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3.5hp 4 stroke Mercury


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I picked up a used 3.5hp 4 stroke Mercury and cleaned out the jets, or so I thought.  I think he let is sit for too long with Calif gas in it. The engine runs fine but I must have missed the Idle Jet. Other OB's I've worked on the Idle Jet is a very small hole. Not sure where the Idle Jet is as I do not have the manual.

So I was trying for a search and came along this  https://www.readysetboat.com/mercury-3-and-a-half-hp-to-5hp-upgrade-kit-carburetor-and-propeller.html

I'm wondering if just a new Carb and prop can make a 3.5 a 5 without any volume expansion?

I still need to find the idle jet so I can see if it is plugged

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MW

Likely your Mercury is a rebranded Tohatsu. There's a good thread in fix-it anarchy on ethanol and Tohatsu and carbs (oh my) from this summer. Plenty of advice about soaking the carb, spraying it, blowing it with compressed air (my fav). See the link below for the service manual. Lark helpfully posted a link. The manual has everything, and the last pages list the differences across the models and years. Check there for the exact differences in carbs between 3.5 and 5 HP. 

His text:

I found the manual by using the same key words on another device with a different search history.   It has excruciating carb details.   At the end (page 243j it discusses variants.   It appears I have a variant with two ‘vent tubes’.    It says to disconnect them, but not what they go to.  :wacko:    I wonder if they actually attach to anything.

http://www.tohatsu.pl/wp-content/uploads/2016/08/pdf/manual/FOUR STROKE/TOHATSU_4B-5B-6B_E.pdf

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Having rebuilt a 4hp  (2cycle ) Yamaha and a 6hp (4 cycle) Merc last winter...if I were you,  after having worked on the carb to the best of your ability...buy a new one.  I do not like throwing good things out, but those carbs are oft times incredibly frustrating to really fix and tune after having been "ethanoled".   And the replaceent carbs, as I was surprised to find out, are very inexpensive.

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I did mine a couple of years ago and finally got it adjusted correctly this year. This helps:

XcuALVH.jpg

The pilot mixture screw is more important than I thought, 1/8 turn makes a huge difference once you get close. Good luck.

 

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Excellent thread timing.  I have a 2008 4 stroke 3.5 Merc that I am reviving. Main problem currently is the gas tank cracked around the filler, so when I tipped it up to sail all the gas drained out. Tohatsu part was Half the price of mercury, hope it works out when it gets here on Friday. Next step is carb clean out. Mine starts and idles fine but dies when I try to give it gas.  

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8 hours ago, Meat Wad said:

I picked up a used 3.5hp 4 stroke Mercury and cleaned out the jets, or so I thought.  I think he let is sit for too long with Calif gas in it. The engine runs fine but I must have missed the Idle Jet. Other OB's I've worked on the Idle Jet is a very small hole. Not sure where the Idle Jet is as I do not have the manual.

So I was trying for a search and came along this  https://www.readysetboat.com/mercury-3-and-a-half-hp-to-5hp-upgrade-kit-carburetor-and-propeller.html

I'm wondering if just a new Carb and prop can make a 3.5 a 5 without any volume expansion?

I still need to find the idle jet so I can see if it is plugged

Imagine if you put tri-power or a Holley double pumper on it.

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3 hours ago, InfiniteElement said:

Excellent thread timing.  I have a 2008 4 stroke 3.5 Merc that I am reviving. Main problem currently is the gas tank cracked around the filler, so when I tipped it up to sail all the gas drained out. Tohatsu part was Half the price of mercury, hope it works out when it gets here on Friday. Next step is carb clean out. Mine starts and idles fine but dies when I try to give it gas.  

Cleaning the main jet should fix that, but you might need to mess with the pilot mixture screw to get it to shift between jets smoothly.

