Johnson ocean runner 150 loss of WOT rpm

Been searching all the previous posts trying to see if I could find a solution or guidance. I was able to in part, but now I am stumped. I have a 1996 Johnson ocean runner. The problem is that at W.O.T. on a Kenner 21 the rpms will peak at 4200 to 4500 depending on motor trim. I had attained 5500 a couple of times, but despite what I do that doesn't seem possible now. It was my dad's and it set up for several years and not run. No problems were ever reported. I have rebuilt the six carbs, and run a can of OMC engine tune. The spark plugs are new and gapped to .030. Although I questioned it the OMC mechanic insisted that QL77JC4 were the replacement. It had QL78YC before, but the same rpm issue existed with them. Today I replaced all the fuel line (again) and reclamped just to be sure there was now leak. The antisiphon is now gone. The VRO is disconnected and premix is being used. I think the fuel supply is good, but maybe I'm missing somthing. The motor idles well although during docking and in out of gear there is a surge from neutal to gear. Compression is 100 in all six cylinders. The 4200 - 4500 rpm sounds good as I can't hear a miss if there is one, but I know this motor can do more. Any suggestions for me or is it time to bring it in?

Re: Johnson ocean runner 150 loss of WOT rpm

Run it on a 6 gallon tank to eliminate problems in the boat side fuel system.

Re: Johnson ocean runner 150 loss of WOT rpm

thanks and I will certainly try the remote tank this weekend. However, I wonder if I have not done that already since the tank (above deck) was drained, marine fuel is used and treated, new fuel line, OMC cone filter and new primer bulb. Anti siphon is out of the tube also. The connections are secured with the compression zip tie. With all this new stuff the engine starts up fine, idles well. Vibrates a little more than I think it should as rpms increase to 1200 or so just to get to end of no wake zone. boat jumps up on plane, but 4500 is most I can get. I read on here that the transition from low to high jets is in the 1000+ rpm range so if that is so would that vibration I mention be normal if in that transition zone?

Re: Johnson ocean runner 150 loss of WOT rpm

I also have a 1996 Kenner 21 with a 1996 Johnson 150 Ocean Pro and mine is doing the exact same thing with the exception of it surges after half throttle to wot. I have also change out the pick-up tube, the bubble, the filter and the fuel line and also installed a water seperator filter. I have change all of the plugs out and made sure every vent was open and every clamp is tight. 5 out of 6 spark plugs look perfect but one is wet and black. Have not done a compression test but will be doing it soon. Is there any obvious solutions that i should be looking for and have missed. I really need some help and am out of clues. Will somebody please let me know your thoughts?
Thank you very much

Re: Johnson ocean runner 150 loss of WOT rpm

I think both mtrs above should have a compression test done on them and then a spark test done with an open air gap spark tester set at 7/16 ". If they pass the tests, then look for fuel probs.

Re: Johnson ocean runner 150 loss of WOT rpm

I forgot I started this thread, but with spring here and boat use coming up I thought I would update. Compression had been done earlier and is balanced at 100 in the 6 cyl. I installed a power pack at the end of last season and didn't get the result I expected. However, when it came time to hurry and get the boat from the dock to trailer I got the 5500 rpm I was looking for three times. I haven't run it since, but my guess is that it will not be consistent at 5500. any thoughts