Johnson 3 cyl 70 hp, 1986. No vacuum at one carburator

[Bernt]

Hi all,
This engine idles Ok but lacks power under load. So after checking zillions of things incl. ignition, spark plugs, fuel system and compression test (even on all 3) we have found the following issue for which we have no explanation:

There are 3 carbs/cylinders. Removing the air inlet box, we tested for vacuum at each carb inlet. We simply used a paper sheet. When covering top cylinder intake, there is strong vacuum, paper sticks to carb and engine dies fast (both at idle and with some gas). When covering mid carb, again paper sticks but engine do not die, but slows down quite a bit. When covering the lower one, no vacuum can be noticed, and no effect on engine idle or with gas whatsoever.

We shifted position of top and bottom carbs (thoroughly cleaned carbs in what seems to be very good condition) to rule out carb issue, and the problem is still the same and on the same cylinders. Gasket between carb and cylinder is new. We also inspected the flaps/valves (vacuum valves or what are they called?) and they seem to be in good condition and sealing well, all though we are not 100 % sure about that.

Finally we also tested with starter gas. When spraying in the lower cylinder, the one without vaccum, the rpm picks up. When spraying the other 2, no rpm increase, rather a bit of hesitation. We tried this both at idle and with some gas, same result.

Since we have decent compression ( 8.5 to 9 bars), strong sparks on all, and carb shift makes no difference, we feel it might have to do with the flaps/valves or some kind of air leak that ruins the vacuum totally on lower cylinder and somewhat on mid cylinder....??

Would be very grateful for some tips and ideas, this is driving us crazy. We even got hold of a OEM service handbook and did all the trouble-shooting tests it offered, but still same issue. So it starts and idle well but has no power, in fact it chokes and dies, when trying to accelerate under load. It does however rev well when in neutral.

Sorry for poor language, English is just my second language. Thanks in advance!

 

[jimmbo]

Spraying Starting Fluid into a 2 Stroke is pretty much one of the worst things you can do to it. Ether, the main Ingredient , is a very effective Oil Remover, and if a carb isn't supplying Fuel, that Cylinder isn't getting any Fresh Oil. Same goes for the Gas you tried, I hope there was Oil mixed in it.
Look along the Joint where the 2 halves of the crankcase meet, Any sign of leakage? An engine that has no Crankcase Compression can still have good Cylinder Compression, so I am beginning to wonder if the Crankcase is not sealed. If the Reed Valves weren't' sealing, or were broken, Fuel would be spit out of the carb when the engine is running

Have you got Spark on all Cylinders that can jump a 3/8" gap?
An engine can run pretty good on one Cylinder in Neutral

 

[Bernt]

Spraying Starting Fluid into a 2 Stroke is pretty much one of the worst things you can do to it. Ether, the main Ingredient , is a very effective Oil Remover, and if a carb isn't supplying Fuel, that Cylinder isn't getting any Fresh Oil. Same goes for the Gas you tried, I hope there was Oil mixed in it.
Look along the Joint where the 2 halves of the crankcase meet, Any sign of leakage? An engine that has no Crankcase Compression can still have good Cylinder Compression, so I am beginning to wonder if the Crankcase is not sealed. If the Reed Valves weren't' sealing, or were broken, Fuel would be spit out of the carb when the engine is running

Have you got Spark on all Cylinders that can jump a 3/8" gap?
An engine can run pretty good on one Cylinder in Neutral

Thanks Jimmbo. OK lesson learned in regards to start gas. No, I can't see any sign of leaks there. But I understand your point. We did change the gasket between case and intake plate while we inspected the Reed valves ( thanks, now I know the name of them). Yes, strong spark, tested with spark jump tool. And no fuel spitting.

I assume the crank case is essentially one vessel, could any leak just affect lower and perhaps mid carbs but not the top carb? Beats me. The difference in vacuum between top and lower carb is very clear. There is also the vacuum connection point and it's short hose that drives the fuel pump, but that seams ok and it drives the pump well.

If it is of any help, we tested the engine on a boat before we bought it and it ran well and was strong at all rpm. Then after one winter of storage this started. And I think it is getting worse and worse too.

 

[F_R]

Trashed piston skirt?

 

[Bernt]

Trashed piston skirt?

Could that reduce vacuum on one carb? Not an expert but assume it's the rings that seals off and creates vacuum. If compression is good then skirt condition would not relate to my issue?

 

[jakedaawg]

Sealing rings on crank, lower main seal, Reed valves, manifold gasket, case halves seal, carb gasket. All possible causes. I'm sure there are others I am missing. It's been a long day and my brain is in pain fog.

 

[F_R]

Could that reduce vacuum on one carb? Not an expert but assume it's the rings that seals off and creates vacuum. If compression is good then skirt condition would not relate to my issue?

