Carbs rebuilt but still running lean

[Darren418]

Hello all,

I have a 1988 Evinrude 48 spl (VE48ESLCCC) that I've had for a couple of years. Runs great at mid-high range but has always been pretty rough at idle. This week, I finally got around to rebuilding the carbs. Both carbs were clean and in good shape although I did go ahead and replace all parts such as gaskets, needle valve, etc.. Upon disassembly, I noticed that the idle air orifices were 27 and 28 where the stock sea level orifice is supposed to be 36. I reassembled the carbs with the proper #36 orifices. It ran like crap at idle on the muffs in the driveway, so I put back the #27 and #28 orifices and it ran much better. I switched back and forth and clearly the richer (#27/28) orifices ran better. With the #36's, it ran well only when I stuck two fingers in the upper carb throat...in the lower carb it didn't make any difference when I stuck two fingers in. Also, with the #36's in, it ran better when I bumped the primer, thus my conclusion is that it is running lean particularly on the top carb.

I can't think of anything else that could be causing this lean condition. I used new carb-intake gaskets, tightened them down thoroughly, triple checked everything when I reassembled the carbs to make sure everything was tight (I was looking for a possible leak when I rebuilt the carbs do was very careful). It seems the top carb is sucking air from somehwere but can't for the life of me figure where from. I tried spraying WD40 around the carbs while running with no effect. Fuel lines checked, no leaks. Could a faulty fuel pump cause the top carb to run lean? Leaking diaphram or something? Didn't think so but maybe...

Should I even be concerned about this? It seems to idle ok (still not great though) with the richer orifices. Is going from a stock #36 to a#28 to get it to idel ok acceptable procedure or should I really try to track down the reason for the lean condition?

This is a great little motor, VERY simple, so it's driving me nuts I can't figure this out!

 

[iwombat]

Re: Carbs rebuilt but still running lean

It's possible to over-torque the flange nuts and warp the flange. Usually when that happens the butterfly shaft binds though. Might want to take one of them off and check the flange against a straight-edge just to be sure. They can leak around the shafts if they're really worn out too, although I've never seen that in an outboard carb.

Just a couple of things to check. Can't think of anything else right now.

 

[lexkyboater]

Re: Carbs rebuilt but still running lean

Disclaimer: I'm just brainstorming. I know nothing about an 88 48 spl, but thought I might throw some ideas out.

From my 2 stroke dirt bike days, seems that high speed/main jet can affect idle as well, right? If it's running good at mid/high, I wonder if it's lean at higher rpms on that upper cylinder? And lean or leaner at low rpms? One way to check gas/air mix at high rpms is to run it WOT and then kill it (at least that's one way on a dirt bike) and then check if plug is white or black. If white, you're lean at the higher throttle openings. Compare to lower plug. Don't know if the marine guys would recommend this - maybe wait for confirmation. One problem might be that water would flood your stern.

On my older 71 Johnson 9.5 HP, there's also only one port/passage from the float chamber and that feeds both the high speed and idle. I wonder if the top carb has some blocked ports?

Have you considered that you might *not* be sucking air on that top carb but instead have a blocked passage/port in the top carb and you're just not sucking enough gas? That would create the same problem. Also, are you running #27 in the top and #28 idle jets in the bottom carb, or vice versa, so they're about even?

 

[Darren418]

Re: Carbs rebuilt but still running lean

Ran down to the lake for some in-the-water testing. Ran like crap at all rpm's. Unsteady, surging, couldn't keep idling, barely made it back to the ramp. Brought my tools and timing light with me, so while I was out, I re-did the link and synch and idle timing. Both were good but redid them anyway.

With nothing left to think of to do, I took apart the carbs again when I got home. Triple checked everything again, float drop, passageways all clear, etc.. Floats (new in the kit) were adjusted to level when upside down and 3/4" drop when upright. But I did notice that the float bowl screws were not real tight. I tightened them down as much as I could and saw the gasket get squished down a bit and some raw fuel get pushed out. Hmmm. Maybe I just didn't have the float bowl screws tight enough? They weren't loose, but certainly took another half turn or so, all of them, before they were really tight. I got those little buggers as tight as I could manage!

While I was at it, I rebuilt the fuel pump with a kit I had sitting around. Of course now I won't know if it was the carbs or fuel pump, but I suspect the carbs since everything was running fine until I messed with the carbs.

Ran it on the muffs just now with #32's in both carbs and it seemed to run much better than before! Lake test tomorrow then followup posting.

 

[iwombat]

Re: Carbs rebuilt but still running lean

Careful on overtightening. They should be snug and then another 1/2 turn or so. You don't want to really reef on those or you'll warp the bowl.

I've seen more damage from overtorquing than I ever did from undertorquing.

 

[Darren418]

Re: Carbs rebuilt but still running lean

Thanks for the heads up on the overtightening. The factory manual calls for 24-36 ft./lbs., which is about all I could do with the screw heads stripped out by a previous tech. The manual also calls for OMC "Locquic" and "Screw Lock" on the float bowl screws. Next time I rebuild 'em I'll put some Loc-tite on the NEW screws.

Also, on the fuel pump rebuild, I noticed the original had two rubber o-rings sealing the filter cap but the OMC kit had a rubber washer that seemed to serve both purposes. No leaks, so I guess it worked.

Thanks for the help. I'll report back after the lake test tomorrow. If all goes well, I won't be back 'til late!

 

[Darren418]

Re: Carbs rebuilt but still running lean

Ok here's the update after a lake test. Fixed. Well, mostly. 2000 rpm and up is perfect. The loose float bowl screws were evidently the post-rebuild problem. It really sings now at cruise, like it always did. But idle is still not great. It works ok, starts, runs through no-wake zones, etc., without stalling but it's still rough. Maybe I'll just need to learn to live with it that way being a big twin and all.

I messed around with the idle orifices some today and here's what I discovered: the old #27/28's that were in the carb prior to rebuild were way too rich after carb rebuild. Not entirely unexpected. Well, the #32's I have run the best and the stock #36's are too lean. I'm saying too lean because the rpm's definitely pick up with primer and in fact I have to bump the primer every 10 seconds or so just to keep it running. The #32's I have seem to run the best, although they are slightly rich. Primer will kill the engine unless it's just for a bief burst then the engine sputters and sounds like it's running on one cylinder for 5 seconds or so and then clears back up somewhat to "normal" rough idle. With the #32's it will idle noticeably better for 10-20 seconds or so after a good 2000 rpm+ run. Thus my conclusion they are running rich with the #32's. How I wish my carbs had adjustable needle valves for the idle mixture instead of these danged orifices then I could find the best spot for each carb and be done with it! I'm guessing #34's might work the best but don't wan't to waste the effort and expense of trying them unless I was pretty sure they'd clear up the idle once and for all. But who knows, maybe one carb is richer?

So 99% good now after the carb rebuild. Wasn't the "magic bullet" yet for my rough idle but it does run super above 2000 rpm. Also learned you'd better tighten down those float bowl screws pretty well during the rebuild or you might end up sucking air.

Thanks for the help.

 

[Dennisanoka]

Re: Carbs rebuilt but still running lean

I know you've done the link and sync over and over but just be sure both butterfly's are truely closing all the way at idle. And also did you check your primer hoses for tightness? Or a crack in a hose fitting on the primer itself. As you know it doesn't take much of a leak. I know what it's like to have everything just right, except this one little thing. I hope you get it.