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Old 03 September 2010, 07:07   #1
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2 stroke yam 20hp d model and 25 or 30 differances

hi ive got a yam 20hp d model and ive heard its exactly the same gear case and power head as the 25hp? just wondering is it also the same as the 30hp......... and if so what parts timing changes etc will i need to convert it to the 25 or 30 thanks for taking ur time to read this
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Old 03 September 2010, 07:28   #2
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The 20 and 25 share the same block. What's meant by this is the cubic inch capacity of the block itself is the same. Manafactuer's use the same size block between different HP models, e.g, 6-15 hp, 20-25 hp and so forth.

The 30 hp model is a completely different motor, and is also a 3 cylinder, as aposed to your 2 cylinder.

The 30 D model uses a 494 cc motor, your 20 D uses a 395 cc motor, which is also used in the 25N.

You need to compare the parts between the 20D, and the 25 N. Your local Yamaha dealer (or any dealer for that matter) should be able to tell you exactly what parts are different with reference to a parts manual.
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Old 08 September 2010, 07:14   #3
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my 20 hp yamaha 2 stroke is identical to the 25hp except for the carbs. Every other detail is 100% identical, even the CDI...
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Old 08 September 2010, 07:17   #4
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really thats interesting? ive rung yam to get a cross referance on parts but they never got back to me? some one on the other post of mine about props said the carbs and also exhaust? do the carbs just have a bigger bore
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Old 08 September 2010, 08:25   #5
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Quote:
Originally Posted by supremesoundz View Post
really thats interesting? ive rung yam to get a cross referance on parts but they never got back to me?
They won't , coz they want you to buy a bigger engine! (and also don't want the liability if you kill yourself "as a result of the modifications")

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Originally Posted by supremesoundz View Post
some one on the other post of mine about props said the carbs and also exhaust? do the carbs just have a bigger bore
There are numerous ways of down-rating an engine. At a high level, you can either mess with the spark timing, or throttle the airflow (in or out). As you are finding out, some engines use one method, others use another, and some use a combo! (and each of them has numerous ways of doing it.....)

Go to Ishop marine, and in their service area you'll find a load of (American) Yam microfiches. Then it's a long evening comparing part numbers......
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Old 08 September 2010, 11:37   #6
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Yam 20 /25 Carbs on E bay

There are 2 listings 1 for each of 2 carbs for Yam 20 or 25 (Brand New)

Looks like seller not sure but has part number.


Maybe of interest to one of you.

Paul
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Old 08 September 2010, 17:21   #7
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The apparent differences are:

Carb Assembly (top)
20D 6L3-14301-00
25N 6L2-14301-01

Carb Assembly (bottom)
20D 6L3-14302-00
25N 6L2-14302-01

The jets etc are listed as the same parts, so I assumed that the carb body itself must have a difference (now it may just be a whole / plug or something?

Exhaust manifold extension
20D 6L3-41131-00-5B
25N 6L2-41131-00-5B
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Old 09 September 2010, 04:56   #8
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Originally Posted by supremesoundz View Post
The apparent differences are:

Carb Assembly (top)
20D 6L3-14301-00
25N 6L2-14301-01

Carb Assembly (bottom)
20D 6L3-14302-00
25N 6L2-14302-01

The jets etc are listed as the same parts, so I assumed that the carb body itself must have a difference (now it may just be a whole / plug or something?

Exhaust manifold extension
20D 6L3-41131-00-5B
25N 6L2-41131-00-5B
Based on that, I will GUESS that there is a cam or WOT stop difference there, if the jets are the same. Have you taken the air box (or it's lid) off & checked if you get a WOT full opening of the valves?

Exhast manifold likely different too - I suspect you may find at this rate you may be as cheap just trading it in for the bigger engine, remembering that as well as the bits, you will need a full set of replacement gaskets too....

Worth checking the throttle linkage as well - there will likely be a cam or lever change to give a linear response to the lever for both engines.
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Old 10 September 2010, 07:15   #9
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when u say the stop do u mean the castings could be different so one allows the butterfly to open fully the other does not?

i was thinking maybe the bore was different allowing more air flow and could have been restricted that way

what do u mean about the linkages?

exhaust tube should be relitivly easy power head off swap the tubes and refit im thinking
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Old 10 September 2010, 07:29   #10
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ive just been out and whipped the air box lid off and opend the throttle fully and the butterfly turns a full 90 degrees so its parrelel with the ground so wide open? linkage seems to move it linearly too
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