'76 Johnson 70 HP, runs great cold, but...

bikodog

Cadet
Joined
Jul 31, 2004
Messages
8
Help!,<br /><br />This motor, which was supposedly rebuilt a couple years ago, starts great all the time. It runs well on first startup, gets boat up on plane, and seems to run full throttle. If I bring it back to idle, or shut it off, once it's warm, it will restart fine, but bogs down and dies before accelerating. It restarts every time, and eventually by feathering the throttle I can get it back running, but it runs roughly at middle rpms, until it gets going. Pumping the fuel bulb doesn't help, have replaced fuel lines and tried different tanks with same results.<br /><br />Would a carb problem only manifest itself while the engines hot? It seems like a fuel problem, but could it be the power pack?<br /><br />Any ideas greatly appreciated....
 

WillyBWright

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Dec 29, 2003
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8,200
Re: '76 Johnson 70 HP, runs great cold, but...

Welcome to iboats.<br /><br />I'd concentrate on the carbs, but don't rule out an ignition problem. Check the throttle shafts. They wear right where they turn in the carb body and leak air causing all sorts of odd problems. The holes in the carb body wear as well, but not seemingly as much as the shafts. Replacing the shafts is usually enough. It's easy to see once the shaft has been removed. The plating wears off and starts to wear into the brass. If one shaft is worn, you can bet all three are.
 

bikodog

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Jul 31, 2004
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Re: '76 Johnson 70 HP, runs great cold, but...

thanks, WillyB<br /><br />I'm pretty sure it's the carbs, I replaced the plugs today just for fun, and it behaved the same, except a little worse, didn't like the brand plugs I think.<br /><br />It will idle and run WOT, but you really have to baby it in between, and sometimes you just can't get it to WOT. <br /><br />Carb rebuild here we come.....
 

bikodog

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Jul 31, 2004
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Re: '76 Johnson 70 HP, runs great cold, but...

well, turns out the throttle shafts were fine, but i had a mechanic look at it, and they did a carb rebuild. they increased the low speed jet from a 31 to a 33 and that almost made the problem go away. it still sputtered a few times, so I might take it back and have them put a 35 in it. <br /><br />Had a nice weekend of tubin, finally!
 

MESS147415

Seaman
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Jul 5, 2004
Messages
61
Re: '76 Johnson 70 HP, runs great cold, but...

Hi bikodog, i have exactly the same prob with my '78 70 hp evinrude. Mine starts fine and runs perfect at wot but inbetween has been a nitemare!.Ive cleaned carbs, changed plugs, new fuel. The only adjustments ive made have been with the throttle/ignition synchronisation - sync and link? Ive now got it so that it is useable, but if i allow the engine to idle or stop for any time, it has to be nursed from fast tickover or it will just die. Once shes hot now she performs ok - i had 2 hrs non stop on sat with kids on kneeboard - great fun but i know its still not quite right!! steve
 

DavidaP

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Joined
Aug 22, 2004
Messages
8
Re: '76 Johnson 70 HP, runs great cold, but...

My '78 70HP does the same thing. I've also had the carbs re-built, new power-pack with no luck. You said that you had the low speed jets increased; how did they do this? Did they have to do any drilling and re-threading?<br /><br />What is WOT?
 

bikodog

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Joined
Jul 31, 2004
Messages
8
Re: '76 Johnson 70 HP, runs great cold, but...

Steve D., Get the low speed jet replaced, I bet that's your problem...<br /><br />No drilling, they just increased the size of the low speed jet. it must be a simple procedure, because they're going to bump it up another size for no charge. <br /><br />My understanding of it is that with an older motor, you need to increase the jet size to give it enough gas for the midrange throttle before the high speed jet kicks in. The size increments are pretty small, but what you get with the larger jets, besides poorer fuel economy, is a rougher idle because there's more fuel in there.<br /><br />My mechanic said that whenever they do a complete motor rebuild, they increase the jet size on the low speed jet. Apparently my rebuilt motor didn't get them.
 

MESS147415

Seaman
Joined
Jul 5, 2004
Messages
61
Re: '76 Johnson 70 HP, runs great cold, but...

thanks bikeodog, i'll pull one of my jets to check size at weekend steve
 
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