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post #231 of 257 Old 01-27-2014
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Re: Nissan / Tohatsu 6h 4 stroke

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Originally Posted by Crazy View Post
I bet its an even chance that the motors which run well have a different carb than the poor running ones.
Might it be that after 24 pages and 2+ years, the answer lies in this brief sentence?

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post #232 of 257 Old 01-27-2014
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Re: Nissan / Tohatsu 6h 4 stroke

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I understand where your coming from about the pitch and prop matching the usage. I understood it before I ever bought the engine. My point is that this is a small engine that tohatsu makes the full line of props for and using any one of "their props) that fit that engine it wont make its rated rpm and there fore isn't making its rated HP based on their specs. The only time I have ever seen it make rated rpm was in neutral. Nobody makes a prop with a lower pitch or at at least at the time I was going through this they didn't. So if that engine does actually make 6 hp . it is a theoretical 6hp that has no relation to any real world usage. I've never said it doesn't work.. just that it doesn't work to it's own performance specs in the real world.


To the gentleman asking about compression. I'm pretty sure it was fine. We tested another engine of the same model and it gave the same test results within about 10 rpm. The engine cranked like a champ and other than power we never had an issue. If we I hadn't had a drop in performance on the boat and then tested the engine we would have never known there was a problem.
Well, it certainly looks like you tried to solve the problem. Some things came to mind, my apologies if you have already tried them:

1. Try both fuel sources, integral tank, & portable tank, make sure both are venting properly. Try another portable tank

2. Have the ignition system checked for both energy output & proper advance

3. Make sure fuel valve is flowing properly, check/replace fuel filter

4. Replace plug, properly gapped.

5. Run the engine in the dark under load, check for any sparks/leaks showing anywhere. Check all wiring with an ohm meter

6. Check all air induction tracts for possible blockages, bugs, etc

7. A stretch, but proper lube and amount in the lower unit?

Be careful if you change the main jet, suggest you only go richer. I would think the stock main jet should be spot on at sea level. Doubt that Tohatsu wants a bunch of seized motors coming back.

Paul T
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post #233 of 257 Old 01-27-2014
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Re: Nissan / Tohatsu 6h 4 stroke

Thats true about seized motors. But being water cooled they can rely on that and push the temps higher. Going up a size or 2 on the jet will bring your temps down.
One thing I forgot to say. The float level is critical on these carbs. That could be the problem and account for the variability among them. Set the float level to the correct point before you vary anything else. Does anyone have a shop manual for these motors?
Incidentally- the needle operated by the float can get hung up. Especially when the float goes empty as when you run it dry for storage. Tapping on the bowl can free it (once the bowl is refilled). I use the handle end of a screwdriver to do that.
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post #234 of 257 Old 01-28-2014
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Re: Nissan / Tohatsu 6h 4 stroke

Absolutely not. If you are near sea level, the correct jets are already in the appropriate carb. If you re-jet richer, you will accomplish nothing but fouled plugs. Period. If you need to convert from the 4 to the 5 or to the 6, you replace the entire carb to get the correct CFM. Period.

Yes, Solas makes great props. The 4-blade models (both 5 and 7 pitch for the Tohatsu/Nissan 8/9.8) are super for sail applications.

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Originally Posted by christian.hess View Post
yeoup, you can get a little more oomph by simply chainging the main jet on this outboard and most alike...

but if one doesnt want to well thats their call huh

in any case, many ways to get a little more oomph

changing to a bigger throat(intake) and changing the main jet acoordingly for WOT or close to it will give you what johnson did back in the 80s and 90s

2 engines in one block

a 9.9 and a 15hp, absolute only difference was this above, throat size, main jet and a few other small things to get about 1000rpm or so more at wot

again correctly prop it for sure...

solas props do a nice job of offering a variety of props for every situation

peace

Paul Van Voorhees
Certified Tohatsu TLDI Technician
Mgr, Obersheimer Sails
Buffalo, NY USA

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post #235 of 257 Old 01-28-2014
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Re: Nissan / Tohatsu 6h 4 stroke

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Originally Posted by Tallswede View Post
I have the 8hp Tohasu on my Hunter 23.5 and it won't rev out either. However it will push my boat at 6 knots in smooth water at half throttle. Trying to open the throttle more doesn't do much. The engine is smooth and quiet being a 2 cyl. But even this larger engine is not producing all it's horsepower on my boat. The old Johnson 4 hp two stroke I had before would push it almost as well but used a lot more fuel and was working harder. The little Johnson works very well on my 16' Sunbird.

Kevin
You have a displacement hull. Your boat is at hull speed. Even a 300 hp OB would not exceed that speed unless you started to dig the stern in and attempt to plane/surf. It's a function of waterline length. That said, you could benefit from either of the Tohatsu/Solas 4-blade high-thrust sail-application props for the MFS8/9.8. There are both 5 and 7 pitch models. In your case, If I wanted to go higher in the RPM band (and get stellar reverse, plus precise speed control), I would start with the 7 pitch, (and verify RPM with a tach. That is the only way to definitevely determine your actual RPM.)

