1990 150 Johnson GT jumping rpm

[92 spectrum 150]

Alright guys, I just bought this 1990 150 GT and after installing it, I took her out for a test run and my tach reads fine till I full throttle. As soon as I full throttle, the rpm will go up to about 5250 and then starts jumping around. Sometimes it'll hold at 5250-5300 for a few seconds and then starts jumping again. I just put in new plugs also. My boat is a 1992 Spectrum aluminum deep v 17.5 footer. Prop is a 15x19p and top speed able to achieve was 48 gps. Previous motor before was a 92 mercury 75 and had no issues with the tach or rpm jumping. The tach is set at 6p. Is this correct? I'll also verify when I'm back at the boat too.

Secondly, while testing the new motor I hit a floating tire in the water and it broke part of my prop off. So I'm looking for a new prop and was wondering if I should pitch up or down. I'm looking to get a SS prop for the replacement. Current prop is the standard aluminum 15x19p.

Thanks!

 

[92 spectrum 150]

I'm looking at a laser 2 SS 13.5x21p as the replacement. Will this work since my old one is a 15x19? My v6 is a 15 spline too I think

 

[emdsapmgr]

Would be interesting to know what you mean by "jumping around." Does the rpm vary? Between what rpm's? Is it just the tach moving (a tach issue,) or does the engine actually feel like it's rpm's are changing? 6 is the correct tach setting. Don't buy a new stainless prop till you get the rpm's sorted out. They cost a lot of $, and if you buy the wrong one, you will live with it for a long, long time. The correct prop should permit that engine run at least 5500 rpm's- WOT and at full trim. Maybe even 5600 or 5700.

 

[92 spectrum 150]

Would be interesting to know what you mean by "jumping around." Does the rpm vary? Between what rpm's? Is it just the tach moving (a tach issue,) or does the engine actually feel like it's rpm's are changing? 6 is the correct tach setting. Don't buy a new stainless prop till you get the rpm's sorted out. They cost a lot of $, and if you buy the wrong one, you will live with it for a long, long time. The correct prop should permit that engine run at least 5500 rpm's- WOT and at full trim. Maybe even 5600 or 5700.

I since then got a SS 14.5x19 and the rpm seems to be a bit more stable but it still jumps. The rpm goes up to about 5k and then falls to like 3k-4k and comes back to 5K and some times goes all the way down to zero. No lost of power because gps speed did not decrease. I'm wondering if maybe it's a rectifier or stator going bad? With the new ss prop, 5K was the highest it'll go to. I think I'll need to step down to a 17p to get my rpm's back up. Will I loose top speed if if go from 19 to 17p?

 

[emdsapmgr]

Likely a problem with the charging system. That system drives the tach and when the tach starts to fluctuate by itself, it could be the combination rectifier/regulator.