need help jetting a merc 2.5

**bfly** · 04-15-2003, 02:07 PM

I have a merc with serial #OG368041. I think it's a 96 260 block. It has wmh carbs on it from a 95 200 motor. The original jets were 66's. When the new block was put on the intake was changed from a 5 petal to seven petal and the carbs were jetted up to 76's for break in and now I'm running 74's in it. Does anyone have any mercury info on what jets should be used in this setup. Any help would be greatly appreciated. If it helps any I found a 3311-821687 c number on the carb.

**patchesII** · 04-15-2003, 09:12 PM

bfly, jetting is something that needs to be done by reading plugs after a high speed run and/or with the help of pyrometers. btw, are you sure your carbs came off of a 200. the reason i ask is because i just bought a set of carbs with that number and was told they were 245 carbs. anyway, i'd like to know how your set-up is running. what hull are you running.

**Hooty** · 04-15-2003, 11:35 PM

Jay Smith wrote a really informative piece on jetting by piston wetness. You might do a search for it or I may have it if you're interested.

c/6

Hooty

**Talon2.5** · 04-16-2003, 05:45 AM

originally posted by jay smith.................

Tom ,
Since nobody has posted I will try and help. The plugs should be a "paper bag" color to be the ultimate in correct jetting. I don't read plugs as there are too many varibles involved, I have had good luck only reading the piston itself. I make a high speed pass and if the hull will allow it without back washing over the splash well kill it clean. Then I roll # 5 piston ( as generally # 5 is the hot hole on a Mercury and prone to burn if the mixture is too lean ) to bottom dead center and look at the piston crown on the intake side ( that will be the side toward the outside of the block ) with a bend o lite , the intake side of the piston at the intake port should be wet and look moist about the size of a quarter out toward the middle from the port. If the wettness is over across half of the crown your too rich and can lean down a bit. If it is dry and ashy in color your too lean and you need to richen up in a hurry. Maybe other people have different methods but this has always been the ticket for me. Some rely on pyros , I have found that when a motor is too lean a pyro will be too late in its report and the piston is already smoked ( a 2.5 Merc with stock stroke comes to top dead center@ 10,000 rpms at a rate of 120 times a second I'm an old man and I can't react that fast maybe some of the younger ones can but my incounters with a lean motor things just happen too fast ) also on MANY occations I have had customers motors be STUPID rich and the pryos are reading flame not egt and they continue to richen even more chasing thier tails. Also if a pyro is "coked" up" it will also read incorrectly . This is why I haven't alot of faith in the pyro system of jetting. I always tell my customers when I build modified motors to start off very rich and SLOWLY work your way to dialing in your jetting. And by ALL means don't listen to JOE BLOW as to where his motor runs best on jetting as EVERY MOTOR is different and what might run great on one jetting curve might cost you another trip to the machine shop and bank !

Good Luck,

Jay @ JSRE

THANKS JAY!!!!!

**ProComp** · 04-16-2003, 09:03 AM

I'll have to check again which carbs I have (w??) but they are the stock ones that came on a 92 245 Carb motor with the accelerator pumps. The jetting in mine is 76.

ProComp

**bfly** · 04-16-2003, 10:46 AM

my carbs don't have the accelerator pumps. I'm sure they came off a 95 200. I bought the motor new and the replacing of the powerhead was an insurance job. I just paid the difference to get a better block. Does anyone know what kind of rpm's this block is capable of turning. Right now it's on a blazer bassboat with a 25 tempest turnind about 6400 rpm's. Bye the way ,I don't have oil injection!!!