Mercury 2.5 2T carb

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Heckler

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Took mine off the other day to clean the jets to cure a hesitation. Noticed that if you open the throttle fully, the throttle slide only goes up halfway, so it only ever runs at max, on half throttle. Now the urban myth is that you can "remove a restrictor plate" and then the engine puts out 3.5 HP. But there isnt a restrictor plate.
I then investigated further, the slide is very long, it tops out so that it is impossible to open the throttle fully. The only way to get full throttle is to cut the top off the slide! Am considering this!
Stu
 
That really needs to be tried. I've got the Mariner version in my shed and it's going to be a little iffy for the intended application - another horse would be much appreciated. Please carry on and report back!
 
Does the control linkage offer more movement that the throttle slide allows? If it's not compressing a spring, say, when the throttle jams at the halfway point, it may need some linkage modification as well to get a fully open throttle. As I have a Mercury 2.5 I'm interested to hear how this goes ...
 
skipper_stu said:
Took mine off the other day to clean the jets to cure a hesitation. Noticed that if you open the throttle fully, the throttle slide only goes up halfway, so it only ever runs at max, on half throttle. Now the urban myth is that you can "remove a restrictor plate" and then the engine puts out 3.5 HP. But there isnt a restrictor plate.
I then investigated further, the slide is very long, it tops out so that it is impossible to open the throttle fully. The only way to get full throttle is to cut the top off the slide! Am considering this!
Stu
Click to expand...

You may be onto something .

AFAICS the only parts of the carburettor that are different between the 2, 2.5 and 3.3 hp models are the throttle valve ( or slide as you call it) and the inlet cover (# 10 and #41 in the diagram below

BUT the cylinder blocks and crankshafts are different too so just modifying the throttle valve on its own is probably not going to upgrade the engine by as much as you hope. I have not checked other parts eg gears and prop.
9dxlS3O8.png
 
I have read posts in which the mod has been done successfully, but believe the jets need to be upgraded too, or it'll run too lean flat out. Not good for a 2T!
 
No Regrets said:
I have read posts in which the mod has been done successfully, but believe the jets need to be upgraded too, or it'll run too lean flat out. Not good for a 2T!
Click to expand...

I've just spent 2 days stripping down and rebuilding a 1980's Zenith carb with a new jet/refurb kit. I had to go through the entire rebuild/re-jet/float-height process 3 times before I achieved an engine which starts well and runs smoothly throughout the entire rev range.

I made the schoolboy error of changing many too variables at the same time .... and then not knowing which change was throwing the whole thing out of whack!

It was worth it in the end though, as the top speed of the old girl has gone up from 60mph to 70mph. :)

If you start chopping down slides, altering throttle linkages and changing jets Stu, make sure that you have plenty time on your hands! :o

Richard
 
VicS said:
You may be onto something .

AFAICS the only parts of the carburettor that are different between the 2, 2.5 and 3.3 hp models are the throttle valve ( or slide as you call it) and the inlet cover (# 10 and #41 in the diagram below


BUT the cylinder blocks and crankshafts are different too so just modifying the throttle valve on its own is probably not going to upgrade the engine by as much as you hope. I have not checked other parts eg gears and prop.
9dxlS3O8.png
Click to expand...
Youre good you are, I googled all sorts to see what the parts differences were. The front plate I figured would need modifying as well because the lever would need to go higher to lift the slide higher. A hacksaw blade would fix that as well. As for the main jet, a bit of suck it and see.
Would be interesting to see if the part number is different for that?
Stu
 
skipper_stu said:
Youre good you are, I googled all sorts to see what the parts differences were. The front plate I figured would need modifying as well because the lever would need to go higher to lift the slide higher. A hacksaw blade would fix that as well. As for the main jet, a bit of suck it and see.
Would be interesting to see if the part number is different for that?
Stu
Click to expand...

You did not give a year or serial number so a bit difficult to check everything across several serial number or year ranges . The ones I did check used the same main jet, and needle, across the HP range
 
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I have the 3.3 Mariner and 3.5 hp Tohatsu versions both with clutch: these Mercury/Mariner/Tohatsu engines share all main mechanical components. I find it hard to believe they build different blocks and cranks for a minor power change from 3.3/3.5 to 2.5 hp. I know some older Johnson/Evinrude 4 hp and 6 hp two-strokes are also the same basic engine.

