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Merc 4.3 thunderbolt ignition tach issue

Wakerider000

Wakerider000

New member
Hi, trying to get my tach working. Bought a 93 maxum w/ 4.3 19’er All online help says tach feed should come from one of two gray wires off neg side of coil. There’s only one grayish yellow wire on neg side of coil and it goes into thunderbolt ignition module.
Already replaced tach. Have good wiring from plug on engine to tach. Took engine plug apart, cleaned and spread pin #2 a smidge. Can’t trace past plug because wire color changes coming through plug. Tach lights up and goes to zero when key is turned on indicating good ground.

Trying to fix tach because boat tops at 33 mph (gps) runs smooth and starts easy. But doesn’t have good hole shot or top speed. Seems sluggish imho.

Wondering if a previous owner disconnected important wires involving timing advance/tach

Additional details:
Boat was owned by elderly couple from Staton Island for the last 10-15 years. Didn’t use it much and has 495 hours. But they kept it at a marina that maintained it.
SS prop 21p
Was upgraded to 4 barrel carb by professional
~3000lbs
Alpha one drive ratio 1.61

Anybody got some advice for me?? I’m new to marine engines.
 
Pin 2 on engine plug should be gray and go to tach (tach should have two (2) gray wires).

V6 1989 to 1993.jpg
 
If you are getting on plane and you trim outdrive up for max performance then at this time I would say it has nothing to do with timing advance. The "ignition Amplifier" in the image is actually the timing control module. If you were not getting full advance the engine most likely would not get above 2400-2800 rpm and the boat would not get on plane.

You should see 4600 to 4800 rpm at wide open throttle with drive trimmed up. Maybe 45-to 50 mph but a V6 4bbl is only about 210/220 HP. so dont expect a racing boat. 42-45 mph may be more realistic.

Once you get the tach working then you will have a better idea
 
Thanks for the quick reply and diagram Kghost! I’m going to try adding a wire from neg side of coil to gray tach wire on harness. Hopefully that don’t fry anything. Everything else looks right on the diagram. mine is missing. Will do that today and update.
 
I thought the early 1990's 4.3's came with a 1.81:1 gearset...if yours has the 1.62:1 gearset, it will likely be slow to accelerate...
 
Added a wire to the neg side of coil and tach now works. Now to drop it in water and see what RPM I’m running.

Not expecting boat to be a race boat but really think it should do better than 33. Haven’t pulled wakeboarder yet but not sure if it’ll pull a 220 lb guy up. I already have to send people to the front of the boat to get it to plane sometimes.

I’m gonna change fuel filter but I’m suspicious of the 4 barrel carb. Installed the year before I purchased boat. What makes a boat run good but lack power?
 
Dropped it in water this morning with tach working and I’m getting 3800-4000 rpm.

looks like somebody jerryrigged a Yamaha marine fuel filter setup into it. Not entirely sure it’s big enough as some of the replacement filters say “for under 115 hp”.

Usual filter issues give rough running in high rpm and cutout. Would an undersized filter give smooth running but never reach full power?

Only other thing I found off is about an inch above full on the oil level. Maybe causing drag on crank?

But got my tach working so THANK YOU kghost!
 
Actually one other thing. Hour meter is a gauge dangling from engine in engine bay. At this point guessing boat was re powered which opens a new can of worms. Potentially mismatched drive etc.
 
Do an oil change and make sure there is no water in the oil.

To much oil will cause foaming and mechanical failure.

I believe the V6 is 4.5 qrts but could be 5. Add 4 and check dip stick and add enough to get to normal level.
Thats if there is NO water.....

As far as rpms, you should confirm outdrive gear ratio.
Spin drive shaft or engine one full revolution and count prop rotation (s).
If drive is actuall 1.6:1 that is what you should see/measure....

As suggested it could be the wrong ratio for that boat.

Most pleasure boats of your size are designed around a 19 pitch prop. +/- 1 pitch. 17-21 pitch. This should get you to ~4600-4800 rpm.

That being said, looks like you have some issues to resolve and confirm.

Do some more investigation and report back
 
According to manual for V6 of that year, 4.5 qrts of oil.

I cannot find in the engine or outdrive manuals what the gear ratios are.
Maybe someone has that info.

Also some additional info may be helpful,

Where do you live, where did the boat come from? Can you get any serial numbers (engine and drive)?

Also if you are checking gear ratio this should be done manually (NO KEY USED). If you lull drive off you can spin drive shaft, if you do this with drive attached, put shift into forward and make sure prop is engaged by spinning C'clockwise then use a socket on crank bolt, rotate as you are looking at front of engine, Clockwise one revolution after marking prop to measure rotation/ratio.
 
Alright thanks for the suggestions might be a week till all that gets done. But I’ll see what I find.

Boat was kept at a lake on the PA/NY border.. I’m in Harrisonburg VA. They bought it early 2000s. So there was a previous owner that I don’t know.
 
Okay went to the lake Saturday.
Here’s my current findings.

Serial number on drive is 0F193143
Haven’t found engine serial# yet.

Ratio appears to be 1.6:1
Every revolution of the crank gives just over half a turn on prop.

Fuel filter definitely says “for engines under 115 hp” thinking this is not the issue since it runs fine. And just changed it out but also ordered the correct filter system.

I get 33 mph trimmed down with rpm of 3500
37 mph trimmed out at 3900 WOT
And 24 mph at 3000 rpm.

Knowing this should I try switching to pitch down from 21 to maybe 17? Any recommendations?

the bolt in one photo was laying down in the engine bay. Not sure where it came from.
 
No photo....

I would look for a rebuilt aluminum 3 blade 17 pitch prop. Dont waist any money on a new one yet.
This should get you about 600 rpm and a much better hole shot. That should get you around 4500 rpm

If the drive ratio was more like 1.84:1 the 21 pitch might be good.

One more detail, Maxum is made by Bayliner, just made across the street from Bayliner plant. 19 ft boat is a bit long for a V6 so this will also affect top speed.
My opinion, once you go longer than 18 ft its time for a V8.
 
wrong upper drive ratio will prohibit you from liberating all the power the engine can produce...and putting a smaller pitch prop is more of a band-aid....and continued use will shorten the service life of the engine...
 

Attachments

  • Stern Drive Unit ratios.pdf
    15.7 KB · Views: 6
Not sure why photos are not uploading right.

Found an 18 pitch SS last night on eBay for $125 ordered it and at least have a closer idea.
I was considering putting a 5.7 mercruiser in as they can be had for 5-6k. Not sure if they’ll fit though. And plate on transom says 220hp MAX. (Not sure how biblical that is). Figured best bet was to at least maximize the performance of my current engine/drive.

I’m gonna look into how hard a gear swap is

Thanks so much for y’all’s input! I live two hours from any lakes and not many people near me have boats and no boat shops around.
 
As far as a gear swap goes,
95% of alpha one have the same lower unit gear ratio so all you would need is the upper gear case. Look on Facebook market place for a upper with a 1.8x:1 ratio.
 
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