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Blowback through top carb 70hp evinrude

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kryck

New member
"I have ovehauled and rebored

"I have ovehauled and rebored a evinrude 1987 70 hp. At first startup i notice that the engine runs a bit rough on idle. After searching through som posts here i figured I might not have cleaned out the carbs enough so i disassembled them and cleaned the thouroughly. Next startup same rough idle. I now notice blowback throug the top carb. This was probably the case at first startup as well. So off with the carbs and checked the reeds. reeds look fine but I see a small hole in the reed plate that matches a hole in the crank cover, like some sort of ventilation or recirculation. I think the blowback is through this hole/path. There is a matching hole in the gasket (aftermarket from GLM) but I'm unsure that this hole in the gasket was in the old one I removed (and threw away, off course).

Is it safe to plug this hole and try the engine again?
Why would there be ventilation/recirculation on the top cylinder?
The hole/path is similar to the one in the lower cylinder crankcase that leads to the air silencer but this one has larger hole and ends right outside reeds (not in a nipple or such)."
 
"None of the reeds should have

"None of the reeds should have a hole in them. You do have a "puddle trap" fuel scavenger system that takes unburned fuel from collectors inside the engine and pumps it to the top bearing (usually) where it drips back to the cylinders for burning/
So sounds like the missing nipple should be found and installed- check the book for hose routing.

You can plug the hole, but sooner or later you will want to hook that recirculation back up."
 
"I can't visulize this hol

"I can't visulize this hole in the reed plate you mention.

The matching hole in the gasket..... are you speaking of the gasket between the carburetor and the intake manifold? If so, that would be a fuel passageway for the fuel primer system.

If you are experiencing blowback thru the carburetor from the downward pulse of a piston, something is wrong with the reed plate (leaf Valves) of that cylinder."
 
"Very fast answers here, appre

"Very fast answers here, appreciated!

Well here's some hopefully better explanations:

The hole/path is not in the reed itself, it's in what i thougt was called the reed plate but maybe manifold is a more correct description, anyway it's the plate were the carburetors and reeds are mounted. There's a hole/path in the crank case cover that matches with a hole in the gasket and a hole/path in the manifold plate. It looks just like the excess fuel/oil vent from the lower crank to air silencer but this one has larger diameter hole and does not end in a hose or nipple, it just ends right on the outside of the top reeds and by this it is (probably) ventilating away almost all the crank case compression and fuel/oil/air mix to the top cylinder. I'll try to attach a photo that shows the manifold on the crank case side with hole in gasket and path showing behind it. The path in the crank case could very likely lead up to the top bearing as william said but I'm not relly up to splitting the crank case again.

I'm not sure but maybe did the old gasket between crank case and manifold cover this hole so that the paths were not connected but I'm not sure and I've thrown away the old gasket. So by plugging it I meant plugging the gasket, not the pathway itself.

I will try switching cylinder one reed box with cylinder two to possibly eliminate reed problem.
287930.jpg
photo of manifold
 
William: I don't think the

William: I don't think the nipple and hose is missing I just thought that this hole/path had similaritys to the one in the bottom crank case. The path ends between the carb and reed on the manifold plate (it's the routing visible behind the gasket shown in photo above).I'll add some more photos when i get home from work.
 
"A suggestion pertaining to pi

"A suggestion pertaining to pictures..... best to keep the pixel width down to 500 when entering pictures.
Otherwise the picture is so large that it tends to be off focus and requires scrolling. I took the liberty to
edit the above down to 500 pixels to give you a idea of the size.
287951.jpg
size
 
"I don't think that this i

"I don't think that this is the problem. I have just rebuilt my 1977 70HP, below the gasket is a channel that connects this top "hole: which passes through the reed plate which is a vent or similar to the top bearing. The channel is connected to the bottom hole & as Joe said, a tube to to the air filter/cover. You could not get blow back from this since it is cming from a very small chanel from the top bearing, sorry can't offer anything else other than what has already been suggested - fault reeds, or faulty reed block maounting to reed plate"
 
"Just switched the reeds on cy

"Just switched the reeds on cyl one and two and still blowback from top crankcase. I guess this eliminates fauly reeds since the "used-to-be" top reeds now are doing their job. Blowback is in fact through this channel that leads to the top main bearing so I guess I must have placed something wrong when I assembled the block. This calls for a new split... Guess the boat won't be seeing water this weekend, huh?

