Low RPM

streetdoc432

streetdoc432

Member
Sep 16, 2010
114
Memphis, TN. Docked on Kentucky Lake, TN River
Boat Info
1988 Sundancer 270, Onan genset, camper canvas, marine a/c, Lowrance HDS-5
Engines
4.3 twins with Cobra drives
I have a 1988 270 with twin 4.3 & cobra drives. When my engines are in neutral & I rev them up, the will hit redline, when I'm under way, my starboard engine won't get above 3200 RPM, port engine runs up to 3900 under way.. What gives? Throttle cable moves like its supposed to, engine doesn't run warm or miss, choke not stuck, carb clean, new 8mm wires & plugs. Any suggestions? Thanks in advance
 
I was having horrible ethanol flashbacks from two years ago but that would be both engines.
Hopefully a simple mix-up in the plug wires - easy fix.
If not it could still be a fuel issue - plugged filter or water separator on that side starving the engine.
Reving at idle may not show that because of the short time at higher RPM.

Just thinking - good luck!
 
If you haven't owned the boat long...possibly an odd prop ?? :huh:
 
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Both of those numbers are quite low. I'd suspect props first, if it has never revved out before, then I'd check fuel delivery. After that I'd say tired engines. We need more detail and history to give a better diagnosis.
 
moparlvr4406 said:
If you haven't omned the boat long...possibly an odd prop ?? :huh:
Click to expand...

Along the same lines I would switch the props and see how it performs.

Ken
 
Iprof said:
Along the same lines I would switch the props and see how it performs.

Ken
Click to expand...

Thanks for the tips! Spun a prop last season, sent them both off for re-hub & balancing. Both engines have around 800 hours, don't smoke or use oil, been running Amsoil past year and a half. Survey when I bought the boat showed good compression overall. I will try to find a plug wire diagram & double check for crossed wires. Looking for caps & rotors to change next, considering going pointless ignition this year too.
Thanks again for all the info.
 
With questionable props, I'd say that's your problem. On the 3900 motor, you could be about 4" pitch off, and on the other one, about 6-8". That's a sh*t ton. Did it ever spin up to 44-4800?
 
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I had a similar problem with low rpm on one engine that turned out to be a clogged fuel filter at the carb.
We had an issue with ethanol in fuel from our marina that we were not notified of then 3 weeks after a fill-up our rpm's dropped on both engines with the power loss after fuel separation. I hope it's not that as all you can do is remove all the old fuel, clean the carbs and replace all fuel filters - big project.

I would check plug wires first.
then check / clean carb fuel filters.
then change water separator filters.

Did it ever spin up to 44-4800?
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Good question - Your rpm's seem low on both engines even with one more than the other.
Another issue though as regardless of the pitch, if both props are the same pitch rpm should be too.
 
Update: this weekend I changed plugs again, swapped plug wires & cap/rotor with the other engine, still no throttle response over 1900rpm on the starboard side. Great oil pressure, norm engine temps. I took the carb off & brought it home with me. Gonna rebuild it this week & hopefully try again next week. I'll let you guys know if that helps. Thanks for all the input.
 
I'm going to assume the carb is a quadrajet, even on a 4.3? If so I'd look at that vacuum choke pull-off, make sure it's releasing and allowing the secondaries to open.
 
My best guess is the floats are not set correctly or the power pump is sticking. Are you about to begin plaining when the rpm's start to drop off?
 
kbsunlovers said:
I'm going to assume the carb is a quadrajet, even on a 4.3? If so I'd look at that vacuum choke pull-off, make sure it's releasing and allowing the secondaries to open.
Click to expand...

Thanks for the tip! Looks like that is the problem! Now if I can just find the part to replace it
 
Well, made it back to the boat this weekend. Put the rebuilt carb & choke pull off back on, fired it back up. Changed the fuel filters & water separator again. Checked for correct firing order on the plug wires, cleaned the inside of distributor cap, checked all the plugs. Now my starboard engine runs very erratic under load. Still won't get above 2400 rpm WOT. Throttle opens fully, choke plate opens when it warms up. I'm about ready to give up & pay a tech big bucks to diagnose for me :-( unless somebody else has a suggestion. Thanks again for all the tips
 
We did some prop changing on a Sea Swirl we used to have and I was very supprised to see the differance a prop can make in the rpm
 
Is it possible the pick-up, in the fuel tank, for the starboard engine is plugged ?
Have you eliminated the possibility of old "ethanol" fuel ?
I have run an engine off a 6 gallon outboard fuel tank to prove-out or eliminate those questions in the past.

You seem to have eliminated everything else - Just thinking :smt017
 
Took a mechanic friend out to the boat today to try & rule out some more possibilities. (aka throw more $ at it)
Checked my new wire location, all plug wires in the right order & connections crimped good. New Accel super coil getting adequate fire.
Timing set good, advanced it a little under load. Installed new plugs. Tried running off of a fuel can just for the heck of it. swapped fuel lines at the pickup on the other engine too.
Just for @#(*% & giggles I bought a new electric fuel pump. The engine idles smooth, runs smooth up to about 3200 rpm, then just runs falls off aka runs rougher & can't keep up with the other engine. Hooked the timing light up while it was running rough, and my # 1 cylinder is not getting fire. The # 3 cylinder gets fire erratically. There is a little play in the distributor shaft. He checked continuity through the electric pickup in the dizzy. Our only guess now is getting a new distributor. I'll update when I get a day to install it once it makes it here.
 
It kinda sounds like your breaker plate is grounding when the advance starts to come in. If you swap the distributors between the two, the problem should move to the other engine.
Travis..
 
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