Johnson 70 hp 3 cyl / 2-stroke dies when giving more throttle

[Faztbullet]

Motor idles to rough..suggest you pull head and inspect cylinders.

 

[racerone]

What pitch prop is installed on this motor ?

 

[JohnnyMat]

1983 J70ELCETE

I just had a very similar problem. Turned out to be my brand new fuel pump failed (torn flapper valve). It would start and idle great, but wouldn't come up on plane and only had half power at full throttle. Never really stalled out though.
I pulled the fuel pump and split it apart...found a split rubber flapper. It was a cheap eBay Chinese knockoff...lesson learned!
Still had the old original OEM, threw it on fired right up... but still had the same power issues.
Turned out the small rubber *** that holds the flapper in place (knock off pump) fell off and got stuck in my lower fuel line right before the 1st "T".
Took that out...runs like a champ. It was the size of half a BB.
There was less than 2 hours of use on that new fuel pump. JUNK!

I'm also currently on vacation with the kids...so figuring this out was a win for the home team!

My top carb spits fuel at me too. I have brand new fiber reeds, good compression, no scoring of cylinders, rebuilt and synched carbs, etc...
I will dive into that more after vacation.


Good luck, I hope you find the issue!

 

[Bernt]

1983 J70ELCETE

I just had a very similar problem. Turned out to be my brand new fuel pump failed (torn flapper valve). It would start and idle great, but wouldn't come up on plane and only had half power at full throttle. Never really stalled out though.
I pulled the fuel pump and split it apart...found a split rubber flapper. It was a cheap eBay Chinese knockoff...lesson learned!
Still had the old original OEM, threw it on fired right up... but still had the same power issues.
Turned out the small rubber *** that holds the flapper in place (knock off pump) fell off and got stuck in my lower fuel line right before the 1st "T".
Took that out...runs like a champ. It was the size of half a BB.
There was less than 2 hours of use on that new fuel pump. JUNK!

I'm also currently on vacation with the kids...so figuring this out was a win for the home team!

My top carb spits fuel at me too. I have brand new fiber reeds, good compression, no scoring of cylinders, rebuilt and synched carbs, etc...
I will dive into that more after vacation.


Good luck, I hope you find the issue!

Thanks for the input! We tried with both the OEM pump and another l, but same result.
Now we wonder about the choke/primer solenoid. The manual do not explain exactly how it works. Is it acting like an accelerator pump, that could explain the hesitation we mostly see, but which sometimes is overcome? Can someone explain the function and what trigger it except activating the choke?

 

[ct1762@gmail.com]

the reason Fazt mentioned bad cylinder should be clear! if it doesnt have adequate vacuum (ie. leakdown), it cannot pull in fuel/air with the correct volume, hence spitting back wasted fuel. pull the head and check.

 

[Bernt]

Point taken, could explain the back spitting. But if such permanent damage exists, how could it be that the motor sometimes accelerates normally but sometimes just dies at mid throttle? It's hard to believe. Intermittent lack of fuel or spark seems more likely? Thanks for all input! I've ordered new gaskets and once they are on I will again test ignition under full load.

 

[racerone]

The electric primer is a VALVE , it is not a pump.------When cranking the motor you push the key in to open this valve.----The fuel pump then pushes fuel through the valve directly into the motor for near instant starting.-----This fuel bypasses the metering circuits in the carburetor.-----When the motor is running the primer valve does nothing !

 

[ct1762@gmail.com]

without having a brand new motor to compare it to, you often won't notice a motor with bad leakdown being low on power . i will say, i've had quite a few of these spit fuel out at low speed (not as much as yours), but as soon as you open the throttle plates, the vacuum increases and the spitting dissapears. make sure the pickup timing is correct, and the idle speed is set to 750 rpm in forward gear.

 

[Bernt]

Hello again folks,
Unfortunately I still have this issue. We now did all ignition related tests specified in the service manual, and we are confident we have a good spark at all op conditions and that timing is correct.

We are thus pretty sure it's fuel related. The service book does not indicate any means to adjust the carburetors, except throttle linkage adjustment. And carbs are clean with new gaskets.

We now managed to test it under load, with the boat tied up to the dock. At about 1/3 throttle it starts to hesitate. If we spray gas at the air intake at that point, it accelerates. So I guess it either 1. get too little fuel, or 2. wrong air/fuel mix.

