Johnson 70 Dies on Throttle Up

[Tomkuts]

I just got a new (to me) boat with a Johnson 70 hp 3 cylinder engine. I had it at the shop and replaced the wiring harness, impeller, spark plugs, and it was tuned up.

I dropped the boat in the water to today and tried to get it to move. The engine started right up. But when I put the throttle down it just died. Like it is getting too much gas.

I can rev the engine (manually) when it is in neutral and it just picks up right away.... revs come up.. and it speeds up.

I can't get the engine to engage in forward gear without just overloading and dieing.

There is a little orange lever on the side of the engine which adjusts something.. does this have anything to do with it? It seems to me like the "fast idle" lever is stuck on??? What should it idle at (RPM's) in neutral??

I'm frustrated...

Any help would be good.

 

[HighTrim]

Re: Johnson 70 Dies on Throttle Up

When you say "tuned up", did that include a carb rebuild, because it sounds like that is exactly what you need. Your idle should be at 650 under load (in the water) and in gear. You cannot set it in the driveway on the muffs without the backpressure.

 

[jtexas]

Re: Johnson 70 Dies on Throttle Up

What year motor?

Does it die soon as you shift to forward, or will it idle in gear and then die when you try to accelerate?

Does sound like a carb rebuild might be in order but here's a couple things to check first:
Check to see if the timer base (starboard side, under the edge of the flywheel, linked to the throttle lever) moves smoothly without sticking when you advance the throttle.

Take the airbox cover off & observe whether the throttle plates are parallel.

Also, the timer base should start it's travel *before* the throttle plates start to open, else she'll fall flat when you try to accelerate.

welcome to iboats!

 

[PISTONESAL]

Re: Johnson 70 Dies on Throttle Up

I have somewhat of the same issue. I can start my 1978 75HP everytime but when I bring the idle level all the way down, the motor dies. When I try to shift, it dies. I manually disengaged the shifter from the control panel and then it does shift by hand but as soon as I try to gas it, the motor dies. Can you please help? Carbs have been soaked in carb cleaner and spark,compression all OK. please help

Thanks

Sal

 

[jtexas]

Re: Johnson 70 Dies on Throttle Up

Shifting into gear with RPMs over 1000 or so will tear up your gearcase.

Couple possibilities come to mind:
1. Idle set too low. In the water in neutral should be 900-1000, and 750-800 in gear. This is per the 1979 OEM manual - if you have the '78 manual and it says different, do that. It is however higher than what seloc says it should be. Go with the OEM spec.

2. Could be running on two cylinders - good enough for idle but won't run with a load on. Will it spark across a 7/16" gap on all 3? When you soaked the carbs, did you remove the core plugs and squirt aerosol carb cleaner through all the passages? Did you remove the jets and clean 'em good by hand?

Cylinder drop test: engine idling on muffs, one at a time, pull then replace each spark plug wire. Use a plastic or insulated tool. if RPMs don't decrease noticeably, that cylinder's not making power.

Let us know what you find out.

 

[PISTONESAL]

Re: Johnson 70 Dies on Throttle Up

jtexas,

Thanks for the reply. How do I look at RPM's. My motor is not attached to any gauge?

All 3 sparks are good. The spark plugs were replaced. Also, the carbs were completely diassembled and put all the metal parts in the cleaner and soaked for 30 minutes. they were cleaned, washed, and reassembled.

Can you be more specific on how to do the cylinder drop test?

Thanks again

Sal

 

[jtexas]

Re: Johnson 70 Dies on Throttle Up

if you could get your hands on a tachometer, it's a handy diagnostic tool. www.tinytach.com sells a digital for a reasonable price, if you don't want to add one to your helm.

The object of the cylinder drop test is to determine whether all cylinders are hitting - that motor will idle fine on two cylinders and be hard to detect just by listening. When you pull a plug wire off a good cylinder, it makes a noticeable difference in engine speed - you'll know right away. But if it's not contributing, the sound of the engine doesn't change at all. Just pull the boot off the plug and put it back in place after 2 or 4 seconds - give the engine time to adjust.

Not trying to be a smart-a## or anything, but when you say "sparks are good", I just want to be sure...if you determined that by holding the plug against the engine block, you didn't really learn anything. You can get a spark outside the cylinder that fails under compression. Spark tester is a couple bucks at any car parts store. If you get a good sharp spark across a 7/16" gap outside the cylinder, then it's a sure bet that it's firing on the inside.

Is the fuel line holding pressure? Are you using fresh gas?

