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Twin AQ260/AQ290A wont go over 3000 rpm

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Indy1K

Contributing Member
1986 31ft Sun runner with twin 350 chevy small blocks. new out drives. The boat sat for a long time with gas in the tank. put in 50 gallons of fresh gas. We ran it down river 70 miles down the Ohio to its new marina. both engines exhibit the same problem. Cannot get the rpms to go over 3000. then the sputtering starts and WOT does nothing. eventually shut down the starboard engine because it dropped down to 1500rpm and no power.
Ok, checked the tank(200gallon aluminum) looked inside thru sending unit hole and it is clean as a whistle. Blew out the lines and replaced the gas filters with RACOR 10 micron, filter/water separators. Rebuilt both Quadrajet carbs. The only real thing I found was a tan colored sediment in the carb filters. It looked like sand but was a fine powder. replaced those filters. Took it out yesterday. Idle seems fine at 900rpm. Put in another 50 gallons of 91 octane ethanol free gas. Added 5oz of Stabil Marine 360. Took it out and ran it up and had the same problem. will not get up on plane. Engines wont go over 3k. fluttering RPMs, no power. throttled back to 2K and engines fairly steady. oil pressure 50psi and 75psi, temps. 180 on both. I am at a loss here. Got back. Starboard engine stalled and could not get her started again. Pulled plugs and they are sooty black. Too rich? Pulled fuel separator on starboard side. not full, but when we checked the bowl it was clean.

My next steps. Rebuild or replace the fuel pumps. new plugs R43s in there, caps, rotors. Wires look new. Replacing the coils and resistors.
Any other recommendations

Question:
My port alternator gauge is only reading about 9-10 volts. My Starboard gauge shows charging at 12-14 volts. Could a bad alternator on the Port side be pulling the ignition down under load?

Thanks in advance for any help.

Kevin aboard the "Russian Princess"
 
charge your batteries overnight slowly,see if motor fires up. A complete tuneup is called for,cap & rotor,plugs. Are the throttle plates inside of carbs opening,secondaries also? Is timing advancing properly? Is the bottom of hull covered with growth?waterlogged? Is belt on port engine alternator tight enough? Most ignition systems need 12 volts to coil to run efficiently. Your coils can be tested with an ohm meter before replacing them! Hope this helps.
 
Thanks much,
The motors will fire up and idle and rev up with no load - curious. I have two Port side batteries and 2 starboard side batteries, but I can swap them.
I will check the Alternator belt, good idea.
Yes, tune up is next. I agree, 12 volts to coils. I can monitor that when I try again.
Thanks, I will keep you posted

Kevin
 
re:"The only real thing I found was a tan colored sediment in the carb filters. It looked like sand but was a fine powder. replaced those filters."

the tan stuff is a residue from ethanol in fuel and water. I know you said you rebuilt the carbs, however, my experience has been that the ONLY way to get the jets clean is by "rodding" them our. Neither compressed air nor any cleaner work. My favorite tool is a 4 inch length of 22ga electrical wire with the insulation stripped off for about 2 inches. Peel back one strand at a time until the remaining wire strands just barely fit. Use the insulated end as a handle. Alternatively as Chris @ Dockside suggests, immerse the carb in an ultrasonic cleaning tank.

I ran a 10 micron filter on my mercarb engine... it was a help but not a cure.
 
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Indy1K said:
We ran it down river 70 miles down the Ohio to its new marina.
OK, so you were able to make a 70 mile trip....correct?

both engines exhibit the same problem. Cannot get the rpm to go over 3000. then the sputtering starts and WOT does nothing. eventually shut down the starboard engine because it dropped down to 1500 rpm and no power.
Several scenarios come to mind for me:
1..... fuel starvation.
2..... issues with ignition system or ignition coils.
3..... detonation that is causing high cylinder temperatures. (see my response in Red Text bellow)


Ok, checked the tank(200gallon aluminum) looked inside thru sending unit hole and it is clean as a whistle.
Have the fuel tank "dip tubes" (aka pick-up tubes) been removed so that the screens can be inspected?

Blew out the lines and replaced the gas filters with RACOR 10 micron, filter/water separators.
Excellent choice! :D I run them also!

Make sure that the fuel tank vent hose and vent fitting are clear.

Rebuilt both Quadrajet carbs.
After or prior to this issue?????

The only real thing I found was a tan colored sediment in the carb filters. It looked like sand but was a fine powder.
I have seen (what several of us determined to be) water plus E-10 in gasoline create an alkaline or acidic mixture. Apparently this mixture pulled aluminum metalic particles into solution. The debris was then deposited onto the internal carburetor components.
I fought this until finally going to the RACOR system.


Took it out yesterday. Idle seems fine at 900rpm.
900 rpm idle speed is high.

Put in another 50 gallons of 91 octane ethanol free gas. Added 5oz of Stabil Marine 360. Took it out and ran it up and had the same problem. will not get up on plane. Engines wont go over 3k. fluttering RPM, no power. throttled back to 2K and engines fairly steady.
Detonation that has not yet destroyed an engine can and will cause these symptoms.
Make dang certain that your ignition systems are delivering the correct Progressive Advance and Total Advance and at the correct RPM!!!!!!
(see your OEM specs for this..... NO Automotive specs!!!!! :mad:)

oil pressure 50psi and 75psi,
75 psi oil pressure is too high. Swap Port/Stbd gauges or Port/Stbd sending units as to verify.

temps. 180 on both. I am at a loss here. Got back. Starboard engine stalled and could not get her started again. Pulled plugs and they are sooty black. Too rich? Pulled fuel separator on starboard side. not full, but when we checked the bowl it was clean.

