External Reeds Merc Mystery

[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

[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