Merc 90 hp 4 stroke - rough idle no power

[ebubeck]

My 2001 90 hp 4 stroke will rough idle after start up. It seems that sometimes I would start the motor and I was missing on a cylinder, very rough with no power. Sometimes it would take just a couple of minutes and then instantaneously the problem dissapeared, like a switch was thrown or something broke loose. The longest this lasted was once for about 15 minutes and then nothing for the rest of the day. I have noticed that the problem is worse on cold days or after I have not used it for a few weeks - and sometimes not at all. Lucky for me my warrantee ran out on this motor 2 months ago and two days ago (after not running for 2 months) I started it and it will not get out of this mode. It is rough and I do not have enough power to get on plane. I am currently diagnosing to the best of my abilities - I have spark at the coils and will check the plugs tonight. After that it's the carburetors I suspect. They have an auto enrichener valve that apparently helps at start up by giving a richer mixture - it works by heating the electrothermal ram and thus closing off the excess fuel. Sounds like something that could easily get stuck on a cold day. Please help me out - I can't afford a trip to the dealer. I will post my final solution.

 

[The Marine Doctor]

Re: Merc 90 hp 4 stroke - rough idle no power

You could be bang on right with the autoenrichner. I believe there is an Ohm's test for the valve. Here it is <br /><br />15-25ohms@68deg F or 20 deg C<br /><br />Piston valve height test.<br /><br />Measure piston valve height<br />Connect it to a 12 volt battery<br />Wait several minutes<br />Measure valve height.<br /><br />Replace the auto enrichner if there is no change. or if it fails the ohm's test.<br /><br /> Have you checked the T-stat yet? These should be changed yearly as they seem to be a prob with the 4 stroke engines.<br /><br />TMD

 

[ebubeck]

Re: Merc 90 hp 4 stroke - rough idle no power

I checked the thermostat - although it is hard to say with a cheap multimeter and a hospital thermometer - it looks to be OK at least in the ballpark as per Merc specs. I could check it more closely with the right equipment but would the T-STAT be the cause of this problem? If it gave an artifically high reading the spark would be cut to a couple of cylinders (not the case) and the overheat alarm should sound (not the case). Other than that I don't see how it could affect the performance unless I am not fully comprehending it's role in the process. Please educate me - I might need to take another look at the T-Stat.

 

[dilligafda]

Re: Merc 90 hp 4 stroke - rough idle no power

I have seen this condition before and found that there was a small amount of varnish/debris in the carbs that would "break loose" or move to allow the fuel to flow properly through the carb. <br /><br />The fuels that are being used today has a large amount of alcohol and evaporates very quickly, leaving behind the solids that cause this problem. I have seen success with using a steady diet of a fuel system cleaner/additive (I use Quick Clean). <br /><br />When the carbs do get a bit gummy, I take the drain screws out of the carbs and use Power Tune into the carb and let it sit for about 20 minutes. It doesn't fix all of the problems, but it is a cheap try prior to a complete teardown/cleaning of the carbs. <br /><br />Hope this helps!

 

[ebubeck]

Re: Merc 90 hp 4 stroke - rough idle no power

OK - auto enrichener checks out, so does T-stat, fuel pump and ingnition coils. Also checked that the auto enricherners are actually getting the 12V they need. Changed all of the plugs - noticed that new plugs quickly become covered in a carbon residue (weak spark or rich fuel). I suspect weak spark but don't have the wiring harnesses or the DVA to check the output from the CDI. I have a spark checking device. I eyeballed .4" as indicated in the manual and saw a spark for each cylinder but am not convinced I saw a consistent spark (1 spark per revolution). I am leaning away from carburetor problems because I have one carb per cylinder and the carbon residue is immediately deposited on all 4 plugs. Anybody have any suggestions on a way forward. Should I pursue further checks on the weak spark theory and if so what? CDI problems? It was replaced 2years ago - different probelm. Do these things have a history of CDI breakdowns? Stator? Regulator? Could any of these have an effect on what is happening? Help please!!!!!!!!

