Mercury Tohatsu 9.8hp 8hp and 6hp comparison

I get a lot of questions about the difference between the 6hp 8hp and 9.8hp Mercury and Tohatsu outboards so though I would do a quick video to explain.
Enjoy 😉

[@filipblaz19]

Hallo, great video! So I have mercury 2 stroke 6hp 2000 year. so I wonder if it is enough to put a 9.8 carburetor for more horse power? or something else needs to be changed?

[@25hpboatracerman]

You can put a 9.8hp carb on it for a bit more power but If you want the full 9.8hp you will need to replace the stator stop and the head and the exhaust cover. Cheers 🍻 Aussie Boat Guy 😎

[@almuhito]

Very interesting video. I am wondering how does the 5 hp compares with the 6 hp. There is a weight difference of 6 kg. So wondering hows the performance differ?

[@25hpboatracerman]

Hi Mate, Good Question 👌 we are talking two strokes here with my answers. The 6hp is way more refined to drive with the twin cylinders, heaps smoother too. It has about 30% more capacity, so it does have a lot more torque. The 6hp is the same capacity as the 8 and 9.8hp. The 6hp will get a boat on the plane with more weight in it and can achieve another few kph over the 5hp. Cheers 🍻 Aussie Boat Guy 😎

[@horsham10]

Great video. You did forget one other part. They call it a coil plate stopper. The 9.8 allows the timing to advance to get about 500 more revs. Easy way to tell is the 9.8 has a little hole in the plate. The 6 and 8 don't.

[@horsham10]

I just rebuilt a 9.8 with oversized pistons. Was getting about 155psi each cylinder...

[@25hpboatracerman]

Cheers mate, I will mention that in the next one. All the best Aussie Boat Guy 😎

[@horsham10]

Where did you get the gearbox nose cone from. I've got and 18 box I want to try it on

[@25hpboatracerman]

@@horsham10 Hi mate, I fabricated it, I should of taken a plug of it. Cheers 🍻 Aussie Boat Guy 😎

[@KrazieKanuck]

What year engines does this apply to? I have a 2000 6hp. Great videos thanks!

[@jetguy1205]

The 6 is a single cylinder so I'm not sure what your are talking about here

[@25hpboatracerman]

Hi Champ, The 6hp 4 stroke is a single cylinder yes but these are two strokes. The two stroke 6hp, 8hp and 9.8hp tohatsu are twin cylinder. Cheers, Aussie Boat Guy 😎

[@Dookey021]

How to convert 2 stroke 6hp to 8hp, is it only the carb?

[@25hpboatracerman]

Hi Champion, I believe it may be the only the timing stop but I’m actually not 100% sure on the exact difference between the 6hp and 8hp variants. From when I have disassembled, I have not visually noticed a difference which makes it hard. Cheers 🍻 Aussie Boat Guy 😎

[@rrrttt1112]

I heard where you said the 9.8hp has 140 psi compression, what is the 8hp? Did you say 90psi?

[@25hpboatracerman]

Hi Mate, Correct 👍 Cheers 🍻 Aussie Boat Guy 😎

[@rrrttt1112]

@@25hpboatracerman I guess this old 1998 8hp I have with 105 psi top & 103 psi bottom isn't so bad a motor after all. I may convert to 9.8hp this winter, so a carb is one thing and I need, and shave the head is the other.

[@suddendilemma5137]

Thanks for the video man. This was a good watch. I’m poking around for a vid on the lower disassembly to replace the water pump and found your gem of a video.

[@25hpboatracerman]

Hi mate, Thanks for your comments, really appreciate it 👌 If you look for my outboard service guide video (it’s on a 15hp Mercury- Tohatsu) it’s nearly exactly the same as the 6,8 and 9:8hp Mercury Tohatsu. I’ve got the water pump replacement as part of that video, good idea to change your gear oil too. Cheers 🍻 Aussie Boat Guy 😎

[@electricvagabonds]

You beautiful man I've been looking for this exact information. Just bought one of these smokey boys and pretty sure its an 8 but not 100%. Was sold in the EU, I think as a 6kW motor. Model is M8B-6 3B2. Serial number 46149. Would love to tweak it a wee bit.