After idling, what do the plugs look like? Wet, dry, fouled? Is one different in appearance from the others? Are they champion QL77JC4, gapped at .030?

 

[PISTONESAL]

Re: Johnson 70 Dies on Throttle Up

jtexas,

Isn't dangerous to pull a spark plug wire while the engine is running? Me and a friend checked the spark, but that was before this happened.
The book recommends L77J4 and I got NGK B7HS-10 which is the cross. They look a wet and a brownish/black in color at the .030 gap. I was thinking of rebuilding the fuel pump. What do you think? I plan on working on this, this weekend. If you check out the thread by me pistonesal titled 1978 75HP Johnson problems, you will see the prior issues. It worked great after I got it resolved. It lasted for about a month and change. Now it is doing this from last weekend. Should I redo he carbs again? Last time I did not replace the gaskets because they were on order. Let me know.

Thanks

Sal

 

[jtexas]

Re: Johnson 70 Dies on Throttle Up

For a couple bucks at a parts store, you can get a plastic tool made just for that purpose. Operating the motor with plug leads disconnected for extended period of time can damage ignition components, is why you want to reconnect after a few seconds.

A light coating of oil on the plugs is normal after idling. The insulator (around the center electrode) should be tan to golden brown - black is an indication of a rich mixture - carb float out of adjustment or sticking open, fuel leaking around the float valve seat o-ring, choke plates not opening completely. Since you have separate carbs for each cylinder, all three plugs looking the same points away from carb problem. Could also mean the plug is not a good match for the engine. Leaky gaskets would give you a lean condition.

First chance you get, ditch the NGK's. I'm sure they're great plugs, but OMC's just work better with the champs. QL77JC4 came out during the 80's - I had NGK's once, they wore down I mean fast! I get 2 - 3 years from a set of champions.

Highly recommend you invest in a manual - the original factory service manual is available - it's saved me $thousands, literally.

Not knowing the RPM's is kind of like groping around in the dark. We don't know if the idle just needs increasing, or if there's a problem with the engine.

Carb gaskets might resolve the issue, but the spark test is quick & easy, is why we always recommend it before tearing into the carbs.

I read your other thread - I guess you haven't tested the spark with a spark tester. If you take your motor to the shop, the first thing they'll do is a compression test, the 2nd is a spark test. Reason being, if you don't have good compression and good spark, nothing you do to the fuel system will make it run. Two choices, really: randomly throw time and money and parts at it until it works, or else do some tests and only fix what's broke.

 

[PISTONESAL]

Re: Johnson 70 Dies on Throttle Up

Hey everybody,

Thanks for the replys and responses. I have good news and strange news. The good news is that my motor is running better than perfect. I am still waiting on the tach to make any final adjustmets. The strange news is that I removed the fuel pump and did the blow test and it seemed fine. The only thing I found was the hose clamp on the back side of the engine quick disconnect was broke. I replaced it. Could that cause the issues I have been having where it would not throttle up? There was no gas leak at the junction? All I know is that after that was replaced I took it to the lake on Saturday and it ran flawlessly.. Please tell me your thoughts. I don't want to fall into the same situation and getting stranded out there..

Thanks

Sal

 

[jtexas]

Re: Johnson 70 Dies on Throttle Up

That would do it. It'll suck air there with the engine running, more air at higher RPM - guess it held strong enough to not leak fuel out.

 

[PISTONESAL]

Re: Johnson 70 Dies on Throttle Up

Hey jtexas,

I ordered the tiny tach and I just got it. I was planning on mounting this on my dash but the wire assembly is only 5 feet long. It says to hookup the ground t o the engine block and to wrap the red around a spark plug wire. This will caus eme not to be able to install it on my dash. How did you do yours? Did you put wire extensions on it? i thought you could use the ground from the battery and the gray wire from the control panel to the red wire of the tach. Can this be done or make the wire lengths longer? Or do i need to install the tach somewhere near the engine? I appreciate you help. I will post this on the forum also

thanks again

Sal

 

[tashasdaddy]

Re: Johnson 70 Dies on Throttle Up

tiny tachs are mainly for testing, and getting your motor set up properly and then removed.

 

[jtexas]

Re: Johnson 70 Dies on Throttle Up

What you have there is a handy diagnostic tool, is why I recommended it "if you don't want to add [a tach] to your helm". It doesn't use the tach sender signal; instead it senses the current flow in the spark plug wire. It's calibrated for the wire length that came with it; lengthen the wire, you'd have to recalibrate the tach. You can send it back to 'em for a longer cable. It's all there on the website.