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

[Bernt]

Hi folks, can't solve this one, help would be highly appreciated! Johnson 3 cyl 70 hp, 1986. Starts easy, runs well on idle and slow speed, responds well and even on throttle when not under load (when tested on land). But when throttle gets to about 50% and under load (running with my 20 foot speed boat), the engine hesitates and mosty dies when I try to pick up speed and dies until throttle is pulled back again. At a few ocassions I managed to pick up speed and then it runs well at high rpm ( + 30 knots, sat 500 rpm), especially if I increase throttle very very slowly and gradually. But mostly, it dies. Following is done:

1. Compression test, OK. About 10 bars/145 psi on all 3 cyl.
2. Carbs inspected and cleaned. Great condition as fast as I can tell, but not an expert on this model
3. Coils measured over 1st/2nd wind, all same resistance 250 ohms. Earthing ok.
4. New spark plugs, gap 0,75 mm / 0,03 inches
5. New fuel pump. Also tried with hand pump, both assisting and by-passing pump. No difference.
6. Spark plug cables measured, one was bad, replaced it, no difference.
7. Tried to manually adjust ingnition advanced by moving the "arm" and it seems to work as it should, so no improvememt. Engine dies if advance is moved back, and pick up again when back as it was. Which is like it should be I guess.
8. When removing one plug cable after another, the idle falls down a bit, same from cylinder to cylinder. So seems it's running on all 3 cyl (idle, no load) but something happens when under load/throttle
9. Removed the air inlet box and looked at throttles under operation. All "flaps" opens and closed evenly when operated.

But:
QUESTION 1: I just noticed that the top carb spits out some gas (backwards, so towards bow)! With the inlet cover removed, I can see a fine drizzle of gas leaving the carb thru air inlet. Nothing on carb 2 & 3. What is happening? My feeling from day one was that one or more cylinders is getting too much fuel, and maybe this is what I see? What could be the reason?

QUESTION 2: is there a fuel/air mixture screw? I can´t see any, is the mix factory set? Never seen carbs like these, no needles, just 3 small holes after the flap.

QUESTION 3: Can I find a free manual online? Googled for a while without result.

THANKS in advance, ran out of ideas, and now half summer is ruined. I am mostly able to fix engines but this time......no luck. Frustration!

 

[racerone]

Run with a timing light hooked up.----One cylinder at a time.----See if there is a spark issue as boat speeds up.-----Running with VRO in service ?---Tested VRO oil output ?-----Or a simple non mixing pump with 50:1 in your tank ?-----Perhaps test run with another tank and hose.

 

[Bernt]

Thanks. VRO pump removed, new pump installed which we think is ok as manual fuel pumping gives same effect. Do oil is mixed in fuel. Fuel tank and line is brand new.

I understand your suggestion, but seems very hard and risky to try to use the lamp, at sea, at speed. And probably very hard to see the marks in daylight. If the timing is OK at no load (at home with gardenhose cooling) at both low and high rpm, I guess it should be OK at load as well?

If I could, I would like to test the quality of the spark under load. But how? Any reason that the spark strength would be OK at no load, but bad at high load? Thanks!

 

[racerone]

You do not understand the concept.----You lay the timing light on a seat so you can observe the flashing of the light !----No flash means you have lost spark !----Does spark jump a gap of at least 3/8" ( 1 cm ) on a test device ?

 

[Bernt]

You do not understand the concept.----You lay the timing light on a seat so you can observe the flashing of the light !----No flash means you have lost spark !----Does spark jump a gap of at least 3/8" ( 1 cm ) on a test device ?

Ok, sorry, Now I get it. English is only my 3rd language so bear with me
Will test these 2 things.
Any thoughts about the "gas spitting" from top carb? A clear difference from the other 2. Some fuel is actually running slowly down the inside of the air liner box.

 

[racerone]

Could be a slight leak / mis-alignment of the reed valves.-----Or an issue with oil recirculation passage way on reed valve mounting plate.----Remove carburetors / inspect reed valves.--Replace gasket.

 

[Bernt]

Could be a slight leak / mis-alignment of the reed valves.-----Or an issue with oil recirculation passage way on reed valve mounting plate.----Remove carburetors / inspect reed valves.--Replace gasket.

Thanks. Will check these points. Is it likely that the Reed valves can just be cleaned and aligned? Or are they often broken? Since they are just flat metal pieces I'm not sure how they would brake.
It is clear that just one carb spit back fuel, I guess this is nit affecting idle too much but affect on load performance, but does it make sense that the engine more or less dies on 50 % throttle( unless I pull it back again) , not just runs bad?

A 2 stroke of the 80s is essentially quite simple, yet I don't get it.. .

