70 hp boggs down when gunning the throttle

[red_chaser]

Hello,

I've got a Johnson 70 hp, 1977. I brought her back to life last year.

I did a carb rebuild, replaced fuel lines, water pump replacement, thermostat replacement, sea foam decarb process......

When first started up, I let my engine warm up a little before I open her up all the way. She responds great, she takes off. After being in a fishing spot for a while I have to do the same. If I gun the throttle from the get go, she starts to lose power and the engine shuts down. I have to warm her up again. Even if I've been fishing for 15 minutes, I have to do the same, let her warm up before opening the throttle. Is this normal? Recently, it seems I have to let her warm up longer to get her going.

Any help to find out what is happening is greatly appreciated.

 

[oldcatamount]

Re: 70 hp boggs down when gunning the throttle

Sounds like you're loosing the prime in the fuel line somehow. Check the primer bulb, fuel line, clamps, etc. If the line is old and cracked, you might consider replacing it. Also check the fuel line to motor connection. There's an o-ring inside and if it's worn out, it would give the same symptoms as you describe. Hard to be exact on this but this is where I'd start looking.

 

[jtexas]

Re: 70 hp boggs down when gunning the throttle

Definately not normal.

Does it restart easily, after the 15 minutes, without using choke or hi-idle lever?

Is the primer bulb still firm when you go to restart?

 

[red_chaser]

Re: 70 hp boggs down when gunning the throttle

1. I have replaced all the old fuel lines with new ones. All the hoses are connected and clamped on tight.

2. The motor strats right up with no issues, choke, or idle lever. I have checked the bulb, thinking that there was no fuel in the line, but when I tried to prime it, it was already firm and full of fuel.

I read on a similar thread that it could be a tank vent issue, could that be the case here?

 

[oldcatamount]

Re: 70 hp boggs down when gunning the throttle

Well, that happened to me with my '88 50 hp Johnson, it would start nice, take off nice, then fall on it's face. Re-built the carbs and tried it again, same result. I swapped fuel tanks and the problem went away. It was the tank vent that was the problem and there is no really good way to test them other than what you're doing now. So, if you have another tank, try it out and let us know waht happens.

 

[jtexas]

Re: 70 hp boggs down when gunning the throttle

One thing's for sure -- it's not cooling down in 15 minutes, especially since it restarts easily, so you know it's not "warming up". Fuel line stays pressurized, means plenty gas in the carb bowls. Dirty carbs would cause it to bog every time (far as I know). You can rule out a blocked vent with a test-run without the gas cap.

I would probably re-check the floats, to be sure it's not flooding a little after you shut it off, although in my experience, when that happens it gets reluctant to restart without a little hi-idle throttle. Make sure the throttle & choke valves are all in sync. Timer base isn't sticking.

Does it idle good at a steady RPM?

 

[red_chaser]

Re: 70 hp boggs down when gunning the throttle

It idles great and has a nice steady RPM.

All this tank talk is making me suspect my gas tank. The cap is cracked and allows some gas to escape when the gas sloshes around. Also, I belive the connection to the tank is bad. It does come off every now and again on its own. It could be that it is sucking some air. I'll fix that and try it out as soon as the weather gets better.

One question, I have the six gallon gas tank with a vent, does that vent normally stay open or closed? I've been keeping it open so there is not a vacuume in the tank.

 

[jtexas]

Re: 70 hp boggs down when gunning the throttle

If the quick-disconnect falls off on it's own, it's ready to be replaced.

I leave the vents open on my plastic tanks, mostly cause fuel can squeeze out the cap if the temperature rises enough. Metal tanks maybe not such a big deal.

 

[h_lankford]

Re: 70 hp boggs down when gunning the throttle

rebuilt carb - correct fuel orifices? even if so, why not just put in next size richer and see what happens. Wish I had done this years earlier on my Johnson 60. Enriching it solved this bogging.