New guy here OMC 25 Help

[arkansasbowhunter]

hey guys when reviewing the parts catalog for my motor a few different parts. I am very new to this stuff and aspiring to learn so any help appreciated. What I found is that the intake manifolds are different, throttle control cam, and bypass cover assembly (obsolete from online stores I looked at).

When I replaced the 25 to 30 carb, the intake holes were a perfect match, is there any other difference in them which would necessitate ordering the 30hp.

It also states different throttle control cams. What would this do for the motor? the butterfly opens all the way forward as is. Does this affect timing or something?

here are some part numbers I found

25 hp 30hp
manifold 396788 396790
cam throttle 334750 326876
bypass coverf 433567 436025

[JohnsonM50]

Im assuming it is but I believe a 25hp OMC triple was made too in '99, are we all on page '2' ?, the 31.8ci 18 to 35?

[JohnsonM50]

[QUOTE=zul8tr;93084]---------------------------------------------------------------------------------------

Using the following specs for the 25 Johnson:

http://www.piranha.com/motor_specs.php?id=4566

the gear ratio is 2.15:1. With a 13"pitch prop at 5700rpm you measured the theoretical speed (no slip and there is slip) is:

Max theoretical speed = 13" pitchx5700 rpm/(1056 convert factor x 2.15 gear ratio)

= 32.6 mph

If you are getting 30 mph the % slip is about 8%. That is darn good for your boat.

Of course there are many variables to consider. And a main one is the pitch of the prop. The formula assumes the average pitch of the prop which is usually close to a manufactures labeling. If a measured pitch by somebody than it probably will not be the average. How accurate is your tach?

You do not state how heavy the boat, motor, driver fuel and is and that will effecr speed and acceleration.

Note that the engine is rated at 5500 rpm so the motor is over reving so you can increase pitch or diameter or some of each to reduce rpm. Might also consider going to 6000 rpm and raise the motor on the transom to reduce drag. As you do that a greater pitch might prove better. The fine tuning is a trial and error process but you are close to the expected speed if you pitch, rpm and gear ratio are correct.[


QUOTE] In agreement, 92% efficient while reaching R's [& then some] is pretty darn good. In order of potential improvement, to do list..
1- Prop, yours is good & it might not be easy to improve but 0.5" to 1" more pitch -might- help but if you over pitch you lose across the board. Within the relm of prop is RPM, with the 31.8's Ive run many hrs over 6g's & some around 7, beyond that you risk losing a rod cage bearing
2- Setup, motor height & tilt can be put up & out so far but the trade-off of traction limits that. You can put a sleeve on the tilt pin to get a little more when the next adjustment is too much.
3- Weight & its distribution, towards the back generally works to a point.
4- Hull condition even tho it isnt advised to make a race hull slippery it works for aluminum & glass at lower speeds. Make sure the hull is straight too- critical. All things said & done you might get 5mph without motor work.
5- Motor work, easier to get a faster motor.. if ya wanna go

[arkansasbowhunter]

thanks for the info. That sleeve idea is great. that may be an easy try b/c with the next pin out from boat it spins out.

my serial number is J25TEEEE if that helps.

What about the difference in the cam throttle linkage. Does that affect timing or something to get a little faster rpm's.

I will work on the wt distribution also.

the tachometer is a tiny tach that I installed. It consistantly fluctuated between 5600 and 5700 rpms WOT.

I have learned a lot from you guys.

thank you very much.

anyone know of an online place to download a service manual for this motor?

the hull wt and spec was in earlier post also.

[JohnsonM50]

hey guys when reviewing the parts catalog for my motor a few different parts. I am very new to this stuff and aspiring to learn so any help appreciated. What I found is that the intake manifolds are different, throttle control cam, and bypass cover assembly (obsolete from online stores I looked at).

When I replaced the 25 to 30 carb, the intake holes were a perfect match, is there any other difference in them which would necessitate ordering the 30hp.

It also states different throttle control cams. What would this do for the motor? the butterfly opens all the way forward as is. Does this affect timing or something?

here are some part numbers I found

25 hp 30hp
manifold 396788 396790
cam throttle 334750 326876
bypass coverf 433567 436025

All the carbs for 31.8 motors will interchange, differences in follower cams might mean you need a matching set of hardware. Otherwise an easy switch. Set the motors timing at what the 35 calls for, Might be different from the 25. The carb might have a different hi speed jet, probably nothing more.
The cam set up syncronizes the timing advance during acceleration, no matter what motor. Ultimately max advance - wide open throttle is the goal. There is an economizer adjustment for setting it up -not- to go full blast but we aint doin that .
Instead of rev-n the crap outta the motor to set time like the 'book' says just disconnect the arm between the carb & the cam. Run at idle, advance timing to the adj.stop, put a time light to it. & set at the desired #. On water testing is easy. too.

[JohnsonM50]

thanks for the info. That sleeve idea is great. that may be an easy try b/c with the next pin out from boat it spins out.

my serial number is J25TEEEE if that helps.

What about the difference in the cam throttle linkage. Does that affect timing or something to get a little faster rpm's.

I will work on the wt distribution also.

the tachometer is a tiny tach that I installed. It consistantly fluctuated between 5600 and 5700 rpms WOT.

I have learned a lot from you guys.

thank you very much.

anyone know of an online place to download a service manual for this motor?

the hull wt and spec was in earlier post also.

BRP parts catalogues will give you part #s & exploded views but I dont know about an online manual.

[arkansasbowhunter]

I just ordered a service manual. can anyone tell me if this was the right one? I see two of them listed so not sure.


service manual
1999 EE
25, 35 hp
3 cylinder

or

service manual
1999 EE
1999
9.9 thru 30

my motor is the J25TEEEE

I am confused.

[arkansasbowhunter]

I am not sure where I just posted this same stuff but can't find it. I am looking to purchase a new service manual for my motor J25TEEEE motor. I am unsure if which one to get. I am confused LOL.

here is what I found so far

Service manual
1999 EE
25, 35
3 cylinder

or

Service manual
1999 EE
9.9 thru 30 hp

[JohnsonM50]

I am not sure where I just posted this same stuff but can't find it. I am looking to purchase a new service manual for my motor J25TEEEE motor. I am unsure if which one to get. I am confused LOL.

here is what I found so far

Service manual
1999 EE
25, 35
3 cylinder

or

Service manual
1999 EE
9.9 thru 30 hp

The 3 cylinder motors totally different. The other one probably covers the 2 cyl ok, they're all similar.