MFS6B Idle Question

[kpgraci]

For reference I had a post last week in this thread about this motor.

Short history: Cleaned carb 3x per this sites recommendation over a period of one week, new fuel and new fuel filter, cleaned tank, flushed fuel lines and fuel pump. Mixture screw is still sealed. Although several years old motor has low hours (claimed <15) and is clean inside and out, looks new in every regard.

Problem: Using a Hardline tack I set the neutral idle at 1300. I let the motor warm up for 3-5 minutes revved a few times and let it settle, then set the idle adjust screw to achieve an average of 1300. Tach samples each ? second and RPM would fluctuate between 1270 to 1330 or so.

After I set the idle on the stand I would rev to 3000 or so for 15 seconds, and return to idle but RPM would hold around 2200 for maybe 2 to 3 minutes or more, then settle down to the 1270 to 1330. It did this several times so I assumed it was OK.

At the boat I installed the motor and tested the engine. After warm up the idle held around 2400 and never went down, either when left alone or when revved. Actually I’m pleased with that because it may mean that I can now bring this idle down to 1300, but I did not spend a long time playing with it and if is it’s going to do the drop in RPM as I observed on the stand I don’t want to lower the RPM or when it does the motor will stall.

Question: Is what I described normal behavior (for a small motor like this) or should I be able to get a consistent 1300 idle speed after warm up? I’m concerned that at 2400 I will not be able to shift into or out of gear, plus the concern of a drop in RPM at ‘random’ intervals.

The only two adjustment on this carb I believe are the mixture screw and float level, I have set neither but have no reason to believe either is out of whack.

Additional:

I don’t know if this is interesting or pertinent, but I had first set the idle by ear before I got the tach. Once I got the tack it showed the idle at a solid 1200, with a 1210 popping up every 20 seconds or so. When I brought it up to 1300 it would float around 1300 and still will not settle (1270-1330 at least wobble). At slightly open throttle (RPM > 1400) the RPM is stable as well. The only place it floats is in the 1250 to 1400 range.

 

[TOHATSU GURU]

Re: MFS6B Idle Question

The float can be adjusted, but if it is level when flipped upside down it correct. The only other adjustment that Tohatsu has on it that you ever need to touch is the idle screw. The engine should not be anywhere near 2400 at idle so something is very off. I would run the engine into any Nissan, Tohatsu, Mercury or Evinrude dealer and let them determine what is wrong.

 

[pvanv]

Re: MFS6B Idle Question

You still have a problem; likely still a pilot circuit problem in the carb. See pps 4-8 and 4-9 of the 003-21034-2 Factory service manual, and you will see a carb flow diagram that explains that you are slightly engaging the bypass circuit of the carb when you are trying to idle. It's likely that the pilot circuit is still restricted (possibly with white oxides from water contamination -- if so, no amount of solvent cleaning will clear it), causing you to increse the idle stop to the point that the bypass circuit is flowing.

Idle in neutral should be about 1300 RPM; trolling speed in gear s/b about 1150.

As a last resort, you could try adjusting the pilot mixture screw. That procedure (no longer documented in the modern manuals, since the EPA has forced manufacturers to plug off access to the pilot screw) requires a good shop tach, and an experienced ear. Ideally, someone who has done the procedure before would be the best tech to do it on a modern carb.

 

[kpgraci]

Re: MFS6B Idle Question

I see. Makes sense. Thanks.

 

[MattFL]

Re: MFS6B Idle Question

Hanging idle can also be caused by an air leak (basically it is a lean condition as mentioned above ). If you had the carb off for cleaning, be sure the gasket was in good condition when you reassembled it.

 

[Sea Rider]

Re: MFS6B Idle Question

Was the Hardline tach set accordingly per it's revolution ? if not, will read double. Digital tachs does not read accurate readings at idle compared to needle tachs, but that's normal. After passing idle speed begins to read more accurate and steady numbers. Check manifold carb gasket is in good condition too, that is, seals well.

Happy Boating

 

[MattFL]

Re: MFS6B Idle Question

I've got a Hardline on mine and it gives a very stable reading at idle. The RPM goes up and down a few RPM at idle as shown by the tach, and you can also hear the difference in the motor if you listen carefully, so the tach is showing what the motor is doing, at least in my experience.

 

[kpgraci]

Re: MFS6B Idle Question

The tach is set to 1 pulse: 1 revolution, and the numbers are not doubled I'm sure so I assume the motor has a sacrificial spark.

As far as the RPM not being real stable at 1300, by ear it does not sound that bad but you can tell is it floating around a bit.

My main concern is the apparent two levels of idle I get (hanging idle) - as I stated after an initial warm up it will settle to an idle which I adjust to 1300, then with subsequent revs it will hold a higher 'idle' rpm for a while (1-2min?), long enough for me to want to re-adjust the idle down, then it will drop to the 1300. After revving I assume there is fuel still riching up the intake so a few seconds for the moto to settle down to an idle seems normal, but 1 to 2 minutes seems a long time.

