115hp/1150 convert to 135/1350 help

I have just heard that an old 115hp can be turned into a 135hp. I have a 1972 1150 thunderbolt that I have to rip apart to find a tapping noise that was recomended to me as a possible piston slap noise! but while I have it apart and am spending the time and money on it I thought I could turn this thing back into what is was made for originaly, a 135/1350. Is this a doable thing and if so what do I need to do it. my boat is rated for 140hp so 135 should be just right. also would you recomend and ignition upgrades that can be made.

I want this thing to live up to its name as the tower of power!

Re: 115hp/1150 convert to 135/1350 help

If it's a true 1972 (Blue Band) you can't easily convert.

'72 and newer blocks with the thick, split-type exhaust adapter plate and (2) rectangular exhaust outlets built into the bottom of the block, to match up to the plate, get their HP increase from larger ports.

Only a '71 or earlier Red Band 1150 can be readily converted to 1350 specs by installing wider reed valve stops (or swapping out the reed blocks for ones with wider stops), changing the plastic carb venturi's, and installing larger main jets (.0785").

The intake & exhaust ports on these Pre-72 blocks are huge, and the 1150 is basically "HP-Restricted" by the "skinnier" reed stop openings, "fatter" carb venturi's, and smaller main jets.

If yours is a later-model 1150, you'd either need to get the block ported (not cheap) or find a 1400 block. Or, find a 1500 block and install power-ported pistons to go with. And of course re-jet the carbs while you're at it. Even better if you can get the carbs that went with the higher-HP motor.

Far as ign goes the smaller ign coil which mounts to the block cover is a worthy improvement over the larger cowl-mounted coil. Not necessarily more spark energy, but a lot easier to work around.

Make sure your dist cap is clean inside, rotor & plug wires in good cond, and distributor bearings nice & smooth. If there's any signs of roughness in the bearings I'd renew them. Check out the snap ring under the copper "chopper" disc, if this corrodes & shatters (as many many have done), the remnants will grenade the rotor and trigger, rendering your very expensive distributor useless! All for a cheap snap ring! Mine broke in the middle of summer one year, luckily the local boat salvage yard had plenty of cheap parts.

Dist parts for these are neither cheap nor plentiful anymore, so if you can find some spares at a good price off Craigslist etc, more's the better!

P.S. the best way to get optimal performance from any motor is to make sure it's properly set up on the boat, tuned & propped correctly. Once you establish a baseline for performance you can look into stainless etc performance props and that'll add quite a bit.

But you never know when your next Inline will be lurking on the Craigslist. I'm trying to resist one right now but not doing so good with that!

HTH.........ed

Re: 115hp/1150 convert to 135/1350 help

Please post your serial #.

Re: 115hp/1150 convert to 135/1350 help

1972 mercury 115hp/1150 thunderbolt ignition SN# 4116865. I hope this helps. also any tips on just rebuilding, like things to watch for or things that should be replaced no matter what, Thank you all for your input

Re: 115hp/1150 convert to 135/1350 help

Measure the cylinder for wear and if over .003 out of round, bore to next size, install the 10/10 reed blocks,restrictor mod's,and if you can find em a set of WMK-19 or 22's carbs(150hp) or WMK-4(135hp) or WMK-25 or 29's(140hp but will have to change throttle arm on carb)replace red coil with black one,

Re: 115hp/1150 convert to 135/1350 help

Your Ser # translates to a 1975 model.

All that's gonna happen if you bolt a set of 1400 or 1500 carbs on your motor is that it's going to run worse.

This block has smaller intake & exhaust ports than the higher-HP motors and carbs (or reed mods) aren't gonna make up for that fact.