115 1976 V4 OMC

[dsinger]

I have a 115V4 but I found some 135hp V4 Carbs I know they fit but will I gain some hp. I have read several things on this some say yes and others say no. I did this on a 1987 90 v4 and it worked well as well as a 80is suzuki. Was just wondering.

Also was wondering about a 35hp v2 replacing it with a 50hp v2 carb.

 

[Chinewalker]

Re: 115 1976 V4 OMC

There's a lot more difference between 115 and 135 than the carbs. The 135 has bigger intake and exhaust ports compared to the 115 and a much bigger exhaust cavity with the "bubble back" exhaust. The old adage that "what goes in must come out" holds true.
- Scott

 

[bowrider]

Re: 115 1976 V4 OMC

So is this a direct swap?

 

[emdsapmgr]

Re: 115 1976 V4 OMC

Your assumption is generally correct. When swapping V4 carbs: you want to increase the throat opening. Usually, more air/fuel=more horsepower. If you are removing carbs that are 1 " throats and installing 1 5/16" throats, you should make more hp. (Look on the front flange face of the carbs-the size will be cast into the flange.) This is bolt-on h.p. Having said that, it would be unusual to have them work perfectly right out of the box. Since you are not installing 135 hi comp heads, exhaust system and changing the block porting, a direct swap with the larger carbs can cause some running issues. While they should run fine, the larger carbs may flow too much fuel and you may not achieve all of the higher hp you would like. I have had to rejet most big carbs in order to get them to work on lower hp engines. (usually have to re-jet downward somewhat.) Summary: you should make more hp, but you will have to tinker to get them dialed in. I have felt the efforts to do so were worth the time spent.

 

[dsinger]

Re: 115 1976 V4 OMC

Well I will find out I guess this weekend. So if I understand right the intake and exhaust will keep me from gaining any horse power. What will happen if I change the carbs out nothing or a little hp jump.

 

[ezeke]

Re: 115 1976 V4 OMC

The exhaust cover system on the 115 will not keep you from gaining extra horsepower. Rather, It will not provide adequate cooling for the extra heat.

 

[Dhadley]

Re: 115 1976 V4 OMC

With larger venturi's you should, in theory, lose a little bottom end & acceleration. The lower your top rpm is now the more that will be true. The higher your top rpm is now the less that will be true.

With larger venturi's you should, in theory, gain some top rpm. The lower your top rpm is now the less that will be true. The higher your top rpm is now the more that will be true.

 

[Dhadley]

Re: 115 1976 V4 OMC

And your hull design will play a big part in that in how well it responds to small increases in hp and / or torque.

A hull like an Allison will respond more than something like a traditional ski boat hull design. Which will respond more than a deep V or a tri hull. On a barge, you'll probably only notice the gas bill.

 

[dsinger]

Re: 115 1976 V4 OMC

What part of the exhaust will I need to change out to get the max performance out of the new carbs.

 

[emdsapmgr]

Re: 115 1976 V4 OMC

392000 is the part number for the exhaust filler block kit. When installed, the back exhaust deck must be re-machined flat on a mill. Also, a new inner and outer exhaust cover must be installed. This is a big job, requiring powerhead teardown and subcontracted machining. Considering you probably have a low port block to begin with, (not a 135/140) this upgrade may not be worth the time and money. In addition to the carbs, consider two other bolt-on things will help your hp quest: A set of 135 or 140 heads and intake filler blocks. Depending on which 115 you have, it may not have them. When you do the carbs, it would be to your advantage to add the 135/140 blocks behind the intake manifold. Part number 322684. You need 4. They are NLA, but can be found.

 

[Dhadley]

Re: 115 1976 V4 OMC

You need to look at your current set up. If you're set up in the mid 5000 range none of this is going to show much improvement at all. A lot depends on what type hull you have. You might gain the most by adjusting your current set up.

If you're looking for the most gain with the least effort, there's a ton of untapped power in the intake manifold.

 

[dsinger]

Re: 115 1976 V4 OMC

Tri hull 17.5 foot only getting around 3400 at WOT. Manual tilt. What can I do to the intake manifold.

 

[Solittle]

Re: 115 1976 V4 OMC

Well you are about 2,000 rpms under where you should be. First step is to get what you have running right. Start by doing the basic troubleshooting.

 

[Dhadley]

Yep, you're a long way from worring about modifing your motor. To be that far off you're probably only running on 3 cylinders. Like Solittle says, start with the basics. We gotta know where we're at before we do anything else. Compression, spark check, etc.

 

[dsinger]

Re: 115 1976 V4 OMC

Sorry I was wrong 5400 at WOT the compression is good 110 on all cylinders I rebuild the powerhead about one year ago. This is a motor I have had for along time and tinker around on it quite a bit. It was at around 5000 until I played with the prop on it. The boat had a 80is 115 on it with T&T and I could get around 55 to 58mph out of it but when I changed it to this motor which is really only about 90hp at the prop really was a let down not to mention no T&T. The RPMs are right on because I tested my tact with my snap on digital light. I will have the carbs around midweek I am still unsure if it is worth even bolting them on or not.