 

[racerone]

Yes they are a very simple motor.----Easy to diagnose and repair.-----The reed valve opens and slams shut up to 5000 times a minute.-----They rarely cause a problem.----I can not see , hear or feel your motor.-----Up to you to have a look at your reed valves.

 

[Bernt]

I now checked the Reed valves. Pictures attached. To my surprise, valve 2&3 (counting from top down) have tiny gaps, enough to see some light anyway. But not number 1, which is at the carb that spits fuel backwards. Strange to me.

All looks clean and in very good shape, sealing was ok. Ideas, anyone?

 

[racerone]

I have lots of ideas of what to check.-----But your reply has been negative on the timing light check etc.------And I stated that reeds rarely cause a problem , so here we are .----What would you try to look at next yourself , bases on what you have seen so far ?

 

[Bernt]

I have lots of ideas of what to check.-----But your reply has been negative on the timing light check etc.------And I stated that reeds rarely cause a problem , so here we are .----What would you try to look at next yourself , bases on what you have seen so far ?

Well you suggested checking reed valves, and I did. I checked spark at all rpms ( no load,) incl gap test, all ok. Load test is not possible right now as the boat is on land and I find it hard to believe the spark will change under load? I ran out of ideas, thus the posted question. Please share your ideas. My feeling is it's ignition related, but since I don't find any problem with it, and I spotted the fuel coming out of carb 1, I'm lost.

 

[racerone]

See post #4 ?----And who cleaned the carburetors ?----Does spark jump a gap of 3/8" on a test device , yes or no ?

 

[Faztbullet]

If its spitting fuel out the idle jet it is highly likely it has scuffed exhaust side of piston due to bridged port.. and compression will be fine

 

[Bernt]

See post #4 ?----And who cleaned the carburetors ?----Does spark jump a gap of 3/8" on a test device , yes or no ?

Yes, it does jump 3/8.
I cleaned the carbs. Only with carb cleaner and compressed air, not ultrasonic. Can do that as well, just in case, but they look very good internally.

Since you seem to be so experienced, could you answer my previously posted questions?
Q1 Do the reed valve gaps pictured seem normal? Can't see the gaps unless using the backight method, so probably just 0.1 or 0.2 mm or so.
Q2 could something affect the ignition at about 3000 rpm, but only under heavy load( boat not yet planing) ? Can't imagine what but I'm mostly working on cars. Like to again mention that in say 25% of our attempts to accelerate, it works and we reach full rpm, although acceleration is a bit slow compared to what I except from this engine /boat combo. In 75% of the attempts, the engine hesitates and dies unless throttle is reduced again. Since we can run full speed sometimes (30 knots with the 20 foot hardtop) I'm certain all 3 cylinders works these times. To me it very likely rules out bad fuel pump, damaged cyl/piston, blocked carb (intermittent block may still be a reason) and also reed valve issues. Not much left, fuel line and tank is brand new and manual pumping don't help. I can only think of ignition issues. Could a coil fail intermittent and only at high load? Or timing?

Thanks a lot for all input and sharing your time!

 

[Bernt]

If its spitting fuel out the idle jet it is highly likely it has scuffed exhaust side of piston due to bridged port.. and compression will be fine

Understand your reasoning. However say 1 of 4 acceleration attempts works and then the engine runs great for longer periods with boat planing, and at high revs about 5000 to 6000. A permanent cyl/piston damage would not allow that I guess. 3 if 4 attempts the engine hesitates and/or dies at around 2500 rpm.

 

[Faztbullet]

Put it back together with new gaskets, sync and link by the book except make it idle with timing and roller loose from cam. Then set a 1/8 gap between roller and cam and test.

 

[Superc125]

Ok, sorry, Now I get it. English is only my 3rd language so bear with me
Will test these 2 things.
Any thoughts about the "gas spitting" from top carb? A clear difference from the other 2. Some fuel is actually running slowly down the inside of the air liner box.

Could be red valves not sealing properly allowing blow back.

 

[Faztbullet]

If engine internals are in mechanically good shape, no. Reason being that when piston starts down stroke the crankcase pressure will push them shut. Also it wouldn't idle worth a dang if reed/s stayed open.

 

[racerone]

There is an oil recirculation passage way.-----Returns oil to the top.----Perhaps that is fooling you to believe there is something wrong ??

 

[Bernt]

Yes, you might be right. Managed to upload videos of the gas/oil spray, hope the links works. Although it's hard to see, check end of video, top carb. Also check the video where we try to accelerate but fail. Maybe a small misfiring on idle? The sound if the engine is also a bit different from other 2t I had, but sometimes it accelerate nicely and we reach 30 knots. Initially I was sure ut only runs on 2 cylinders but spark is there, all (new) plugs looks the same in colour , and it runs well sometimes ......

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