1970 25hp deceleration after run at WOT

[davidmbv]

Hello, I have a 1070 25hp Johnson that’s been running great. Recently when I took it out, after running it for 5-10 min at WOT I noticed it began to decelerate. It didn’t seem like it was missing but instead just felt as though I was turning the throttle down. After I dropped the throttle and ran it at lower speed for few min, it would resume running good at WOT for few minutes then the same issue would happen-loss of power. It idles great and doesn’t seem like it’s running on only one cylinder. I checked the linkage in the carb and it seems good. The throttle plate has some movement back and forth when it’s off and I open the throttle to wide open. However I really need to force the movement. Otherwise everything seems right.
Any ideas what would cause this? I rebuilt the carb few months ago and replaced the high speed jet but only soaked the carb and shot out passages. Didn’t remove plugs yet. Next step I plan to do is clean rebuild the carb again. I do notice the smell of gas or exhaust when the issue happens however there are no leaks on or around the motor or gas tank/line. New tank and line

 

[racerone]

Water pump impeller ?----Fuel pump diaphragm ?

 

[davidmbv]

I replaced the impeller and checked the fuel pump screen few months ago. I’ll check the fuel pump again. I will also plan to pull the thermostat (recently replaced) once it warm and make sure it’s getting water….

 

[oldboat1]

Post compression numbers. Touch the top of the cylinder head when it stalls. Too hot to touch means it's overheating (160F +).

 

[racerone]

Not the screen.-----Check the diaphragm in the fuel pump.----Mixing gas and oil at a 50:1 ratio ?----Checked gear oil in the lower unit ?-----Any water in the gearcase ?

 

[kbait]

Did you pump the primer bulb when it slowed? Could be a weak pump..

 

[clemsonfor]

I'm thinking it may be over heating..but other suggestions are good like weak pump, could have a coil dropping out also when it gets hot, where you only running 1 cylinder. Once it cools it starts to fire again untill hot.

 

[racerone]

If coils have never been replace it is due for a set.----Pull the flywheel.

 

[davidmbv]

Thanks for the responses. I will plan to check the fuel pump, I have a 71 25hp that runs good and I had replaced that motor’s fuel pump last year with a new OMC one. I’ll swap that fuel pump on the 70 that’s giving me issues and see if that works.
I replaced the fuel line and new primer bulb and the gas tank is newly rebuilt.( replaced the fuel connect and pickup). I will try to pump the bulb when it’s losing power.

I pulled the flywheel when I picked up this motor few months ago. I set the points following the OMC service manual and inspected the coils etc. all looked good but the coils appear to be original (blue). They looked brand new. All wires looked good and wraps look new. I have two new OMC coils for spar parts but figure I would keep the originals until they begin to fail.
I changed plugs, gear oil and impeller when I first got it few months ago and it has been running great until now. It spun a prop when I took it out for its maiden voyage and have since replaced the prop with a new OMC prop (factory replacement for the motor).
I do notice it sounds like it runs at higher RPM’s at WOT than my 71 25hp Johnson. Even when running the same prop. ..?
The linc and sync appear to be set correctly and it idles down nice and smooth.
The issue I’m having now doesn’t sound like an ignition issue. I had to work through that issue with my 71 where I ended up going through the ignition replacing coils, points, condensers, plug wire, top seal etc.
The 70 seems to just be decelerating...?. I took apart the throttle handle few weeks ago and cleaned and lubed it and also greased all the zerts.
I will plan to pull the cowling this week and take it out on the lake and check the throttle plate/cam when it begins to decelerate and see if something is slipping. It doesn’t seem to be missing (dropped cylinder) when I lose power, just seems to slow down....
Oh, and I checked compression when I first got the motor and I can’t recall the exact numbers but it I believe it was around 150 and both cylinders where the same or very close. I recall it was well within factory specs and better compression than my 71.
The motor appears to be low hours and in much better shape than my 71 .
I’ll pull the flywheel again and recheck compression if the other tests this week check out.
Pulled Both plugs few days ago and they look the same black but not super oily. I had replaced them few months ago with J6c’s. I can’t believe plugs would go that fast. They don’t appear to be fouled too bad.
Thanks again. Super fun motors that fly my 13’ fishing boat.

