84 Johnson 70hp - problems getting up to speed

[kehops]

Hi,

Before I explain my issue, I just want to say that this is my first boat over 15hp so I'm somewhat new at this. Thanks.

I recently purchased a 16ft Sunray with a 70 Johnson outboard. When I picked it up, the motor started right up and idled nice and smooth. I took it out for the first time 3 weeks ago and had no problems with it.

This weekend I took it out for the 2nd time and flooded the engine when trying to start it (I used the choke and a bit too much idle gas when it would of started fine without it). Anyway I finally got it going and it seemed fine at first but after running a bit I noticed that the throttle response was suffering a little bit. I shut her down and anchored for 30mins or so, when I started it up and took off again, the engine ended up dying on me when I gave it full throttle. We had 4(max capacity) people in the boat so it would sometimes take a few seconds to start planing properly but usually be no problem. Basically I can hear the engine wanting to shutdown when I give it too much gas. I literally have to creep up the throttle really slowly in order for the engine to turn faster and not die out on me. Eventually I got up to speed and the engine ran great but getting to this point is an issue.

I decided to dock it and take the cover off to have a closer look. I noticed that the 3 bottom screws for the carb cover were missing!!! As a result of this I noticed that a bit of fuel was dripping down(nothing major). I'll be replacing the screws tonight and wanted to know if I should be looking at other things while I do this. Honestly the amount of fuel dripping isn't over-dramatic so I'm not 100% that this is causing my problem. As I said, the boat starts like a champ everytime and idles great, but once you give it too much gas to quickly, it dies-out.

A few more things to point out:

I'm usually mixing around 40-1 and was running out of gas yesterday. I only had non-mixed fuel on board for some reason so added a little bit which would have leaned the mixture to probably 25-1(edit: 55:1 :redface.

Someone also mentionned I should make sure that I have air circulating from the gas tank. If no air is coming in then the engine has a harder time getting the fuel right? My tanks are metal(will be purchasing plastic soon) and when I put the cap on, I make sure it's on good. Is there something I'm missing or should be looking at to ensure air coming in?

Is it better to be using premium gas for these older engines? I was using regular and someone mentionned that I should use premium...

All in all, the engine has been very well maintained by the original owner I bought it from. He had it shrink wrapped every winter and kept it maintained regularly and apart from this new problem mentionned above, the engine runs great.

Any opinions or suggestions welcomed! Thanks you very much.

JF

 

[jtexas]

Re: 84 Johnson 70hp - problems getting up to speed

welcome to the forum!

There has to be a vent on the gas tank, the metal ones usually have a thumbscrew. Without a vent, the pump will be pulling against a vacuum, which it can only do for a little while.

50:1 means 50 parts fuel to 1 part oil. 40:1 is more than you need, you may have fouled the spark plugs. Never, *but NEVER* add straight gas to a nearly empty tank. Your mixture would have got higher, not lower, but that's not the point. Would you drain the oil out of your pickup then drive it down the road? Then don't run your boat motor without oil! Not even in an emergency.

The missing airbox cover screws aren't a big deal, pick three at random and put them in the bottom holes to keep oil from leaking into the lake. 1/4" nutdriver is the best tool for that, it won't line up just right, but you can force 'em in.

Your symptom sounds like a link & sync issue. Look on the starboard side of the engine...the cam needs to hit the roller right on the mark...

if that doesn't help, I recommend a carb rebuild. Might as well order your service manual now -- it'll pay for itself many times over.

 

[kehops]

Re: 84 Johnson 70hp - problems getting up to speed

Thanks for the response.

I ended up opening her up and noticed that the top Carb was missing the 2 front screws where the gasket is on the float bowl (this was also the cause of my leak). The air/fuel ratio was all screwed up and was much more obvious at WOT.

I bought some replacement screws and put them on the carb. It now seems to be running much smoother so I think it's ok.

As for adding "non-oiled" fuel, I'm talking minimal amounts in a 1/2 full tank so I don't think the ratio was that high. You mention that 40:1 is more than I need...you mean more than I need oil or gas? I read in my manual that 50:1 is the correct ratio but this might have been for engines with VRO. Do you know what the correct ratio should be for an 84 70HP Johnson w/o VRO?.

As for the gas tank, mine is metal and I just tighten up the gas cap until it is snug and stops turning. When I open it up, there is a clear air presssure release sound that occurs, does this mean my tank is not vented? And if it does, does this affect performance or does it only wear down the fuel pump prematurely?

Thanks for your help!
JF

welcome to the forum!

There has to be a vent on the gas tank, the metal ones usually have a thumbscrew. Without a vent, the pump will be pulling against a vacuum, which it can only do for a little while.

50:1 means 50 parts fuel to 1 part oil. 40:1 is more than you need, you may have fouled the spark plugs. Never, *but NEVER* add straight gas to a nearly empty tank. Your mixture would have got higher, not lower, but that's not the point. Would you drain the oil out of your pickup then drive it down the road? Then don't run your boat motor without oil! Not even in an emergency.