Paul Van Voorhees
Certified Tohatsu TLDI Technician
Mgr, Obersheimer Sails
Buffalo, NY USA
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post #236 of 257 Old 01-28-2014
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Re: Nissan / Tohatsu 6h 4 stroke

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Originally Posted by Crazy View Post
Thats true about seized motors. But being water cooled they can rely on that and push the temps higher. Going up a size or 2 on the jet will bring your temps down.
One thing I forgot to say. The float level is critical on these carbs. That could be the problem and account for the variability among them. Set the float level to the correct point before you vary anything else. Does anyone have a shop manual for these motors?
Incidentally- the needle operated by the float can get hung up. Especially when the float goes empty as when you run it dry for storage. Tapping on the bowl can free it (once the bowl is refilled). I use the handle end of a screwdriver to do that.
Forgot about the float level, good point. I would guess that unless someone fooled with it, the factory setting should be spot on? However, it is easy to check.

Last time I ran my generator it was running rich. Tried the tapping method which didn't work. Lowered the float bowl, "tickled" the float, back to normal.

Paul T
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post #237 of 257 Old 01-28-2014
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Re: Nissan / Tohatsu 6h 4 stroke

Yes the factory level is the best setting. It allows the correct amount of fuel to be picked up by the jets. Whatever their size. And yes it is sometimes necessary to go in there and free the metering needle manually.

pvanv: you are correct about hull speed. But notice audio____'s info- he hadn't been able to reach the speed given previously by the 4hp motor. I don't know how you formed your opinion on carburetors but I suggest you do some research about motorcycles. Carbs are indeed rejetted all the time.
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post #238 of 257 Old 01-28-2014
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Re: Nissan / Tohatsu 6h 4 stroke

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Originally Posted by pvanv1 View Post
Absolutely not. If you are near sea level, the correct jets are already in the appropriate carb. If you re-jet richer, you will accomplish nothing but fouled plugs. Period. If you need to convert from the 4 to the 5 or to the 6, you replace the entire carb to get the correct CFM. Period.

Yes, Solas makes great props. The 4-blade models (both 5 and 7 pitch for the Tohatsu/Nissan 8/9.8) are super for sail applications.
do you want me to show you the page where johnson SPECIFICALLY says this is so Im not arguing...I agree....the bes way is to change carb completely however on some engines, specifically the one I mentioned

the johnosn 9.9 and 15 series its simply carb jet size and manifold...NOT CARB

Like others have posted before and have experience in this(I restore motocrcyles too) you can absolutely depending on where adjust your jet sizes for performance where you need it(most carbs come jetted for sea level naturally) if you are in a lake up in tahoe it makes sense to do your due dilligence and rejet...

the correct jets however are almost NEVER installed in your carb, especially if you live in countries like the good ole use or europe where epa and other nuissances DICATATE what power and emmissions you can get from a certain engine especially when its sold commercially to major areas.

so i disagree...but thats ok...to disagree....

this also comes from experience in small engine rebuilding...on water and off

for example most carbs especially on motorcycles these days have specific kits called smog block off kits, evap removal kits or in some cases even pilot jet cap removal kits as some carbs are soooooooo EPA compliant that they wont even let the owner fine tune the slow jet for easy starting depending on your altitude and climate

lets not even go there...its very easy to get a little more oopmh from these simple engines...especially once you have propped right

here is another tidbit

saying the jets that come with all outboards are perfect makes no sense...especally since many dinghy outboards for example used at idle when they were designed to run at wot or 3/4 foul plugs up naturally, especially if 2 stroke and hence why most older outboards had nice rich lean knobs to fine tune your idle mixture

if yo were a diligent owner you did this every time you started and stopped the engine

just sayin

not here to argue just offer my point of view...and experience

cheers

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Re: Nissan / Tohatsu 6h 4 stroke

here is a fun little page for those of us who like to read its good for any outboard, johnson or not...2 stroke on 4 strokes you have other stuff to worry about but carbs are carbs mostly intercheangable

Maintaining Johnson 9.9 Troubleshooting

for example I used a mikuni 2 stroke carb on my 4 stroke honda no prob...just need to know what you are doing...thats all

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post #240 of 257 Old 01-28-2014
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Re: Nissan / Tohatsu 6h 4 stroke

I don't know anything about motors and rarely use them but I thought I would just throw this out there. I have a 2005 Johnson 6hp 4 stroke. My boat is 22' long full keel and displaces 7400lbs on a light day. At half throttle I get 4.5 knots out of my engine at 13 mpg. I have never fully tested it but I did half test it. last year I missed my tide cycle by half a mile. it was blowing a steady 20 knots on the deck and we were experiencing wind over tide. We dropped the jib and motored straight into it at half throttle and 2.4 knots. The main was up but we were dead into the wind. I guess what I am saying is that I am quite surprised the the power.
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