Tohatsu 50D - Running Problems

[Craig LaForce]

So, any update? Is the motor back in gear so to speak?

[hvaldezz]

UPDATING @ May 13th

The Tohatsu mechanic got the carb parts in and did all 3 carbs and checked everything. Also replaced the hose going from the inlet for the gas and the hose going from the outlet on the fuel filter to the juction where it's mixed. Checked spark on all plugs and guess what?

STILL THE SAME PROBLEM!! Idles and runs fine without a load (prop in the water and in gear). As soon as he runs it up, it starts dying out.

The choke is not being engaged when it dies out and is all throats are clear. The plugs have plenty of fire, the fuel filter fills up and stays full, and all indications are that everything is normal.

I know I'm repeating myself but whatever suggestions he's getting from Dallas we've already tried.

When he pulls out a whole plug (#2) the engine doesn't stall out very much at all, and probably #3 would do the same. If he covers the throat on #1 carb it stalls badly, the other 2 don't much at all. I asked him about timing and he says that's the last thing he wants to try but we are basically out of options.

I'm going to go check compressions again tomorrow but they were 135 for all 3 cylinders last time we checked. He thinks it's not a compression problem. He also checked out the lower unit and the oil to make sure it wasn't binding or anything and it's fine. We never hit anything with the prop and really hadn't run the motor for a few months before all this happened.

I know we're missing something but I can't put my finger on it. It's not that difficult a motor to work on but we're stumped. So here's the synopsis:

1. New gas tank, fuel (super), fuel line, fittings.
2. New fuel filter and hoses.
3. New plugs, and gapped correctly
4. New carb rebuilds.
5. New fuel pump rebuilds.
6. Coils have been checked and are good.
7. 2 cycle oil is good and full.
8. Spark is very good.
9. Compression +/- 135 on all cylinders.
10. No compression leaks obviously because of 135 lbs. in cylinders
11. Lower Unit is fine, oil is fine
12. No bent prop shaft.
13. When running, water pump is working fine.

WHAT HAVE WE MISSED?? No one has touched this engine except for the carbs (cleaning and then rebuilds). Plugs I installed but are the correct type recommended by Tohatsu.

HELP with any suggestions if you see we've missed something. This is getting serious, very serious.

Henry

[kmclemore]

I'm grasping at straws here but.... here's a couple of other possibilities....

After trying the power-on load test, and having it stall, have you pulled the plugs immediately afterwards and checked them? Just curious as to if they look overly wet with fuel - discovered best with a 'smell' test. I ask because if you've got a vacuum leak in the intake manifold there's a possibility that the idle mixture has been adjusted to compensate for this leak and it will idle fine, but then when you try and run it up the mixture is horribly off and it will die.

Other possibility may be that the jets in the carb may still be clogged, despite your cleaning. It's been a while since I had my carbs apart, but aren't they progressive orifice carbs? If so, one of the progressive orifices may be clogged and cause a flat-spot or dead-spot, which would cause the symptom you see. Whenever I rebuild a carb I use a set of tiny wire-drills and a finger-chuck to clear the jet orifices....



Also, have you tried hooking up an exhaust gas analyzer to see what's happening as you run the motor? I'd be curious to see what the output is of your motor versus another Tohatsu 50.

Finally, I'd think about checking the exhaust gas temperature as you come on load.. this would pinpoint the bad cylinder(s) immediately if there is a significant difference between them. You can check the temperature using an infrared thermometer.

[pokerrick1]

Kevin, you know waaayyyyyyy tooooooo much - - -waaaayyyyyyy more than you are supposed to know

Rick

[Jack Sparrow]

Sounds like spark advance to me, is it electronic advanced or mechanical

Jack Sparrow

[kmclemore]

Kevin, you know waaayyyyyyy tooooooo much - - -waaaayyyyyyy more than you are supposed to know

Rick

Oh yeah... so how come I can't figure out my 14-year-old?

[kmclemore]

Sounds like spark advance to me, is it electronic advanced or mechanical

Jack Sparrow

Agree, but I thought they checked that?

[Graham Carr]

I had the same problem with my Mercury 50 Bigfoot a few years ago.

