Fun with 25hp to 30hp conversion

Hope I do not get beat up to bad on this thread.I know this subject has been brought up many times and I found plenty of info on it, but not anything on this question..Acouple of years ago I converted my 20hp(J20CRCOS) to 25hp by changing out the intake and it has been running fine.Last year I bought a 30hp(E30RCCE) very cheap for a parts motor.I know the intake on the 30hp has more area on the reed side compared to the 25hp.I wanted to experiment just for my own knowledge to see if just putting the nice clean 30hp carb. on the 25hp intake would work.Timing was stamped at 30* BDC on both motors.Set link and sync and set idle /low speed screw up and hit the water.Got the motor started easily and it idled just like the 25hp carb and thought it would be good to go.Had boat on trailer backed in while setting the idle, then tried to go for a run.Everything seemed great until I tried going from about 3/4 to full throttle and it fell on it's face.Did not stall just bogged way down like it wasn't getting gas or too much air.Not sure? and it did this repeatily.Would "Not" changing out that intake cause this.?Just wanted an opinion on this.I did see the CDI has a different # also than the 20hp.Was not sure if that was my isue either?Next move would be to swap the intakes and or the CDI and try again.

Nice report, I appreciate your experimentation. I like to do what I'm not supposed to do all the time. I don't think the CDI pack will change things. Probably gonna have to change or modify the intake. Is the old 20 hp powerhead your working with in good condition compression wise?

after doing parts break down comparison, looks like intake and both gaskets are the same, reeds and exhaust tubes also, in the carbs, the high speed jets a substantially different, 47D vs 70D, and your right the general opinions from the grues is its a lot of work with little results
the power packs are different numbers from OMC but CDI replacements are the same, good luck with your experimentation

timguy said:

Nice report, I appreciate your experimentation. I like to do what I'm not supposed to do all the time. I don't think the CDI pack will change things. Probably gonna have to change or modify the intake. Is the old 20 hp powerhead your working with in good condition compression wise?

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Thanks for the response.Have not checked the compresion since last year and it was close to 120 I think .I did put the 30hp intake on tonight and when I get a chance I will give it another go on the water.Hoping that the correct intake is all it takes.That is probably the issue and if that is the case it was worth just experimenting with to see what happens since I just was uncertain of the exact problems that would arise with the 25hp intake.Plugs may have been just getting an overdose of fuel at full throttle.

scalhoun506 said:

after doing parts break down comparison, looks like intake and both gaskets are the same, reeds and exhaust tubes also, in the carbs, the high speed jets a substantially different, 47D vs 70D, and your right the general opinions from the grues is its a lot of work with little results
the power packs are different numbers from OMC but CDI replacements are the same, good luck with your experimentation

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Thanks.