My next steps. Rebuild or replace the fuel pumps.
Before doing that, take fuel pressure readings as part of your P or E and Forensic Work!


new plugs R43s in there
What measurement are you using to set the electrode gap?
Equally important.... what Ignition systems are you using????

, caps, rotors. Wires look new. Replacing the coils and resistors.
Any other recommendations

Question:
My port alternator gauge is only reading about 9-10 volts.
I think that you mean Port Volt Meter.
9 to 10 volts would indicate a Dead Battery!

My Starboard gauge shows charging at 12-14 volts.
Charging occurs at voltages at/above 13 or so.

Could a bad alternator on the Port side be pulling the ignition down under load?
A bad alternator is not charging nor contributing to the battery SOC, therefor that battery voltage will drop.
While I do not see how this would cause an issue with the Port Engine...... it certainly needs to be repaired.



Be careful and do not just throw a bunch of parts at this.
Instead, use the tried and proven P of E (process of elimination)!

Test/check/replace one suspect item ONLY and one item ONLY at a time.
Move on the next suspect item ONLY after having either found or eliminated the previous suspect item.

When used systemactically and methodically, the P of E will not let you down! :)
You will also be better prepaired for future issues.
Click to expand...


.
 
Ricardo,

1. Yes, 70 miles. Shut down starboard engine after 15 miles because it would not go over 1500rpm. cruised down on starboard engine at about 2000rpm(8mph) long day.
2. I suspect the same. since both engines, either fuel or ignition.
3. blew through the dip tubes, but will pull and check screens.
4. Racor, yes
5. Will check the venting
6. Rebuilt both quardajets, problem still there after rebuild. Wondering if the secondaries are opening up. Will check too. I am thinking about switching to the Edelbrock 1409 carbs. Your opinion?
7. Running very rich. the engines will stall much below 800-900rpm
8. I will check the timing and advance. I have a timing light and a tach and dwell meter. The ignitions are stock. I will check to make sure I have 12 volts and a good ground on the coils. What would be the symptoms of a Detonation Damaged engine? Starboard engine sounds loose, like a coffee grinder.
9. Just replaced the oil sending unit on the starboard engine because it was reading zero. I will swap them to test.
10.What kind of fuel pressure should I see. Why not replace the diaphragms in them?
11.Plugs are black and sooty, not wet. I will replace both sets and gap accordingly.
12.I meant volt meter. yes, I will check for a dead battery.

Thanks
 
What are the compression numbers? Use a piston stop tool to confirm the damper TDC mark is correct then check the base timing and progressive advance curve. The outer wheel on the damper will slip on those older motors and you cannot time it properly until you confirm true TDC the damper can be as far as 30 degrees out. Be sure and disconnect the battery when you put a piston stop tool in you dont want to punch a hole in a piston.
 
I will be doing a compression check next time I am there. I will also check the TDC piston to damper mark, never thought of that. Is there any info, other than the top of the Flame arrestor to indicate the advance curve? I have heard 30deg @ 3000rpm.
On the same issue. If the gas tank vent line was clogged, not allowing negative pressure in the tank to breath, could this cause the fuel pumps and carbs to starve during higher RPMs? The reason I ask is when putting fuel in, it always clicks off full even though the tank reads 1/4 or 1/2 and visual look inside showed same.
 
Indy1K said:
f the gas tank vent line was clogged, not allowing negative pressure in the tank to breath, could this cause the fuel pumps and carbs to starve during higher RPMs? The reason I ask is when putting fuel in, it always clicks off full even though the tank reads 1/4 or 1/2 and visual look inside showed same.
Click to expand...

The "quick check" for this is to loosen or remove the fuel filler cap
 
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Indy1K said:
I will be doing a compression check next time I am there. I
Good!

will also check the TDC piston to damper mark, never thought of that.
The procedure involves a PPS..... Positive Piston Stop. That is the ONLY accurate method for you at this point.
Don't let anyone tell you that a dial indicator is going to be as accurate!!!!!!

Is there any info, other than the top of the Flame arrestor to indicate the advance curve?
The flame arrestor info will typically mention BASE or Initial Advance ONLY.

I have heard 30deg @ 3000rpm.
Not with the GM full dished pistons!

On the same issue. If the gas tank vent line was clogged, not allowing negative pressure in the tank to breath, could this cause the fuel pumps and carbs to starve during higher RPMs?
Only after some time, and this is very dependant on the fuel tank level.
The fuel pump is designed to pull a vacuum against the fuel.
In the event of a restricted vent hose or vent fitting, a full tank being drawn down will create more negative pressure earlier than a nearly empty fuel tank.
Generally speaking..... if you started out with ambient tank pressure, and if the line/fitting was restricted, the engine will typically run well until the negative pressure over-comes the pump's abilty to pull!

The reason I ask is when putting fuel in, it always clicks off full even though the tank reads 1/4 or 1/2 and visual look inside showed same.
It is certainly worth investigating!
Click to expand...
 
Update on doing a compression test on the Starboard engine. Readings range from 120 to 140psi. This is cold and dry. no oil squirted in cylinders.
Manual states 146 to 152psi.
Putting in new plugs AC Delco. Manual says gap to .028". Old plugs gapped at .035" to.038" wet with fuel, but not burnt or sooty or oil soaked. I would normally gap carbureted small block chevy to 0.035". Advice?

The distributors appear to be refitted with the Pertronix Ignitor SS ignition. Plate says 1581-82 and the sensor says 38W-V4. Coil has a ballast resistor. I will measure my grounds and 12 volts next to confirm that I have the correct current to the Ignitor. I have not gotten to the timing yet. Could these pertronix units be causing my limited rpms?
 
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