On land with a water hose hooked up, the engine would idle (no load) and run fine at higher RPM. In the water (under load) I could only idle along, she would not speed up. Sometimes she would die or sputter. As you have done, I checked everything. It turned out the problem was the water pump impeller. At the time I did have a great pee flow so at first I assumed my pump was working fine. Turned out that the t-stat looked good but had just a slight bit of corrosion on the back side (block side), so it reduces the flow. It is possible to have a t-stat that opens when tested but still restricts the flow enough to cause the engine to over heat.
A new t-stat solved my overheating problem. The old one looked almost as good as the new one.

The pee flow only shows the pump is working or the water inlet is not blocked. You can have a great flow (as I did) but still overheat. On the Bigfoot, the water flow splits (see below in the flow info). Also you can have a low pee flow with out the engine overheating, but you can have the (Bigfoot) fuel cooler and fuel pump over heat. They are water cooled.

I found out that the owner’s manual is wrong on temp for flushing. They state (opens at 120 deg) only idle the engine while flushing and that is enough for the t-stat to open. So I followed the flushing as specified for two years. That is when my problems started. This is what I found out after the stat was replaced and I install the “Smartcraft” system monitor (this is not the Guardian warning sounds). This is a gauge system that monitors engine functions.

I found that when the garden hose is hooked to the engine and running at an idle, the temp NEVER reached the opening temp of the t-stat. I have to run the engine a little faster to reach 120 deg. So it seams that I had only flushed part of the engine. Salt water must have remained at the back of the t-stat. The Guardian system will not allow the engine to run at higher RPM only enough to limp (sometimes) back to port.

Hope this helps.
Graham Carr

See test below.

Testing for the Mercury Bigfoot 50

T-stat testing:
1. Place the t-stat in a pan of water
2. Place a thermometer in the water and heat slowly.
3. 118 - 123 F (48 - 51C) starts to open it
4. AT 145 F ( 63C) you should have a minimum lift (opening) of 0.12 in. (3mm)


Engine coolant test (ECT):

1. Place the ECT in a pan of water.
2. Place a thermometer in the water and heat slowly.
3. Measure the resistance at different temp. if the reading is off, you will need to replace the sensor


30 F = 32 (KΏ)
60 F = 15 (KΏ)
90 F = 7 (KΏ)
120 F = 4 (KΏ)
180 F = 1 (KΏ)
240 F = .5 (KΏ)

Water pressure test:

This should be done on a warm engine with the thermostat at 140 F (60 C). You need to remove the test access port plug from the base of the thermostat housing and screw an adapter in so you can attach a pressure gauge.
Water pressure measurement:

1000 RPM = 5.5 psi
2000 RPM = 8.5 psi
3000 RPM = 12 psi
4000 RPM = 14.5 psi
5000 RPM = 17.5 psi
6000 RPM = 20.5 psi


Water flow info:

This info is based on the 40/50/60 EFI (4-Stroke) Water flow diagram which can be found on the last page of the Mercury service manual.

The numbers reference the parts shown on the water flow diagram.

The cooling water flow is as follows:
1. Enters water inlet-
2. enters water pump then exits to-
3. Driveshaft housing into-
4. Water tube which channels the water into the-
5. Adapter plate and into the-
6. Cylinder block. At the base of the block the water splits. Part of the flow is directed into the base of the water jacket cover. The water exits the base of the jacket via tubing and is routed to the (10) Fuel cooler, then the (11) fuel pump and then exits the (12) Tell-tale (pee hole). The rest of the cooling water flows up through the cylinder block channel (water jacket). This channel routes the water into the-
7. Cylinder head, then it flows back into the cylinder block around the cylinder walls until it exits the block at the top via the
8. Thermostat and then back into the water jacket. This flows down to the-
13. Exhaust water discharged.

[Kelly Hanson East]

I dont think spark advance problem would cause stumbling, it would just make the engine anemic. If its actually cutting out and getting rough, I would still think its a fuel problem.

I confess Im stumped on this one though.

[hvaldezz]

Gentlemen: Thanks for the inputs, all of them. At this point we'll continue to see what else we can try. I think the intake manifold leak may have some merit although with new gastet there it shouldn't be a problem.....I think.

