Engine Dying After Backing Off WOT

[macowneril2015]

The 2 times I have run the boat no issues starting or getting up to wide open throttle but as soon as I go to back down to cruising or back down at all it seems to bog down and die. After that initial bog down the RPMs seem to bounce which makes me think some sort of fuel issue. I am planning to buy new tanks and set up better filters. Last year I rebuilt the carbs and inspected the fuel pump (wrong gaskets were sent but the pump seemed fine). I had this issue come up briefly late last season but seemed to resolve itself. I need to still get idle and timing dialed in but it is pretty close and I do not think that would cause this issue. I am struggling to brainstorm and fix this issue so any thoughts would be helpful.

[Highlander]

#1 how old is the bulb on the fuel line if 10 yrs or more replace it & remember the fuel flow is directional if the check valve inside it is defective when u throttle down u,ll loose all ur fuel line pressure as the fuel drains back into the fuel tank
#2 check the fuel pick up lines inside the fuel tanks make sure they r not broken some times they will break off 3-4" from the bottom of the tank then when u slow down the gas surges inside the fuel tank & the pick up line starts sucking up air pockets
start there to begin with

J

[NiceAft]

2001 50 Hp Nissan (NS50D2)

I thought I’d post your motor. It may help others supply an answer.

[Be Free]

I don't know that engine but carbureted engines often have a mechanism to keep the throttle from shutting too quickly. When the engine goes from a high RPM to idle this mechanism slows the closing of the throttle valve in the carburetor. It's a "shock absorber" of sorts for the throttle valve. The ones I'm familiar with work with a combination of vacuum and spring tension. If it is not doing its job you would get a symptom similar to what you describe. Look for something with a shaft coming out one side that is connected to the throttle linkage and a vacuum line on the other side. It will fail if either the vacuum line or the internal diaphragm is leaking.

[macowneril2015]

#1 how old is the bulb on the fuel line if 10 yrs or more replace it & remember the fuel flow is directional if the check valve inside it is defective when u throttle down u,ll loose all ur fuel line pressure as the fuel drains back into the fuel tank
#2 check the fuel pick up lines inside the fuel tanks make sure they r not broken some times they will break off 3-4" from the bottom of the tank then when u slow down the gas surges inside the fuel tank & the pick up line starts sucking up air pockets
start there to begin with

J

I had weird issues like this last year and replaced the primer bulb and line so that shouldn't be the issue. I think I have a bad tank or something so I am replacing them as both are 20 years old and will see if that resolves. If not I think it may be a carb sync issue potentially as I don't have a vacuum leak from my inspection. It almost seems as when sloshing it dies but if it starts to stumble and I pump the bulb it rebounds and gets back. My service manual also notes that it could be running lean on oil which I will check but I have not touched the oil pump or adjustment lever from its last tune up. I will report back after additional testing. I appreciate all of the help and input.

[adudinsk]

correct about that "shock absorber thing" that prevents the engine from throttling down too fast
Typically one for the entire carb stack.
Often called a DIAPHRAM.

With the motor off.. manually with your hand give it full throttle by pulling the linkage.
Let it go..
it should not slam shut.... it should soft close... as the diaphragm is releasing air slowly.

on my merc, not sure about your nissan... it looks like a 2" silver metal frizbee.. with a hose going into the top.
Not sure what Nissan does.. maybe springs?


AD

[adudinsk]

If stumbling.. and you pump the bulb.. and it recovers..
could be the carb float is set too high.. and not enough fuel in the carb bowl.

AD

[Sheppie62]

I had the same exact motor. It was extremely finicky to tune. I finally paid a professional to tune it, he checked everything! Took him 2 try’s. I immediately sold it got way more than I ever thought it was worth. Think around $3000 ? Turns out a guy wanted that exact engine for his rib inflatable. He said because it’s super light and easy to make more power. There is a racing class that uses these engines with a shorter mid section. Have you tried tuning it with back pressure (engine in barrel of water). That is very important for the proper tuning.

[OverEasy]

Hi Sheppie62!

Your Nissan and our Tohatsu are brothers of the same mother but different last names!

Our Tohatsu has had similar issues (as do a lot of older 2-strokes).

>We did the professional tune-up which helped some.

>We redid the fuel tanks as part of an upgrade for further cruising range. No change in engine persnicketiness.

> We replaced all our existing fuel lines from 1/4” diameter to 3/8 diameter, got rid of the spray less quick connects on the tanks with hard mounted full through flow fittings, new non collapsible weighted in-tank siphon lines routed to the aft inboard tank corners, a 3/8 dia fuel tank selector valve, new 3/8 dia primer bulb, new fuel fuel filter separator, and a fuel pressure limiter valve system. This helped some.

>We made sure we got rid of all our old fuel and had fresh clean marine grade non-ethanol fuel from a reputable dealer who weekly changes their fuel filters. No perceptible change.

> We added several cans of “Seafoam” fuel additive to several tanks of clean marine grade fuel. This helped some.

> We upped the idle speed setting of the engine.
This helped stop the bog-die aspect when throttle down and at gear shift change issues at the expense of a bit faster idle ( from original of about 700 rpm up to our current 1000 rpm). This helped a lot (as far as the annoyance issue of having the engine die…particularly when conducting docking maneuvers with currents and winds ).

We started last season (2021) at about 25% power and ended at about 80% power. Our reliability improved dramatically over the course of our endeavors.

Our next steps for this particular engine is to remove it from Over Easy and rebuild/replace the fuel pump, carbs, reed valves and gaskets. Test and tune in a water tank. Then either buy a used engineless flats boat or … build one from scratch (but there are several other home projects to come first 🫣).

We have a new 4-stroke DF60AV Suzuki engine that will be installed this Summer for Over Easy. This is to support our goal of cruising the ICW North and South of Beaufort SC in the future. The new 4-stroke has dramatically better fuel economy, better cruising thrust at lower rpm’s, and should have definitely more reliability than our older 2001 2–stroke 50 hp Tohatsu.

While I don’t doubt that our Tohatsu will get back in shape and reliability we really feel that for the extensive cruising we’d like to accomplish that we’d travel more confidently with the new engine.

Hope this helps!

Best Regards
Over Easy

[Sheppie62]

I think you meant “ Brother from another Mother”

[bubby-joe]

check to make sure the cap vent is open, also clean the ground on the cylinder block if 2 boxes make sure one doesn't ground thru the other unless suposed to older mercury is like that, 1 box grounds the other