Early Big Twin 25 performance hop-up....

[waukeganomc44]

Greetings all! I'm new to this forum & looking to increase performance on my '55 tiller model. I also have a nice, unmoletsed, totally original '51 BT25 but rather not alter it. I run the motor on a 14' Crestliner Commodore (mainly) & would consider such alterations as reed, carb or prop replacement, and perhaps... cylinder head work / replacement. I'll likely remove the head to install a new head gasket anyways. Appreciate any tips, lessons learned, etc.

 

[nwcove]

Re: Early Big Twin 25 performance hop-up....

jmo....but i dont think theres much you can do to make the old 25 in to a "screamer"......other than make sure the whole package is working in perfect harmony. i.e. carb set perfectly, timing exactly on the mark, motor set to the ideal height and the right prop for your application. ( but you could always feed it some racing fuel.....it would be fun for the two minutes before it turned into a grenade!!)

 

[HighTrim]

Re: Early Big Twin 25 performance hop-up....

What is a BT25?

Most mods are not noticable to the human body, unless you are Spiderman of course

Best bang for the buck? Upgrade to a 35hp

 

[F_R]

Re: Early Big Twin 25 performance hop-up....

Easiest mod would be when you remove the head, replace it with a 30hp head.

 

[nwcove]

Re: Early Big Twin 25 performance hop-up....

What is a BT25?

Most mods are not noticable to the human body, unless you are Spiderman of course

Best bang for the buck? Upgrade to a 35hp

BT25.....BigTwin 25? ...and just a thought on spiderman......wouldnt the cartoon and the movie have been much more entertaining if ol spidy would have shot webs from where real spiders do, instead of his wrists???

 

[kbait]

Re: Early Big Twin 25 performance hop-up....

If you made more compression out of a rope-start Big Twin.. Well, Atlas would break his arm starting it. If yours is running well, you may not pass anyone, but you'll pull the Titanic away from the 'berg!

Never heard of a BT25? the suspense is killing....

 

[1946Zephyr]

Re: Early Big Twin 25 performance hop-up....

What I want to try, is mounting a powerhead from the 1972 40hp Norseman, to a 1957 BT35 midsection. Since the 1971 and later 40's are basically a 33 Skitwin punched out, then it should work. Of course, I'll have to manufacture cowl mounts to go on the head, but that's no biggie for me.

Yes, I believe when he said BT25, he meant Big Twin.

 

[samo_ott]

Re: Early Big Twin 25 performance hop-up....

Does that mean the '60-'70 40's will not fit? I've never tried.

 

[F_R]

Re: Early Big Twin 25 performance hop-up....

They will not fit.

 

[1946Zephyr]

Re: Early Big Twin 25 performance hop-up....

Yea, 60 - 70 40 had a different powerhead mounting, but I am curious about the later 40, that was basically a punched out 33 Skitwin. Then again, I could match up base gaskets and see how things look.

 

[HighTrim]

Re: Early Big Twin 25 performance hop-up....

Gaskets are different. I dunno if that would work.

 

[1946Zephyr]

Re: Early Big Twin 25 performance hop-up....

Darn. Oh well, the thought was good while it lasted. Might try just boring it out then and using the pistons.

 

[waukeganomc44]

Re: Early Big Twin 25 performance hop-up....

Appreciate the info, and not surprised that replacing with a BT (Big Twin) 35 power head results in the most bang for the buck! My assessment is to bite the bullet & snag another complete motor, that being a 35... a Big Twin or a Lark.
Thanks, Ron S>

 

[1946Zephyr]

Re: Early Big Twin 25 performance hop-up....

Yea, the '57 Big Twin is maybe only a couple pounds more than the older 25's, but that would most likely come from the starter. Size for size though, they're pretty equal. The 35 has a bigger bore and heftier gears in the lower unit, and that's pretty much the only difference.

 

[waukeganomc44]

Re: Early Big Twin 25 performance hop-up....

Regarding my '51, I have earned the early BT25 has a more slender lower & thus a less drag. BTW, did the earlier 25hp OMC motors (the '51 vs. '55) also produce more power? Noting the silencer induction / intake incorporated in the latter model 25's, I'd guess the 1st gen motors might have had more oomph. Being a vintage Honda bike guy, I know the earlier variant of most models had more power than the latter due to EPA regs, quieter induction systems, etc. Presume the same holds true with 1st vs. latter model of a given outboard model...??

 

[waukeganomc44]

Re: Early Big Twin 25 performance hop-up....

RE: race fuel being added, I fully understand that too much is NOT a good thing! However, adding a little octane booster or (let's say) 10-15% race fuel by volume (on occasion only!) might make that grin a little wider?

 

[F_R]

Re: Early Big Twin 25 performance hop-up....

Nah, no significant difference in power. The silencer doesn't slow it down. But I'll tell you what---the pre '55 will SEEM a lot faster because of the noise and vibration. They tamed those horses in the '55 models.

 

[samo_ott]

Re: Early Big Twin 25 performance hop-up....

The pre '55 will SEEM a lot faster because of the noise and vibration. .

Ha!