Logo
FULL SITE MENU

Poor W.O.T 1989 Mercrusier 4.3

F

Forolio

New member
I have a 1989 Thundrcraft Magnum 220 with a 4.3 Mercruiser Alpha 1 drive.
It is a heavy boat at 4000 LBs
When I purchased the boat it had a 14 D 19 P 3 blade Piranha (replaceable blade)prop.
The boat never achieved WOT with that prop. and with a load would not even plane.
I changed to the spare prop that came with the vessel which was a 14 D 15 P.
It now rides and handles much better and cruises at a speed of about 25 MPH. at about 3200 RPM
When I research all that I could most recommendations come back at a 19 P. prop
Is a 14 P prop seriously under propped for that boat motor & O/D?
The motor is solid, starts good and runs good except when approaching WOT.
When hitting about 3500 RPM it seems to lag and almost seems to stall.
I have never pushed it above that point.

I have now had the following done by a Mercruiser mechanic and still cannot hit WOT, used all OEM parts
I emptied the boat of all non essentials for weight.
Still using a low pitch prop!
  • New plugs / wires / distributor / full tune up all OEM parts
  • Had the carb taken out and cleaned rebuilt with a carb kit ( Holley 4 barrel)
  • Changed primary and secondary inline fuel filters.
  • Did notice almost a black jelly substance on the primary bowl filter which i changed.
  • Thought the fuel line might have been deteriorated causing the sludge so I put on a new fuel line.
  • Changed the ignition coil
  • Set the timing which had been slightly advanced
  • Compression checks within spec. I'm told.
  • Hot wired from the battery to the coil to ensure full voltage.
  • The boat starts well/ idles well/ under low power runs well.
  • Trying to accelerate the motor bogs down and will not hit WOT
  • Even took a buddy to spray gas down the carburetor while accelerating in an attempt to isolate a carb problem under acceleration. No better.

I will pull the boat out and ensure the out drive has the proper ratio for that motor just in case.
I just bought the boat in May it did run OK at trial (believing a tune up was required)
I will also check the exhaust to ensure it is not restricted. One of the shudders may have broke and is restricting flow

Before all the above was complete I could get up on plane and hit 32-3500 RPM when trimmed.
It now does not go above 2700 RPM and almost seems to perform even more poorly than it did.
When accelerating the motor just bogs and will not accelerate.
Really perplexed as to what other conditions may contribute to this.
Any additional assistance brain storming would be appreciated as we are running out of potential causes.
Starting to lean towards ignition amplifier, Here's my logic behind the ignition amplifier. (hope I'm on the right track)
On the 19 pitch prop it would only max at 2500 RPM (consistently at WOT)
When i changed to the lower pitch prop revs would only get to 3200 RPM (consistently at WOT)
When checking the timing it had been advanced so it was adjusted back to 8 degrees (from 12 degrees)
Once timing was adjusted it now only gets 2700 RPM (consistently at WOT)
I am speculating that the previous owner may have advanced timing to get higher WOT perhaps knowing the module was bad.
I'm hoping that by checking timing while accelerating the timing may not advance correctly or be inconsistent.
I will check all other suggestions as these modules are very pricey..
 
My best guess? Broken flapper valves blocking the exhaust outlet. The outdrive has to come off to find out.

Jeff
 
Further Bt Doctur... I'm running a 17x15P 3 blade on my 23 ft 4000# deep V ( 22 degree dead rise) with a 5.7L/260HP engine via a 1.61 reduction. The online calculators indicated it would turn a 21x15. On line calculators are ok for a first guess only. If engine is OK and bottom clean, the correct prop is that prop that at WOT allows you to get to within the WOT throttle RPM range specified for that engine.

Re: fastjeff's post... It's worth a try. I gained 450RPM @ WOT when I went from thru drive exhaust to thru transom with mufflers.
 
Re: Timing... Do you have a Thunderbolt ignition??? IFSO, timing needs to be set at idle after grounding a wire, restarting the engine therby putting ignition system in base timing mode.
 
1. 1989 thunderbolt will be IV, so no jumping anything to set timing.

2. No such thing on an OEM mercruiser 4.3 as a holley 4 bbl carb.
You said duel feed??? Your carb has two fuel lines to it?
.if so get a new carb. That most likely is a 750 cfm "double pumper" commonly used on high performance 454 and bigger big block V8's.

That stalling if above is true may be because its flooding out, too much fuel.

3. Drive serial number (should be very close to engine and transom serial number) or confirm ratio in drive. Should be 1:84-1. And probably should have a lower that makes it 2:1.


15 p - 23 p props can be normal based on many factors.

Most run abouts are typically set up for 19-21p props.

Something may have been changed that has caused this issue.

Typically when the timing module is not working it runs like crap above 22-2400 rpms.

Timing advance can be seen with timing light just by increasing rpm in nuetral. If base is 8 btdc and by~24-2800 rpm it should be fully advanced..~32*.(dont hold me to the spec as i am answering this from phone and no manual to refer to right now)

Maybe someone else can give the advance rpm and amount in degrees....
 
Last edited:
I needed a timing light that could see the degrees when reving the RPM higher, the one I used could not give me an actual degree reading, hence I don't know that timing was advancing properly to 30 degrees at higher revs.
The carb is a holley and may have been changed. No dual feed lines.
The carb was rebuilt and sonic cleaned, do not believe the carb is a contributing factor.
Going at it again today, with a better quality timing light.
Hence my rationale on ignition module if timing is not advancing under revs.
 
A Full In advance of 32* @ 2,400 RPM may cause severe engine damage.
For data as important as ignition advance, play it safe and always refer to your OEM manual via your particular engine and serial number.


.
 
Disregard prior post!!!!
Advance is 100% controlled electronically for Mercruiser thunderbolt ignitions!!!!
There is nothing a human can mechanically do to change it.
 
Disregard prior post!!!!
Click to expand...

Post #10 was written as to elaborate on info in post #7.

post #7 said:
Timing advance can be seen with timing light just by increasing rpm in nuetral. If base is 8 btdc and by~24-2800 rpm it should be fully advanced..~32*.
(dont hold me to the spec as i am answering this from phone and no manual to refer to right now)

Maybe someone else can give the advance rpm and amount in degrees....
Click to expand...

As said;
post #10 said:
A Full In advance of 32* @ 2,400 RPM may cause severe engine damage.

For data as important as ignition advance, play it safe and always refer to your OEM manual via your particular engine and serial number.

Click to expand...
 
Again disregard prior post.

For testing all will be good.
Just make sure base is to spec ~ 8* BTDC

If prior poster had the manuals that may help him in inderstanding.

No one gives an F about when or how much advanca.
The issue for this topic is...

DOES TIMING ADVANCE OR NOT.

NOT HOW MUCH OR WHEN.....ugh!
 
Last edited:
No one gives an F about when or how much advanca.
Click to expand...


I certainly do, and I'm not alone!
In order to fully understand the gasoline internal combustion engine and to tune one correctly, one also must be educated and fully understand how spark lead plays a very important role.
Too much lead and too early.... and damage may occur!
 
Problem is you are the only one who thinks this way.

NO ONE ACTUALLY CARES WHAT YOU SAY OR HAVE TO ADD.
You seemed to have been away from the Mercruiser forum.

You should have and or continue to stay away.

Again your posts are off topic and not valid especially with no actual specification and only, what you and you alone feel is highly desired words trying to demonstrate some kind of superior technical expertise which you do not posess with Mercruiser products.

So share your "expertise" on the other forums!
With ZERO Mercruiser experience you dont belong here!
 
Back
Top