[@25hpboatracerman]

Thanks Mate, Yep 8hp. Keep in mind that the 9.8hp has a different timing stop too ( it allows more advance in timing) The 8 goes well too. Cheers 🍻 Aussie Boat Guy 😎

[@bhishamramesar8571]

My tohatsu 9.8 hp gave me on compression test 124 psi for cylinder 1, and 127 psi for cylinder 2. Is this a good range?

[@25hpboatracerman]

Hi Mate, Sounds fine 🤩 remember some compression testers read low and some higher and also depends on how the compression test is done. Those numbers are fine. Cheers 🍻 Aussie Boat Guy 😎

[@jurreoudega6948]

Hi, can you also switch a mercury 1998 2 stroke from 8 to 15 and if it can, what do you need to make it a 15?

[@25hpboatracerman]

Hi Mate, If it’s the USA Built 8hp, you cannot, I know the 15hp looks exactly the same but it has more capacity (cc). the 6hp and the 8hp are the same capacity and the 9.9hp and 15hp are the same. You could probably try a 15 carb and re-jet to suit your motor but it will take a bit of tweaking to get the mixture safe for your motor. Also not sure how much hp you will add, maybe 1/2 or so. Cheers, Aussie Boat Guy 😎

[@qldshane]

I’ve owned a 8hp tohatsu and 2 x 9.8hp. I know both 9.8hp Comp test 150psi to 155. I didn’t test the 8hp although top speed in my tinny was about 33km with the 8hp and 37km in the 9.8hp.

[@25hpboatracerman]

Hi mate, Yeah great motors 👌 Yeah, I’ve owned a truck load of each 😁. Unfortunately comp gauges read very different from gauge to gauge, what brand is your one? I really should get another gauge as well to compare. Cheers Aussie Boat Guy 😎

[@25hpboatracerman]

Hi mate, Yeah, the 6 and 8hp are around 100prs comp. And your correct, not a massive top end speed difference between the 8hp and 9.8hp, you can feel a bit more torque due to that higher compression of the 9.8hp though. Cheers 🍻 Aussie Boat Guy 😎

[@nunomascarenhasribeiro]

Is the Mercury 1998 2 stroke 9.9 a downgraded 15hp?

[@25hpboatracerman]

Hi Mate, Yes I believe it is Champion. Cheers, Aussie Boat Guy 😎

[@jameslaird8324]

Hey mate like your work, I have the 8 hp and want to machine the head down to a 9.8 hp to increase the compression, would anyone be able tell me the shave measurements I need to achieve this for best results, Cheers.

[@25hpboatracerman]

Hi Mate, Thanks, I really appreciate you comments. I have shaved a few 8hp heads to increase compression. I never measured the amount I took off but can explain where to stop. the 9.8hp head has only about a 1mm or slightly less of a step down from the gasket mounting surface to the cylinder squish band, the 6hp and 8hp have over 2mm, possibly close to 3mm. I just shaved my 9.8hp head to only leave 0.1mm and it raised the comp to 155psi. I feel you are after about 140psi. cheers 🍻 Aussie Boat Guy 😎

[@presentlypresentingmyprese7058]

So whats the difference between the 6 and 8hp? Simply the carb?

[@25hpboatracerman]

Hi Mate, From what I have disassembled it seems that the carbs are nearly the same but possibly the timing is different. Cheers 🍻 Aussie Boat Guy 😎

[@presentlypresentingmyprese7058]

@25hpboatracerman thanks mate. I've watched a couple of your vids. Not sure I've ever seen you play with timing on the ones ive watched. Is this something you do on outboards? Would love to see you do a video on it to see any difference.

