Tohatsu 50D - Running Problems

[hvaldezz]

I need help, please.

I have a 50D engine that I had the carbs done on. Once they were done it seemed to work fine but hadn't taken it out on the water thus putting a load on the engine.

Did that today and it conked out and would not stay running. For the record, it has new plugs, gas filter, fuel line, fittings and even a new tank. The bulb stays firm when running (idling) and high revs in neutral. Once in the water it will idle, when in gear low revs seem to be ok but then when I give it higher revs it like powers down and quits running. I tried slowly work the revs up but at around 50 percent it starts the quitting. I believe it is not a fuel issue, it is something else.

Does anyone have any suggestions? I've already laid out good money on the carbs what now. By the way, compression is at least 135 lbs compression on the cylinders, oil is fine, etc.

Help!!

[Frank C]

I see that you have replaced your fuel line and fuel tank. Seems you've covered some key sources of fuel starvation, but the fuel pump can be sucking air (instead of pure liquid gasoline), thru hose clamps, primer bulb, brass QR fittings, etc. (The brass fittings include rubber o-rings that can fail.

Could be lack of fuel, for example, from a dirty fuel filter. Sometimes there's a secondary fuel filter under the hood ... might want to check.

A vacuum leak might also be the culprit ... admitting too much air effectively reduces quantity of fuel.

The symptoms you described are still fuel starvation "classics."

[Craig LaForce]

The top of the new fuel tank fitting might have an air leak. There is a small fuel filter on the engine itself that you might also check.

Does it quit totally or just slow down? The overheat sensor on the motor was defective on a bunch of them and dropped it to the slow setting when loaded a bit.

Is the manual choke off?

Is the gas tank vent open?

Is the deadman switch still alive?

Just a few random thoughts.

[Trouts Dream]

Is the valve on the fuel tank all the way open?

[hvaldezz]

I see that you have replaced your fuel line and fuel tank. Seems you've covered some key sources of fuel starvation, but the fuel pump can be sucking air (instead of pure liquid gasoline), thru hose clamps, primer bulb, brass QR fittings, etc. (The brass fittings include rubber o-rings that can fail.

Could be lack of fuel, for example, from a dirty fuel filter. Sometimes there's a secondary fuel filter under the hood ... might want to check.

A vacuum leak might also be the culprit ... admitting too much air effectively reduces quantity of fuel.

The symptoms you described are still fuel starvation "classics."

Fuel filter is clean and works fine. When I took off the fuel line going into the filter and pumped the bulb it shot plenty of gas out. Checked all hoses and doesn't look like a leak anywhere in there.

Could it be spark advance?

[hvaldezz]

The top of the new fuel tank fitting might have an air leak. There is a small fuel filter on the engine itself that you might also check.

Does it quit totally or just slow down? The overheat sensor on the motor was defective on a bunch of them and dropped it to the slow setting when loaded a bit.

Is the manual choke off?

Is the gas tank vent open?

Is the deadman switch still alive?

Just a few random thoughts.

Craig, it quits totally not just slowsdown. It sounds like a deep sound with no firing of the plug...just my description. Manual choke is off, the gas tank vent is open. Deadman switch is fine and works. I don't think it's fuel starvation because the little bowl for the gas filter remains full. What about spark advance? Could that be?

[hvaldezz]

Is the valve on the fuel tank all the way open?

The valve on the fuel tank is open. In fact I switched tanks and the same thing happened. Please read my responses to Frank and Craig. Thanks.

[Trouts Dream]

I had atank where the valve only opened a bit, thats why I suggested the valve.
Out of curiosity did you ever run it up to nearly full throttle when on dry land. I ask to try to define the problem better. Is it the high RPM or the load causing the problem?

If the carbs were done by someone else, maybe they should be having a look at it.

[Frank C]

Craig, it quits totally not just slowsdown. It sounds like a deep sound with no firing of the plug...just my description.

Okay, that's a different symptom. Fuel starvation causes missing & stumbling at the half-throttle setting, causing a max of only 50 percent revs. This IS NOT a total kill ... rather, the engine simply starves-out at a middle throttle setting. It runs find at lower-half of throttle demands, then stumbles when the throttle is pushed higher.

Fuel starvation happens because the fuel demands at a higher throttle setting cannot be met ... the increasing pump vacuum begins sucking air thru a suspect hose connection. Spark advance would feel more like fuel starvation, not a total kill.

If it just totally dies, then I'd look at electrics and electronics. Faulty coils can cause something like this, maybe even the distributor condenser. This should've been easy for the dealer to troubleshoot ... doesn't even sound like fuel problems (starvation or carbs) if it just totally dies.

[Don T]

Hello,
A few thoughts:
1. There was a problem on some engines where the temp overheat sensor malfuntions and kills every other spark to the plugs.

2. The trick to getting it to idle properly is adjusting the carb sync, mixture and spark advance linkage to the factory spec. This is tricky and most repair guys set the full advance but not the idle. You have to futz with it until it idles at -3* (yes that's ATDC) and full advance at full throttle (+28* IIRC / I can look it up when I get home). The carbs are even more tricky. There is only one idle screw so the other carbs are set by the linkage. Also there is a screw to set the part throttle sync on the middle and lower carbs. The only way to get it right is with a flow testing tool (like the uni-syn). I installed idle screws into the lower 2 carbs because the linkage does not adjust fine enough. The final adjustment should be made while tied to a dock, in gear, so the mixture and idle speed is set under load.

