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power drop under load mercruiser 3.0

S

Sasha

New member
I have been having issues on my mercruiser 3.0 1992. Serial D706324
When idling or anything up to about 1/3 throttle it seems great, but after that there is a noticeable drop in power and the engine seems to stumble/bog down and actually drops speed. Can't get it past say 10mph. Not prop related for sure as that would not cause even lower engine rpm above 1/3 throttle, but would stay flat.

Symptoms/specs/things done:
  • Was not run for over 7 years - bought it this way.
  • no real idea how many hours on motor as hour meter shows 70 hrs but I don't think it was a stock item as other photos boats of same year don't have it installed where mine is installed (below instrument panel at left of steering wheel)
  • Carb was all corroded inside and I cleaned it/restored it.
  • starts a bit hard ( I know the starting procedure. turn over 5 secs to load bowl, pump 3 times throttle, then start). It takes about 10-20 secs to start.
  • idles good.
  • low power in gear good.
  • rpm in idle good.
  • tach not working (new one on order) so I don't know what it could indicate.
  • sometimes backfires/spits through carb on acceleration.
  • starts harder when warm after sitting for a few minutes
  • timing is currently 0 degrees.
  • timing advances on increased rpm.
  • plugs black, sooty indicating rich fuel.
  • Carb mechanicals all smooth and clean
  • all carb passages tested with air and by pumping fuel through with hand pump.
  • carb power piston plunger was seized so I cleaned/polished it but unsure if it is operational at high vacuum/low rpm - this does not however explain lack of high speed as plunger would be at maximum extension and pushing on high speed valve at bottom of bowl anyway.
  • accelerator pump is good and pumps nicely on throttle.
  • flame arrestor clean
  • spark on all cylinders checked using timing light
  • cleaned distributor cap and rotor
  • fuel pump and filters all good and clean
  • compression 130 on all cylinders
  • emptied fuel, added bottle of seafoam and new fuel
  • cleaned carb
  • new gaskets on carb
  • adjusted float to specs
  • cleaned jets
  • did carb 5 times to eliminate any possible issues
  • the carb is a mercarb also. 1 mixture screw at bottom. (I did not adjust this or touch it during rebuilds)
  • spark cables look in very good condition
  • no visible or audible arcing of sparks
  • distributor appears solid, clean and in good condition (can't feel any play)
  • distributor is electronic - no points or fly weights, just rotor and module
  • coil plugs to distributor at center post.
Here is what I did not do:
  • did not test for air leak at carb since gaskets are new and there is no fuel leakage around gaskets. (I could do this with spraying wd-40?)
  • did not check for intermittent spark (I could do this with timing light on each wire?)
  • did not test fuel pressure as I have only a vacuum pump (hand operated) and compression gauge. (any other hack/ideas on what I could use so I don't have to buy a $100 tool I will never need again? I have an a/c gauge, could I somehow use that?)
  • did not test giving it extra fuel into carb when in water as I was testing alone at the time.
  • did not record high speed advance since no tach for rpm.
  • plugs are not changed but look to be in good condition other than the soot.
the carb was varnishy smelling for sure. It is perfect now. the fuel in the tank, oddly enough actually ran fine in both my lawn tractor as well as in my truck (thought I would try) but I did flush it all and used new fuel and Seafoam as per my post. Third, the carb was rebuilt 5 (FIVE) times just in case. Fourth, The entire fuel line system was cleaned, flushed and checked. Nothing was ever plugged, varnished or in bad shape all the way down to the fuel pickup screen in the tank. Fifth, the only thing I never touched was the idle mixture screw and all components/holes etc were cleaned with air and fuel pumped through manually (again, read the post).

When the engine is at idle with the white jumper wires connected at distributor and shift lock disabled, the timing is at 0 Degrees. What is the timing for this engine serial D706324?
Am I missing something obvious? I have read every advice column, forum etc on this motor and nothing else works so far.

Please help? I know it's easy to say "take it to a mechanic" or "throw more money at it" but that is not really an option if I can help it because funds are low and non-helpful, self-serving comments like that are of no use to anyone, so please save those for you private "in your head" comments.

Any positive advice would be very helpful please. I live in BC Canada nowhere near a big center that has loads of resources.
 
