johnson 40 hp 50 hp carb size and main jet size

[slx12001]

I installed a set or big bore 50 hp carbs and Boyeson reeds on my j40tlenm 40hp. The stock main jet in the 40 hp carbs is a 47d. The big bore 50 hp carbs use a 49d main jet. (50 hp carbs are from the same 1992 year engine with early build carbs) My question is how is it possible to run such a smaller main jet as the earlier 40-50hp small and big carb engines ran a 56d main jet. I'm struggling to get some rpm on top. I can blow out a 15p prop but can't even rev 5k on top. (more like 4700 rpm) I don't want to switch to a 13p prop unless absolutely necessary. Anyone try a 56d or even a 52d main jet on the newer 40-50 with the small 47d-49d mains? Plugs look lean on top but not white or chalky. I have been building 2 strokes for many years. You can't simply drop 7 jet sizes without a lean seizure all being even. The big bore carbs seem to be the same size bore from older to newer. Why 7 sizes smaller mains? Something to do with emissions may be running on the edge? Engine is very healthy with great compression. Any method to this madness would be helpful. I would like to be close to 5500. Thanks

 

[jimmbo]

If you're not getting into the recommended RPM and the engine is in tune and the engine setup is fine, you are going to have to go to a lower pitch prop. Jetting is only a small part of a fuel delivery curve. Air bleeds, venture shape are also factors.

I'm sure the OMC engineers who chose the smaller size jets knew what they were doing

 

[flyingscott]

Is the timing the same, are you sure they are the same block, same porting, same exhaust tuner, same reed cages. The 40 could be a choked off 50. Usually not as simple as changing carbs. I believe Evinrude will list 2 different blocks for the 40/50 motors

 

[slx12001]

Is the timing the same, are you sure they are the same block, same porting, same exhaust tuner, same reed cages. The 40 could be a choked off 50. Usually not as simple as changing carbs. I believe Evinrude will list 2 different blocks for the 40/50 motors

Yes the 50hp has a different block. I asume the port timing is higher on the 50hp model. I gained alot of bottom end torque with the 50hp carbs and boyesen reeds but top end is similar. I read alot about people turning 15-17p props with similar setups. I may just end up getting a 13p prop but I blow the 15p out so bad now off the line I will have to really ease into the throttle off the line with a 13p. The timing is the same components including cdi 40/50. What is really strange is that I can be with myself running 4600-4700 rpm 28-29 mph or a full load 3 people in some chop and run about the same speed and rpm. If the motor was over propped you would think it would run 2-3 mph slower loaded. Unless the larger carbs and lower porting created a torque monster but if that was the case then why would it not rev higher. Thinking I need to find a triple and be done. Boat is rated for 75hp.

 

[slx12001]

I have a Quicksilver QA3084R 12-1/4 x 15p prop that I bought new when I started this project. I sent it to the prop shop to get measured and see if it was defective somehow due to blowing out off the line and not reving. I just got off the phone with the prop guy and the prop measures over 16.5p overall due to cupping and design. WTF. That makes more sense. So what prop is actually a 15p these days as all seem to be cupped? Does that mean if I but a 13p I may get a cupped 13p making it closer to a 15p or what.

 

[slx12001]

If anyone is reading this or looking for similar help I bought a new Quicksilver QA3082R 12-1/2 x 13p prop. down from the Quicksilver QA3084R 12-1/4 x 15p prop that was new as well. The 13p came looking like a pontoon prop with huge blades. The 15p looks like a normal prop. The 13p gave 175 rpm more at most and dropped top speed by 1.5-2 mph. What a joke. Slip calculator with this 13p says -9 at 4800 rpm. Strange thing is that I don't blow the 13p out at all off the line but the 15p blows out like mad. Not happy with quicksilver