More HP+TQ from omc V4

Can any one reccommend a larger jet size for my 1980 100h evenrude. Its been completely rebuilt .030" over. I have ported (removed access material) the intake already. It has #25s in it and someone once told me either newer or older heads would pick up a few HP can someone please clarify. The boat it is on is a 1990 19.5' sea nymph Alluminum of course. The boat is rated for a 140hp. And also could you guys reccomend a prop that will run good the one on it is shot and I dont know what size it is or what to get. All of my experence has been with 60hp and lower im in the blu on this one. Also just found this site, Great site deffinatly an aggravation reducer Thanks, Paul.

Re: More HP+TQ from omc V4

Almost forgot, I think the twin 2 barrles are non Fuel/air mix ajustable, will different jets screw up the mixture

Re: More HP+TQ from omc V4

Re jetting won't do anything other than make the engine run rich. You must also increase the carb size if you have "truly" ported the engine (this means increasing port size and location). If you have jus "polished" or deburred the ports, nothing is neccessary as far as carbs go. The carbs are matched to a mass air flow, the more air, the more fuel also till they reach thier peak mass flow at a given manifold pressure. Another consideration is the reed induction manifold, more HP can be gained with modifications here too. The heads will help but be carefull of your timing, could cause detonation when you raise the compression. Heat of plugs also comes into play here too. One of the best things you can do is run Boyesen Reeds. This alone will give 200-300 RPM at the top end and better idle with better acceleration.

Re: More HP+TQ from omc V4

might have a point with the older heads. Older engines ran with higer compression, and might wring out more of the burn energy of the fuel. Does that engine have the 3.5 inch pistons? maybe you can get carbs for a greater hp engine like a 135 if the block is the same.

Re: More HP+TQ from omc V4

Lets start with the basics to make sure you are getting the most out of what you have. Whats the top rpms now? Whats the water pressure?<br /><br />Good luck!

Re: More HP+TQ from omc V4

I dont know top RPM or water pressure I have no gauges. I think a previous owner moded the reed induction manifold. I dont know what a stock one looks like. There are semi circle cuts in it. If they are left fresh cast from omc they have been modified. It looks as though someone went at it with a mill. It has brand new reed valves I dont know the manufacturer. Is there a way to tell the manufacturer. So buying new jets is pointless? I had asumed that the .030 overbore would create a few more ccs which might run stronger with bigger jets. No big deal Im just trying to get a few more MPH. If I polished the carb throats would that help flow enough air to go up a jet size or two. Also I have excess to a mill can I bore out the throats without ruining the carbs? And if so how much? I dont know what to do to the cylinder ports im afraid of ruining my engine. Also can If I put carb risers on the manifold with that increase the Fuel/air velocity enough to give a few extra. I also heard that takeing the airbox off leans the engine DANGEROUSLY if I jet the carb will removing the airbox compinsate. Thanks for the help!

Re: More HP+TQ from omc V4

Also guys i am doing this partly because i enjoy working on and modding engines i quess you could say a hobbie, I just have no clue about V4s

Re: More HP+TQ from omc V4

One often overlooked rule of hopping up a motor is that what goes in, must come out. You can add all the carbs and intake ports you want, but unless you also address the exhaust side of things, you're going to drown it. The exhaust manifold and tuner from an old cross-flow 140 will go a long way towards letting her breath a lot deeper...<br />- Scott

Re: More HP+TQ from omc V4

I figured because the exhaust flowed all the way down the lower unit and came out the hub the back pressure would stay the same anyway. I am just theorizing though, If im wrong it wont hurt me a bit OK this is what we have so far. I dont know but I thought backpressure was a good thing with a two stroke<br /><br />1 Bigger jets without bigger carb, run rich with no gains <br />2 The Reed IM is ported and the intake has been completly deburred.<br />3 older higer compression heads would help <br />4 An old 140 crossflow exhaust manifold would help<br />5 I can bore the carb throat out and that would help with the jet job <br />6 The reedvalves are brand new so i will stick with them regardless of brand.<br />I still think if i took off the airbox i could jet the carb, any thoughts. <br />I dont know how to change the timing can you guys please explane.<br />Thanks again

Re: More HP+TQ from omc V4

Bigger bore carbs may help a little on the top end but you'll pay for it coming out of the hole.<br />Are the reeds metal or fiber? If they're metal, get yourself a set of Boyesen reeds and follow the installation instructions to the letter. It'll idle better, accelerate better and pick-up 300-400 revs on top end.<br />I've ran with and without the intake air box and can't tell any difference. <br />As far as exhaust, I'm attaching a website with a animated, running two cycle engine. It'll tell ya a bunch about exhaust.<br />If you exceed the manufacturers timing spec, you're gonna start holing pistons.<br /><br /> Running Engine <br /><br />c/6<br /><br />Hooty

Re: More HP+TQ from omc V4

DHadley's posted on milling the manifold divider down to join the passageways side to side. Pick some free hp, still idles well. Search the archives.<br /><br />He knows his stuff on these V4s, BTW, and he's right about in looking at your setup first. You want it propped to turn 5500+ rpm, trimmed at the best angle, and mounted as high as you can without losing water pressure or blowing out (cavitating). You find these points by experimenting. There's cheaper speed there than anywhere else.