I'll bet your have a plugged anti-syphon valve at the fuel tank.

If that's NOT it, you need to hook up a fuel pressure gage and see what is going on.

Jeff
 
>>>snip<<<. There's a good chance that the fuel lines from tank to carb are not ethanol rated. If you put ethanol fuel in the tank, it tends to soften and eat the inside of the fuel lines, but they look OK from the outside. Now the fuel pump has to suck through a little tiny passageway, so the carb starves for fuel. As Jeff says, test the fuel delivery with pump disconnected from carb.

Now, I'm not saying that you need to spend a lot of money, but boats are not cheap toy to own or operate. Spend your money wisely with an eye towards safety above all other considerations. Having a reliable, easy starting engine that maintains a well sorted electrical system is the primary safety focus. Without that you are risking yourself, and others on the water.
 

Went to lake last weekend. I could get on plane, but it takes a few seconds longer. At WOT, I only get 4000rpm (with a reduced pitch prop). Pulling a tube, it bogs until exactly 2400 RPM at which point you could feel the acceleration kick in hard and it goes right up to 4000rpm. This is exactly the rpm each time. Definitely not carb, but timing. I spoke with a marine mechanic and he said that the module is working. What is not working at this point is that it is stuck in base mode. I have followed the procedures over and over again and cannot leave base mode. I have unfortunately both of my available modules both stuck in base mode. Is there any way to reset them? The timing in base is absolutely correct. It just does not leave base mode. How can I tell? It stays at 1DBTDC. Only advances to about 12Degrees at 3000 rpm. I am clearly missing 1000 rpm at WOT and rpms struggle until about 2400 rpm under load at which point it goes but only to a max of 4000 rpm with a reduced pitch prop. With the stock prop, it only goes to about 3500 rpm max. With engine running, put shift interrupt switch back to normal, then removed jumper at distributor. Did this countless times as per manual.
Checked voltage at jumper and it is 12V+.
Ideas? No sense saying I am overpropped, carb etc. This is clearly a timing issue and solely to do with lack of advance.
It never leaves 1DBTDC at idle.
It should read 12DBTDC at idle when not in base mode.
Someone suggested to set it at WOT to whatever the advance is, but that is not good because it still doesn't have the proper advance curve.
Buying yet another module might be an option but I just feel that there must be a way to set the ones I have back to regular timing mode.
icon1.gif

Went to lake last weekend. I could get on plane, but it takes a few seconds longer. At WOT, I only get 4000rpm (with a reduced pitch prop). Pulling a tube, it bogs until exactly 2400 RPM at which point you could feel the acceleration kick in hard and it goes right up to 4000rpm. This is exactly the rpm each time. Definitely not carb, but timing. I spoke with a marine mechanic and he said that the module is working. What is not working at this point is that it is stuck in base mode. I have followed the procedures over and over again and cannot leave base mode. I have unfortunately both of my available modules both stuck in base mode. Is there any way to reset them? The timing in base is absolutely correct. It just does not leave base mode. How can I tell? It stays at 1DBTDC. Only advances to about 12Degrees at 3000 rpm. I am clearly missing 1000 rpm at WOT and rpms struggle until about 2400 rpm under load at which point it goes but only to a max of 4000 rpm with a reduced pitch prop. With the stock prop, it only goes to about 3500 rpm max. With engine running, put shift interrupt switch back to normal, then removed jumper at distributor. Did this countless times as per manual.
Checked voltage at jumper and it is 12V+.
Ideas? No sense saying I am overpropped, carb etc. This is clearly a timing issue and solely to do with lack of advance.
It never leaves 1DBTDC at idle.
It should read 12DBTDC at idle when not in base mode.
Someone suggested to set it at WOT to whatever the advance is, but that is not good because it still doesn't have the proper advance curve.
Buying yet another module might be an option but I just feel that there must be a way to set the ones I have back to regular timing mode.
 
