1976 Johnson 70 hp

[Coltrickle]

I have been tinkering with this motor for months now.. I have several other threads for timing, carbs, and other issues.. I have replaced all the gaskets. Rebuilt the carbs completely but I still have a dead spot in acceleration. About 1/3-1/2 way through the throttle. If I hammer down off the start it will bogg and die, if I ease through the throttle and get past this point I can push the throttle wide open and it will take the throttle but takes a few seconds to plane out but once it does it will fly!! While on plane I can back down on the throttle and go right back to wide open and there is no hesitation at all.. However if I let it come off plane I have to ease back through the throttle to get it going again!! Very frustrated.. The only thing I haven't done was replaced the carb jets. I took them out and measured them with pin gauges and all were good.

So I call a mechanic yesterday to just let the knowledgeable fix my boat.. He ask me if I had check my compression and I said yes it was 124,124,and 119. He said that was my issue.. To much difference in the cylinders.. Is this true?? I felt as if he was being honest but have seen people post worse on here and been told it shouldn't be a problem.. Any advice is greatly appreciated before I drop some money?? Thanks!

 

[Bosunsmate]

nope i wouldnt agree with him yet.
How your plugs look?

 

[Coltrickle]

The plugs are always wet with fuel. I have probably put 3-5 hours of run time on them and they have no carbon build up whatsoever!! Very weird to me.. I can wipe them off with a rag and they will look almost new besides a little oil residue inside the groove around the electrode..

 

[racerone]

On my compression tester I would expect to see 140 psi or higher.---It does not take months to figure out what is wrong on these simple motors.------The cheapest thing for you to do is remove the cylinder head for inspection.----I have to agree with your mechanic !!-----Then again I can not see, hear or feel your motor from 1000 miles away.

 

[Bosunsmate]

The plugs are always wet with fuel. I have probably put 3-5 hours of run time on them and they have no carbon build up whatsoever!! Very weird to me.. I can wipe them off with a rag and they will look almost new besides a little oil residue inside the groove around the electrode..

Thats not right as you say. Have you checked you have a very healthy spark, jumping a 7/16 gap?
A weak spark can lead to dead spots when motors under strain and thus wet sparkplugs.
Ive a ten dollar multimeter sof if it was me id resistance test the stator, Even voltage test it if you can.

 

[Coltrickle]

It has a very good spark while using my spark gap tester I can get 5/8" gap bright blue.. But I have read where they may not fire the same inside the cylinder sometimes.. I did do a resistance test on my stator and borrowed a dva for the voltage check. The ohms from orange to brown were 635 and the 3 white trigger wires individually to the black white was 18 ohms. Both in spec per cdi. As far as the stator voltage cdi states 150v AC from brown to brown orange but i only get 95v AC. So is that 150 at idle or wot? My mercs had high and low speed voltage checks and the voltage decreased with more throttle. This one is very low at idle. I took it to the lake a while back and after some trial runs I took it out of the water and walked up the hill to the marina.. The mechanic told me that it didn't really matter what the voltage was as long as I was getting the 7/16 or so spark jump. I told him it was about 5/8" and he said if it's jumping that gap then the electronics should be fine.. So I abandoned the stator theory.

 

[Bosunsmate]

Yeah if your jumping that gap id say that was fine too. However wet plugs mean something isnt right in there. Other possibilities are too much fuel or that isnt entirely fuel on the plug, ie it has water contamination.
Hows the piston tops look through the sparkplug hole, do they look carboned up (normal) or do they look as if they have being steam cleaned by a bit of water in the engine?

 

[MokiCruiser]

I did just get my 1975 70 HP Johnson back from my mechanic Monday, and he did a compression test on my motor and said that all three cylinders were between 130 and 140, and also said that the rule of thumb is no cylinder should be more or less than 10% from the others. That being said, even though it would be better if the compression were another 10 to 20 pounds higher in all three cylinders, all are within 10%.

 

[Coltrickle]

Thanks

 

[Coltrickle]

When I first got the boat and run it after it sat a while 2 weeks or so I removed the plugs and they had a little grey oil/water contamination on the middle cylinder. I replaced all gaskets and resurfaced the exhaust plate parts and head.. There is no longer any evidence of water on the plug. Just oil and wet fuel on all 3.

 

[Coltrickle]

Also , piston tops look good and carboned up

 

[Bosunsmate]

wet plugs, try lighting them to check that it burns