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Old 27-05-2020, 14:56   #1
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3.5HP to 5.0HP Mercury OB carb upgrade

I understand the 3.5HP Mercury OB has deemed unreliable by some even if it is the same engine as the Nissan

Mine has given me the fits and I am thinking of just buying a new carburetor and start from scratch and pamper it with ethanol free clean gas, and run it dry, never lay on its side, etc.

The question I have is whether it makes sense to get one of those upgrade carburetors promising to take it from 3.5HP to 5.0HP?

Logic may tell me the higher the HP the larger the nozzle jet diameter and in theory less chance to clog and not perform. I already have the AL propeller

Thoughts?
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Old 28-05-2020, 05:03   #2
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Re: 3.5HP to 5.0HP Mercury OB carb upgrade

No thoughts
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Old 28-05-2020, 06:08   #3
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Re: 3.5HP to 5.0HP Mercury OB carb upgrade

I have a 4HP Merc that started giving me issues. Dropped it off with the local lawnmower mechanic. Valves were way out of adjustment.



I asked him about the carb upgrade from 4 to 6 HP. He said it works fine but may need a new prop to get the full effect.



I did the upgrade, however, I haven't tried it out to see how it works yet.
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Old 28-05-2020, 06:11   #4
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Re: 3.5HP to 5.0HP Mercury OB carb upgrade

Hmmm. Do you need the additional horsepower? At that size motor you are not planing your dinghy, you are just putting along at something like 4 knots, and 3.5 sounds fine.

Second hmmm. This conversion is dependent on the 3.5 and the 5 being the same engine with different carbs. That's done, I remember a 200 HP that was the same engine as a 225, but I don't know whether this is the same, or if there is a different bore as well.

Have you pulled and cleaned the existing carb? It's not a difficult task.

The most frequent small engine problem for me is an engine that has been laid up long enough for the gas to lose its volatility. The dead fuel left in the carb effectively blocks starting. That's resulted in a hate-hate relationship with them, but also running them dry and marking each newly filled gas can with a Sharpie for month and year. After a few months, any left-over gets dumped into my truck.

Those are my first thoughts. I do hope you find a route out of this frustration.
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Old 28-05-2020, 06:47   #5
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Re: 3.5HP to 5.0HP Mercury OB carb upgrade

Quote:
Originally Posted by tkeithlu View Post
Hmmm. Do you need the additional horsepower? At that size motor you are not planing your dinghy, you are just putting along at something like 4 knots, and 3.5 sounds fine.
I have a 3.5 hp Tohatsu, which is basically the same engine as the Merc. It easily planes my 10' portabote with one (fat) person in it, and will plane with two adults in calm conditions.

But it would be nice if this actually works. I wouldn't mind having the additional hp, but I'd be concerned if it means sacrificing fuel economy. At lower throttle the little gas tank lasts a long time. As soon as I bump it up (start planing), my fuel efficiency drops considerably.
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Old 28-05-2020, 07:26   #6
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Re: 3.5HP to 5.0HP Mercury OB carb upgrade

Many engines are sold in different HP levels, if the carb is the only difference, then sure why not, often there may be a restriction plate too though.
You not going to use more fuel unless you use more HP, then of course you will burn more fuel.
If the upgrade in power requires a higher pitched prop, then you may cruise at slightly lower RPM and slightly lower noise and vibration if you run at the same speed as you did.
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Old 28-05-2020, 07:55   #7
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Re: 3.5HP to 5.0HP Mercury OB carb upgrade

Wow, Mike, I'm really impressed. Just what kind of dinghy are you using? Have you checked actual speed when planing?
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Old 28-05-2020, 08:25   #8
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Re: 3.5HP to 5.0HP Mercury OB carb upgrade

Quote:
Originally Posted by carlosproa View Post
I understand the 3.5HP Mercury OB has deemed unreliable by some even if it is the same engine as the Nissan

Mine has given me the fits and I am thinking of just buying a new carburetor and start from scratch and pamper it with ethanol free clean gas, and run it dry, never lay on its side, etc.

The question I have is whether it makes sense to get one of those upgrade carburetors promising to take it from 3.5HP to 5.0HP?

Logic may tell me the higher the HP the larger the nozzle jet diameter and in theory less chance to clog and not perform. I already have the AL propeller

Thoughts?

Wouldn't do it.

Agree the base Mercury motor to ~ 40hp is Tohatsu. The Tohatsu2.5/3.5 series isn't made for the higher rpms to get 5hp while the 4-6 hp motor is made for the higher rpms. The 4-6hp difference in carb jetting (higher rpms) to develop the increased hp.

