1977 Johnson 35 jerks at full throttle

[scrimper]

Our 77 Johnson outboard has been having issues. Someone replaced the water pump, and we weren't able to water test it for a while, however when we hooked it up to the hose the engine ran beautifully. We took the boat out later and when it was running in forward and reverse there was an intermittent JERK every couple of seconds, which was more frequent at higher throttle. We replaced the fuel tank (it was the original one) and the fuel lines, the fuel filter looks very clean. We took the boat into a mechanic and he said we needed a new lower unit and that the gears were worn. He put in a rebuilt lower unit. Lots of money later we put it in the water again, the mechanic had disconnected the water pump and the jerking was STILL happening. We took it back, he reconnected the water pump (he did) and said he fixed the jerking.. blah blah. He didn't, it still jerks but now only at high throttle. One of the mechanics said there was something that needed to be adjusted to fix the jerking, something needed to be turned. I don't know what he was talking about, the drive shaft or something to do with the dogs maybe but we're not taking our boat there AGAIN. Does anyone have any suggestions as to what this problem could be. It's shrimping season here in Charleston and we're a boat down!

 

[Daviet]

Re: 1977 Johnson 35 jerks at full throttle

OK here we go, it sounds like what every body is talking about is the shift dog in the lower unit. If it is jumping in and out of gear sometimes you can turn it around and get a little more life out of it.

There are a couple of things that can cause sudden jerk every few seconds, one is the shift dog and the other is an ign misfire. By swaping the lower unit hopefully we can assume the shift dog is OK, yea I know, never assume anything.

Have some one operate the boat, use an inductive timing light and check spark on the cylinders and see if you loose spark when the engine jerks. If so you have an ign problem that needs to be repaired.

Normally a fuel problem does not jerk, it is a slower responce, like a surge, but try pumping the primer bulb when the problem occours and see if it helps.

There are high speed jets in the carbs, but there again if it is a fuel problem you would have more of a surge than a jerk.

Just some thoughts.

 

[jonesg]

Re: 1977 Johnson 35 jerks at full throttle

check for mis-routed plug wires, old wires can cross fire if they are touching.
Ya never know, at least its a cheap date.

 

[asdasc]

Re: 1977 Johnson 35 jerks at full throttle

I had this problem once and found that the problem was due to misadjustment of the shift cable not allowing it to get fully into gear. Check that first. An easy way to check it is to disconnect the linkage at the motor once you get underway and shift it manually at the motor.

On my 18HP there is a shift lever disconnect about half way down the leg under an oval cover plate. If yours has that, make sure it is connected solidly or it won't go all the way into gear.

Lastly, when I was having the jerking issue, all my buddies told me the jerk was at the other end of the steering cable, not at the motor!

 

[shrimpwhistle]

Re: 1977 Johnson 35 jerks at full throttle

Could this be a shift-rod adjustment problem?

 

[Jeff_G]

Re: 1977 Johnson 35 jerks at full throttle

Those engines were notorious for eating gears and clutch dogs. Adjustments didn't really help as you would lose the other gear. The problem in reality was the main prop shaft bearing design in the front of the gearcase.
The only solution is to replace the clutch and gears, not cheap.
I don't know of any company that re-manufacturers those gearcases.
Check your shift linkage that it is correct, if it is then the only solution is replacing the assembly or the gears/clutch.
Bait shrimping is probably the hardest thing on gearcases.
I'm about 40 miles south of Charleston if you need further help PM me.

 

[scrimper]

Re: 1977 Johnson 35 jerks at full throttle

Thank you for all of your replies.

I'll try to check the spark and see if that's the problem.

The entire lower unit was replaced. There's a chance he replaced it with one that was already bad (he's either a crook or an idiot, can't decide), but presumably the gears should all be like new. Is it possible that when he put the new foot on he didn't line something up and that's why we're getting the jerking? This boat and engine was kept in amazing condition since 1977 and the jerking started happening after someone replaced the water pump.

 

[shrimpwhistle]

Re: 1977 Johnson 35 jerks at full throttle

So you have not been shrimping with the boat yet?