MERCURY 2.5 LITER Carberator Information

HP	Year	Serial Number Range	Model
150		C239553 - Up	WMH-1, 2, 3B, 5, 7, 8, 8A, 11A, 12, 12B, 13, 13B, 14A, 15, 16, 18A, 21, 22, 23, 24, 25, 28, 90, 31, 32, 33, 34
150	1978-1990		WH-2, 12, 21, 23, 27, 29, 35, 38, 40, 48 with 2 needles and seats
150	1980-1982		WH-7A
150	1994-1995		WMH-31
150	1996-1997		WMV-2
XR6	1994-1995		WMH-32
XR6	1996-1997		WMV-3
175		C239553- Up	WMH - -1, 2, 3B, 5, 7, 8, 8A, 11A, 12, 12B, 13, 13B, 14A, 15, 16, 18A, 21, 22, 23, 24, 25, 28, 90, 31, 32, 33, 34
175	1976-1990		WH-1, 4, 6, 7, 13, 17, 30, 34 with 2 needles and seats
175	1980-1982		WH-7A
175	1994-1997		WMH-33
175	1996-1997		WMV-4
200		C239553- Up	WMH - -1, 2, 3B, 5, 7, 8, 8A, 11A, 12, 12B, 13, 13B, 14A, 15, 16, 18A, 21, 22, 23, 24, 25, 28, 90, 31, 32, 33, 34
200	1978-1990		WH-3, 14, 18, 20, 22, 26, 28, 31, 39, 46 with 2 needles and seats
200	1994-1995		WMH-34/39
200	1996-1997		WMV-5
225	1980-1981		WH-15 / 20
225 3L	1994		WMH-19A
225 3L	1994 1/2		WMH-46
225 3L	1995		WMH-47
225 3L	1996		WMV-7
225 3L	1997		WMV-13
245	1996		WMH-32/36

Jet sizing and part numbers for Mercury

Jay Smith on the subject of correctly jetting a motor

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 encounters 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. Another thing, 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 ! - Jay Smith of Jay Smith Racing Engines

Always be conservative and build a cushion as you NEVER know what is in the gas pump at the local gas station. They have maybe just a load of 87 octane in the 91 tank.The cushion of 1 to 2 jet size may pull off a couple of HP BUT may save you a piston if the above mentioned thing occures.