Jetting a 20 HP Mercury

[TTG]

Hi there, I have a 2001 20 HP Mercury that I would like to jet to a 25.
The 20 and 25 have the same cubic inches, bore and stroke, lower end, ect.
After researching parts lists on the two , I found I would need to order a bigger jet , a "valve throttle" (that sits between the carb and the head ) and some gaskets.
These are the only parts that seem to differ from the 20 to the 25.
I would like to get the extra 500 RPM .........Has anyone done this? Are there any other parts that I would need?
Great Site! ......George

 

[Laddies]

Re: Jetting a 20 HP Mercury

The jet will not help without the tune pipe it will just use more fuel.

 

[TTG]

Re: Jetting a 20 HP Mercury

Were is the tune pipe located? Is this something I can change?

 

[Laddies]

Re: Jetting a 20 HP Mercury

It's the tube right below the powerhead on the driveshaft housing adapter plate

 

[TTG]

Re: Jetting a 20 HP Mercury

Hey , thanks for the info, now its a matter of finding one I tried Crowley Marine, any oyther good sites for looking up those type of parts?

 

[8075mercury]

Re: Jetting a 20 HP Mercury

One thing I would double check is the intake and exhaust port size. I know you can not do the swap you describe on a 7.5 and 9.8 because not only is the carb different, but the intake and exhaust ports are smaller on the 7.5 than the 9.8. I thought, no problem, just drill them out, but the centerpoint is different too. So, my 7.5 stayed a 7.5.

I would bet that the 18/25 and 20/25 is the same situation. At least I would check by opening the intake and exhaust covers before getting all of the conversion parts.

Good luck!

 

[Laddies]

Re: Jetting a 20 HP Mercury

8025. bet your wrong

 

[8075mercury]

Re: Jetting a 20 HP Mercury

Well that is good news if that is the case because you could do a hp increase pretty easy.

Good to know.

Thanks!

 

[Laddies]

Re: Jetting a 20 HP Mercury

8075, the problem is that Mercury has made those items about as scarce as hen's teeth, so you would have to find a older 25 for a donor engine.

 

[TTG]

Re: Jetting a 20 HP Mercury

8075, the problem is that Mercury has made those items about as scarce as hen's teeth, so you would have to find a older 25 for a donor engine.

Yep, I cant find hens teeth anywhere!!

 

[25xs]

Yes, the jet is all you need... Tuner pipe is same from 15hp thru 25hp

Yes, the jet is all you need... Tuner pipe is same from 15hp thru 25hp

You can easily make your 20 into a 25 by rejetting and letting the max timing advance travel as far as possible... You pretty much can not "over time" this engine.

 

[rtse66]

Re: Jetting a 20 HP Mercury

wow, if i'd seen this i wouldn't have posted my question. i assume this will also apply to an 84 18hp? sounds like what 24xs said is that all you have to do is change a jet and adjust the timing stop. i'm a newbie when it comes to outboards so anything will help. sorry didn't mean to hijack the thread but i'm in the same "boat" . rob.

 

[25xs]

20hp & 25hp tuner pipes are the SAME in 2001 Rejet the 20hp to make a 25hp

20hp & 25hp tuner pipes are the SAME in 2001 Rejet the 20hp to make a 25hp

Part No. 99702A7 is the tuner plate adapter which includes the exhaust tuner pipe... It's the same between the 15XD/15SeaPro/20hp/25hp motors in 2001 and surrounding model years. You can make a 25hp motor from a 20hp pretty easily.