09-30-2014, 04:03 PM #1
- Join Date
- Sep 2014
787 not achieving max RPMs - '97 Challenger
Got a '97 Challenger w/ single 787 - 127 PSI compression at 3,500 ft elevation.
Problem: With 4 people in boat, engine will only rev to 5500 RPM and will not plane.
What I've tried:
- New plugs (NGK BR8ES stock gap)
- Cleaned carbs - no sludge, didn't seem worn, but no tools to test pop-off etc., set low/high screws at factory specs
- Cleaned fuel selector,
- Trimmed 1/4" off spark wires,
- Cleaned rave valves (lots of black oil but minimum carbon)
- Disconnected water regulator from exhaust for short while (same top RPM)
- Disconnected rectifier (~200 RPM gain) and test voltage at battery w/ rectifier connected (held steady at 14.0v at 2500 RPM)
- Connected jumper cable from negative battery lead to engine to test ground (no difference)
At one point with the rectifier unplugged and the springs off the raves, I hit ~6900 RPM, but it then leveled off at 6500 on plane. Couldn't repeat. Also, my tach is at 500 RPM with the engine off (see 1st photo), so perhaps all these RPM #'s are minus 500.
I think it's time for a new wear ring (see 3rd photo of clearance) but I think the impeller looks pretty good (2nd photo), especially if it's original. This should help get on plane, but a worn ring should mean higher and not lower RPMs so there must be another engine problem. No wobble or noise from the impeller bearings that I can detect.
09-30-2014, 09:33 PM #2
Service the RAVE valves?
oops i see you did...
10-01-2014, 07:44 AM #3
Have you looked very closely for exhaust leaks? Exhaust gas in the engine compartment can cause this... maybe you have the compartment open the day it hit 6900?
You have 127 compression on BOTH cylinders? Both plugs look the same color? What do the plugs look like, black? Chocolate? White?
Also, your jumper cable method on the grounds check won't work... check and clean the grounds on the front of the mag cover, they have to be clean and getting a good connection, using a jumper cable like that doesn't help the coils, mpem, and rectifier get a good ground. The jumper cable method only diagnoses a problem in the main large ground cable. It doesn't test the ones on the front of the engine going to the electrical box.
10-01-2014, 10:27 PM #4
- Join Date
- Sep 2014
That's very perceptive of you wkuadam07. I did have the engine compartment open when it hit 7000, though it was also open for the other tests and I could not repeat it. I forgot to mention, however, that there was often a bit of exhaust smoke in the engine bay. I asked about it on another forum and didn't get any feedback so figured it was normal.
Is there some sort of pressure test or other diagnosis for the exhaust system? I should definitely mention my RAVE gaskets are shot (black oil around them on block, could also leak exhaust) - new ones get here tomorrow.
Today I had 4 people and eventually got on plane, had it running pretty steady at 6500 RPM with the blower on, but it suddenly dropped 500 RPM and was steady at 6000. Seems more RAVE related too no?
As for mag side grounds, what color/ how many wires am I looking for? I've attached a picture of the plugs - one is chocolate, one is a bit darker (both high speed screws are closed and last ran at WOT except for docking). Bad needle?
10-02-2014, 07:08 AM #5
Exhaust smoke in the engine compartment is unacceptable, it will NEVER run right with exhaust gas in the engine compartment. Look for dirty spots like the one around your rave housings, usually exhaust leaks are easy to find. Work out all of the exhaust leaks and make sure your raves are 100% right and you should be in business. Leaky rave housing gaskets will cause problems, the vacuum that opens them runs through the housing and that gasket you say is crappy, so it's likely a leaky gasket is affecting how and when the raves open. Casting plugs in those pipes are a likely spot for exhaust leaks as well. Also, look carefully at your j-pipe and manifold gaskets, if there is any doubt replace all the exhaust gaskets, if these leak you can get water in your motor and that's bad too... Exhaust leaks in the engine compartment are most likely your culprit, if it's just leaking from the raves you got real lucky...
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