Mercury 2.5 variations

[leifbert]

Years ago I bought a 1992 Mariner Magnum III 2.5 litre 150 for a great price thinking I could make some easy horsepower. I drilled exhaust reliefs, threw away the can, swaped in a bob's tuner, velocity stacks and Boyesen reeds. End result... It runs exactly like it did before only much LOUDER. The top carbs say WMH 2241 and the bottom is WMH 2251. The carbs are clearly necked down to a smaller size at the throttle blade. My question is: What is the difference between a carb 2.5 150 and a carb 2.5 200?
If the porting is different can it be done by just pulling the heads. I am an experienced mechanic but i know nothing about 2 strokes. The motor runs great but it really lacks power. In fact, the 2.0 ported 150 I ran on the boat previously had a little more power. Thanks for any advice.

[O/B Dude]

I have never had a 2.5 150 but know a litle bit about them. Here goes... Its my understanding the porting, exhaust chest volume, exhaust outlet, tuner, and carbs are the difference. You cant port it without full disassembly, and it would cost more to do that than selling the powerhead and buying a used 200. What you can do is open the exhaust passages in the bottom of the block where it meets the exhaust adaptor, open the adaptor passages also. Use a 200 gasket and the Bobs tuner as a template. Then get a set of 200 or 245 carbs, cut the heads for 145 psi, and you should see a noticeable gain. I think I read that some 2.5 150's have the smaller restricted exhaust passages and some do not. I'm really not sure. Maybe someone else can jump in and clarify?
The mods you did should have made it idle better and have snappier throttle response but no real world speed gains. Also, how is the jetting? V-stacks and (sometimes) a tuner change can increase flow enough to warrant jetting it up.

[steve]

Also have the compression relief slots in the exhaust ports. If yours has them , don't bother trying to mod it . Sell it. You'll have to pull a head or use a small probe mirror/ light to see if they are there. The 200 is an easier place to start in any case IMO. Do you have a set of 4 petal rubber coated reeds yet? Have you milled the heads at all to increase compression? :cool: