External Reeds Merc Mystery

[Fastjeff57]

Reference: Merc 350 twin, 30 cube, with twin external carbs.

I’m ready to drag this damn thing outside and set fire to it! Everything I've tried to date has seemed to make it run even worse. It starts reluctantly (and with LOTS of choke), idles poorly, and is hard restarting after it blubbers out.

The reed valve assemblies seal perfectly—and I’ve messed with them several times now--yet a visible cloud of gas droplets blows out of both carbs. And we’re talking enough gas to wet your fingers!
Ironically, it runs like a bear when I crack the throttle. Took it to the pond yesterday for a trailer test. Motor had excellent power and took the gas instantly (though it was running way too lean). It has good compression, excellent spark, and lots of new parts.

I'm truly baffled! How can gas blow out the carbs like that with tight fitting reeds? Obviously THAT is what's causing the lousy starting and crappy idle, but what is causing that? It's possible that the reeds are too close to the flywheel and the shock wave generated by the crank whizzing by the reeds is popping them open? (I'm gonna add a 1/4 inch spacer tomorrow and see if that helps.)
Anyone know what’s going on here!
Thanks in advance for any help.
Jeff

[88workcar]

Put the air box back on it.

[Mark75H]

Put the air box back on it.

He didn't get it

[Fastjeff57]

What "air box"?

Jeff

[iwanajohnson]

What "air box"?

Jeff

Little platic box that covers the carbs, has air inlets on the sides and a solid plastic piece in front of the carbs so air comes in the sides. I don't run one on my 56ci motor and it is a mess every time i bring it back from the river, if you don't have one then make one out of aluminum or something but that'll fix your problems just make sure and put a good seal on it. I had a mod 50 front half on a 56ci motor and it shot more gas out of the front like than than the motor actually took. Simple fix either way weather it's worn rings but an air box on front will help a lot.

[Tim Kurcz]

The spit back you describe is most likely caused by a badly worn or loose labyrinth seal between cylinders on the center main (former reed cage).

Did you open up the clearance between the crank and existing reed cage? How did you assure a gas seal between cylinders?

Looks like a fun project. Good luck!

[Fastjeff57]

Thanks to all. I'm digesting your replies.

Jeff

[Fastjeff57]

I may have stumbled onto somthing. Instead of down angling the reed boxes (which most makers do, to move the carbs away from the transom clamps), I aimed the reed boxes straight backwards. This produced a "pond" area below each reed assembly where oil collected (and leaked out as I pulled the reed assemblies out).

When the oil collected--and reeds are designed to seal air, not liquid--the collected oil blew past the reeds and out the front of the carbs. (Why didn't it do this before, you may ask, when the reeds fit poorly? Good question, but the better they sealed, the more the oil blew out.)

I rigged up a drain system with check valves to suck out that oil accumulation. We'll see if that's the fix (and tomorrow 'cause I "wounded" myself in a finger and have to rest up a bit).

Jeff

[JohnsonM50]

Id think if the rings were worn & the blow by would pressurize that pistons crankcase section, it would tend to slam the reeds shut even more.. but probably not leak.. Unless it blew thru the center seal & out the the opposing 'open' reeds.. again even more probably not, a long way for lost blow by to go since upon rings being bad they would produce less compression to begin with & your comp. reading would show it.. You might be onto it with the puddling fix, working thru the external & easy possibilities 1st makes sense, Id think if that doesnt do it then Tims right about the center seal. Good Luck.. It aint easy to motor outside the box.

[Fastjeff57]

Here's the latest on the *~@#^ little darling (it might be listening):

Still blows out the carbs (though a little less, I think). Idle speed is even lower than before, so much so that I have to use over 3/4 spark advance to maintain a 1,000 rpm idle. (It idled MUCH faster before I improved the reed's sealing, by the way.)

One thing I've noted all along is that I can't seem to over richen it at idle. That might sound weird with all that gas blowing out, but it's true. And it needs LOTS of choke to restart, even when up to temperature. (There's a clue there but I'm not tuning in on--yet!)

Thanks for all the help--I appreciate it!

Jeff

PS: Motor has excellent compression, a tight reed block/ crank seal, and runs like a bear above idle.