Runs rough at low speeds - 1998 Merc 90 ELPTO

[john craig]

I'm running out of ideas on this one. Compression is 120 in all 3 cylinders. Very low hours on this motor. Carbs rebuilt with OEM kit and new jets - no improvement. New spark plugs of course. All CDM's produce a 7/16 inch spark at cranking speed. Timing is right on. Link & sync adjusted per manual.
Runs perfect at higher speeds but at 2000 rpm and under it's very smokey and runs rough so I'm guessing it's a too-rich mixture because as soon as the throttle opens to admit more air everything is fine. Thought the enrichment valve might not be shutting off but it checks out. Idle needle valves are all about 1-1/2 turns out and tweaking them doesn't help. If there is too much fuel, where is it coming from? Maybe recheck the float levels?
A bad fuel pump wouldn't cause a rich running condition, would it?
Any advise will be very much appreciated!

 

[Silvertip]

Re: Runs rough at low speeds - 1998 Merc 90 ELPTO

Improperly adjust float levels most certainly can cause overly rich conditions. An inlet needle(s) that are improperly installed or not sealing will also cause the problem. If the floats are shutting off properly the fuel pump won't be an issue. If they are not operating properly the fuel pump will have no restriction to flow and will pump fuel at a rate up to its capcity. When you finish checking this, before reassembling each carb, tip it upside down and blow into the inlet fitting. If you can, the needle and seat are not functioning properly. Link & sync can be touchy so make sure that's spot on.

 

[Texasmark]

Re: Runs rough at low speeds - 1998 Merc 90 ELPTO

I have one of those critters, had similar problems, and it responded very well to a decarb with Sea Foam and subsequent addition of it to every tank ful. Other contributing factor could be lugging.....what are your WOT rpm's and where do you do most of your running (rpm)?

Mark

 

[WillyBWright]

Re: Runs rough at low speeds - 1998 Merc 90 ELPTO

Those are their roughest idlers. But above idle it should smooth-out OK. To smooth the idle, they recommend adjusting the top carb idle jet at about half of the others, about 3/4 turn. That leans-out the top cylinder so that it doesn't fire at idle. It'll still get enough lubrication, and above idle it all kicks in. But I find that holeshot suffers badly when doing that.

A torn fuel pump diaphragm will leak fuel into one or two cylinders and make it run rich on that/those cylinders. It usually shows on the sparkplug(s). But kits are cheap and fairly easy to install. If this is a new condition, it's definately worth a try.

 

[john craig]

Re: Runs rough at low speeds - 1998 Merc 90 ELPTO

Really appreciate your input, friends!

The idle is somewhat rough but acceptable, it's the 1500 to 2000 rpm range where something is not right. I actually have tried adjusting the top idle valve 1/2 turn out but it didn't solve the problem. I'm sure I'm not lugging - the motor will rev right up to 5500 with a light load but I normally run it at 4000 or so. It's on a 17ft Boston Whaler Montauk with a 21 inch aluminum prop.

My plan of attack: (1) check the float levels, (2) rebuild fuel pump, (3) get some Sea Foam.

Thanks again for sharing your expertise!

 

[Silvertip]

Re: Runs rough at low speeds - 1998 Merc 90 ELPTO

Nearly all two strokes, including my current 75HP have what I call a "burble" at about that rpm. Its a transition point in the fuel/ignition advance range and there isn't much that can be done. As for rough idle, my 75 idles just fine. It's not as smooth as a four cylinder but definitely smoother than a twin.

 

[Texasmark]

Re: Runs rough at low speeds - 1998 Merc 90 ELPTO

Sounds like you have her propped just right; considering your numbers and relating to mine.....same engine, 17' boat, but lighter, prop pitch higher as a result.

5500 WOT and 4000 for cruise on that rig should work just fine on that rig.

One thought I just had. If you don't have a thermostat your idle will be rougher also. I had a bad stat and for a short time I ran without one (before I installed a new one) and I found that the engine ran considerably rougher when cold....(with no stat).

The pee spiggot on yours should come from a rt angle connector on top of the exhaust manifold cover. This part of the engine gets cooling water as soon as the impeller gets the water up there. So this isn't where you can test for stat operation. No real way to know if you have a stat without popping the cover and looking. You could possibly feel the water jacket cover adjacent to a sparkplug after you have been running slowly for a few minutes. Stat on mine is 120 F so it should feel hot where you would have to remove your fingers after 30ish seconds of touching it kind of heat.

If you get up on plane or run over a couple thousand rpm's the bypass valve (right next to the stat under the same cover on the upper left of the cylinder's water jacket cover) opens and shorts out the effect of the stat and you can't tell anything. Gotta be going slow to test for it.

Mark

Mark