[@rrrttt1112]

@@25hpboatracerman Hi, I have two 1998 Nissan 8HP motors, and both carbs have 17mm throats, so I am wondering if the 9.8 carb is the same as the 8hp, with a different Jet for this year model?

[@25hpboatracerman]

@@rrrttt1112 Yeah, ok. I have not had any 8hp with a 17mm throat but I have only disassembled the Mercury snickered tohatsu. Keep in mind that the jets in a carb are very specific for the throat size of that particular carb. putting larger gets in a carb will loose power as it will be running too rich. So your particular 8hp probably just needs the 9.8hp head, the exhaust cover plate and the stator stop off the 9.8. Cheers 🍻 Aussie Boat Guy 😎

[@rrrttt1112]

@@25hpboatracermanThanks! Does the exhaust cover plate need any gasket sealer with the gaskets? Seems like the head has some gasket sealer too, is that normal?

[@vonbaronsosiskin]

Thank you for this video. What is the cylinder liner made of? Is it a cast iron liner in an aluminium block?

[@25hpboatracerman]

Hi Champion, Yes it is 👍 I really appreciate your comments mate. cheers 🍻 Aussie Boat Guy 😎

[@rashad-tm4zm]

? I have the 4 hp 4 stroke merc. I added the 6 hp carb and notice the difference. Wil any of the parts ya mentioned if added increase the hp of y 4 hp any more?

[@25hpboatracerman]

Hi Mate, Unfortunately not, these are two strokes and will not fit the 4 stroke versions. cheers 🍻 Aussie Boat Guy 😎

[@wilsnich02]

Great video, I have the 8 HP and my mate the 9.8 HP so it was really interesting to watch

[@25hpboatracerman]

Thanks mate 👌 I also was just informed that there is a different stator stop on the 9.8hp which allows more timing. I still need to check that out though. cheers 🍻 Aussie Boat Guy 😎

[@rittzbyitch3682]

Also, is the Tohatsu you're working on a 2 stroke?

[@25hpboatracerman]

@@rittzbyitch3682 Yep two stroke 👍

[@rrrttt1112]

@@25hpboatracerman Yes, the 9.8 has a different coil plate stop.

[@douger1517]

Very interesting vid. How long ago did Tohatsu start making outboards for Mercury?

[@25hpboatracerman]

Hi mate, I think it was around 1988 - 1990 that Tohatsu started making outboards for Mercury, most of the Mercury outboards under 30hp where re-branded Tohatsu. Love the Tohatsu 👌👌

[@rafaelminetto3642]

Oque tenho que fazer para deixar meu 8hp igual ao 9.9hp?

[@25hpboatracerman]

Olá Campeão, você não pode transformar 8cv em 9,9cv, apenas pode atualizar para 9,8cv. usei todas as minhas peças de 9,8 CV. felicidades 🍻 Cara do barco australiano 😎

[@rrrttt1112]

Great info! I can't wait to see what you do hot-rodding one of these motors!

[@25hpboatracerman]

Cheers mate, Modifying the 9.8hp over the next few weeks 😁😁👌

[@G029er]

The 9.8 moves more air. Brilliant.

[@25hpboatracerman]

Sure does and as you see in the video it also has more compress and advanced timing with a different exhaust cover. Cheers 🍻 Aussie Boat Guy 😎

[@clockk2008]

Good job !

[@jctecson]

Hi I jave question ? I have a 2019 tohatsu 6hp can I replace it with a 8 hp carb? any modifications needed? I'm not satisfied with wy 6hp as if it's not reving as much? anyway can I do to make it rev a little more? Thank you very much

[@25hpboatracerman]

Hi mate, Firstly is your 6hp a two stroke, that’s what I’m showing in this video. I would put the 9.8hp carburetor on, you will not notice a difference with the 8hp carburetor. Also if you can put the 9.8 head on. Cheers 🍻 Aussie Boat Guy 😎

[@TheOvacado]

Hey bro what prop do you run on your 9.8?