You can start one with the "control panel" part 41 removed, as long as you keep the kill-cord switch connected - that would both remove any airflow restriction and might allow you to open the throttle slide fully.
 
jwilson said:
I have the 3.3 Mariner and 3.5 hp Tohatsu versions both with clutch: these Mercury/Mariner/Tohatsu engines share all main mechanical components. I find it hard to believe they build different blocks and cranks for a minor power change from 3.3/3.5 to 2.5 hp. I know some older Johnson/Evinrude 4 hp and 6 hp two-strokes are also the same basic engine.

You can start one with the "control panel" part 41 removed, as long as you keep the kill-cord switch connected - that would both remove any airflow restriction and might allow you to open the throttle slide fully.
Click to expand...

I cannot comment on your Mariner and Tohatsu but as far as Stu's Merc is concerned when I looked at the parts catalog i found different part numbers listed for the components I mentioned and a few other minor components I did not mention I don't of course know what the differences are but I am sure that if they list them with different part numbers in the same list there must be some differences

As far as 4 and 6 hp two stroke Evinrudes are concerned I have a 6hp it is a 164 cc engine while the 4 hp is only 87 cc! The 8 hp however is a 164cc engine. I looked into the differences between the 6 and 8. IIRC different carb jets, which could of course easily be changed, but looking into it further I found that the manifolds, reeds and crankcase where different. Presumably the porting is different between the 6 and 8 HP engines
 
VicS said:
I cannot comment on your Mariner and Tohatsu but as far as Stu's Merc is concerned when I looked at the parts catalog i found different part numbers listed for the components I mentioned and a few other minor components I did not mention I don't of course know what the differences are but I am sure that if they list them with different part numbers in the same list there must be some differences
Click to expand...

It seems likely, but it may be simple obfuscation. The windscreen for a Triumph Spitfire is precisely the same as the windscreen for a TR4 or TR5, but has different part numbers for the two applications. It cost about three times as much under the TR number. Similarly, I am told that parts with Porsche part numbers for the Cayenne cost a lot more than the identical parts with VW numbers for the Touareg.

Do the 2.5hp and 3.5hp engines have the same bore and stroke? If so, it's hard to see why the crank and cylinder would need to be substantially different.
 
skipper_stu said:
Took mine off the other day to clean the jets to cure a hesitation. Noticed that if you open the throttle fully, the throttle slide only goes up halfway, so it only ever runs at max, on half throttle. Now the urban myth is that you can "remove a restrictor plate" and then the engine puts out 3.5 HP. But there isnt a restrictor plate.
I then investigated further, the slide is very long, it tops out so that it is impossible to open the throttle fully. The only way to get full throttle is to cut the top off the slide! Am considering this!
Stu
Click to expand...


Urban myth? Perhaps, a lot of stuff is repeated in this place and becomes - "known fact".
I am sure that if you doctored your carb, or bought new parts, you could ginger up the performance of your 2.2. But I also think:

The needles are different as well - at least on later models
The final drive ratios are different.
The crankshafts are different with the more powerful engine being supplied with a splined shaft to cope with the extra output.

Plus the obvious differences in the water pump and gear mechanisms.

This is an excellent resource:

http://www.outboard-engine.com/searchbymanufacturer.php?recordID=TOHATSU
 
VicS said:
I cannot comment on your Mariner and Tohatsu but as far as Stu's Merc is concerned when I looked at the parts catalog i found different part numbers listed for the components I mentioned and a few other minor components I did not mention I don't of course know what the differences are but I am sure that if they list them with different part numbers in the same list there must be some differences

As far as 4 and 6 hp two stroke Evinrudes are concerned I have a 6hp it is a 164 cc engine while the 4 hp is only 87 cc! The 8 hp however is a 164cc engine. I looked into the differences between the 6 and 8. IIRC different carb jets, which could of course easily be changed, but looking into it further I found that the manifolds, reeds and crankcase where different. Presumably the porting is different between the 6 and 8 HP engines
Click to expand...
I think the difference in the crank etc might be to do with the fact that the big ones have a neutral, so the end of the crank may be different.
Stu
 
skipper_stu said:
I think the difference in the crank etc might be to do with the fact that the big ones have a neutral, so the end of the crank may be different.
Stu
Click to expand...

2.5s have a square driveshaft and square ctank end. 3.5 due to having impeller on driveshaft are round and splined crank end. Bore and stroke are the same. Difference in engine blocks are the length of lugs where the cdi unit bolts on to. Hope this helps
 
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