The overheat suggestion is interesting although after running on and off the block is real cool and plugs can be pulled with no problem (not hot at all after a few minutes). That's kinda' weird but I think it's ok.

So the consensus tends to be fuel problems although spark advance could be our next direction. Mechanic says that shouldn't be a problem because it is a "mechanical advance"?? Don't quote me on that but I think that's what he said. He did mention something about it not being a pneumatic advance.

I've emailed him most of the suggestions and he does read them although most we've tried already.

Will advise on findings, hope today turns out to be a good one. In my mind I'm almost to the point of looking for a good used engine and having it mounted but I've got so much into this project that I probably won't. I just wish we could all get together at some location and tear into it once and for all. Is my frustration showing????

Henry

[Hardcrab]

I also think the spark advance will end up a dead end. But it should be looked at, and eliminated. A part of the mech linkage could be at fault.

I'll suggest that if fuel delivery seems to be the most likley culprit, then,

FOR TEST ONLY, trickle some fuel into one or both of the "dead" acting carbs. Run the rpm's up into the dead range. It might go rich as hull for a short while, but the right mixture will be seen at some point.

Perhaps use a drinking straw, with fuel captured by your thumb, so that you can control the amount delivered. Become your own carb, grasshopper.

"Known" fuel in the throat should either make it better or worse.

Needless to say, this needs to be done with the utmost care, but it should focus your efforts between fuel or ignition.

No Smoking during test.

(Typing this made me ponder if the carbs Hi Speed circuit could be delivering to much fuel, and you could have "rich mixture misfire". Although wet plugs, pulled quickly, should show this. Food for thought. An exhaust gas sniffer would be priceless, as kmclemore has brought up).

[Don T]

Hello,
1. Under load, manually open the throttle without moving the timing plate (push on the cam that actuates the throttles). See if the engine runs ok. If it is a carb / fuel problem the motor will die when the main jets start working. The engine will not develop full power but it will disclose a problem.

2. Disconnect the timing plate to carb actuator arm and advance the timing while under load, leave carbs at idle. If there is a wiring problem the motor will cut out. NOTE: You wont be able to get to full advance without the engine sputtering.

3. Even though you have replaced the fuel line and fittings do not over look a fuel flow test. As I said before, the new replacement parts don't always work correctly. The lines and filter will stay full even if there is insufficient flow to keep the engine running. I replaced my hose and fittings with factory parts and ended up with fuel problems. Basically I replaced a bad hose with faulty parts (traded one problem for another) so my starvation problem persisted.

4. Under load, run up the throttle until the motor dies or turn off the ignition when the problem is at it's worse (do not let the engine idle). Loosen the fuel bowl drain screw and check how much fuel is in each carb. Pull the plugs and read them.

5. It is easy to overlook rechecking things when you are convinced they have been checked. The reality is, if everything was perfect it would be running OK. Assume that it is the first time you have looked at it, begin from the beginning and check everything step by step.
Determine which of the 3 things (fuel, fire & compression) the motor is losing. It seems like your mechanic is a parts changer, not a diagnostician. No disrespect intended but he has now changed every obvious part and still has no idea what is going wrong or which system is failing.

Added on edit:
6. Be sure to try running it up with the over temp sensor disconnected. It is the sensor on the head near the top spark plug with a wire going to the CDI unit and a ground under a bolt close by. There were some factory defective units on some engines.

[hvaldezz]

Don T: Thanks for the suggestions, raining today so tomorrow will try to methodically and logically try some of them.

Thanks to all for their suggestions. I'm sure we're missing something in our approach. I guess we've been around the problem too long that we're going around in circles....who knows?

Henry

[Rich Walton]

Had the same thing on a suzuki df 50 with 20 hours on it, couldnt find a thing, changed the plugs (the old ones looked new and worked fine at low speed and load) after the new plugs all fine. A long shot but a cheep thing to try.

[The Arabella]

These engines have carbs like a motorcycle, and use different jets/metering mechanisms for different throttle settings. I bet your guy put some jets in backwards, or the carbs have a passage blocked somewhere.

I would take it to someone else.