1985 70HP Johnson - stalling at low RPM

[gibletts]

Ive got a bit of a problem with above motor, it will start, idle and and run well over 2000 rpm, however between idle and 2000 rpm it will bog down and stall if let run in this range for more than say about 30 seconds, if I accelerate through this range quickly theres no problem. If it does stall it starts up again in neutral without any hesitation but then needs some coaxing to get past that 2k mark again. Note VRO disconected and fuel pre-mixed. thanks dm.

 

[Dhadley]

Re: 1985 70HP Johnson - stalling at low RPM

Has the motor been sitting? Or was it fine one weekend and this started the next?

 

[gibletts]

Re: 1985 70HP Johnson - stalling at low RPM

I have owned it for about a year, bought it as "rebuilt" and have done about 25hrs on it, this problem has always been there. I have had it back in the shop but couldnt replicate the problem in the test tank.

 

[gibletts]

Re: 1985 70HP Johnson - stalling at low RPM

Given that the motor runs fairly smoothly untill it just stops in that RPM range makes me wonder if it is running out of gas for what ever reason, the fuel pump appears to be aftermarket (VRO dissconnected etc), and from memory looks pretty much the same that was on my old 1974 135HP. FYI plugs correct type and wearing similiarly, just from an eye-ball inspection sync & lync etc looks ok, carbs were rebuilt in shop (I will assume this was done properly for the moment).

So with the weekend comming up I have the opertunity to sneak her over to the marina in town (early in the morning untill someone gets pi@$$d off with me and kicks me out) and on the nice sheltered concrete ramp and comfy floating tie up jetty, I will be able to start her up and idle it in gear in the water teathered to the jetty or sitting on the trailer and have a close look at what is going on with fuel delivery etc.

Anyone have any suggestions as to what to look out for with regard to assessing fuel pump etc.

 

[LongLine]

Re: 1985 70HP Johnson - stalling at low RPM

Pump the bulb while in the trouble range & see if it still bogs down.

Tom B.
(LongLine)

 

[gibletts]

Re: 1985 70HP Johnson - stalling at low RPM

There is a filter of some kind on line just before the fuel pump, it is in a see-thru plastic tub, if fuel is being delivered correctly should this remain filled to the top during operation? thanks dm

 

[gibletts]

Re: 1985 70HP Johnson - stalling at low RPM

The weather has been a bit hopeless so I have not managed to look at this thing in the water again however..... getting back to my observations regarding the on-line filter, and before starting motor I give the primer bulb a squeeze and this fills the fuel filter to the top, once motor is running (and I have only looked at this on the muffs) the fuel gets sucked right down to the last 0.25 in the bottom of the bowl (of the fuel filter) within 30 seconds or so, is it reasonable to assume from this that more fuel is being sucked out than is able to get in? and if so we have a problem with hoses etc before the filter? if I squeeze the bulb again it fills back up, and again emties. I made up a straight hose from the tank without a primer bulb and still that fuel is sucked down so i assume this is not a fautly primer bulb. I can replicate this on either of my 3 x tanks?

 

[CharlieB]

Re: 1985 70HP Johnson - stalling at low RPM

Since the fuel pump delivers adequate fuel for WOT running, then it is more likely that you have a small air leak INTO the fuel system somewhere between the tank and the fuel pump.

Most all fuel pumps are more than capable of providing far more fuel than the engine requires as a safety margin to prevent leaning the carbs and melting pistons. At least as long as the pump is in fair shape.

At low speed the fuel pump is sucking enough air that the carbs can be running low, rapid acceleration gets past this area of 'minimum' flow.

As a test, install a piece of clear hose immediately after the fuel pump and watch for any bubbles, indicating air leaking into the line before the pump.

If the filter flows enough fuel for WOT then it is not a contributor to your problem unless one side of its fitting is your air leak.

 

[gibletts]

Re: 1985 70HP Johnson - stalling at low RPM

Thats kinda what I thought, I can see bubbles going though the opaque plastic of the filter.

 

[tedious]

Re: 1985 70HP Johnson - stalling at low RPM

An air leak between the primer bulb and the fuel filter should drip fuel when you squeeze the bulb - get out the paper towels and a flashlight! The o-rings in the OMC connectors are notorious for going bad after a time, so check where the fuel line plugs into the motor.

Tim

 

[gibletts]

Re: 1985 70HP Johnson - stalling at low RPM

Sorted, motor now runs through all ranges of RPM. I ended up replacing some fuel line and the primer bulb, this helped some but the real weird thing was the fuel filter was located as such that it was at the highest point on the fuel line and above the fuel pump (filter only half full with fuel when running), I re-positioned it so it was at the low point and below the level of the fuel pump (now runs full!) after that no more problems? Maybe still a air leak around the top of fuel filter, but anyway running so well at the moment I am reluctant to mess with it anymore. thanks dm