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On 10/21/2020 at 11:12 AM, snubber said:

MW

Likely your Mercury is a rebranded Tohatsu. There's a good thread in fix-it anarchy on ethanol and Tohatsu and carbs (oh my) from this summer. Plenty of advice about soaking the carb, spraying it, blowing it with compressed air (my fav). See the link below for the service manual. Lark helpfully posted a link. The manual has everything, and the last pages list the differences across the models and years. Check there for the exact differences in carbs between 3.5 and 5 HP. 

His text:

I found the manual by using the same key words on another device with a different search history.   It has excruciating carb details.   At the end (page 243j it discusses variants.   It appears I have a variant with two ‘vent tubes’.    It says to disconnect them, but not what they go to.  :wacko:    I wonder if they actually attach to anything.

http://www.tohatsu.pl/wp-content/uploads/2016/08/pdf/manual/FOUR STROKE/TOHATSU_4B-5B-6B_E.pdf

The Tohatsu 4hp is almost identical to the Merc 3.5. I'm gonna take the carb off again. I think I know what I missed.
After posting this I found a good diagram. It's hard to find shit anymore all these search engines suck.

And what's the deal with Sea Foam? I see it in lots of videos. Is it a Better Carb cleaner / solvent?

16 hours ago, SloopJonB said:

Imagine if you put tri-power or a Holley double pumper on it.

I'm afraid of burning it out. There is quite a difference between 3.5 and 5.

I also have a 2 stroke Suzuki 5hp and it was easy to find the idle jet and clean it. I like the Suzuki but it is a short shaft and the 3.5 is a long shaft and I don't have to mix gas and deal with a smoker.

waterworldjet3.jpg

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On 10/22/2020 at 1:39 PM, Meat Wad said:

The Tohatsu 4hp is almost identical to the Merc 3.5. I'm gonna take the carb off again. I think I know what I missed.
After posting this I found a good diagram. It's hard to find shit anymore all these search engines suck.

And what's the deal with Sea Foam? I see it in lots of videos. Is it a Better Carb cleaner / solvent?

I'm afraid of burning it out. There is quite a difference between 3.5 and 5.

I also have a 2 stroke Suzuki 5hp and it was easy to find the idle jet and clean it. I like the Suzuki but it is a short shaft and the 3.5 is a long shaft and I don't have to mix gas and deal with a smoker.

waterworldjet3.jpg

The Merc 3.5 and Tohatsu 4 aren’t the same motor. However the Merc 3.5 and Tohatsu 3.5 are 100% identical (excluding stickers and color)

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 Yes. Merc or Tohatsu 2.5 and 3.5 are essentially the same motor. 4 through 6 are essentially the same motor. Just look at the displacement and weight specs. You may be able to make your 4 into a 6 with a carb or something, but I doubt you are making your 3.5 into a 5.

just to follow up on what I mentioned earlier, I have a Merc 3.5 (2008) 4 stroke and saved 60 bucks using the Tohatsu internal fuel tank rather than a Merc part, not to mention gaskets and other stuff was about half price. Parts fiche on boats dot net shows the same diagrams Merc vs Tot, but part numbers are different. 
 

Also I cleaned out the carb (diagram above was not the same as mine but close enough to give me confidence) and now it runs much better above idle when I give it the juice. 

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Had exactly the same issue with a 2-stoke.  Finally caved and just got a new carby, runs perfectly now in all rev ranges.

Wasn't as expensive as I thought.  If I had a 4 turned into a 6, I'd do it anyway.

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48 minutes ago, Bugsy said:

Ish, 

I have 15 hp Merc with a carb that seems a bit 'off'.  Where did you get the service manuals that you reference above?  They look really well done....

It's the official Mercury service manual, so yes it is well done. I bought mine off Ebay, less than $20.

There are also some decent vids on YouTube which help.

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I, too, have a Merc 3.5. I picked it up used late last season and used it extensively this summer.