Then after all that and my decision to go with the idle I had set at the boat the idle never did go back down after warm up to the 1300. To me this could indicate that the low speed passage was getting un-gunked with use (I do have seafoam in the gas) and now what I'm seeing is the actual idle speed - which would be great, but I don't like that it does different things at different times.

I need to go spend some more time seeing if it will stabilize with more use and maybe I can safely reduce the idle. Problem is its back on the boat out at the marina which is a bit of a drive.

EDIT: if an air leak can cause a hanging idle due to leaning the mixture, then a blocked low speed passage (which supplies gas I assume) would also lean the mixture, so...maybe my assumption is backwards, would a blocked low speed cause a higher idle?

 

[MattFL]

Re: MFS6B Idle Question

The answer is both. An air leak actually causes a lean mixture, plus more air.

At the right mixture idle is stable. Too rich and it will fall, too lean and it will fall (a 2 stroke can race). But if its too lean causing it to fall, and you then increase the idle speed, effectively open the throttle, then you get to a condition where it's too lean to be stable, but enough air and fuel to make it run fast for a little while. This condition can be caused by either a plugged up passage or jet combined with an increased idle setting as mentioned by pvnav, or by an air leak. This is my experience anyway.

 

[kpgraci]

Re: MFS6B Idle Question

hmmmm...carbs are such simple devices too, go figure. Armed with this knowledge I'll fool around with it a bit more. All the time I'm putting into it (and expense too, tach, carb cleaner, service manual) and fustration, if I can't get good results I can purchase a new carb from defender for just over $100 - seems like a bargin to get the motor running perfect.

 

[pvanv]

Re: MFS6B Idle Question

There still could be other problems, but from experience, as I previously mentioned, it seems that you have a restricted pilot circuit. That forces you to open the throttle plate a little more at idle, putting you "on the edge" of engaging the power circuit. That "edge" is probably what you are experiencing. If the carb cannot be cleaned with organic solvents, the restriction may be metal oxides, which basically are permanent damage. If so, a new carb is in order. IMO, do not replace with a B or C version carb, unless you are married to the EPA ratings and more sensitive/tricky idle of the newer carbs. Instead, use the 3R4032001M, CARBURETOR ASSY, MFS/NSF6A2, MSRP $116.52, available from any dealer, including several forum members (who have been helping and advising you for free). Most dealers will match price as well. Even Elvin and I have them available.

 

[MattFL]

Re: MFS6B Idle Question

I ordered a hard to find part from pvanv in the past and he got me exactly what I needed.

 

[rsnyder518]

Re: MFS6B Idle Question

Paul,

What causes the "metal oxides" that you mention and is there away to avoid it? I was adjusting my idle earlier this season on my MFS6B and experienced the same problem that kpgraci descibed. I eventually found a spot where the idle was in spec and it didn't seem to "hang up" after I revved it. But I'm pretty certain that I'm also right on that hairy edge of engaging the power circuit because once in a very great while the idle will get "hung up" after revving the motor. My idle adjustment screw is very sensitive and there seems to be three settings within a small fraction of a turn of the adjusting screw. Those settings are: "Occasional stalling at idle", "Good","Power circuit".

Thanks for the tip about not replacing witht he B or C carb, I'll keep that in mind should I decide that I do in fact have a problem with metal oxides restricting the pilot circuit.

Also, Paul, how would I contact you outside of this forum if I need t order parts?

 

[kpgraci]

Re: MFS6B Idle Question

There still could be other problems, but from experience, as I previously mentioned, it seems that you have a restricted pilot circuit. That forces you to open the throttle plate a little more at idle, putting you "on the edge" of engaging the power circuit. That "edge" is probably what you are experiencing. If the carb cannot be cleaned with organic solvents, the restriction may be metal oxides, which basically are permanent damage. If so, a new carb is in order. IMO, do not replace with a B or C version carb, unless you are married to the EPA ratings and more sensitive/tricky idle of the newer carbs. Instead, use the 3R4032001M, CARBURETOR ASSY, MFS/NSF6A2, MSRP $116.52, available from any dealer, including several forum members (who have been helping and advising you for free). Most dealers will match price as well. Even Elvin and I have them available.

What a great asset this forum is, you guys are doing us a great service out here. I am by no means married to the EPA standards and will take you up on that offer if it comes to that. Great info.

 

[pvanv]

Re: MFS6B Idle Question

Paul,

What causes the "metal oxides" that you mention and is there away to avoid it? I was adjusting my idle earlier this season on my MFS6B and experienced the same problem that kpgraci descibed. I eventually found a spot where the idle was in spec and it didn't seem to "hang up" after I revved it. But I'm pretty certain that I'm also right on that hairy edge of engaging the power circuit because once in a very great while the idle will get "hung up" after revving the motor. My idle adjustment screw is very sensitive and there seems to be three settings within a small fraction of a turn of the adjusting screw. Those settings are: "Occasional stalling at idle", "Good","Power circuit".

Thanks for the tip about not replacing witht he B or C carb, I'll keep that in mind should I decide that I do in fact have a problem with metal oxides restricting the pilot circuit.

