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1996 25 hp Johnson power increase with Carb

U

U.S.S.GUPPY

New member
Hi guys,
Does anyone know if there is a difference other than the orifice sizes and float valve between these three carbs. (439380, 439377, 439378). Ive done the research and all three carbs come from a mix of years and engines sizes from 25-35 hp. all have matching part number for bowel, gaskets, carb cover, fittings and ext. They look the same but I'm not sure if the body is different or if the venturi is different between the models. :confused:
 
I know that I need to match the intake and the bypass cover to the larger carb, but I am unable to find a clean used 439377 carb which is the recommended one for my motor. I can find a couple of the other model numbers but I don't want to buy a 25 hp carb that looks like the 30-35 hp carb. If the venturi is identical and the body is the same then I can replace the orifices to match the setup I want and also to fine tune. Any input is greatly appreciated!
 
I have looked up the part #s for all of the carbs. The orifices and float valve are the only different part numbers that I can find between the carbs. The research that I have done shows that if I change the bypass cover it will change the timing for the increase of HP. the only difference in the intake is the size in the opening. I can machine that to match the carb. I have ordered a 28 hp Carb that I found for 40$ once I get it in my hands I will be able to determine if the only difference in the 25-35 carb is jetting. The body of the carb does not have a listed part number which leads me to believe the are all the same for the three carbs (439380, 439377, 439378). Hopefully I have get my hands on the 439377 and take some measurements to see if the venturi is infact the same size on all models.
 
I have looked up as many forums on this discussion that I could. I'm honestly new to the 2-stroke world and don't completely understand how they function. But I do know that the 20-25 and 30 shows a different part number for the bypass cover for the 1996 J25Teleda. In multiple forums people have stated that putting the the correct bypass cover on will change intake timing. With that being said I crossed referenced the bypass cover part number from the 25 and the 30 and Johnson replaced the two numbers with with 0439912. So they must be the same or the change is not significant enough for such a small change in HP. If you could tell me the purpose of the cover that woul help me.
 
Honestly I'm just trying to get a little more power from my 25 hp without buying a new motor. My Boat is only rated for a 35 and already gets 32mph with two people and runs great any small gain for a couple hundred dollars would be huge.
 
I was under the impression the difference in the 25-35 carbs was just the venturi opening.Intake opening should be the same.I see people upgrade the 25's to 35's by just changing the carb.Not sure of any other difference, but maybe there is?
 
The bypass cover is there to machine the intake ports on the cylinder. You could probably just put a manifold and carb off of a late 80s 35hp and gain a couple ponies. Have you checked WOT rpm? Sounds like your pushing a fairly light rig you may gain a little speed with more prop pitch if the rpm is up over 5500 wot rpm.
 
There is a different intake part number from 25 hp to 30 hp in the 1996 model. I've honestly heard both that there is no difference and that there is a difference in the intake opening. The 1996 25 hp carb is completely different model from the 30 hp and definitely has a smaller venturi, you're right. The 28 hp 30, and 35 hp carbs all seem to have the same main frame body and vetnuri with different combinations with jets. Its hard to get straight answers because if you look up the engines part numbers so many of these parts are interchangeable and some of the 25 hp engines come stock with 30 hp parts. The 30 hp carb also has the same part number for carb to intake gasket as the different model 25 hp carb, which leads me to agree with you that the intake opening might be the same size. I'll just have to size them up and see what happens. I bought a 28 hp carb. I'm going to buy the jets that match the size for the 30 hp carb that is recommended for my engine and hopefully get the extra power. I'll record the results.
 
The powerhead block crosses with a 1988 35hp same with the exhaust tuner. The only difference is the intake port and carb. For the 1989 specs in my book the 28hp has the same WOT rpm as the 25hp. Keep a eye on the spark plug color and WOT rpm you may need to change prop pitch for the best performance. As long as the plugs burn a nice toasty brown you should be fine.
 
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