Can I put gt120 carbs on my gt100?

[smkur50]

I have heard that the only difference between the 1989 GT100 and GT120 is the carbs? Is there anyway to modify my carbs to 120hp specs or can I just buy some GT120 carbs and bolt them on? If so what years of GT120 would work? Anyone know where I can get the parts?

Thanks.

 

[Dhadley]

Re: Can I put gt120 carbs on my gt100?

Never really heard of a GT120 but a 120 looper is a whole different motor than your 100 crossflow.

 

[Chinewalker]

Re: Can I put gt120 carbs on my gt100?

I've never heard of a GT120. The only 120s I'm aware of were big block loopers. The looper carbs will not work on a crossflow V4, which your GT100 is.

The GT100 has the aggressive porting of the 115 crossflows (post 1985, same as a 1984 & prior 140) but lacks the bubble-back exhaust tuner. I am using a 1988 GT-100 block as the basis for my 1982 140 build-up.

So, bottom line would be to find some 1 5/16-inch venturi carbs and add a bubble-back exhaust and you should gain about 15-horses. Whether that translates to much speed depends on your set-up and where you're running now.
- Scott

 

[smkur50]

Re: Can I put gt120 carbs on my gt100?

I see, there might not be a GT120. Someone told me there was but I can't find it on any searches. Basically I have 89 hydrasports x260 bass boat, I was a bit disappointed with how fast it really ran after I clocked it. 39mph. It has the factory prop which I am guessing is 13 1/4 x 15 and it reaches top speed at 5700rpm. I am wanting more top end, so I figured by the prop calculator that a 13x21 prop should hopefully get me close to 48-50mph, or is that too much to expect? I was wondering if the extra 15-20hp might get me mid 50mph speeds which would suffice me for a while. I'm used to working on big deisel trucks so the outboard stuff is somewhat new to me.

 

[Chinewalker]

Re: Can I put gt120 carbs on my gt100?

Your motor is already in the sweet spot as far as RPM goes. Adding 6-inches of pitch will HURT your performance, not help. It's akin to going the speed limit, towing a trailer, up hill, in third gear and dropping it into fifth and expecting the maintain your speed... Also, don't guess - remove the prop and read what is stamped on the hub.

I'm not familiar with you boat. Length? Weight? You might gain some speed by adding a jack plate and raising the motor upwards. The addition of extra height MAY allow you to go up to a higher pitch propeller.
- Scott

 

[Dhadley]

Re: Can I put gt120 carbs on my gt100?

Judging from those numbers I'd bet it doesn't have a jackplate. Those boats will benifit greatly from one. However, if you're looking for that kind of speed I'd suggest a 60* 150 (1993 & newer). The V4 looper might do it but the 150 looper would do it much easier. No need to re-rig the boat, everything will work as is.

 

[smkur50]

Re: Can I put gt120 carbs on my gt100?

The x260 supposedly weighs 800lbs dry. add the 2 trolling batteries, trolling motor, motor, 2 people...I am guessing 1600lbs. There really isn't a whole lot to this boat. It's 16 1/2 foot long. No jackplate. I want the best bang for the buck. Would a 6' jackplate be good and a 12' jackplate be too much? I would just love to get an extra 10 or so mph out of the boat. I hear the stainless props give an extra 2-3 mph but if a $250 jackplate will give me more, then I wanna spend the $ wisely. The boat is rated for 120hp max. I would put a 150hp on the boat to make it really quick but what are the consequences of that? Too much weight on the rear of the boat or what?

 

[Dhadley]

Re: Can I put gt120 carbs on my gt100?

The weight difference between a 90* 120 / 140 looper and a 60* 150 / 175 looper is slight. Stay within the 120 hp spec on that boat.

If it were me I'd get a 6" Bob's convertable jackplate. I'm sure you'll use the full 6" with the crossflow. You can buy 2" spacers so now with one plate you have 3", 5", 6" and 8" setback options.

With the plate you'll probably need to go to a ss prop for the most efficiency.