[@25hpboatracerman]

Hi mate, At this stage it’s the stock 8.5 pitch alloy. Will be trying a few after this motor is back together 😁 Spoiler, it not going back together stock 😎 it will be ported, probably change the exhaust tuner and possibly a bigger carburetor

[@TheOvacado]

@@25hpboatracerman cant wait!!!!!!

[@bradleyfarquhar2023]

love it mate I'm still stoke mines got the 18hp block from the other we talk about it good job on this video mate cheers.

[@25hpboatracerman]

Cheers mate

[@boatsandoutboards9070]

Nice video! I wonder if this tohatsu 9.8 (or merc 9.9) engine power is limited by any means? You know they are intentionally designed to be below 10.0hp to bypass some license or registration requirements. I don't expect it to be anything like suzuki 9.9/15/20hp which are same engines in reality. But maybe 9.8s can be upgraded to 11 or 12???

[@25hpboatracerman]

Hi mate, I’m modifying the 9.8hp now, I’ve put some videos up on this if your interested. The 9.8hp is the same block as the 6 and 8hp so it’s kinda already the most hp from that small block and Tohatsu does have a 9.9hp - 15hp block along with an 18hp block which looks the same as the 9:9-15 block also got a vid on them too. I have achieved more power so far from the 9.8hp but it’s taken a lot of modifications and still going. When I modified the 5hp the gain was more than what I’m achieving from the 9.8hp. Cheers 🍻 Aussie Boat Guy 😎

[@boatsandoutboards9070]

Thanks. I had a look in those upgrade videos and realized that you are talking about the old 2stroke models. My mistake. Let me rephrase my question Do you think mfs 9.8 can also be upgraded to some point ..not 18hp perhaps

[@perhagglund9173]

Great information, thanks!😊👍

[@25hpboatracerman]

Hi mate, Thanks 😊 And glad it was helpful. Cheers 🍻 Aussie Boat Guy 😎

[@magiccarpet3.5]

Thanks for another great video. I knew about the carb and the head but not the exhaust cover. Are you interested in porting a 5hp tohatsu for a fee. I have sorted the rest but the barrel porting is a bit tricky. I see a new 5hp race class in the making here. Cheers

[@25hpboatracerman]

Hi mate, Thanks 😊 I was just informed that there may be a difference in the stator stop on the 9.8hp too to allow a bit more timing, I will have to check though. I’m not that keen on porting another 5hp, pretty time consuming. I am now onto the 9.8hp, don’t know if my porting tool will even fit 🤷‍♂️. Cheers 🍻 Aussie Boat Guy 😎

[@magiccarpet3.5]

@@25hpboatracerman cheers mate I'm looking forward to your super 9.8 vid.

[@pontusgoth2582]

Is ther any porblem to uppgrade an 8hp to a 9,8 just by replacing to a 9,8 carb? will it over heat or other things happen?

[@25hpboatracerman]

Hi mate, I have done it many times. You need to change the head, exhaust cover, carburetor and stator to get the full 9.8hp. Just changing the carb to a 9.8hp one will make a small difference but not sure it’s worth it. Cheers 🍻 Aussie Boat Guy 😎

[@pontusgoth2582]

@@25hpboatracerman Thanks a lot for your answer. Im not a mechanic only a lazy sailboat (2000 kg) owner that wants to do a easy fix to get the motor some extra power to get that extra knot. So the only skill I have is to change the carburetor. I have changed to a hi thrust 4 blade pitch 5 propeller. I think it’s worth it to just change the carb if it delivers 10-15% more power. Do you think it will do that?

