787 wont reach max RPM

[c-D00]

Was out on the lake this weekend and doing more testing on my 97 challenger twin 787. At the begining of the day on saturday, i idled out of the no wake zone, pushed the throttles forward and both engines went up to 7k rpm without issue.... throttled back down and idled around for a little while and then, for the rest of the day, could not get one engine past 4800RPM. This is my second tank of fuel and have put in seafoam both times. I have owned the boat since december and the previous owner had the same issue. Got in the boat sunday morning and the boat performed much better. The one engine would run up to 5k rpm and as the boat got up on plane it would punch forward to 7krpm. The previous owner only had the boat out on the water about 3 times over a 2 year period. Compression is 150 on both cyls..... Do you guys think I need to go ahead and bite the bullet and rebuild the carbs (never done that before) or should I just continue to run seafoam thru the engine to see if the performance continues to improve?

Thanks!!
Scott

 

[parrothd]

Rebuild the carbs, replace the needles and seats, and rebuild the accelerator pumps, and make sure the boat has been upgraded to the newer rave vales and carbs jets. You can search to find the answers to these before you destroy the engines by running it in it's current state....

 

[c-D00]

Thanks for the quick reply. I was thinking that rebuilding would be the way to go. I am rather new at this so I have a few questions about your post:

1. Do carb rebuild kits normally include the needles, seats and acc pump rebuild parts?
2. I cannot seem to find info on upgrading Rav valves.... I just see standard valves. Where can i find upgraded rave valves?
3. Probably a dumb question, but how does running it in its current state destroy the engine? Is it running to lean or something like that?

Thanks again for all of the help and education!!

 

[parrothd]

Thanks for the quick reply. I was thinking that rebuilding would be the way to go. I am rather new at this so I have a few questions about your post:

1. Do carb rebuild kits normally include the needles, seats and acc pump rebuild parts?
2. I cannot seem to find info on upgrading Rav valves.... I just see standard valves. Where can i find upgraded rave valves?
3. Probably a dumb question, but how does running it in its current state destroy the engine? Is it running to lean or something like that?

Thanks again for all of the help and education!!

1. Nope, here's the accelerator pump, you need a rebuild kit, and 1.5 needle and seat. http://www.parkeryamaha.com/mikunicarbacceleratordiaphragmpwc.aspx

2. Search the forum at pwctoday, http://www.seadooforum.com/showthre...-97-challenger-1800-rpm&highlight=update+rave

3. Yep, there's something wrong, running it is probably only to make things worse, you need to fix it...

 

[seadooya]

Was out on the lake this weekend and doing more testing on my 97 challenger twin 787. At the begining of the day on saturday, i idled out of the no wake zone, pushed the throttles forward and both engines went up to 7k rpm without issue.... throttled back down and idled around for a little while and then, for the rest of the day, could not get one engine past 4800RPM.
Thanks!!
Scott

Remove the water regulators off the muffleers, and switch them. That rpm range your seeing, leads me to a bad regulator.

 

[DonoBBD]

Remove the water regulators off the muffleers, and switch them. That rpm range your seeing, leads me to a bad regulator.

Can you detail this fix. I have a totaly new motor and carbs. This 787 will do the same thing. IF I feather the throtle I can get her to catch and rip but is intermitant.

Don.

 

[c-D00]

Just now getting around to starting to dig into this and wanted to run past you guys what I found last night.... When I removed the air box and the filter plate off of the carb, I moved the throttle lever all the way forward. I then went back to the engine compartment and was able to move the throttle linkage a good 1inch forward. After examining the throttle cable it appeard that there was some slack in the line so I tightened it up and will test in the water this weekend. My question to you guys is do you think this could be my issue with not reaching max RPM? After making the cable adjustment, the throttle linkage will only have about a .25 inch play forward when the throttle at the console is all the way forward.

Thanks for the help!

 

[dsw222]

before you tear the carbs apart, did you clean the RAVE valves?

 

[c-D00]

Just wanted to update for anyone that experiences the same issue. Cleaned Rave valves and no change. tightning the cable helped some but that made it to where I couldnt get it to idle below 2000rmp in the water. Pulled the carbs and found that whoever messed around with this before wasn't very good at it. There was a missing bushing at that goes on the end of the throttle shaft (shaft that goes thru carb and buttefly is attached to. That was causing the throttle to stick very badly. The screw heads on the butterfly are completly striped so I cannot fix just yet. Secondly, the screw that sets when the accelerator pump engages was completly missing. This kept the plunger for the pump depressed all of the time. When I replaced, that completly fixed my issue with reaching max RPM. Just an FYI. Now it is extremely quick out of the hole and I dont have to gradually advance the throttle to keep it from dieing. thanks for all of your help.

 

[c-D00]

Oh one last thing.... the person before me apparently tried to tune the carbs to get around not having that screw there.... the low speed screw was 3 turns out and so was the high speed screw! Guess they were trying to compensate for the lean condition created by the accelerator pump never dumping extra gas to the carbs when advancing the throttle.

 

[Murph_13]

c-DOO,

I'm with seadooya...your problem sounds like a leaking water regulator (on the waterbox/muffler...looks like your RAVE valves)...engine runs fine up to 4500-5000RPM.

I've had a leaking water regulator on a 787 engine and that's exactly how my boat acted...race up to 4500-5000RPM and no further...fixed leaking water regulator and all fixed!

Quick check to rule out the water regulator is to swap them like seadooya suggested.