2.5 two-stroke OB

[Georgio]

When buying my faithfull Merc 2.5 two stroke about 4-5 years ago I did a really stupid thing - I SHOULD HAVE BOUGHT THE 3.5!!!


I realise it's the same basic engine so how hard/expensive is it to upgrade it to a 3.5, would it need a whole new carb?

Am I waisting my time...

 

[misterg]

This information is based on the Mariner 2.2 / 3.5 engine, but I believe it to be identical to the Mercury ones:

It's restricted by the throttle slide being over-long for the carb body, so the throttle can never open fully (look into the carb & see). The 3.5 has a different (shorter) slide that will open fully, and some differences in the throttle lever / fascia plate to allow the full travel. You can cut down the slide on the 2.2 (2.5) so that it can open fully, and a bit of judicious bending will allow it to do this with the existing lever / fascia plate.

Unfortunately, none of this gives you the biggest advantage of the 3.5 which is the gearbox /forums/images/graemlins/frown.gif

Having done it once, I wouldn't bother again - Pushing the rubber duck, it just makes more noise without going any faster.

Andy

 

[Georgio]

hmm, not sure if that is the same as mine. The throttle lever on the front will only go from the slow setting to 70% of the way to the fast setting (hope that makes sense). Under the carb I can move the linkage up and down without using the lever but it stops at the same place so I don't think the throttle lever length/shape will make any difference.

Looks like a carb or jet change may be required?

2.5 to 3.5 that's a 40% increase so it should make a small light dingy a bit more fun....

 

[misterg]

[ QUOTE ]
Under the carb I can move the linkage up and down without using the lever but it stops at the same place so I don't think the throttle lever length/shape will make any difference.

Looks like a carb or jet change may be required?

2.5 to 3.5 that's a 40% increase so it should make a small light dingy a bit more fun....

[/ QUOTE ]

No, no, no.... That's the whole point /forums/images/graemlins/smile.gif - the actual slide inside the carb is too long to allow it to open fully - it hits the top of the carb (the bit that unscrews) before getting to full throttle. You would need to shorten the slide (saw about 6mm?? off the top) so that it can rise far enough to get full throttle (or buy the slide for the 3.5). Then you'd have to fiddle with the lever to make sure it moves the slide through the full range through which it can now move.

If you unscrew the cap on the top of the carb, you can raise the slide up until it gets to fully open (look into the carb - full throttle is when the whole venturi is open - i.e. when the bottom of the slide makes a nice smooth shape with the carb venturi.)

Once you do this, you can see how much the slide needs to be shortened - check the top in case there is a spacer, or other easily removable piece there, but in my case, the whole brass slide was in one piece, and needed to be sawn off.

Not still under warranty, is it ? /forums/images/graemlins/grin.gif

Someone (probably VicS /forums/images/graemlins/wink.gif ) posted a parts diagram for these engines here a while ago, and the only difference on the carb was the throttle slide, fascia panel & maybe the lever. The jets, carb body, etc. were otherwise common across the different power ratings.

We used ours on a 2.4m slatted floor inflatable dinghy, and more power just seemed to make it more difficult to steer, and make more noise... /forums/images/graemlins/ooo.gif


Andy

 

[Georgio]

Thanks Andy, what you said makes sense now. Since posting I have found a complete s/hand carb from a 3.3 so I will fit that and see how I get on, If all else fails I can put the original one back on.

Thanks for your help.

A parts diagram/exploded diagram would he handy if you know where to get one on the interweb? I did a google but without luck...

You are correct, no longer under warranty.