Carburettor Butterfly Valves Not Fully Open at WOT

[MY70TLRS]

Hello all,

I recently completed a link & sync on my 1994 3 cylinder 2 stroke 70hp TLRS after a complete powerhead rebuild and noticed the carburettor butterfly valves aren't rotating to the horizontal (fully open position) when the throttle lever (on the control box) is in the WOT position. I'm not sure if this is normal and I can't run the motor to test it just yet.

Movement of the throttle cam seems to be limited by the timing stops on the CDI unit however the throttle lever on the carburettors requires significantly more rotation to open the butterfly valves fully after the cam control lever hits the the fully advanced stop. Hence the butterfly valves are a long way off horizontal at WOT, which seems odd.

I'd appreciate any comments on this before the initial start up.

Thanks

[99yam40]

apparently some motors are de-rated by Yamaha by limiting the throttle plate opening.

not sure it that is true for your motor or not, but here is a thread that shows it
http://www.yamahaoutboardparts.com/f...arburetor-swap

[ChuckB1954]

Until someone more knowledgeable comes along, input from an amateur. Did it open fully before you made adjustments? This would obviously point to the adjustments you made.

How do you know the butterfly's are not wide open ? I know on my carbs they are not visible.

Some engines HP is rated by the amount of throttle opening, so this could me normal.

EDIT: Looks like a someone more knowledgeable came along while I was typing this.

[ChuckB1954]

apparently some motors are de-rated by Yamaha by limiting the throttle plate opening.

not sure it that is true for your motor or not, but here is a thread that shows it
http://www.yamahaoutboardparts.com/f...arburetor-swap

I just noticed this:

Join date: Aug 2019, Posts 12000 ! A new record!!

CaptureJoin.JPG

[ChuckB1954]

I went to the Yamaha website and based on the available 1994 70HP TLRS carburetors shown for your year, I would think that horse power is not regulated by the amount of throttle opening. In other words, I think your carbs should open fully at WOT. Sounds like an adjustment is needed in my humble opinion. Others may come along and correct me.

[99yam40]

I just noticed this:

Join date: Aug 2019, Posts 12000 ! A new record!!

CaptureJoin.JPG

I tried to get that fixed some time ago, but they still have not helped.
there are other problems also with boxes that open up way out of wack for me.

it does not bother me too much.

every time I open the web site it updates my join date

[99yam40]

I looked at the carb#s and only find they fit 70 hp motors along with a 2001 60hp.

OP do you think someone put the wrong carbs on your motor?

Some ahole drilled out the main jets on my C40 before I got my hands on it .
took me over a year to find out why I had that high speed miss

[zenoahphobic]

I looked at the carb#s and only find they fit 70 hp motors along with a 2001 60hp.

OP do you think someone put the wrong carbs on your motor?

Some ahole drilled out the main jets on my C40 before I got my hands on it .
took me over a year to find out why I had that high speed miss

Interesting, who would ever measure their jet orifaces ; and how would you do that anyway, are there hole gauges like feeler gauges out there? They almost all of them are stamped with a number (sometimes hard to know what they actually mean as a measure anyway).

[ChuckB1954]

Interesting, who would ever measure their jet orifaces ; and how would you do that anyway, are there hole gauges like feeler gauges out there? They almost all of them are stamped with a number (sometimes hard to know what they actually mean as a measure anyway).

Somebody probably did it seat of the pants.. I have tried it before on Harley carbs with mixed results.

They sell carb jet gauges that will tell you in millimeters the size of the hole and they also have jet charts that will translate the numbers in millimeters based on the number stamped on the jets.

[99yam40]

I used drill bits and a caliper to measure them.
found what would fit into them and what would not.
could not find anyone or any thing to reference what they should be,
so I bought 1 new one to measure.
the new one was smaller

I bought all new ones and installed.
high speed miss went away

what lead me to looking at them was when I had the cowling and silencer off and running the boat on the water into the wind the miss went away, but came back when I ran with the wind.
told me it was running too rich
and I had been thru the carbs and everything else I could think of many times trying to figure out what was wrong.

[ChuckB1954]

I used drill bits and a caliper to measure them.
found what would fit into them and what would not.
could not find anyone or any thing to reference what they should be,
so I bought 1 new one to measure.
the new one was smaller

I bought all new ones and installed.
high speed miss went away

what lead me to looking at them was when I had the cowling and silencer off and running the boat on the water into the wind the miss went away, but came back when I ran with the wind.
told me it was running too rich
and I had been thru the carbs and everything else I could think of many times trying to figure out what was wrong.

The plugs should have told the story.

[99yam40]

It ran OK except at WOT and I normally never ran at WOT except the get up on plane.
plugs looked OK to me.
only run about 15 minuets to where I normally fish and run low RPMs easing along the flats

strange thing today I pulled the plugs and found #2 had the gap almost bent closed.
noticed a miss fire last Thursday coming back to cabin.

did not see anything out of order on the plug except the gap closed up some ,so I ran a compression test and all were 115.
cleaned, re-gaped, and installed
ran it in the drive way and it seems it is running better.

hope it was just a piece of carbon the came loose and hit the plug
I will take it out later this week, headed fishing with someone else tomorrow

[ChuckB1954]

Yes.. I am learning that it is a lot easier/convenient to analyze carburation on a land vehicle verses a boat! Loads are a big factor.

[MY70TLRS]

Firstly congrats to 99Yam40 on your milestone. I agree with ChuckB1954 that these carbs don't need regulation. Can I get this post back task and ask if there is someone with a Yamaha 70hp from a similar era to check the movement of the throttle valves at WOT? It would be interesting to know if Yamaha designed the carbs to operate this way.