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Sporadic RPMs above 3000 Chrysler 360

D

dbrahms

Contributing Member
"since doing an oil change the

"since doing an oil change the other day, one of my 360's acts funny above 3000rpm. I can get both motors to 3400ish RPM but the starboard engine backs itself down to 3000 on its own. Sometimes it will jump back up but not often. I back both engines down to 2800 to cruise and it's smooth. Pressure hovers around 40psi. temp is 160ish and I used 25w40 weight oil. It could just be a coincidence that this is happening after the oil change...or not. Fuel is less than a month old. and again, it's only happening on one engine. issue presents low-level vibrations and hum when it happens...just as when engines are out of sync before I trim them both even. any thoughts?"
 
"I suspect a bad plug or ignit

"I suspect a bad plug or ignition wires. Try looking at the plug colors--if one is darker than the rest, there's your problem. Also, you may have bumped a wire loose at a plug during the oil change.

Jeff"
 
"Jeff - could secondaries be s

"Jeff - could secondaries be stuck and not opening at 3000rpm? first thing i was going to check was how free the 2ndaries are by hand. if it's sticky, i was going to clean it - maybe spray it with a tiny bit of carb cleaner.

if you dont think this is the issue, i'll go right to the wire and the plugs."
 
"Dan:

If the motor was up t

"Dan:

If the motor was up to speed, then backed down, the secondaries aren't the problem. They might not open up, but if they did open, they won't close on their own.

Jeff"
 
"Dan,
Could be bad plug wire

"Dan,
Could be bad plug wire arcing to chassis under load.... Don't know which distributor type you have but could also be defective or wimpy condensor. Again, depending on distributor type, could be mechanical advance not moving freely. It sounds coincidental with oil change unless you hit or moved a plug wire. It definitely does not sound like a carb problem...sounds electrical. Just my guesses....Ed"
 
"There are several potential c

"There are several potential causes of this condition. Many of us have chased this gremlin. I just found my problem in the last place I would have expected. The new ECU that I had installed this spring was bad. I had eliminated all other potential causes by replacing the distributor cap, rotor, wires, plugs, resistor, advance springs, fuel filter, collapsed exhaust hose and rebuilding the carb. As a last resort I replaced the new ECU with a $12 unit I had bought as a backup. Instant fix. Try swapping out components from the port to the starboard motors to see if the problem follows the part. But don't assume that the new parts are reliable."
 
"ok,I changed the plugs and to

"ok,I changed the plugs and topped of with fresh fuel. Tightened the wires too. Now I'm thinking fuel pump maybe ? One engine still loses Rpms when around 3500 for a few seconds. I back it off and it smoothed out. Then I throttle up again and it holds for another ten seconds before giving up a few hundred Rpms again."
 
"Hi Dan.} I just had the same

"Hi Dan.} I just had the same thing happen to me on my port side engine, when I went to full throttle the RPM gauge would flex and the engine RPM’s would drop off. I won’t bore you with the details of what I went through to locate the problem but at the end I had my father drive the boat and I went in the engine compartment. As I had my father throttle up and down I felt a fine mist on the left side my face, and as it turned out my front water pump was dipping ever so slightly, the drops would hit the inside of the bell housing and at full throttle it became a mist that landed on the alternator which for some strange reason caused the RPM’s to drop off. I replaced the water pump and had no more problems. I don’t know if that is even close to your problem but it’s worth investigating. Good Luck."
 
"Dan:

I hope you're not

"Dan:

I hope you're not running the motor too lean due to fuel starvation, too hot a plug range, timing off, etc. That would NOT be good!

Jeff

PS: I hope you don't have a FINE fuel filter BEFORE the pump! If so, that can cause fuel stavation and burned pistons."
 
"Had the marina's mechanic

"Had the marina's mechanic out with me on a run last weekend and he thinks it's head related. He thinks it could be a hung valve, broken spring or something like that. boat gets up to 3500 and even 3800 but after 5-8 seconds, it gives up a few hundred RPMs and vibrates until i back it down a bit. Still able to bring it back up but then it vibrates again after a bit. I think he took off one of the plug wires while running and said "taking this off doesnt change how the motor runs"....as if that was the problemed cylinder? He tried that with all wires...I think. "
 
Sounds to me like you have a v

Sounds to me like you have a valve issue. It sounds like a valve spring is either broken or getting weak and its floating that valve when you get up in the rpms.
 
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