Also, Paul, how would I contact you outside of this forum if I need t order parts?

The carb is an aluminum alloy. It's coated in a sort of anodizing from the factory, which helps stabilize it from oxidizing. If you run water droplets (even encapsulated "stabilized" E10 fuel)... if that is allowed to sit in the carb for long (say a week), some corrosion will typically start. Likewise, if run in salt water, the air is laden with salt fumes, and everything corrodes faster -- much faster than equipment that is used in fresh water.

The problem is that... if you are suffering from corrosion, no solvent will eat the oxides off. Some acidic cleaners might, but the passages in the carb are so bloody precision, that even cleaning a jet with a copper wire can cause micro scratches which can lead to problems and inconsistencies in the flow -- where you may have some stability of flow at a specific flow rate (RPM), but at others, the mix could be either too rich or too lean.

In every instance where I have had a 4/5/6 carb that was "on the edge" of a good solid idle, the pilot circuit was restricted. In 90% plus of cases, I have been able to clean the carbs... but I work on boats of the [fresh water] Great Lakes.

If anyone needs to contact me, you can drop a PM. iBoats doesn't like posting email addresses or website URL's.

 

[kpgraci]

Re: MFS6B Idle Question

Not trying to beat a dead horse here...just tyring to understand my syptoms more clearly. In looking at the carb flow diagrams on pgs 4-8...of the service manual it seems clear to me that since I run ok-ish at off-idle, the 'problem' exists beyond the bypass port which is the fuel/air mixture screw (that is covered up) leading to the idle port. Seems to me it is not even possible to properly clean this carb without removing that needle valve to at least give the cleaning fluid a change to penetrate those passeges.

Another observation, and I'm going to need help here, poor idle is an annoyance but my motor quite at 3/4 throttle, If the low speed passages (which also supply fuel/air at open throttle) were to be suddenly blocked at 3/4 throttle, do you think that would be sufficient of a fuel/air disruption to cause the motor to die? I would not think so but that seems to be the explanation that fits. I would expect a change in engine output, but as stated these carbs are tuned to strick EPA guidelines and anything less than perfect is a problem.

 

[pvanv]

Re: MFS6B Idle Question

Some have indeed [illegally, btw, though I have never heard of the EPA police raiding an outboard shop] pulled the pilot mixture screw when cleaning... Actually, the cleaning fluid will get there regardless, but the cleaning may be more thorough with the screw out. At issue is resetting it afterward. If you do go that route, be careful removing the brass plug/cover, so that your drill doesn't twist the screw, damaging it or disturbing its initial setting. Then precisely count the turns-in until the screw is lightly seated, so you can return to where it was originally. Do not tighten the screw too hard, or it and the seat will probably be damaged beyond use.

When working on such a carb... resetting the pilot screw can get tricky. You need a very good shop tach, and a very good (preferably very experienced) ear... set the mixture to the original turns, start the motor and get it fully warmed, and adjust the idle stop to the proper speed, both in and out of gear. Then adjust the pilot screw. Once you get an RPM increase, readjust the stop to specs, and adjust the pilot again. Repeat a few times, until you have the smoothest idle at the minimum throttle plate opening. Then richen the screw about 1/8 to 1/4 turn, to provide stability to the setting. If you are too lean, the motor can stall when rapidly coming back to idle from WOT.

Be forewarned that even an experienced wrencher can goof up this procedure, or at least spend a lot of time getting it right. The motor must be in the water, and you must be checking both the neutral idle as well as the in-gear trolling idle, as changes in manifold vacuum will dramatically alter how the fuel / air emulsion flows in the carb.

Yes, the pilot has some effect at all throttle positions... but it is a miniscule amount at 3/4 or more throttle... literally a "drop in the bucket".

 

[MattFL]

Re: MFS6B Idle Question

Did you mean that your motor quit (stopped completely) while at 3/4 throttle? If yes, then that's a whole new problem. As pvanv mentioned, the more you open the throttle the less the low speed jet has effect. At 3/4 i would expect the low speed jet to have negligible if any effect, so if it's cutting out at 3/4 throttle then your problem is elsewhere.

 

[kpgraci]

Re: MFS6B Idle Question

Yes. It did quit at 3/4 open, to which I cleaned the carb and was left with the idle problem I've been working on. So it seems the main jet was obstructed, after cleaning is was OK, but the low speed side still has issues after 2 cleanings. There is no visible varnish or corrosion on the visible internal parts of the carb, it looks new and perfect - it just does not idle that way. The passages after the slow jet lead to the bypass port, then the air/fuel mixture needle valve, then the idle port, so I'm deducing that is where the problem is. I will remove the cover and be careful to seat and count turns, clean and reset to original position - if that fails I'll just go with the older carb and be done with it. Thanks for all the help.

 

[MattFL]

Re: MFS6B Idle Question

Ok so cleaning the carb fixed the problems at high speed but left you with idle problems, got it. Have you blown compressed air through the low speed circuits? That can also help dislodge junk that gets caught in the tiny little places.