2 stroke yam 20hp d model and 25 or 30 differances

supremesoundz · 03 September 2010, 11:07

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

Bus_Boy · 03 September 2010, 11:28

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.

simsy · 08 September 2010, 11:14

my 20 hp yamaha 2 stroke is identical to the 25hp except for the carbs. Every other detail is 100% identical, even the CDI...

supremesoundz · 08 September 2010, 11:17

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

9D280 · 08 September 2010, 12:25

Quote:

Originally Posted by supremesoundz

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")

Quote:

Originally Posted by supremesoundz

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......

TUFTY · 08 September 2010, 15:37

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

supremesoundz · 08 September 2010, 21:21

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

9D280 · 09 September 2010, 08:56

Quote:

Originally Posted by supremesoundz

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.

supremesoundz · 10 September 2010, 11:15

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

supremesoundz · 10 September 2010, 11:29

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

simsy · 10 September 2010, 13:19

My 20 hp 2005 yamaha outboard opens fully 90 deg on both carbs.

I think the bore of the cards is bigger on the 25 hp....

What sized bore does a stock 25 hp yamaha 2 smoke have?? (around 2005)

9D280 · 13 September 2010, 08:54

The fact they are carrying differnet assy part numbers would indicate something is different. The down side is that the spares lists will not go down to the individual componet depths, so if it opens all the way, you probaby have different castings, or as you say, a bigger / smaller bore, although putting my manuifacturing efficiencty hat on, I would be surprised if it were two totally differnet castings, purely dwn to economies of scale. Having said that, Yam also mke bikes,so goodness knows whether one of them is shared with a two wheeled machine or not.

Remember you also have differences in the exhast side. I still reckon if that's the case, you may be as cheap just trading in for a bigger engine.

Only way to find out for sure is go measure one.

Northern Pike · 21 September 2010, 14:13

I don't know anything about Yam's but I went through this with a '89 Evinrude 20. Its all about fuel/air in, exhaust out, and maintaining the proper balance. I spent hours at a cooperative dealer examining and cross referencing microfiches and part #s. I also examined a same year 25. When looking into the carb throat of my 20 I saw a restricted diameter casting behind the carb. 25 didn't have that: manifold casting dia. equaled inner dia. of carb. Only part # differences were manifold and main jet. Using a Dremel I carved the hole in the manifold to equal that of the 25 (after stuffing the bore with a grease coated rag to prevent chips from entering the engine), then I installed the bigger main jet specified for the 25. Voila! Noticeable 25% increase in HP.
Same block was also indicated for 30, but exhaust had a different part #. Through this process I also consulted with a mechanic friend who used to race and hotrod outboards. He showed me 2 exhausts from similar Mercs, but with different HP ratings. The only difference was a couple of 1/4" holes drilled into the higher HP one.
Balance: More fuel air in; more exhaust out.
To up my 20 to 30 I would have had to remove the powerhead and alter the exhaust. Too much work, but I happily ran my new 25 for several years with no problems.

supremesoundz · 23 September 2010, 14:07

il have a lil look and c if the bore is reduced but like the parts list suggests summat is different with the carb bodies must be diam, the main jets are the same on the 25 and the 20 so im assuming reduced flow at both ends i know from my bike experience when u have a bike restricted to the 33bhp limit for ur test its normally washers between the carb and manifold? and on mopeds tht have to be restricted to 30mph they usually have the oppisate end restricted a washer in the exhaust