And thanks for the ups on image sizes."
 
"Powerhead is now split. The p

"Powerhead is now split. The path from otside the reeds goes up to the top bearing as william and richard said.

I think the function is that a portion of fuel/oil is sucked up before the reeds and goes through path and into the top bearing and then down into the crankcase where it mixes with the fuel/oil that took the regular through-the-reeds-path. My problem here is that when the piston moves down and the fuel/oil is supposed to be pushed into the cylinder, a good portion of the fuel is pushed back through this top-bearing-lube-path.

This is not connected to the air silencer as I guess richard says. Richard says blowback through this path i impossible because it comes from a very small channel in the top bearing. This "very small channel" is 5 mm at the bearing and 8 mm through the crank cover (see pictures). I don't consider this a very small channel at all. In fact the 8 mm hole in the crank cover looks very much as it's suppose to hold a one way valve (check valve?) and this would really solve the problem. But my seloc manual doesn't mention such a valve for a 3-cyl and browsing through various websites gives no clue either. The only other solution as I see it right now is that the top bearing is suppose to hold back the blowback but I think this is a strange funcion for a needle bearing with no lower seal.

History: Bought this engine and it was supposed to work, had no chance to check function then. It later showed that the top piston was scorched, rings split and serious scratches in the cylinder wall. This is the reason i rebored it. The engine now runs very lean and "pops" a lot on the top cylinder 'cause much of the fuel is spat back, but it runs and revs up so guess maybe the previous owner didn't mind the rough idle and lean popping until i broke down. Well this last part i just guessing, I have now way of talking to the previous owner other than through a medium...

Long post but my question i really short: Is there nobody out there that have seen a 3-cyl with a check valve on the top bearing lube path?"
 
"<[img]"http://www.marineengin

"
287987.jpg
Manifold plate - reed side."">
Manifold plate - reed side.
287988.jpg
Crankcase cover - manifold plate side."">
Crankcase cover - manifold side.

Sorry for the bad quality, I only have the cellphone camera at the moment."
 
"<[img]"http://www.marineengin

"
287991.jpg
Crankcase cover - crankcase/bearing side."">
Crankcase cover - crankcase/bearing side. This is where I think a one way valve would fit. Hole is 8 mm."
 
"I think you have it-- a small

"I think you have it-- a small one way valve in there. OMC and chrysler used them. They were a small brass pellet with a rubber valve inside. Might have fallen out during assembly. While I'm not "right there" you may have the solution. Now if you can find a crankcase break down on one of those parts web sights, it may show up. Would start with this web site and then "Crowley" and "Franzmarine".

As they say "The answer is out there"!!!"
 
"Christian, although mine is 1

"Christian, although mine is 1977 model, the hole that you have circled is in the "reed" plate which is much larger than mine - see photo. I guess that you would need to get hold of another similar to your year level to compare.
Regards
288042.jpg
reed plate hole
 
"No luck yet in finding any on

"No luck yet in finding any one way valves... No description that I have found regrading intake manifold or crankcase shows that there should be a valve her. Not sure what to do - guess it's inrepairable..?

Richard: On the 77 model the manifold plate is a two part assembly. One where the reeds are mounted and one where the carburetors are mounted. These bolts to the crankcase cover with a gasket between. On the 87 model there is just one plate with the carburetors on one side and the reeds on the other se my first photo.

I guess one way of fixing this would be to make a plug, that fits in the crankcase cover - bearing side, and this plug can have a small hole in it so it acts as a restrictor. Unfortunatly this would restrict the flow in both directions and lubing of the top main bearing would suffer."
 
"Christian, Sorry this is way

"Christian, Sorry this is way out of my league. I would try to get a look at a part from an outboard wreckers, or speak to a dealer! Good luck"
 
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