 

[davidmbv]

And yes running 50.1 OMC outboard oil

 

[racerone]

There is no difference between a 70 and 71 model except for cosmetics / color of paint.---Run with timing light hooked up and see if spark goes away on one cylinder.-----It should be easy to determine the loss of power.-----Plastic pin on bottom of vertical shaft still in one piece.

 

[davidmbv]

Yeah it’s weird, the same motor (70 vs 71) but the 70 sounds like its running way higher top end RPM’s.

I’ll borrow a timing light and read up on how to test it with light.
Forgive my ignorance recerone, but could you provide me with a part reference number or another description for the “plastic pin on the bottom of the vertical shaft”?
If what you mentioned is the impeller pin, I replaced that pin (#16 parts diagram) with a new one when I did the impeller.
I’ve been self taught on working on these motors and with the help of this forum, service manual, youtube, and online parts diagrams— doing my best to learn how to work on these motors.

 

[racerone]

???---Look at the shaft on the port side of block.----And I know that the impeller drive pin is not plastic !!--I did not mention a pin on the driveshaft.-----Some simple trouble shooting needs to be done.----Good luck with your motor.

 

[davidmbv]

Racerone, I think your talking about the control lever pin. If so, that was a good idea to check that. However I just checked it and that “plastic” pin is still in one piece as well as the impeller pin.
Thanks Racerone

 

[davidmbv]

Ok I checked spark as recommended using open air spark tester. Both leads jumped gap greater than 1/4”. Also noticed bright blue sparks.
I checked fuel pump diaphragm by blowing into inlet side and heard a “horn” sound. Blew into outlet and it was solid. Although the gasket appears bit old and I have a spare new one I will replace it with.
I pulled carb, disassembled, & popped out all the core plugs. Found bits of grime inside the carb and fuel pump screen. Also yellowish brown sticky residue inside the carb. I have the carb soaking in lacquer thinner and will soak it over night. I plan to blow out all fuel lines with compressor (air) . Will also spray carb with carb cleaner, then air and reassemble.
If I’m still losing power at WOT after carb work, can someone please advise me how to test using a timing light when it’s running on the lake at WOT. I have a timing light, do I need to check the timing marks on the flywheel to see if I’m dropping a cylinder? That sounds a bit tricky on the lake…Or will the light stop working/blinking once the cylinder drops?
Sorry about the question but Honestly never used a timing light and this is new to me.
Thanks

 

[tphoyt]

No need to worry about timing marks.
You are correct the light will stop blinking if you drop a cylinder. It sounds like you carb was in need of a good cleaning and you may find that’s what the problem was.
Best

 

[davidmbv]

Thanks. Got the carb rebuild kit today and carb is nice and clean. Will plan to take it out for test ride soon.
I did test the fuel pump again by doing the “blow test”. I noticed I can blow somewhat into the outlet -it’s not completely blocking the entry of air. Curious if this is a sign it may be going out. Should the outlet be completely blocked not allowing any air to enter when blowing into it? It doesn’t make the horn sound but air does enter….
I purchased an OMC fuel pump rebuild kit but hate to tackle the project unless it’s needed. Was only 15$..Worst case I’ll test spark with timing light if the carb clean didn’t resolve the power lock, then if issues continue can attempt the rebuild.

 

[clemsonfor]

I would bet the carb rebuild does it. But I can't remember without going back does it run faster then after awhile slow down....if so I don't think the carb will fix that. It wouldn't run well at first then stop running fast and smooth. I think if it ran fine for 1 minute or 10 it should stay good, unless it's not filling the bowl for reason like the bad fuel pump or sticking float needle valve. This is where pumping the fuel line ball will help diagnose to little fuel.

 

[davidmbv]

Thanks. It runs great for few min then starts slowly decelerating. If carb rebuild doesn’t solve it, I’ll try pumping the primer bulb before rebuilding the fuel pump to see if power increases. Also will test with timing light before and when it starts losing power. Thanks for all the input everyone.

 

[Erseverkontr]

When it started to slow down, did you remember to pump the priming bulb? The pump may be defective.