Scenario 1, we tried with a new fuel pump, no difference. We checked fuel lines. When we pump fuel manually before start, we hear the carbs fill up and the floats then stop the flow as they should. And at sea test, we do reach full speed sometimes, so at least them it get enough fuel.

For scenario 2, my question is if the mix be adjusted? Seems not.

End of ideas. Leak at intake kind of ruled out as we installed new gasket and all flanges look great. But hard to be 100% sure.

Input please?

 

[ct1762@gmail.com]

time to remove the cyl head and have a peek. i'm suspecting shot skirts as mentioned above.

 

[oldboat1]

Might be an overheating issue affecting your '86, unless already modified: https://forums.iboats.com/attachments/photo279501-jpg.264424/

 

[Bernt]

time to remove the cyl head and have a peek. i'm suspecting shot skirts as mentioned above.

Could be, but I,m hesitating since I´ve never done that on a motor like this and might cause further issues. As I asked above, how could my problems be intermittent if the cyliner/piston is dagamed? Sometimes it do not hesitate the same way, the boat accelerates well and the engine deliver good power and revs and we reach a very normal top speed of this kind of boat. In addition, the compression rate is high and even on all 3 cyl. Beats me but me experience with these engines is a bit limited.

 

[Bernt]

Might be an overheating issue affecting your '86, unless already modified: https://forums.iboats.com/attachments/photo279501-jpg.264424/

Thank you! Will investigate. The problem however occurs also with a cold engine, and comes and goes without any obvious link to temperature.

 

[Bernt]

without having a brand new motor to compare it to, you often won't notice a motor with bad leakdown being low on power . i will say, i've had quite a few of these spit fuel out at low speed (not as much as yours), but as soon as you open the throttle plates, the vacuum increases and the spitting dissapears. make sure the pickup timing is correct, and the idle speed is set to 750 rpm in forward gear.

Thanks. You are right, the spitting stops when throttle opens a bit. And we did check the reed valves and they are in good shape. So I don´t think the spitting is releated to the hesitation/loss of power. About power, I had many engines of this power rating and on similar boats. Alltough I can´t tell exactly, the power delivered (when it actually runs well good about 1/4 of the times we try to accelerate). And top rev is normal. We get around 33 knots with the 20 foot cabin speed boat, which is in line with the boat specs and what I have seen on these boats before. I jjust find it hard to see that the reason is some permanent damage, it has to be some intermittent issue with either fuel, ignition or a mix of both....

 

[oldboat1]

Move the indicator elbow to the top as described in the service bulletin, in order to avoid/correct an air lock in the cooling system.

 

[Bernt]

Move the indicator elbow to the top as described in the service bulletin, in order to avoid/correct an air lock in the cooling system.

Thanks, OK we will try. So the idea is that the air lock from time to time triggers a high temp signal which makes the engine run on low rev /emergency mode only? No overheat alarm is heard from the controller but it might be broken.

 

[racerone]

Are the high speed jets ( orifice plugs ) in the carburetors bowls clean,---- yes , no or not sure where they are ?

 

[Bernt]

Are the high speed jets ( orifice plugs ) in the carburetors bowls clean,---- yes , no or not sure where they are ?

Yes, they are clean. We where certain after first check/ cleaning of all 3 carbs, but after still having issues we checked them one more time. All still clean and in very good general condition. But your question is very valid, dirty carburetors seems to be a likely reason. And the engine worked fine before it was moved from one boat to another, just being stored 4 months in-between. Issues noted at first test after storage.

 

[racerone]

Have you replaced all the hoses on carburetors ?-----Have you had float valve seats out of carburetor body to check for blockages ?-----Did you change the propeller if you went to a bigger / heavier boat ?----What pitch is the propeller on this motor ?

 

[Bernt]

Great questions. We replaced some but not all fuel lines. They are not blocked and not leaking, but some are old and stiff. We will replace all.

Yes floats and seats/needles are OK.

Propeller not changed but the boat is larger, point taken. Propeller is 13,75x 17, but if that would be an issue, why would it hesitate so sudden and distinctly (see video) and at quite low rev? And not all times? When it works we accelerate normally and reach normal speed
.
My feeling is still a fuel issue, or ignition but can't pinpoint it. Maybe air leak that comes and goes? Or spark that comes and goes? New plugs, plug cables, coils measured, spark gap test ok at idle and high rev (running on shore with no load, where no hesitation is present).
Maybe we drop the darn thing at deep waters. ....a full season ruined .....