3. Replacement fuel hoses and fittings do not always push the check valve open far enough. The original factory ones work OK but even replacement ones bought from Tohatsu exhibit this fault. The ball bearings that secure the fittings are smaller and the depth of the groove they set into is different. I ended up turning new ones out of stainless on my lathe. I machined them with a longer nose to fully open the ball bearing check valve (in the fitting). This solved my starvation problem.

4. For a test you might put a length of clear fuel line at the engine fitting so you can see if there are air bubbles in the line. Have someone watch it as you throttle up. If the tank fitting is not opening properly the hose bulb will collapse. If the motor fitting is at fault there will be no indication (no air bubbles / bulb stays full / filter is full).

5. When you start throttling up the first part of the power increase (about 1/4 throttle) is achieved by spark advance alone. After that, the linkage begins opening the carbs. Look to see if that is the point where the motor begins to stumble. If so, then the carb off idle circuit may be at fault (this is the passage way under the chrome plate on top of the carb near the mount).

It is not rocket science but it will take some sleuthing - GOOD LUCK!!

[Craig LaForce]

incorrect Spark advance wouldn't cause it to die if it idles and starts to run up OK. It is a simple mechanical adjustment however and no timing light is needed. Doesn't hurt to check it.

I would add some oil to the gasoline manually to test if the oil injector pump is working. Scenario might be that when it is cool, the piston is loose enough to move, but as it heats, without oil it seizes up. Then when cool it will let you start up again. By the way, the manual tells you to add oil to the first gas tank of the season anyway.

To test Frank's coil idea, if you have a timing light, putting it on a plug wire and repeating the rev up and die while watching the light flicker might show a loss of ignition at some point.

I would also visually inspect the linkages on the carbs to make sure the mechanic put them back together right. With the plastic inlet air thing removed I would make sure also that the choke is indeed not engaged. (possibly the choke linkage was left off?)

Obviously it's something, so just keep checking the fuel, ignition, air operation until you find the culprit. Carb float level would be another thing to check at some point. How confident are you in the mechanic that pulled and cleaned the carbs? Did he do anything else while fiddling around under the cowling? Sometimes it's the last thing that was serviced. Did it have this problem before the carb was cleaned?

[hvaldezz]

I had atank where the valve only opened a bit, thats why I suggested the valve.
Out of curiosity did you ever run it up to nearly full throttle when on dry land. I ask to try to define the problem better. Is it the high RPM or the load causing the problem?

If the carbs were done by someone else, maybe they should be having a look at it.

Had the mechanic come out and check them and they apparently were fine when he left, of course the engine was run with ear muffs on only, no load. NOTE: I assume that when the engine has a load on it the fuel consumption goes up????

[hvaldezz]

Hello,
A few thoughts:
1. There was a problem on some engines where the temp overheat sensor malfuntions and kills every other spark to the plugs.

2. The trick to getting it to idle properly is adjusting the carb sync, mixture and spark advance linkage to the factory spec. This is tricky and most repair guys set the full advance but not the idle. You have to futz with it until it idles at -3* (yes that's ATDC) and full advance at full throttle (+28* IIRC / I can look it up when I get home). The carbs are even more tricky. There is only one idle screw so the other carbs are set by the linkage. Also there is a screw to set the part throttle sync on the middle and lower carbs. The only way to get it right is with a flow testing tool (like the uni-syn). I installed idle screws into the lower 2 carbs because the linkage does not adjust fine enough. The final adjustment should be made while tied to a dock, in gear, so the mixture and idle speed is set under load.

3. Replacement fuel hoses and fittings do not always push the check valve open far enough. The original factory ones work OK but even replacement ones bought from Tohatsu exhibit this fault. The ball bearings that secure the fittings are smaller and the depth of the groove they set into is different. I ended up turning new ones out of stainless on my lathe. I machined them with a longer nose to fully open the ball bearing check valve (in the fitting). This solved my starvation problem.

4. For a test you might put a length of clear fuel line at the engine fitting so you can see if there are air bubbles in the line. Have someone watch it as you throttle up. If the tank fitting is not opening properly the hose bulb will collapse. If the motor fitting is at fault there will be no indication (no air bubbles / bulb stays full / filter is full).

5. When you start throttling up the first part of the power increase (about 1/4 throttle) is achieved by spark advance alone. After that, the linkage begins opening the carbs. Look to see if that is the point where the motor begins to stumble. If so, then the carb off idle circuit may be at fault (this is the passage way under the chrome plate on top of the carb near the mount).

It is not rocket science but it will take some sleuthing - GOOD LUCK!!

Thanks for the hints, I believe the spark advance is not the problem either because I never had a problem with the engine before this. We ran some old gas through it but could have had air leaks in the system at the tank. That's why I went new tank line, plugs, etc.

It could be something the mechanic did in the linkage, possible but don't know. Having someone else very familiar with Tohatsu looking at it this morning (Saturday). Thanks for the hints.

[Kelly Hanson East]

The work done by your engine comes from the fuel of course -

More work done (load) means more fuel burned, correct.

Just to prove how bizarre I am, I once let my Saab idle in the driveway all night and burned about 8 gallons of fuel - so used about .7 gallons per hour.

At 60 mph, same car would get about 25 mpg, so in one hour it would burn about 2.3 gallons per hour

So loaded it was burning about 3x more fuel.

[K9Kampers]

KHE says:

Just to prove how bizarre I am, I once let my Saab idle in the driveway all night and burned about 8 gallons of fuel - so used about .7 gallons per hour.

Well Steve, unless you're willing to fill in the blanks as to WHY the car was left running so long, we'll have to contact Paul Harvey for The Rest Of The Story!