Yamaha C80 to C90 HP Conversion

[Nuclear-Inc]

Here is my 1997 80HP. I found some Pre 1992 90hp carbs to transfer all the jets over to my current carbs. I have cross checked all PN’s and only differences between 80 and 90 seems to be carb jetting. Venturi sizes are the same. Exhaust tuner are same as well. Has anyone done this conversion on a 1140cc engine? What type of RPM increases can I expect with an extra 10hp?

[boscoe99]

Is the thinking that just going to a smaller jet size will result in 10 more HP?

[Nuclear-Inc]

Is the thinking that just going to a smaller jet size will result in 10 more HP?

I think you misread my post. I’m installing 90hp jets on my 80HP.

[boscoe99]

C90 shows the use of a #78 jet. C80 shows the use of a #80 jet.

Even if the C90 used a larger jet, simply adding more gasoline does not result in more HP.

[Nuclear-Inc]

C90 shows the use of a #78 jet. C80 shows the use of a #80 jet.

Even if the C90 used a larger jet, simply adding more gasoline does not result in more HP.

I used the wrong nomenclature. I used a 1991 Carbs. The C90 carbs are the squared auto prime. Your correct about about the #78 jet. However Pre-1992 90HP used #165 main jets vs my #160 stock 80hp.

https://www.crowleymarine.com/yp/KB3...tlrp-1991-90hp

[boscoe99]

Just adding more gasoline to the same amount of air does not result in more power.

[Nuclear-Inc]

Just adding more gasoline to the same amount of air does not result in more power.

Both carbs have the same Venturi, exhaust tuner, block.

[99yam40]

do the throttle plates open the same amount?
is the timing set and advance the same?
Is the RPM the same on both motors?
how about the pistons, rods, heads, and crank
something has to be different to mix more air and fuel together to get the proper mix to keep burning as it should

[boscoe99]

What about the manifolds?

[boscoe99]

[ausnoelm]

This never gets old does it?

[Nuclear-Inc]

Once again I'm going to state that I'm using a pre 1992 90HP engine as a reference since it was the last years that the 90 used the old style carbs. You are comparing a non-oil injected model with a oil injected model manifolds. Aside from the jets, timing must be advanced to 22* vs 20* for an 75/80 HP. I also spoke with Chris Carson Marine (CCM) who referenced same blocks just different jets/ timing. They recommend their model 333s reeds. To supplement the increase in fuel the reeds will also increase airflow to the engine. The 75HP does have a different tuner. Overall I look forward to finishing my project and sharing the results.

[boscoe99]

I compared a C80 to a C90 because that is the subject of this thread. You are talking about removing the C80 jets and installing some 90 jets. I think. If just changing the jets will give you 10 more HP then well and good for you.

Maybe install some 100 HP jets and get 20 more HP.

[99yam40]

Once again I'm going to state that I'm using a pre 1992 90HP engine as a reference since it was the last years that the 90 used the old style carbs. You are comparing a non-oil injected model with a oil injected model manifolds. Aside from the jets, timing must be advanced to 22* vs 20* for an 75/80 HP. I also spoke with Chris Carson Marine (CCM) who referenced same blocks just different jets/ timing. They recommend their model 333s reeds. To supplement the increase in fuel the reeds will also increase airflow to the engine. The 75HP does have a different tuner. Overall I look forward to finishing my project and sharing the results.

Can you dyno the before and after to see if HP increases and by how much?

someone drilled out the jets in my C40 before I got my hands on it,
it took me over a year to figure out why it had a high speed miss.
jets had the correct numbers and I could not find any info on hole sizes.
had to buy new ones to compare.
they did fix my problem.
throwing just extra fuel does not always work as some think