Did you know there was a fuel pump issue in the 2.5/3.5 tohatsu motors? Maybe this has been the problem all along.

I would go for the fuel pump issue before buying a new 3.5hp carb. New carbs for a 3.5 on ebay were ~ $70.
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Old 28-05-2020, 09:40   #9
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Re: 3.5HP to 5.0HP Mercury OB carb upgrade

I would be specious of a claim of 40% increase in HP by just bolting on a carb. I assume it's a bigger bore carb but simply upping the CFM and main jet is not likely to increase the performance that much in any engine I know of. 5% to 15% maybe but not 40%. I could be wrong but this seems a bit more marketing hype than anything else. You would have to compare cams, valve sizes, exhaust size compression etc. between the 3.5HP and the 5HP to really tell.



Quote:
Logic may tell me the higher the HP the larger the nozzle jet diameter and in theory less chance to clog and not perform.
False logic. There is more to a carb than the main jet and the increase in size will be minimal, like .06 mm at the most. Big jets still clog up If they increase the bore size on the carb they need to up the air idle jetting, idle jet and emulsion tubes too.



More power? Sure. How much more? Only your dynomometer can tell.
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Old 28-05-2020, 10:06   #10
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Re: 3.5HP to 5.0HP Mercury OB carb upgrade

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Originally Posted by tkeithlu View Post
Wow, Mike, I'm really impressed. Just what kind of dinghy are you using? Have you checked actual speed when planing?
I haven't ... or rather, I did years ago after first getting the engine, but I've long forgotten the speed number. It's plenty fast, but it's also pretty inefficient at that speed. My fuel consumption probably doubles, or more (haven't actually measured it either .)

I think it's mostly to do with my dinghy. Mine is a 10' portabote. These boats (or botes) are fairly light, with a solid, smooth bottom. They are also nicely streamlined. They behave a lot like a standard rigid dinghy.

There's no way this engine could plane an inflatable dinghy.
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Old 28-05-2020, 17:04   #11
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Re: 3.5HP to 5.0HP Mercury OB carb upgrade

We've just retired our 3.5 Hp. Tohatsu, Nissan -(Brand will be no longer made after 2020) Mercury engine after many hours of operation. I'm sure you know they are all the same engine. These engines are known to have some serious carburetor issues. When the engine is tilted, the carburetor becomes flooded and will not start. It usually takes a few hours for the engine to clear itself and be in a position to restart. You can manually drain the fuel off but you'll upset the idle screw.

The method I found to work is; Never tilt the engine with the fuel valve open. Just don't. Keep it vertical. Let the engine stay vertical and stop the engine by closing the fuel shut off valve and the engine run out of fuel usually-about a minute. You'll find your reliability will increase as the engine will re-start.

Forget the larger carb! This engine does not have the cooling capacity to handle this increased operation. Just because it available, does not mean your engine will take it.
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Old 28-05-2020, 17:25   #12
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Re: 3.5HP to 5.0HP Mercury OB carb upgrade

I guess the most obvious question from your original comment “OB giving you fits” is it giving you fits because of a carb issue. If it’s something else giving you problems, just changing to a different carburetor is not likely to make this OB a winner for you.
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Old 28-05-2020, 19:08   #13
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Re: 3.5HP to 5.0HP Mercury OB carb upgrade

Thanks iall for the feedback.
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Old 07-06-2020, 17:08   #14
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3.5HP to 5.0HP Mercury OB carb upgrade

So the saga continues

The carburetor change was too easy, then instead of living the OB alone, I decided to check the impeller. In all my previous outboards is a straight forward exercise.
This particular engine has an extension wedge making it longer shaft, and this is where I am stuck.
Being a to hasty engine with neutral and forward only gear, there is no special antilock mechanism to first release
What seem strange to me is the usual 2 bolts at the gear housing assembly does not look right. In fact there is a nut above the propeller instead of the usual bolt ( see pics). My theory was that this is so to accommodate the 6 inch wedge extension. The problem is that I cannot dislodge the lower unit. Maybe the long bolt I seized due to corrosion?

Where you see my finger is where I feel the rotation axis so I cannot wiggle that part down

Does anybody has this type of configuration for the leg?Click image for larger version

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Old 08-06-2020, 00:40   #15
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Re: 3.5HP to 5.0HP Mercury OB carb upgrade

I also have had to change the carb. on my Merc 3.5. (with a Tohatsu carb and a fraction of the price of the Merc carb)


Just be sure to run the carb dry by shutting of the fuel valve, if the motor will be left standing. I also use seafoam in my fuel to keep it clean. Seems to work well.
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