97 Challenger performance issue: Fuel or Electrical problem?

[IDoSeaDoo]

Hey guys! I just put a new prop on this boat but can't tell if I resolved my cavitation issues because I don't feel that the boat is outputting like it should. It has a fresh set of pistons/rings on it, compression at 150 and 160Mag. Carb set to stock specs except the highs are just a little rich, like 1/4 turns I believe. So here's what it's doing. It will start and run up to 6500rpm. It will plane out and maintain that rpm for maybe 2-3 minutes, then it will all the sudden drop 500rpm and maintain at 6000. If I pull the throttle back to 80% it will maintain 6000rpm. When I slow to idle, it feels sluggish to get back to speed. It seems that it's better to slam the throttle forward than to pull it fwd slowly. When I accelerate, the acceleration feels like it's in steps.

The plugs looked a shade of dark chocolate and dry. Voltage during operation was about 12.6, but that was judging from a fish finder. I don't trust it 100%. I put in a set of brand new gapped plugs and had the same result, except the rpms would go up to 6700. The whole boat feels sluggish and weak and loses rpm dramatically when I turn. Does this sound like a blown a rectifier, or do I need to delve back into my carbs? PS once it's warmed up, it doesn't like to start right away. I have to play with the throttle quite a bit to make it fire up. Thought I'd ask the forum before I rip stuff apart again.

 

[Dr Honda]

If you are only seeing 12.6v while running (about 4000 RPM) then you aren't charging... and that can effect the other electrical.

With cavitation... it's like spinning the tires of your car. SO... if you had it... the RPM's would be very high (7000) and you wouldn't be moving. So... if you can jump on the throttle... and it doesn't instantly bounce off the rev-limiter... then that's not the issue.


When RPM's drop for no reason... that's not good. Generally it's a fuel issue. Couple that with the hard starting... and I would think you either have a carb issue... or you have a small leak in the feed hoses going to the carbs.

 

[IDoSeaDoo]

The reason I mentioned cavitation is this boat has always been plagued by it. Once I thought I had it fixed, but really, the rave caps were just melted and not opening. I didn't have enough power to bounce off the stutter box. This time the Raves are clean, plungers and O-rings are new. One rave assembly has new valve and housing as it was leaking before.

The problem with the RPMs dropping is repetative. If I shut it off for a while and start it up, it'll rev to 6500, and approximately 3-5 min later drop back down to 6. The boat was fully loaded at the time.

If I disconnect my rectifier red wire, would that rule out a rectifier, a stator, or both? I did see some dark spots on the rectifier circuit board though that gel...

I've heard that you can get upgraded rectifiers for the 951. Would they work for the 787?

 

[IDoSeaDoo]

Took the boat out yesterday to test it with the rectifier unplugged. I charged the battery to 13v. Started it and took off. At 6500 rpm, I was reading 13.86v. Disconnecting the rectifier seemed to have no effect on max rpm. I took a reading of the rectifier with my digital voltmeter by connecting the pos lead to the red wire coming out of the unit, and black wire to ground. The voltage was all over the place, reading as high as 17v and as low as 4. It changed as fast as the voltmeter could refresh. Is this normal?

Yesterday, the boat was not as loaded. It had me (180lbs), my friend (guessing 115lbs), and maybe 100lbs of misc. stuff. It planed out a lot faster and maintained 6500rpm. I experienced no cavitation, but feel that with a 14.5/21.5 pitch prop, I should be turning 7k easy. I checked the high needles on the carb and they were quite a bit turned out. I closed the MAG and opened the PTO 1/4turn. Did not check plugs at the end. I had my water injection regulator turned out two turns to try and cool the stupid-hot expansion pipe, which once almost melted my oil return line. I wanted to maybe turn it in some, but the spring broke when I took the cap off. Even with two turns out, the pipe was still running really hot. Can't keep my hand on it. Easily the hottest part of the engine. Is that normal? Any suggestions as to where my missing 500rpm are?

 

[Dr Honda]

If the regulator is bouncing around... and the RPM's are steady... I would be suspect that the regulator isn't happy.

But... if the RPM's are dropping... you need to figure out why.

The 800 engine will run 6700 to 7000 rpm as a "Normal" top RPM. It depends on the pump and impeller. Not to mention... the OEM tach's aren't always accurate. So... if you are tuning... you need a good, aftermarket tach to check things.


If you are loosing RPM's... I would remove, clean the carbs... remove/clean the water regulator... remove/clean the orifice on the head pipe that the regulator feeds.... I would make sure the carbs open all the way (a common, low RPM issue)... I would also, make sure, 100% that the temp sender is working, and that the buzzer functions.

 

[IDoSeaDoo]

Funny thing was, it maintained at 6500. It didn't drop rpm to 6000 like the day before when I had 3 adults plus 100lb of misc. stuff onboard. Will check all those things Doc. Not sure of the temp sender, but the beeper works like a charm. Bought a new radio shack unit for all my boats. Also, got a cheap aftermarket inductive tach which agrees 100% with stock one. The boat doesn't seem to have the power it once did when I first rebuilt the first engine.

 

[IDoSeaDoo]

Problem solved: the plastic rave plungers got completely stripped out. They looked perfect from the top, but when I went to take them apart, I realized they were completely stripped out. Replaced them and the performance is back. Puts a grin on my face

 

[IDoSeaDoo]

Well, that was short lived. The rave plungers melted. I HOPE it's just worn housings, and now worn RAVE slots in teh cylinder. Though I know for a fact that I bought them from SBT last year... I also know that I put a crap load of hours on this boat, but there's no telling how many hours are on the actual cylinders themselves. How common of a problem is it for the actual slots to wear out??

Anyway, I found precise bushings that I plan to insert into the Rave housings. There was a thread about it on PWCtoday I think. The bushings are brass with steel backing and measure exactly the diameter of the valve stem. I plan to upgrade a few worn housings and try it out this weekend. Weather's supposed to be 86 or 89 (depending on what meteorologist you go to). Wish me luck!!