tohatsu mfs6b quits under load

[kpgraci]

for ref i had posted this a while ago: http://forums.iboats.com/honda-tohatsu-nissan-outboards/mfs6b-idle-question-627120.html

long story short, motor has low hours, looks showroom new, carb was fouling with little gel balls, paul said this was bad gas, symptoms were poor idle and dying while under load (main jet fouled, i assumed). after many cleanings and fuel system flush I bought a new card from paul, a 'a' model with largers ports (pre-3 star emmission standards), flushed everything again, tank, lines, new filters, everything. I bought fresh gas yesterday from shell, 0% ethonol.

At the boat I ran motor for 1/2 hour at various RPM's, adjusted idle on new carb, sounded great. Put motor in gear with boat tied up in slip to about 1400 prm, forward and reverse, motor never missed a beat...good power even at idle with gear engauged.

Took the boat out for a spin around the harbor, idled out of slip area, when I reved up to around 1600 rpm under load it misfired, backfired, and quit. Restarted eaisly but I could not appy any significant power under load or it would do the same thing, every time. This is exactly what it was doing before all of this, but this time the idle is perfect.

So...my current problem is NOT a fuel issue, no way, no how. When not in gear it will rev and idel with no indication of a problem, under light load it is ok also, under medium load it backfires and dies.

So, experts, what do you think is going on here?

 

[Sea Rider]

Re: tohatsu mfs6b quits under load

Could be experimenting some sort of an electrical overheat issue on CD, ignition coil when passing middle throttle ? If it were an old engine, probably a head gasket failure when at more load, but it's not your case.

Happy Boating

 

[kpgraci]

Re: tohatsu mfs6b quits under load

Could be experimenting some sort of an overheat electrical issue on CD, ignition coil when passing middle throttle ? If it were an old engine, probably a head gasket failure when at more load, but it's not your case.

Happy Boating

I have a theory, see if this sounds right. Weak spark...if the spark were weak it could perhaps ignite the mixture under no load conditions, but under load more fuel is introduced (for a given rpm) and on the compression stoke it fails to ignite the mixture, on the exhaust stoke it fires again (sacrafical) and it ignites, causing a backfire and smoke out the intake screen (as observed) since both valves are partially open. No sure why it would die in this case, it's just a miss, but I suppose the blast through the carb would disrupt the flow of air/fuel on the next stoke (sure, I'll go with that), I had inspected the plug, which looks fine, but something wrong with the CDI would explain this also.

In any case I put it in a shop I trust...well see what they come up with.

 

[pvanv]

Re: tohatsu mfs6b quits under load

Hmm. Could be almost anything, but probably not the carb
I'm glad the new carb solved the idle problem.

There are ohm meter tests to run on the ignition system. They may reveal something. Intermittent problems can be a bear to find, but if the problem is easy to duplicate, it should be possible to diagnose.

Many many problems will only show up under load. The motor isn't required to make much power to rev in N, but does do a lot more work in gear in the water. A puff through the intake can indeed disrupt flow (and the contents of the intake will then be mostly smoke), so that can cause a stall.

If the ignition system passes all electrical tests, another possibility is a sheared (or partially sheared) magneto woodruff key, causing the timing to be off. A super-rapid stop of the engine (such as submersion, or being radically high on oil can cause that). Easy to check timing with an inductive timing light. Relatively easy to change the key if needed.

While on that subject, please be sure to always verify oil level on the dipstick (with the motor level). High crankcase oil can masquerade as other issues.

 

[rsnyder518]

Re: tohatsu mfs6b quits under load

I had an issue last season that may be related. I had sailed all day and started the motor to come back to the dock. It ran fine for a a little while and then suddenly quit. It restarted easily and idled ok, but as soon as I put it into gear it would quit. I tried several more times to get it going, but anytime I put load on it then it would quit. I brought it to a local repair shop. If I recall correctly the replaced the igniter...apparently it was not functioning properly when it got warm.

Not sure if this is your problem or not but I figured I'd chime in as it's something else you could check into. Hope it helps!

 

[kpgraci]

Re: tohatsu mfs6b quits under load

Hmm. Could be almost anything, but probably not the carb
I'm glad the new carb solved the idle problem.

Yup. Very happy with the new carb as the idle problem is gone. I thought it very strange after I cleand the old carb that it would die under load with 1/3 throttle or so, and the main jet showed no sign of fouling that time - but I was stuck on all symptoms being caused by a carb problem, seems I had two problems. fuel and electric, two weak links.

When I first got the motor it ran fine, the little I used it. No sudden stops that could shere anyting while under my care, but I've got the pros looking at it now, well see how good they really are :

 

[kpgraci]

Re: tohatsu mfs6b quits under load: UPDATE

Re: tohatsu mfs6b quits under load: UPDATE

Shop called me and said it's fixed. I'll need to see it working on the boat before I say its fixed, but that will have to wait.

When I brought it in I told the lady up front what I'd done (new carb, etc.) and she was writting as I was talking, I'd have perferred to speak to the mechanic but ok.

When they called I asked the mechanic what he did and he sid he blew out the card, drilled out the adjusment screw and adjusted it, and replaced the sparkplug. My guess is it was the sparkplug that 'fixed' it.

So I'm paying $230 for a sparkplug, and sure, an adjusted carb.

If it actually works I'm not complaiing. I knew going in it could be as simple as a sparkplug, but franklly I was tired of messing with it, so do it yourself or pay someone else - not a problem.

I'll see this weekend how it runs.

 

[TOHATSU GURU]

Re: tohatsu mfs6b quits under load

The carburetor did not need to be adjusted. It just needed to be cleaned correctly. The plugs may or may not have been an issue as well. Put an inline fuel filter on your engine just before the carburetor and that will help you immensely.

 

[kpgraci]

Re: tohatsu mfs6b quits under load

The carburetor did not need to be adjusted. It just needed to be cleaned correctly. The plugs may or may not have been an issue as well. Put an inline fuel filter on your engine just before the carburetor and that will help you immensely.

There is an inline filter before the carb, just replaced it too. I put the motor on the boat and cranked it up...it sounds a whole lot better.

 

[pvanv]

Re: tohatsu mfs6b quits under load

The carbs are set by instrument at the factory, so likely don't need a pilot adjustment. Some shops just set them too rich (masking some other issues). That will kind of work, but will foul plugs faster.

 

[TOHATSU GURU]

Re: tohatsu mfs6b quits under load

There is an inline filter before the carb, just replaced it too. I put the motor on the boat and cranked it up...it sounds a whole lot better. [/QUOTE



I'll be more specific. Replace or add a filter that will filter out the water that you are getting into your carburetor. The factory filter does not filter out enough water before it overloads and allows it to pass.

 

[kpgraci]

Re: tohatsu mfs6b quits under load

I'll be more specific. Replace or add a filter that will filter out the water that you are getting into your carburetor. The factory filter does not filter out enough water before it overloads and allows it to pass.

Right. I was looking at those, they all seemed pretty bulky and pricey too (the water seperating kind) (this from a guy that just spent $200 on a spark plug!). Any suggestions?