 

[hondon]

Re: Merc 90 hp 4 stroke - rough idle no power

How much water do you have in that fuel? Are you useing a water separator? If not ,you should have been all these years.Take a fuel sample and get back to us.

 

[ebubeck]

Re: Merc 90 hp 4 stroke - rough idle no power

No water in the gas. I'll be makeing wire harnesses and ordering the DVA. I'll let you know what I find. Any ideas until then?

 

[Mucka29]

Re: Merc 90 hp 4 stroke - rough idle no power

check coils and change plugs

 

[dilligafda]

Re: Merc 90 hp 4 stroke - rough idle no power

LA, Not that it's a bad idea to check out the ignition, but why? I can almost garauntee you that your problem is being caused by the carbs. The ignition on these engines is just about bullet-proof. I have seen a suspected ignition problem turn out to be the carbs several times. Try using the powertune or seafoam in the carb bowl, before spending a bunch of money on the ignition system!

 

[ebubeck]

Re: Merc 90 hp 4 stroke - rough idle no power

UPDATE:<br />Hope you don't mind such a long string - I want A-advice B-to make this a learning experience for anyone who cares to read the posts.<br /><br />OK - I spent a little time on her this weekend. I discovered that if I pulled plug wires to #3 and #4 there was no detectable change in rough running. Hmmmm - spark looks OK, I even checked the CDI output and it is within speck. Also checked the CPS and it looks to be within speck. Gotta be the carbs right? I ran seafoam through with no luck so I took the carbs off and have dissasembled #3. It looks great and I have not seen one speck of dirt anywhere. I guess I will just blow everything out and reassemble. Anything I should do while I've got them off? I've never gotten into a carberator before so I want to be sure and do a proper job. Should I go through #1 and #2 while I'm at it, I wasn't planning on it. Also, I can't get out one of the needle valves (it's got a blind plug over it). It looks good from the top. I was planning on just leaving it, maybe soak what I can with seafoam and then blow it out. Heckles and advice appreciated.

 

[dilligafda]

Re: Merc 90 hp 4 stroke - rough idle no power

Hey there LA, long thread? No problem here!<br /><br />Yes you are on the right track. Clean (even if you see nothing) and adjust the carbs to factory specs (would have to look up!). Make sure that the carbs are balanced after you install them. Carbs that aren't balanced will cause a very poor running condition! You will need a manometer to do this, and unless you work on multi cylinder motorcycles, you probably don't have one. There are several on the market that use mercury, water or dial gauges to make sure that the vaccuum is equal on all cylinders. If you get the one that uses water or mercury, be sure to use extreme caution when revving the engine. If you "chop" the throttle, you can suck that medium into the cylinders (bad thing). Follow the procedures outlined int he service manual and you should do real well and your engine should purr like a kitten again! <br /><br />Good Luck!

 

[ebubeck]

Re: Merc 90 hp 4 stroke - rough idle no power

Went fishing yesterday with my smooth running four stroke pressing me onward!!!! As many of you suspected it was indeed the carbs. I rebuilt all four and tuned it using a mercury carb synchronizer ($40 - JC Whitney online). A word of caution for all of you novices out there (myself included) -When I first restarted she was running terribly. I was about to give up and turn her and my wallet over to the dealership but I took out the auto-enricher for one last look and found that the needle had been offset when I replaced it thus the problem. No big deal to straighten it back out and now she runs better than from the factory.<br />Thanks everyone for your help!

 

[ebubeck]

Re: Merc 90 hp 4 stroke - rough idle no power

One final note - Don't be afraid to take apart the carbs on these things. I have very little experience with carbs and had them out, rebuilt and back in within 3 hours. Get a can of spray carb cleaner, compressed or canned air and an assortment of flat head screwdrivers (to get the jets out) and a clean working area. No problem. The fuel lines can be hard to remove be careful not to tear them up - you might want to get a few feet from the auto parts store in case you do. That's it clean everything. I never saw any dirt but apparently I unlodged something.