1976 Johnson 70hp will only run low rpm after warming up.

[Casey J Pieretti]

Hi, Ihave a 1976 70hp. Good compression, good spark and runs like a champ for about 3-5 minutes. Then it will only run at low speed starts just fine, revs in neutral. Fuel ball Ball is hard. Even if it site for a few it will not run at higher speeds. New gas, New gas tank, New fuel lines. I don't know how long it sat before I picked it up. Looks to have New coils and wiring. I don't know plugs. But they are clean. Motor is not over heating. Impellor is new. Block is not hot.

Any ideas?

 

[CaptnKingfisher]

new gas, New gas tank, New fuel lines

Any history on the motor? Has it had this problem since you got it? what prompted the new gas tank and fuel lines? I would suspect previous owner might have gotten water in the fuel. I have had this same condition from having water in my carbs.. starts and idles fine at low ROMs, stalls when going into gear or increasing throttle.

If you have water in your carbs the solution is easy. Drain your carburetors and pump fresh fuel through it. If you have a fuel water/separator you may need to replace or drain the contaminated fuel out. Idk if I'm making this last part up or not but I do personally.. I fear water has gotten into cylinders when I get water in carb so I take out all the plugs and turn over the engine so the water can be expelled from cylinders.

Other thoughts.. if boat has sat awhile might be due for a carb rebuild. I hate rebuilding carbs tho so I do as thorough a diagnosis as possible. Measure for strong spark with an adjustable spark tester,
read plugs for damage, gap, fuel mixture, measure for good, even compression, measure fuel pressure, make sure fuel tank is vented properly and using quality fuel.. if all the above checks out then I rebuild carbs.

 

[Casey J Pieretti]

I have no history on motor. Only it sat in a garage for a few years. What gets me is it jumps out of the hole, but after about 3-5 minutes of running at speed it just will not run in gear at anything more than an idle. I can shut it down and let it sit for a few minutes and it will start right up, but will not move in gear at anything over an idle. If you gas it , it will die. But if I bring it home and try the next day it repeats the same problem.

It was drained of all fuel when it sat. The new lines and tank are because I am putting it on a "63 dorsett. It had no tank or hoses.

I am not familiar with venting the tank. It seems the fuel system is sealed. Could it be creating a vacuum and starving fuel?

 

[Casey J Pieretti]

Also, since the motor sat these first 6 gallons are at 40:1. Though I put some oil in the cylinders before turning it over. Compression is great and even. Idles well, starts right up at sea level.

 

[flyingscott]

Did you clean the carbs? Looked at the fuel pump diaphragm? What happens if you pump the primer ball?

 

[Casey J Pieretti]

An update, tok off carb cover housing. Floats are working. All looks clean. Found a hose nightmare, see photo. Pumps up ball and leaks fom every hose. Will check fuel pump next. But I will restate. The motor runs great for several minutes than will only idle while in gear. I can shut it down and start it up without issue. Idles fine throughout.

 

[Casey J Pieretti]

Next update, the three way splitter is too small for the hose. Definitely allowing air onto the fuel system. As well as it being cheap plastic....and my favorite, the zip tie hose clamps!

 

[CaptnKingfisher]

An update, tok off carb cover housing. Floats are working. All looks clean. Found a hose nightmare, see photo. Pumps up ball and leaks fom every hose. Will check fuel pump next. But I will restate. The motor runs great for several minutes than will only idle while in gear. I can shut it down and start it up without issue. Idles fine throughout. View attachment 358562

Looks to me like you found the issue

 

[Keyboardman]

If you're tank is not vented, you will be starving for fuel. I'd check that out.

 

[racerone]

You might find that 5/32 ID is the right hose.-----And use ty-wraps made for hose.

 

[Casey J Pieretti]

Well, reworked the lines.... same issue upon test run. I did find the pump ball was soft after running. I will next check the fuel pump then move into electrical.

 

[racerone]

The primer bulb is a simple manual pump to fill the carburetors for start up.------It saves the starter motor from doing that.-----Or on a manual start motor it saves your shoulders.-----And the bulb does nothing when motor is running.-----And the bulb does not stay hard when motor is running !!

 

[Keyboardman]

Racerone is right, the bulb does not stay hard while running. Seriously, have you made sure you're tank is vented? My 75hp Stinger lets me know in just a few minutes if I forgot to loosen the cap on my portable tank.

 

[Casey J Pieretti]

So I ran with loose cap and had no change. I am now going to rebuild the fuel pump as it is $20 for th kit.

 

[racerone]

Are compression values up around 140 PSI ?-----Does spark jump a gap of 3/8" on each lead ?----If you have original steel tanks they are sealed.-----But when you attach the fuel hose there are 2 small pins that are pushed in.----They are valves.-----One lets fuel out of tank and other is the vent.

 

[Casey J Pieretti]

Compression all equal at 120 on one guage and 145 on the other. I have a new plastic tank. No venting. I pumped up carbs and left cap cracked on a 3/4 tank. Good spark, gap is spec. Like I said it runs well for a few minutes than only idles under load. Does not overheat. Shuts down and re-starts easily. When running had great power. I am switching to your above described tank, and rebuilding pump. I will also check all coils and wires and replace plugs.

 

[racerone]

There may be a service bulletin regarding carburetor issues on that motor.

 

[Keyboardman]

Those carbs had a problem with a flat spot on acceleration, I have the TSB somewhere. Is the timing base sticking? I've had that problem before and the engine would not run over idle.

 

[Casey J Pieretti]

I also know it ran well before it was sitting in garage. It was also at altitude in the sierras, sitting dry for two years.

 

[Casey J Pieretti]

UPDATE: RE-read the gas tank documents and it is a new west marine tank that vents. I found poorly connected fuel line that was leaking. Waiting for fuel pump kit. Then test run number 4.