dsfsdfsdf
icon1.gif
Went to lake last weekend. I could get on plane, but it takes a few seconds longer. At WOT, I only get 4000rpm (with a reduced pitch prop). Pulling a tube, it bogs until exactly 2400 RPM at which point you could feel the acceleration kick in hard and it goes right up to 4000rpm. This is exactly the rpm each time. Definitely not carb, but timing. I spoke with a marine mechanic and he said that the module is working. What is not working at this point is that it is stuck in base mode. I have followed the procedures over and over again and cannot leave base mode. I have unfortunately both of my available modules both stuck in base mode. Is there any way to reset them? The timing in base is absolutely correct. It just does not leave base mode. How can I tell? It stays at 1DBTDC. Only advances to about 12Degrees at 3000 rpm. I am clearly missing 1000 rpm at WOT and rpms struggle until about 2400 rpm under load at which point it goes but only to a max of 4000 rpm with a reduced pitch prop. With the stock prop, it only goes to about 3500 rpm max. With engine running, put shift interrupt switch back to normal, then removed jumper at distributor. Did this countless times as per manual.
Checked voltage at jumper and it is 12V+.
Ideas? No sense saying I am overpropped, carb etc. This is clearly a timing issue and solely to do with lack of advance.
It never leaves 1DBTDC at idle.
It should read 12DBTDC at idle when not in base mode.
Someone suggested to set it at WOT to whatever the advance is, but that is not good because it still doesn't have the proper advance curve.
Buying yet another module might be an option but I just feel that there must be a way to set the ones I have back to regular timing mode.


icon1.gif

Went to lake last weekend. I could get on plane, but it takes a few seconds longer. At WOT, I only get 4000rpm (with a reduced pitch prop). Pulling a tube, it bogs until exactly 2400 RPM at which point you could feel the acceleration kick in hard and it goes right up to 4000rpm. This is exactly the rpm each time. Definitely not carb, but timing. I spoke with a marine mechanic and he said that the module is working. What is not working at this point is that it is stuck in base mode. I have followed the procedures over and over again and cannot leave base mode. I have unfortunately both of my available modules both stuck in base mode. Is there any way to reset them? The timing in base is absolutely correct. It just does not leave base mode. How can I tell? It stays at 1DBTDC. Only advances to about 12Degrees at 3000 rpm. I am clearly missing 1000 rpm at WOT and rpms struggle until about 2400 rpm under load at which point it goes but only to a max of 4000 rpm with a reduced pitch prop. With the stock prop, it only goes to about 3500 rpm max. With engine running, put shift interrupt switch back to normal, then removed jumper at distributor. Did this countless times as per manual.
Checked voltage at jumper and it is 12V+.
Ideas? No sense saying I am overpropped, carb etc. This is clearly a timing issue and solely to do with lack of advance.
It never leaves 1DBTDC at idle.
It should read 12DBTDC at idle when not in base mode.
Someone suggested to set it at WOT to whatever the advance is, but that is not good because it still doesn't have the proper advance curve.

Buying yet another module might be an option but I just feel that there must be a way to set the ones I have back to regular timing mode.
icon1.gif

Went to lake last weekend. I could get on plane, but it takes a few seconds longer. At WOT, I only get 4000rpm (with a reduced pitch prop). Pulling a tube, it bogs until exactly 2400 RPM at which point you could feel the acceleration kick in hard and it goes right up to 4000rpm. This is exactly the rpm each time. Definitely not carb, but timing. I spoke with a marine mechanic and he said that the module is working. What is not working at this point is that it is stuck in base mode. I have followed the procedures over and over again and cannot leave base mode. I have unfortunately both of my available modules both stuck in base mode. Is there any way to reset them? The timing in base is absolutely correct. It just does not leave base mode. How can I tell? It stays at 1DBTDC. Only advances to about 12Degrees at 3000 rpm. I am clearly missing 1000 rpm at WOT and rpms struggle until about 2400 rpm under load at which point it goes but only to a max of 4000 rpm with a reduced pitch prop. With the stock prop, it only goes to about 3500 rpm max. With engine running, put shift interrupt switch back to normal, then removed jumper at distributor. Did this countless times as per manual.
Checked voltage at jumper and it is 12V+.
Ideas? No sense saying I am overpropped, carb etc. This is clearly a timing issue and solely to do with lack of advance.
It never leaves 1DBTDC at idle.
It should read 12DBTDC at idle when not in base mode.
Someone suggested to set it at WOT to whatever the advance is, but that is not good because it still doesn't have the proper advance curve.
Buying yet another module might be an option but I just feel that there must be a way to set the ones I have back to regular timing mode.
 
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