1979 Mercury 7.5 Crankcase Vent

[Pripyat]

I broke the mounting ear off my original carburetor on my 1979 Mercury 7.5 by being careless. I bought a new (used) carburetor frame off ebay. The new carburetor from ebay is from an older engine but is identical, minus a nipple for what I believe to be the crankcase excess oil drain hose. On the original carburetor the hose in question attached to the carburetor just before the powerhead. When I run the engine with the hose attached to nothing it blows out oil/fuel mixture.

What can I do with this hose? plugging it seems careless but drilling the new carburetor body to install a nipple seems excessive. Any input would be greatly appreciated. Why did the older engines not have this vent on the carb? Why do the newer ones?

 

[jimmbo]

Up until the last 20 years all 2 stroke engines fed a gas-oil mixture through the crankcase on the way to the combustion chamber. Not all of it makes it to the combustion chamber and builds up in the crankcase. In time enough builds up and does get to the combustion chamber and then that can cause misfires. Old engines had drains to discharge the excess puddling out of the engine, usually into the exhaust system. Some felt it made Outboards real polluters, so in the late 60s or early 70s the manufacturers designed systems to either recirculate the condensate to various bearings, or back to the induction system to try another shot of it making it through to be burned.

Plugging the hose will cause the condensate to accumulate and will make for a engine that will not run at low speeds for very long before loading up, run rough and even stall. The best thing to do is get the correct carb, or if mechanically inclined, drill and attach a hose fitting. There are some who run the hose to a jar or other container and save the discharge to put back into the fuel tank, messy

Edit. That carburetor has the fuel pump built into it?

 

[racerone]

Remove the nipple from the old carburetor.-----Install it in your replacement carburetor.------Best thing for you to do.

 

[Pripyat]

Thanks a lot for the feedback. Any tips on removing the nipple from the old carb?

 

[Pripyat]

I missed the edit up above - Yes, it does have the fuel pump mounted on the side of the carb.

I got the nipple removed, there was only a very short piece pressed into the carb body. Came right out.

There is a trough on the bottom of the older carb. I was about to drill the hole when I realized that I would be drilling right through the trough, affecting the right port (right if you are looking into the carburetor as if you were the engine). I epoxied the trough, leaving the port open. I'll drill through it once the epoxy hardens.

Is there any reason this won't work? In the picture you can see the trough filled in, right port plugged temporarily until the epoxy sets, and a mark where I will drill the hole once the epoxy hardens.

 

[Pripyat]

Here is the carb next to the original before I applied epoxy -

 

[Pripyat]

No Title

I ordered a diaphragm kit and can't figure out if I should be using the gray, brown, or both of them. If both, is the order pictured correct?

 

[jimmbo]

That's odd, the exploded parts picture(page 4-25) show 2 slightly different parts, one a pump diaphragm, the other the gasket. but the two you have look identical except material

How were they arranged when you took the carb apart?

http://www.boatinfo.no/lib/mercury/manuals/mercury_1965-1989_2-40.html#/54 page 4-23

 

[Pripyat]

No Title

There was only 1 I believe. I'm not confident it was correct before. Every diagram I've found only pictures a single part. If I do use one I'm not sure which would make more sense. Using both did not work well..

The orifice of the idle tube on the newer carb is easily twice the diameter of the older carb. The idle mixture screw is very different also. Currently I have the newer (larger) idle tube in the older carb. I have to use the older mixture screw with the older carb body. Adjustment is not right - it will idle OK but 1/8 turn in either direction will cause the engine to shut off. Is this because the mixture screw is not paired with the correct idle tube?

Why is there such a vast difference between the orifices? See attached.

 

[jimmbo]

So much for the carb being Identical. When you refer to the 'older' carb, are you referring to the carb that was on your motor, or the carb that supposedly came from an older motor?
What engine was your recently purchased carb allegedly from?

 

[Pripyat]

Older carb is the one I bought off of an online auction site. It was listed as an "early 70s" 7.5hp carburetor. My motor is a 1979.

Engine idled well at .5 turns out with the large idle jet. I put the small idle jet back into the carb and it seems to idle better after some adjustment. I didn't count the turns. It does seem to smoke more, though.

Next chance I get I'll take it to the lake and see how it runs as it sits.

 

[Pripyat]

The other carb is a pre-1978 according to the seller. Is anyone aware of a resource that explains the differences between pre and post 1978 carbs?

 

[jimmbo]

In the 1970s the first of many 'Gas Crises' occurred. Mercury made changes to the carbs on many of the engines in efforts to burn a little less gas. I can't say if the small fishing motors were subject to that.
Hers a link to a page that was intended for rejetting for altitude. However it shows differences in main jetting over various years, which means something else in the carb that affected the fuel delivery curve was changed. I there might be other info on the web but...
http://www.boatinfo.no/lib/mercury/m...2-40.html#/474

 

[Pripyat]

So I finally made it to the lake/creek. I still have some work to do with tuning but the carb is quite usable just like it is.

I was running fine for about 10 minutes when the engine shut off. The powerhead was very hot. I let it cool and pulled the brand new thermostat out. Engine started right back up and was peeing fine. After it ran a while longer I stopped the engine but when I restarted it wasn't peeing at all. I let it cool fully and restarted but it would only blow out some water mixed with air/steam. I paddled back to the dock.

The engine has a brand new water pump.

 

[Pripyat]

So I pulled the foot and blew air from the pickup tube up to the thermostat housing. Seems like it may be slightly restricted. Air is getting through but it is definitely not a straight-through passage the same diameter as the tube. The copper pickup tube was also bent slightly, the opening was maybe 15% smaller than it should have been. I used a screwdriver to get it back to round and lightly flared it out. The tube from the thermostat housing out the side of the engine is clear.

Should I pull the exhaust manifold and take a look inside?

 

[racerone]

The original PLASTIC washer at the top of the water tube often melted on severe overheat.-------It then blocks water flow to the engine.---------Powerhead has to be removed to inspect / change it.-------The factory replacement is a better material today.

 

[Pripyat]

What about the exhaust manifold? Should I go ahead and check under there while I'm at it? The bolts look like they will be impossible to remove if I leave them there much longer. They are rusted pretty good. I have them soaking in KROIL at the moment.

 

[Pripyat]

I removed the powerhead from the lower cowl and sheared off one of the bolts on the exhaust manifold. Interested to see how difficult it is to get out the rest of the way. I'm about to order new gaskets for the exhaust manifold and powerhead.

While I've got everything apart - Does it make sense to do anything else to the engine?

 

[Pripyat]

No Title

The baffle plate is very warped. But it costs $22. Should it be replaced or can I just clean up the existing one? Also - The washer racerone mentioned is definitely misshaped.

 

[racerone]

That is the replacement washer, originals were black.-------------Looks like there is a score mark on top piston.