[@25hpboatracerman]

@@pontusgoth2582 Hi champion, I do think it will add some extra power. You did the correct thing changing to a high thrust prop. I don’t know if it will add much speed though, if your getting around 4 to 5 knots now don’t know how much more you will achieve by just a carb 🤷‍♂️ If you can get one cheap, it is an easy conversion though so worth a try 😁 Keep me posted too. Ps: the 9.8hp carb has a 17mm throat. Cheers 🍻 Aussie Boat Guy 😎

[@pontusgoth2582]

@@25hpboatracerman Thanks for your thoughts Im doing 6 knots now with the standard propeller have not yet received the hi-thrust propeller. But I found a new OEM carb for 9,8 hp on amazon for 28 USD so that is a bargain. Maybe it’s a crap Quality it's worth a try. i will let you know the difference.

[@25hpboatracerman]

@@pontusgoth2582 Worth a try 👌👌👌

[@pietlopsa9164]

Hey mate great video! I have a couple of questions: My Tohatsu M8B had problems, no water from the thermostat cooling passage, just exhaust gases. I removed the head (massive pain in the as but i did it) to replace the thermostat, cylinder head, changed the cylinder head gasket, engine base gasket (all oem from japan) and sanded down the corroded block a little with sandpaper glued on a glassplate only to find out, that it still has the exact same problem. Motor is overheating (95C) and not putting out cooling water from the hole in the back. Sometimes a little water mist, but not actual water. Telltale works fine and all the water passages are clear (tested it by drilling a tiny hole in the old cylinder head at the thermostat housing and then putting a water hose to the pickup in the leg, I had water up there). Nobody here in germany can help me, they say it is not worth it but it bothers me so much, i cant sleep without thinking about this problem. My guess is that the block is a little uneven and therefore the exhaust pressure press back the cooling water. BUT could it also be something different, what are your guesses? And also, how much Nm do you do for the head bolts? Somebody said 6Nm but in my opinion way too loose. I did 15Nm and they were far from breaking( i know because i broke 5 out of 11 :) )

[@25hpboatracerman]

Hi mate, It is a very common problem on the 6hp 8hp and 9:8hp, you have done exactly what I would do. It is nearly always the thermostat. I would change impeller at the same time. Keep in mind that the head is supposed to get pretty warm, not over 100 degrees though. Water is only supposed to come out the top leg outlet when the thermostat opens, if your water is very cold that may not happen often at idle. If you are traveling along at full throttle, you should see a steady flow of at least on and off flow from that outlet. My thoughts are 1) I would check running with a decent amount of throttle to check flow (not for over 3 mins after warmed up though so I don’t cause damage) 2) if no good, I would check the outlet for blockages. 3) I always check the new thermostat before installing on a cup of boiling water to see if it is faulty. 4) Remove head, check inlet and outlet in water passages to ensure no blockage. 5) worst case I would remove power head and visually check all passages. The Tohatsu m6 bolts are only supposed to be torqued up to 6.5 ft pounds, 15 is way way too much, you will start stripping the alloy block threads out or snapping bolts. If you damage the threads in the block, don’t worry, just Helicoil them (stronger than original) I have a video on the channel for that. That are great little motors and worth the effort mate. Keep me posted Cheers 🍻 Aussie Boat Guy 😎

[@pietlopsa9164]

@@25hpboatracerman Thank you heaps! Will keep you updated! Cant wait to zip around in my little achilles dinghy :)

[@pietlopsa9164]

@@25hpboatracerman Water passages are all clear and water is getting up there. The Problem with no cooling proceeds to exist.. :( I made a short video: kzbin.info/www/bejne/inacnWibgqmGn7c

[@NinoSpinner]

Il carburatore del 6 hp è 13 mm? Sicuro? Perché sul 5 hp è da 19 mm

[@25hpboatracerman]

intendi 4 tempi? sui due tempi il 6 CV è 13 mm e il 5 CV è 15 mm, il 9,8 CV è 17 mm. saluti

[@NinoSpinner]

@@25hpboatracerman ah ok Pensavo fossero 4 Tempi Vorrei cambiare il carburatore ad un 3.5 hp (4tempi) e montare quello del 4 o 5 hp

[@25hpboatracerman]

@@NinoSpinner Oh va bene, Credo che il terzo sia un kit di aggiornamento aftermarket per il quattro tempi da 3,5 CV. coni con un carburatore più grande e un'elica migliore. non so dove trovarlo, dovrai scoprirlo. ne parlano molto negli USA. ciao 🍻

[@NinoSpinner]

@@25hpboatracerman io ho già comprato l'elica, sto cercando di capire qual'è il carburatore perché ne ho trovati parecchi

[@25hpboatracerman]

@@NinoSpinner 👍

[@brendanbarker2095]

Interesting vid , thanks.

