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 :rolleyes:

[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.

[MHARDEE]

Since Tim has HAS covered THE CRANK SEALING RING --- FROM PIC Looks like yu added a lot of crankcase area, if so could cause low crankcase compression. Hence hard to crank,weak idle signal, etc. OR if carbs are too big ,and or crankcase compression is low , Carb ventura velosity will be to low to pull fuel at lower rpm's or combining the two multiplies problem. JUST A THOUGHT. ----------- CHEERS Mitch ---- I'm no guru but i did sleep in Holliday End Express for a week once:D:)

[Fastjeff57]

I like that, Mitch! Tried to keep everything tight, but... Some epoxy will now be added to remove as much of that area as possible.

Thanks again!

Jeff

[JohnsonM50]

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.

If.. the labrynth seal were leaking a little, on throttle the incoming charge would overwhelm the leak + as the Rs raise the time the seal has to work is so short that hardly any could pass by time alone. It woulds have to be real leaky. How bout the crankcase, block seal, or a bolt hole or 2 that go thru? Those wouldnt explain the carb spit tho. Can you pressure test the crankcase? Piston down, pressurize & listen for leaks? [or if your like me get someone who can hear to listen] Is one leaking more than he other?, could also be a clue.

[Fastjeff57]

I like that, Mitch! I did try to keep everything tight, but... Some epoxy will now be added to remove as much of that open area as possible.

Thanks again!

Jeff

[Fastjeff57]

Update! Found that I could add at least ¼ of an inch of epoxy to the walls above and below each reed cage, to stuff it, so I did. Motor runs a bit better now, but it still starts a bit hard and idles inconsistently (too big a carb?)

I also found out why the gas blows out the carbs at idle: It’s called “reversion” and is caused by a shock wave bouncing off the reed valve at the end of the suction stroke. My buddy witnessed a big block Chevy running on the dyno with Hilborn injector stacks (one per cylinder) that had a gas cloud half a foot high ABOVE each stack at certain rpms! Using a manifold—where one cylinder is pulling after another stops—doesn’t do that like individual cylinder/ carb combos. For that reason I added a ¼ “ ID tube between the two carbs, and it seemed to help.

Jeff

[Mark75H]

Reversion is common on 2 strokes with intakes other than reeds. Its possible that your large carbs are causing it, but unlikely its the reeds.

[Fastjeff57]

Could be the large carbs but, mathematically, they are the same relative size as the engine they came off: a 50 cube OMC triple. We're looking at 49 cubes/ 3 cylinders vs my 30 cubes/ 2 cylinders.

Gonna pond test it soon, now that I have adjustable main jets. Should be interesting! Made lots of power before, even though it was running (gasp!) lean.

Jeff

[Fastjeff57]

First of all, thanks to all who assisted me on this frustrating project. Here's what has transpired since my last posting:

On gas blowing out the carbs at low speed (inversion, it's called): I added a crossover passage to the carbs that seemed to help, but better still was DELIBERATELY screwing up the reed sealing! (I noted that, the better I adjusted the petals, the worse it ran. Go figure!)

The idle is MUCH better after I added an idle speed stop to the carbs, sort of like what 4 strokes use. Apparently, not enough air was coming in to maintain an idle without gobs of spark advance. Weird, I know, but it's working.

Power wise, the thing seems pretty strong while pond testing it strapped to my trailer. (It throws back the same amount of water as my 50 hp 44 cuber, and that is a strong running motor.) I now have adjustable main jets on both carbs, so my lean problem at WOT is history.

Soon as the water gets a bit warmer I'll be running the boat at the lake and see what she'll do. Can't wait!

Again, thanks for all the suggestions.

Jeff