I had the 2 stroke version and someone absconded with it. I had made some mods to it, most notably a metal prop instead of plastic. It made a significant difference and I was able to get my AB 9 AL to plane with it.

Having done all the same research noted above, these things are rebranded Tohatsus and the parts are interchangable. The lower unit is exactly the same as the 2 stroke and impeller kits from one or the other should interchange.

I was excited by the 5 hp "conversion" kit @Meat Wad mentioned and put it on, having had success with similar projects in the past. I didn't find the motor to run materially faster - does not get my dinghy on plane like the high revving 2 stroke would. It also idled uncomfortably high, resulting in very hard shifting and an annoyed wife when pulling away.

After having the fuel tank break, exactly as another poster mentioned, I got water in the tank and needed to clean the 5 hp carb. The gasket is exceptionally hard to get into place and didn't seat properly. Of course, I didn't figure that out until the tank had emptied itself after having motored out to my boat. In the course of a week, I broke the fuel tank, smoked the impeller and made a humungous fuel slick.

I cleaned the original carb, put it all back together and it's run flawlessly ever since.

 

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Well, Saturday yanked the carb, dropped the bowl off squirted with cleaner and found another jet that is recessed. Needed some specs to see the slot for the driver. Yanked the Main Jet again and pushed the tube out. Didn't know that came out.  Also saw the little rubber stopper in a hole, yanked that and there was a tiny hole.

So I found extra holes to squirt and use the Air Compressor to blow them out. Put it back together and what a nice idle. What a pain in the shoulders to work on something that is above your height.

 

But there is still a rough patch. I think It is a bit too lean. When I open the throttle a bit it stammers like it is starving for furl but does mellow out a little.
At about 10 sec, I throttle up a bit. I'll have to play with the mixture. Otherwise it seems to run nice. I even put it in gear at idle and there was no hesitation.

 

Thanks for the input.

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12 hours ago, justsomeguy! said:

Why do people use that tri-split screen?

Sorry, this is the first video I took with my phone. I'll have to look at the settings to see if I can turn it off.

 

When I download the videos to my computer, they look fine. I wonder if I uploaded from my computer?

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12 hours ago, justsomeguy! said:

Why do people use that tri-split screen?

I think YouTube does that to vertical shots so they fill the horizontal screen. The actual video doesn't have the effect.

Fix: Shoot horizontally.

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12 hours ago, justsomeguy! said:

Why do people use that tri-split screen?

16 minutes ago, Meat Wad said:

Sorry, this is the first video I took with my phone. I'll have to look at the settings to see if I can turn it off.

 

When I download the videos to my computer, they look fine. I wonder if I uploaded from my computer?

10 minutes ago, Ishmael said:

I think YouTube does that to vertical shots so they fill the horizontal screen. The actual video doesn't have the effect.

Fix: Shoot horizontally.

The phone setting by default is 16:9, there is also a 1;1 and Full. I switched to full and will have to test it.

 

 

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On 10/22/2020 at 2:39 PM, Meat Wad said:

>> And what's the deal with Sea Foam? I see it in lots of videos. Is it a Better Carb cleaner / solvent?

There's probably some religion around products, but I had my best results cleaning the carb with Berryman's B12 and then adding Sea Foam every time I filled the tank.

 

 

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On 10/26/2020 at 9:35 AM, Ishmael said:

I think YouTube does that to vertical shots so they fill the horizontal screen. The actual video doesn't have the effect.

Fix: Shoot horizontally.

On 10/26/2020 at 9:47 AM, Meat Wad said:

The phone setting by default is 16:9, there is also a 1;1 and Full. I switched to full and will have to test it.

 

 

Ishmael is right. the phone setting does not matter, it's the phone orientation.

Upright = 9 x 16

horizontal is 16 x 9

You would think the the camera would shoot 16 x 9 all the time.

 

 

 

 

 

 

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3 hours ago, Meat Wad said:

 

 

Ishmael is right. the phone setting does not matter, it's the phone orientation.

