1974 70 HP Johnson Skips Under Load

[Bay Boater]

I've searched the forum here and have not found quite this problem.

The motor starts easy and idles well. Idles OK when in gear. Runs strong and steady up to about maybe 1/3 to 1/2 throttle. Above that it starts skipping / running rough. Maintains about the same power as at part throttle when it's smoothbut no more.

Occasionally skips at idle also but under part load it runs and sounds very good.

So far on my list is to check the main jets, full throttle timing, and the high tension wires.

All electronics - stator ring, sensor ring, power pack, and all 3 coils are about 3 years old with maybe 50 hours on them.

The powerhead is a fresh overhaul with all accessories transferred from the previous one which ran just fine until it failed with a cracked cylinder sleeve.

What else should I be looking for?

 

[Joe Reeves]

Skips just doesn't do it.... Need a better description.

If it feels like you;'re hitting a submerged object every so often, the engine's jumping out of and back into gear. If this is the case, it's either a bad shifter dog and forward gear...OR... the shift cable needs adjusting (as follows)... OR someone has turned the lower unit shift out of adjustment (see below)
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(Centering Shift Cable)
(J. Reeves)

When all is as it should be, the proper method to adjust the shift cable is to disconnect the cable from the engine. Move the shift linkage on the engine to find the center of the play in neutral, and when found, leave it centered.

Now, grab the end of the shift cable sleeve, push and pull it to find the center of the play there, and center that play.

Adjust the trunion on the threaded portion of the shift cable so that the centered play of the cable lines up with the centered play of the engine's shift linkage. Install and lock the shift cable with the retaining clamp in that position. That's it.
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If anyone has turned the shift rod out of adjustment, it will be impossibly for the engine to shift properly. The rod will require readjustment, measuring from the top surface of the gearcase to the center of the shift rod hole. The critical measurement is as follows.

1973 - 1974 - 65, 70hp (S) 16-11/32 - - (L) 21-11/32 plus/minus 1/32". Have shift rod in neutral, facing forward.
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[95hd]

I am having a similar problem with a 7.5 Johnson except if I take the cover off it runs just fine. Just wondering if you have tried running yours with the cover off?

 

[fireman57]

If it runs with the cover off you have an exhaust leak at the base of the powerhead most likey.

 

[95hd]

That might be a the problem as the exhaust note is different sound than the other 7.5's here. Thanks

 

[Bay Boater]

Skips just doesn't do it.... Need a better description.

It's not the shift cable, not like it's jumping in and out of gear.

Up to 1/3 to 1/2 throttle it's running smooth and steady. Above that throttle setting it's like it's dropping a cylinder intermittently. You can feel the power pulsation but it still pushes the boat at about the same speed as the lower throttle setting. The more throttle, the more pulsation. It's not a hard pulse as if there were mechanical impact. By ear, about the same RPM from the start of the skipping to full throttle. Definitely NOT a hard bump as if it were slipping a gear.

I've checked the timing on the starter with the advance lever up against the rubber snubber used to set the full throttle timing. 15 deg BTDC. The book spec is for 20 deg under full throttle load and considering the advance the power pack adds in at high speed the 15 deg seems about right.

High tension wires are all snug and tight.

Fuel pump screen was about 1/4 blocked with debris and some sort of yellowish translucent gummy material. It doesn't seem enough to restrict fuel flow to that extent but anyway it's clean now.

I learned that the motor spent the winter with gas in the carbs which seems a recipe for gunk to form (maybe like what was in the pump?). Tomorrow I check the main jets. If the idle and low speed jets (which sit higher up in the carbs) are clear but the mains (at the very bottom of the carb) were blocked maybe that would give me enough gas to get to part throttle but then starves it at full throttle.

The main jets are my final hope at this point. Anything else to put on my list?

Thanks to all.

 

[fireman57]

If you have slimy yellow stuff in there just take your time and tear the carbs apart and give them a good cleaning. Also, make sure there is not more of the slime in your tank.