1987 etlh 70 hp issue

[medicineman]

I have a 1987 2 stroke 70 hp Yamaha outboard that sat several years before I got it. Compression 130,130, 125. I cleaned carbs well getting all jets open. Put new fuel pump and fuel lines on. Replaced water pump and thermostat. It cranks and idles great. It runs very smooth from 2/3 idle up, and if backed down once it accelerates it runs smooth at about half throttle up. I set the carbs according to manual (I think 1 7/8 but don’t have book in front of me now) and have tried tuning carbs about 1/2 round in each direction with no change in idle or performance noted. Have tried 2 new sets of plugs gapped to proper specs with no difference. My problem is a rough acceleration up to point pontoon planes—-or as much as a pontoon planes. Engine almost sounds like it is skipping. I have a very flat pitch prop (11) so even thou it is a big pontoon should push it ok. If I gun the engine to 2/3 on takeoff it does fine, but I am hoping someone can clue me in on what I have missed. I did not buy carb kits—-as they are pretty pricy and my gaskets all appeared fine—so I realize that may be an issue. Appreciate the Yamaha outboard gurus pointing me in right direction!

[boscoe99]

I would suspect a passageway within the carburetors is clogged or partially clogged.

Try spritzing some gasoline into the air intake system when it starts to run rough and see what happens. If it picks up and runs better than is a good sign that something is amiss within the carburetors.

[medicineman]

I would suspect a passageway within the carburetors is clogged or partially clogged.

Try spritzing some gasoline into the air intake system when it starts to run rough and see what happens. If it picks up and runs better than is a good sign that something is amiss within the carburetors.

I did try leaving airbox off and covering one carb opening at a time thinking that would richen mixture and did not notice any improvement. However your idea may be better and I will give it a try.

[boscoe99]

Blocking a carburetor off might result in more gasoline flowing but at the same time air flow is lost. Power might go down as both air and gasoline are needed to produce power.

[medicineman]

Blocking a carburetor off might result in more gasoline flowing but at the same time air flow is lost. Power might go down as both air and gasoline are needed to produce power.

Thanks for the education. I appreciate you sharing your expertise. Will give your suggestion a try tomorrow.

[medicineman]

Spraying gas did not seem to make a difference. Any chance a coil could cause those symptoms?

[medicineman]

Any other ideas? I tried syncing the carbs and noticed no difference. It cranks and idles perfectly, just accelerated poorly unless I give it about half throttle. Once it gets to steady speed runs smooth at any throttle level.

[99yam40]

my guess is off idle ports in carb are plugged

[medicineman]

I have cleaned arbs 3 times and feel fairly certain all pets and jets clean. I did do a de carb today with a can of sea foam in a gallon of gas— a lot of smoke but no real difference in running. I have not bought carb kits yet and wonder if that might be a good next step. I checked for air leaks with carb cleaner but never heard a change in idle, but maybe a slight air leak exists.

[TownsendsFJR1300]

Did you actually PULL ALL the jets out and check if you can see light thru them?

If not, just a spray of carb cleaner, it don't count..

https://www.boats.net/catalog/yamaha...tlh/carburetor

[medicineman]

Yes. First cleaning I missed jets under the plugs on top of carbs, but got them on second try. Or to the best of my knowledge I got everything. Since it is not running perfect, it is obvious I have overlooked something and I appreciate all tips offered here.

[TownsendsFJR1300]

Is the boat / engine "new" to you? As in did it ever run "correctly" in that RPM range and under those conditions or is this something new?

[panasonic]

What do the plugs look like when you remove them? Do they all look the same?

You said ". Once it gets to steady speed runs smooth at any throttle level"

This means you can go back to idle and then accelerate normally for the rest of the day?

[medicineman]

Not really. I can slow and it runs pretty smooth. However if I come to a complete stop and engine has to start moving this heavy old 28!foot pontoon it runs rough if I only give about half throttle to get it going again. However if I give it 3/4 throttle or better it accelerates smoothly. It acts, from what I have read, as if carbs are not in sync and I have synced them three times and looked in carb throats to make sure throttle plates were completely closed—so I do not think that is issue.

[pstephens46]

Not really. I can slow and it runs pretty smooth. However if I come to a complete stop and engine has to start moving this heavy old 28!foot pontoon it runs rough if I only give about half throttle to get it going again. However if I give it 3/4 throttle or better it accelerates smoothly. It acts, from what I have read, as if carbs are not in sync and I have synced them three times and looked in carb throats to make sure throttle plates were completely closed—so I do not think that is issue.

Strange that misting fuel into the carbs made no difference when this was occurring.