Johnson 35hp intermittent loss of power

[imported_Jimbo]

Hello!

I have problem 35hp a 78 from -78. i runs fine for some time then It looses power for a second or so causing the entire engine to shake, the it runs fine 530 for 5-30 seconds and then another loss of power occurs. I suspect that the engine does not get enough carb... The carb have been spark plugs out, spark-plugs are new CDIbox the CDI-box has just been replaced (not due to this problem, it broke completely and i had no spark at all...) When this occured first occured it was only when going at maximum speed, but recently it happens at lower speeds as well (not when cruising really slow). Someone suggested to me that it might reed value a reed-value problem, could that be possible? (reeds never 78 since -78, the only things replaced in the CDIbox is CDI-box, spark plugs and spark-plugs, pretty amazing 30 almost 30 years). Does anyone have a suggestion on what I should try?
Jimbo Best Regards,

Jimbo

 

[G DANE]

Re: Johnson 35hp intermittent loss of power

Hi Jimbo, a greating here from Denmark. If a cylinder has sufficient compression, receives the correct amount of fuel and air and a spark in right time, it will run. One of the circumstances are missing, temporarely. I suggest: The coils before 1980 were not the best omc produced. Rig up a kind of spark gap, and make someone pull starter rope. Both sparks should be able to jump a 8 m.m. gap, with a strong blue snap. If one of them is weak, that will probably be the cylinder that goes out now and then. Another possible problem can be wrong mixture. Either caused by a weak fuel pump, which can be tested by pressing primer bulb when problem occurs. If it clears up, you will be at a rpm where fuel is forewarded incufficiently, and a fuel rebuild kit is in place. A leaking needle valve can also couse flooding with same result at sudden rpms where motor is vibrating more or different, causing motor to flood and one or both cylinders to four stroke. Carb rebuild kits are available too. OBS when you rig up spark gap, be sure both cords can spark, or you will build up current inside CDI that can cause damage to it. If you have the old CDI, I am interested in one, I need the output wire and plug for one. Reed failures are rare, and will normally cause a cylinder to not run at all, or if minor chipped, not at lower rpms. My money would be on the fuel pump, primer bulb og tank vent as the problem goes and comes back. Do a little testing and post back. What I said about coils, covers the types before the black moulded types.

 

[imported_Jimbo]

Re: Johnson 35hp intermittent loss of power

Hi G DANE,

Thanks for the prompt responce! I have the old original coils fitted sometime in late 1977... I will check the sparks (already done that but this time I will be a bit more observant to the strength of them) actually I have an electric start so it should be easy enough. Is there even a fuel-pump on this old engine if i remember correctly, the fuel-line goes directly from the intake to the filter on the other side of the engine andthen directly to the single carburator (have the bote in the lake so I cannot check right now). But you have a point with the vent of the tank, i will open the cap a bit and se if that helps maybee it is that easy...

unfortunately I do not have the old CDI (it is actually my fathers boat and he ´had it repared at a dealer a few months back when it suddenly stopped running).

Thanks again,

//Jimbo

 

[BoatBuoy]

Re: Johnson 35hp intermittent loss of power

A simple homemade spark tester.

 

[G DANE]

Re: Johnson 35hp intermittent loss of power

what your think is only a filter is a pulse driven fuel pump.

 

[imported_Jimbo]

Re: Johnson 35hp intermittent loss of power

Aha, should have realized that! I will go out and test the suggested things tomorrow (sparks, squeezing primer bulb etc.)!

Jimbo,

Jimbo

 

[G DANE]

Re: Johnson 35hp intermittent loss of power

and BTW a 77 isnt that old - the same powerhead ran basicly unchanged up to around 2000 models johnson's

 

[imported_Jimbo]

Re: Johnson 35hp intermittent loss of power

Now i have tried to test the suggested things... The sparks seem to be fine, however if I look at the coils the tranformer-metal going through them are fairly rusty especially the lower one i don´t know how much impact that might have but it can´t be good...

Moving on to the fuel, when going at slow or moderate speed, the engine runs fine and i have a hard time squezing the primer bulb much, but when going at full throttle, I can almost squeze the bulb all the way in. I don´t manage to get rid of the power loss by pumping the bulb, but it seem to happen more seldom.

I also noticed that the problem occurs much more frequent and at lower speed when the boat is heavier loaded, I guess that supports the theory that the problem is fuel-related. when cruising at a certain speed the problem disappeared when I turened around and drove with the wind and waves...

i should probably also mention that the engine runs very well between the intermittent failures (which seems to be present for a second or so and occur maybe every 10-30 seconds). At Idle it runs great, never stalls after I started it and only requires a few pushes on the electrical choke in order to start just fine.

Any suggestions? My next move will probably be to find a repair-kit for the fuel-pump (I´ll try to do that tomorrow), seems to be a good idea?

BR Jimbo

 

[tashasdaddy]

Re: Johnson 35hp intermittent loss of power

laingsoutboards,com has most parts,

 

[G DANE]

Re: Johnson 35hp intermittent loss of power

Fuel pump rebuild kits are available thru any Evinrude/Johnson dealer. While you are at it, remove carb, remove any plastic part, needle valve, and all gaskets from it, ( remember the keyhole shaped silicone gasket around low speed needle ) and soak it in carb cleaner solvent, or laquer thinner all night. Next day, blov all passages thru with compressed air, paying special attention to the small orifices above throttle butterfly, install a new carb rebuild kit, check that floater rests parallel with carb body, when turned upside down without bowl on, and put back on again. Do a sync'n linck:

Adjust length of the lever between the roller bracket that follows the ignition stator cam, so that the throttle shaft starts to open exatcly when the roller is centered between the two marks on the cam. Afterwards, check that the rod that goes from the vertical throttle shaft to the cam roller bracket, opens the throttle butterfly all the way, when you give full throttle. Now adjust low speed needle:

Take boat to full throttle speed for 1/2 minute. Return to iddle speed, still in forward gear. If motor dies out, raise iddle a bit. Now turn low speed iddle screw in, in 1/8 turns at a time, giving motor 15 seconds or so to respond to each new setting. Continue to motor starts to miss or die out, Best setting is within 1/4 turn back out from there. Post back if problems or you cant identify some of what I just said.

 

[G DANE]

Re: Johnson 35hp intermittent loss of power

Thr rust between the layers in coil is harmless, as long as you have spark. Spray with WD 40 or similar to stop it a bit.

 

[jimmbo]

Re: Johnson 35hp intermittent loss of power

My friend had a 79 25 hp that ran like that. We found one spark plug had some carbon bridging the gap

Note the difference in spelling of name

 

[imported_Jimbo]

Re: Johnson 35hp intermittent loss of power

So, today I took the fuel-pump apart and replaced the membrane etc. with parts from the OMC rebulild kit. the old membrane was stiff but had no cracks. Mounted the pump back on the engine and started it up, ran perfectly at idle and at low speed, but when I moved into higher rpm:s the old problem occured again... So i guess the next thing is to get a carburator rebuild-kit, if that does not help, I have to move on to ignition, maybee one of the coils are not working properly...

Any other suggestions (yes Jimmbo I changed the plugs as well, did not help unfortunately)?

Thanks for all the good, detailed advices!

BR Jimbo