[@25hpboatracerman]

Glad you enjoyed it mate. cheers 🍻 Aussie Boat Guy 😎

[@michaelsimpson9779]

Good info there. Thanks

[@25hpboatracerman]

Cheers mate 🍻 All the best, Aussie Boat Guy 😎

[@markolsen-f5w]

Thanks for your videos! Looking at schematics i do not see the 9.8 curved exhaust flow plate. If you ever have a minute,could you give me the part numbers for that? As well as the others to transform my 8 to 9.8. Thank you!

[@25hpboatracerman]

Hi mate, Thanks for your comments 😊 The 9.8 has the curved exhaust plate, the 6 and 8hp have a flat plate. Unfortunately I don’t have the parts numbers. Sometimes the schematics show for the most used parts but will have other part numbers for different hp models. I did do a video on the difference between the 6-8 and 9.8hp if you’d interested but forgot to mention that the 9.8hp has a different stator atop plate to allow for extra timing as well. Cheers 🍻 Aussie Boat Guy 😎

[@markolsen-f5w]

10/4...ill keep digging...excited to see your mod 9.8 build...thank u sir!

[@25hpboatracerman]

@@markolsen-f5w Just getting the jetting correct 😁😁 Cheers 🍻

[@markolsen-f5w]

Mmmmmm delicious!!

[@random_dude_99]

Is the tohatsu 9.8 crankshaft the same ase the tohatsu 9.9?

[@25hpboatracerman]

Hi mate, The 9.8hp crank shaft is totally different to the Tohatsu 9.9hp. The 9.9hp shares the same crank with the 15 and 18hp. Cheers 🍻 Aussie Boat Guy 😎

[@random_dude_99]

@25hpboatracerman thanks . I was hoping it was because it looks the same but i think i will be able to find one

[@Phoenix_1269]

Hey mate, great Video but I have a question. The Tohatsu part list says that in the engineblock is also a difference. Do you know what that could be?

[@25hpboatracerman]

Hi mate, Thanks 😊 I really cannot see a difference at all, ports are the same, bore, reeds, reed cover all the same. Maybe the block is stamped differently from factory because it has the different head and exhaust plate. I am currently building a 9.8hp with an 8hp block, obviously with the 9.8 head, carburetor and exhaust cover. Also realized that the stator on the 9.8hp has a different stop, it allows the timing to run more advanced at full throttle. Cheers 🍻 Aussie Boat Guy 😎

[@Phoenix_1269]

@@25hpboatracerman Thank you. I want to tune my outborder too and now I know 👍 Also thanks that you told me that I have to change the stator too. Cheers mate 🍻

[@rittzbyitch3682]

​@@25hpboatracermanAny solution for the different stop on the stator, can the stator be swapped from the 9.8, any solution to complete the full conversion?

[@25hpboatracerman]

@@rittzbyitch3682 The 9.8 hp is the same stator but has a different stainless plate that can be swapped out on the 8hp stator. Just order the part. Cheers 🍻 Aussie Boat Guy 😎

[@rittzbyitch3682]

​@@25hpboatracermanwould this carb upgrade works on the 4 strokes version?

[@KennethCarroll-d6i]

Can I turn a 6hp to an 8hp ?

[@25hpboatracerman]

Yep, think it’s just the carb mate. Cheers

[@yusufalikausmaully4725]

What to change to upgrade my 6hp to 8hp