Upright = 9 x 16

horizontal is 16 x 9

You would think the the camera would shoot 16 x 9 all the time.

 

 

 

 

 

 

It would certainly make the video thing easier to watch, so many people shoot vertically and waste 2/3 of the screen.

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43 minutes ago, Ishmael said:

It would certainly make the video thing easier to watch, so many people shoot vertically and waste 2/3 of the screen.

It took me an embarrassingly long time to realize that the vertical video shooting thing only exists because the current generation of teens and 20-somethings watch videos almost exclusively on their phones in a vertical orientation.

In that context, it makes sense even if it's still a bad idea for 90% of video subject matter.

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6 hours ago, IStream said:

It took me an embarrassingly long time to realize that the vertical video shooting thing only exists because the current generation of teens and 20-somethings watch videos almost exclusively on their phones in a vertical orientation.

In that context, it makes sense even if it's still a bad idea for 90% of video subject matter.

Having used my GoPro to shoot video, I was excused from the phone.
It was a nice short lesson.

Thanks for the engine and video input.

I can't wait to see how far the 1 liter gas tank will take me.

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So the rule of thumb I've heard for 2 strokes is 10hp is 1 gallon per hour at WOT, 15hp is 1.5 gph, etc. My experience with my new-to-me 2 stroke 15h Merc bears this out.

A 4 stroke is ~20% more efficient so by that logic, a 3.5hp 3 stroke should be about 0.25 gph or about 1 hour at WOT for a 1 liter tank. I wouldn't take it to the bank without testing but it'll give you a rough order of magnitude.

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41 minutes ago, IStream said:

I should clarify that the 3 stroke engine referenced above is, of course, a 4 stroke engine in its late teens. 

;)

"A three-stroke internal combustion engine completes a complete combustion cycle of exhaust, intake, compression, ignition, and expansion within a single revolution of a crankshaft by a single stroke of a first piston and a single stroke of a second piston within a single cylinder."

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On 11/1/2020 at 9:17 PM, IStream said:

So the rule of thumb I've heard for 2 strokes is 10hp is 1 gallon per hour at WOT, 15hp is 1.5 gph, etc. My experience with my new-to-me 2 stroke 15h Merc bears this out.

A 4 stroke is ~20% more efficient so by that logic, a 3.5hp 3 stroke should be about 0.25 gph or about 1 hour at WOT for a 1 liter tank. I wouldn't take it to the bank without testing but it'll give you a rough order of magnitude.

This lines up reasonably with my '94 Merc 5hp 2-Stroke. The internal tank has got me 7 miles @ WOT @~12kts with a little bit to spare. I think it's about 1/2 a Gallon. My 3 Gallon external tank outlasts whatever beer and ice I can fit in the cooler.

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On 11/2/2020 at 9:38 AM, IStream said:

I should clarify that the 3 stroke engine referenced above is, of course, a 4 stroke engine in its late teens. 

I remember when it only took me 3 strokes.......................:)

 

On 11/3/2020 at 9:14 AM, solosailor said:

I've had several 3.5HP 4-strokes.   Full tilt is .5-.6GPH.   The internal tank runs out in 30'ish minutes at full throttle.

 

Thanks, Sounds like just enough to get from Ventura Harbor to Mandalay starting area.

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10 hours ago, Meat Wad said:

I remember when it only took me 3 strokes.......................:)

 

Thanks, Sounds like just enough to get from Ventura Harbor to Mandalay starting area.

It's just enough to get from where you are to where you're going minus a mile. Despite my best intentions I always end up filling it in a nasty chop on the dinghy instead of on the mothership.

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8 hours ago, Ishmael said:

It's just enough to get from where you are to where you're going minus a mile. Despite my best intentions I always end up filling it in a nasty chop on the dinghy instead of on the mothership.

Don't feel so bad. I'll bet you remember to put your oars aboard just in case. I often don't.

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