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1985 merc 18hp

L

lee m

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just bought one of these in very nice shape but am wondering about boosting it a bit. I have a 1988 and a 93 25 horse that have scored cylinders but was wondering if the carb would fit onto the 18 horse to turn it into a 25. does anybody know or is there a conversion kit. I havent got the two together yet to see but have heard that the 18,20 and 25 are all the same just better carburation. Also would the powerhead from the 18 fit into the 25 engine. I know they went to nicosil coated bores in the aluminum blocks in 1988 but the pre 88 powerheads might go into a 88 to 94??
 
No the 35 hp parts won't fit. You can turn it into a 25 hp by getting the carb and exhaust tuner. Here are the differences bear with me it is confusing. The 18/20 hp share a carb but the exhaust tuner is different. The 20/25 share the exhaust tuner and the 25 has it's own specific carb. The blocks pretty much stayed the same all the way through just how the they were plated. The ignition is the big difference around 93/94 they no longer had a mechanical advance so the tiller set-up is different and the carb advance arm is different.
 
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Didnt think it could be that easy. One thing though don't think it will change anything - wasn't a 35 hp i was trying to rob the carb from. It is a 1988 25 hp. Looks like I should just look for a parted out 85 25 hp and get the carb or do you think a merc dealer would have a carb/exhaust tuner or is there a jobber outfit you would recommend. One last thing - the blocks are all the same through those years so conceivable the powerhead from the 18 might fit the 88 25hp? Thanks for your help and explanation.



flyingscott said:
No the 35 hp parts won't fit. You can turn it into a 25 hp by getting the carb and exhaust tuner. Here are the differences bear with me it is confusing. The 18/20 hp share a carb but the exhaust tuner is different. The 20/25 share the exhaust tuner and the 25 has it's own specific carb. The blocks pretty much stayed the same all the way through just how the they were plated. The ignition is the big difference around 93/94 they no longer had a mechanical advance so the tiller set-up is different and the carb advance arm is different.
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Yes the powerheads interchange just drop the 18 hp powerhead on the 25 hp lower it will have the exhaust tuner you need. Then put the 25 hp carb on it. Sorry a mistake on my part about the 35 hp. It is an easy conversion to do. The blocks are not quite the same there is a design 1 and design 2 one has a thermostat one doesn't. So make sure you keep the serial # for the powerhead and the lower for when you need parts. Also make sure you make sure all the timing parts from the 18 hp stay with that block. I will double check later today to make sure there is not a timing change to make.
 
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