Yes, the piston skirt blocks the exhaust off from the crankcase. Compression can still be good if the rings are ok.

 

[jimmbo]

The inlines don't have Sealing Rings on the Crank. Those are used on the V4s and V6s. The inline engines basically rely on Oil to keep the crankcase sections separate from each other

 

[racerone]

If a piece broke off the piston skirt it pops a hole in the crankcase.----No crankcase compression.------If the bottom seal is bad then no / little crankcase compression.----Remove powerhead and start inspection.---- Bearings may have failed wiping out the labarynth seals.

 

[jimmbo]

I know Merc used Labyrinth Seals on their Reed Blocks which didn't have any Roller, Ball or Needle Bearings in them, but didn't know OMC used them

 

[racerone]

The 3 cylinders OMC motors used them from 1968 to 2001.----Machined into the block.----Have inspected the insides of many of them myself !

 

[stackz]

you have good compression. thats a big one. you are down on power. thats a big one. you've exhausted all your other options as you have stated including reed valves.

you likely have an issue in the short block. possibly a spun bearing.

 

[Bernt]

Thanks to all. What I don't understand is why issues like spun bearing, leaking crankcase or leaking skirt only ellimate proper vaccum at One carb inlet? Isn't the crank case just one vessel, so a leak would kill suction thru all carbs?

Getting back to the Reed valves, assuming I'm wrong and one section is bad, would it eliminate the vacuum? Or just suck in/spit out fuel?

What a mystery (to me). Fuel arrives to all carbs but the lower cylinder refuses to suck it in. The other two do. Shifting carbs changes nothing.

 

[jimmbo]

This is a 3 Cylinder 2 Stroke, think of it as 3 separate Engines, with a Common Ignition System, and Block. Each Cylinder has it's own Crankcase, otherwise it would not run.
If a Reed Valve isn't functioning, Air-Fuel will just be inhaled and exhaled through the Carb.
If the lowest Cylinder is not inhaling, then you have to find out why.
You can swap carbs, reeds, till the cows come home. It was stated in an earlier post, #9, that you have to inspect the Block, the lower crankshaft seal. It is time to S***, or get off the Pot.
Perhaps you need to read up on 2 Stroke Engine Fundamentals

 

[Bernt]

This is a 3 Cylinder 2 Stroke, think of it as 3 separate Engines, with a Common Ignition System, and Block. Each Cylinder has it's own Crankcase, otherwise it would not run.
If a Reed Valve isn't functioning, Air-Fuel will just be inhaled and exhaled through the Carb.
If the lowest Cylinder is not inhaling, then you have to find out why.
You can swap carbs, reeds, till the cows come home. It was stated in an earlier post, #9, that you have to inspect the Block, the lower crankshaft seal. It is time to S***, or get off the Pot

Ok, now I understand. Thank you. Sorry for not getting it first, but this is why I posted a question. Trying to grasp tech terms in a foreign language doesn't help.....

 

[jimmbo]

Copy the Texts and run them thru Google Translate. It can also translate what you want to say, into English to

 

[racerone]

Each cylinder has a sealed crankcase.------No vacuum means ----A hole in the block.-----Blown crankcase lower seal.-----Bad lower labarynth seal.----Fuel pump diaphragm ( air motor on VRO ) bad -Are you saying there is nothing happening with a piece of paper in front of the lower seal ?

 

[Bernt]

Each cylinder has a sealed crankcase.------No vacuum means ----A hole in the block.-----Blown crankcase lower seal.-----Bad lower labarynth seal.----Fuel pump diaphragm ( air motor on VRO ) bad -Are you saying there is nothing happening with a piece of paper in front of the lower seal ?

Correct, nothing. The paper is not sucked in, nor pushed out. No impact on rpm. On the other 2 cylinders, it is clearly sucked against the carb air inlet and rpm drops. ( we test without air box installed, but it is very open and no filter, so it makes no impact on engine behaviour I assume?).

The problem has gradually worsened. VDO is removed. New fuel pump installed. All fuel and air hoses replaced. If starter gas is sprayed into the lower carb, rpm picks up.

 

[racerone]

Remove power head.----Inspect crankshaft lower seal.-----If that seal is good.----Take engine apart.----Should be easy to find the damage then.

 

[Bernt]

Remove power head.----Inspect crankshaft lower seal.-----If that seal is good.----Take engine apart.----Should be easy to find the damage then.

Thanks for all wisdom. How is the design of the vacuum tube going to the fuel pump? It extends from the lower part of the block, is it therefore (only) connected to the lower crank case? If it would be leaking, that would explain the issue?

Also like to better understand what I assume is a breather line/tube that hook up to the air box. Can that line cause my issues? I know the engine ran very well when we bought it, then bad once installed on a new boat. It seems unlikely that a major internal issue happens just then, although it could. Appreciate your help.