably unlikely. Check to make sure the link arm between the spark advance lever and the timer base is still connected. That ball and socket joint can come disconnected and the outer nylon clip retainer can break with age-causing it to become disconnected often. Running the engine high rpm's in neutral is not a good test. It'll run high rpm's on only 2 cyls. It would be interesting to load the engine (in forward in the lake) and watch the timing advance with a timing light shining on the #1 cyl marks on the top of the flywheel.

 

[karl3419]

Re: 1978 70 HP Johnson Running Problem

It's always possible that when the throttle arm is advanced and the timer base is dragged forward, that the timer base wiring may be contacting ground during it's movement. If the timer base is new, that's probably unlikely. Check to make sure the link arm between the spark advance lever and the timer base is still connected. That ball and socket joint can come disconnected and the outer nylon clip retainer can break with age-causing it to become disconnected often. Running the engine high rpm's in neutral is not a good test. It'll run high rpm's on only 2 cyls. It would be interesting to load the engine (in forward in the lake) and watch the timing advance with a timing light shining on the #1 cyl marks on the top of the flywheel.

I put the engine in the water on the 10th and tied it to the dock and davits and when I opened the throttle while in gear the engine would die above 1300rpm. I pulled from the water and all plugs were firing properly. The next day I was going to check the spark as you suggested and put it in the water and but the engine did not show a problem. The rpm increased as I opened the throttle while in gear. I stopped at 3000 rpm as the davits were creaking too much. I had made no changes.

 

[emdsapmgr]

Re: 1978 70 HP Johnson Running Problem

Not sure what your conclusion is from the above test. Once you got the engine to run 3000, did you take the boat out for a spin? Do you think the engine will run normally above 3000 now-or do you think there is still a problem?

 

[karl3419]

Re: 1978 70 HP Johnson Running Problem

Not sure what your conclusion is from the above test. Once you got the engine to run 3000, did you take the boat out for a spin? Do you think the engine will run normally above 3000 now-or do you think there is still a problem?

The point I wanted to make is that this engine is not consistent. Remember the night before it would not run above 1300rpm so the next day I connected a timing light to #1 and wanted to see if it lost spark when I opened the throttle above 1300rpm. But it ran normally so I could not determine if loss of ignition was the problem above 1300rpm. I did not take it out because my current problem is having the other engine repaired. It froze up and the powerhead needs to be remanufactured. Further analysis on this engine will have to wait until the other engine is repaired so that I have at least one good engine when I go out into the water.