The missing airbox cover screws aren't a big deal, pick three at random and put them in the bottom holes to keep oil from leaking into the lake. 1/4" nutdriver is the best tool for that, it won't line up just right, but you can force 'em in.

Your symptom sounds like a link & sync issue. Look on the starboard side of the engine...the cam needs to hit the roller right on the mark...

if that doesn't help, I recommend a carb rebuild. Might as well order your service manual now -- it'll pay for itself many times over.

 

[jtexas]

Re: 84 Johnson 70hp - problems getting up to speed

ok, didn't mean to get preachy about the fuel/oil thing. 50:1 is the proper ratio. 40:1 is more oil (40 ounces of gas for each ounce of oil, instead of 50), only real downside is more smoke and eventually, fouled spark plugs.

If pressure releases from your gas tank when you pop the top, then it's not vented. It'll run until the vacuum just gets too strong for the pump to overcome. Forget to vent a plastic tank, it'll suck the sides of the tank in before it finally quits. Don't ask me how I know.

 

[kehops]

Re: 84 Johnson 70hp - problems getting up to speed

Ya, the ratio made sense in my head using my special math

What can happen to the fuel pump in this case? I'll make sure to have it vented next time but could this have cause damage to the pump?

I took it out last night and still had the same problem as before. It seems to have a really hard time when I give it too much throttle too quickly. It just keeps trolling along until finally, slowly it recognizes that the throttle is on and starts picking up. Then at full WOT, it starts to intermitently stop and go, stop and go. Almost as if I was pumping the throttle back and forth when in fact I was WOT the whole time.

When colder, the motor goes. When it has been running for 5mins or so, it has a really hard time. Almost as if the gas can't get to it fast enough.

Thoughts?

Thanks for your help.

ok, didn't mean to get preachy about the fuel/oil thing. 50:1 is the proper ratio. 40:1 is more oil (40 ounces of gas for each ounce of oil, instead of 50), only real downside is more smoke and eventually, fouled spark plugs.

If pressure releases from your gas tank when you pop the top, then it's not vented. It'll run until the vacuum just gets too strong for the pump to overcome. Forget to vent a plastic tank, it'll suck the sides of the tank in before it finally quits. Don't ask me how I know.

 

[jtexas]

Re: 84 Johnson 70hp - problems getting up to speed

lack of vent won't hurt the fuel pump any.

Just to be clear, when you had it out last night, the tank still wasn't vented? If not, I kinda think you need to run it with the tank vented before proceeding - it might be masking other symptoms. You can loosen the cap, or take it off entirely, shield it from getting water or debris in there.

If you did vent the tank last night, then you've got a cylinder cutting out. In fact, these motors can sound pretty darn happy idling along on two cylinders.

Were it mine, I'd do a compression test and spark test, and if those checked out, a carb rebuild. She just always seems happier with fresh carbs.

I'm not talkin bout a "spark plug" test - get an inline spark tester (couple bucks at autozone), set the gap for 7/16" and look for a sharp blue spark.

If one or more won't jump a 7/16" gap outside the cylinder, check back for the next step.

 

[freddyray21]

Re: 84 Johnson 70hp - problems getting up to speed

still sounds like a non vented tank or not vented enough. When cold the fuel tank has been sitting long enough to get air in the tank. As it warms up it draws gas out of the tank and if no air can enter the tank because of a closed vent or plugged vent it creates a vacuum and it gets harder and harder for the pump to deliver fuel to the carbs. When you first open it up it needs the most fuel it will ever use when running. after it gets on plane it takes less fuel to run the same rpms. Run it once with the gas cap off or just barely on

 

[kehops]

Re: 84 Johnson 70hp - problems getting up to speed

lack of vent won't hurt the fuel pump any.

Just to be clear, when you had it out last night, the tank still wasn't vented? If not, I kinda think you need to run it with the tank vented before proceeding - it might be masking other symptoms. You can loosen the cap, or take it off entirely, shield it from getting water or debris in there.

If you did vent the tank last night, then you've got a cylinder cutting out. In fact, these motors can sound pretty darn happy idling along on two cylinders.

Were it mine, I'd do a compression test and spark test, and if those checked out, a carb rebuild. She just always seems happier with fresh carbs.

I'm not talkin bout a "spark plug" test - get an inline spark tester (couple bucks at autozone), set the gap for 7/16" and look for a sharp blue spark.

If one or more won't jump a 7/16" gap outside the cylinder, check back for the next step.

I actually tried running it with the cap lossened and removed completely, it unfortunately didn't change much. It seems to pick-up a little better when I've let it idle for 1min or so. Only for a few secs though, then it starts sputtering and lagging again...

Is there a write-up for doing a proper compression test and spark test?

Thanks

 

[b_lish1966]

Re: 84 Johnson 70hp - problems getting up to speed

I had some similar problems with my 85 70hp johnson. Two things that I fixed to correct the problem were to replace the center screw on the fuel pump, mine was stripped out and also made sure the tank was vented and she now runs much better. I did also check the compression and spark and those things did check out fine, so it may be worth your while to do the same.