Merc 43XL Thunderbolt IV incomplete fuel burn

"I sure as heck hope someone h

"I sure as heck hope someone has some idea's on this. I'll try to give short version. Last summer boat started to "surge" slightly when on plane, especially when hiting a wave creating extra load. Gradually got worse over the summer until it would not make on plane and would "bog" when applying full throttle.

I have replaced fuel pump, carb, fuel line,fuel filter, tested anti-syphon valve, etc, fuel is good no water so its not the fuel system.

So must be ignition, replaced as maitainance items, rotor, ditrib cap, wires, and plugs. Nothing. But plugs are black on all cylinders. Replaced coil. NO. Just replaced ignition module as the Merc test indicated w\G wire should spark to ground when coil tower grounded and it didn't. It still does not spark to ground, but volt meter shows drawn down. Exaust stinks and there is carbon deposits in the water and unburnt fuel.

Did compression check and 150 psi on all cylinders. Did manifold vacum check and 18 lbs at idle with no drops and 24lbs at 3000 rpm with no drops. Seems engine breathing well.

Oh, I had my mechanic do a timing set mid summer last year. Going to do it again this week to be sure.

HELP!!!!!!!!!!!!!!!!!!!!

Terry}}"

"Just an idea, but check the s

"Just an idea, but check the spark arrester. Seen them get clogged and completely dog a motor out.

Jack"

One of the first things. I ran

One of the first things. I ran it several times with hatch up and arrester off.

Thanks Jack

"Terry:

Black plugs indicat

"Terry:

Black plugs indicates too much fuel coming from the carb. "Exaust stinks and there is carbon deposits in the water and unburnt fuel" is evidence of too much fuel. It seems the float may be out of adjusment and/or the needle valve is worn causing a near flooding condition.

The spark issue is apparently part of the problem. I use a "Flash Tube" spark tester that is held next to the spark plug to indicate the type of spark at the plug. It will let you know how each plug is firing.

Start by checking the choke setting and operation. Next, verify the float level is to specs. and needle valve cond. and adj. or replace as needed. Then clean or replace the plugs due to the carbon and soot condition. Fuel old or contaminated with water? Pump it out if tank is full or if 2/3rds or less full, dump in two cans of SEA FOAM and fill with quality grade of fuel w/o alcohol. Hope these suggestion help.
Guy"

"As Guy says, you are running

"As Guy says, you are running way rich. Suspect you need a new needle and seat. They don't last forever. Fairly easy to change out. Is it a Weber 4 bbl?
Rod"

"Hi Guy & Rod,

That&#39

"Hi Guy & Rod,

That's what I thought at the first. I put a new carb on late last year. Rochester 4bbl.

I ran the fuel pretty low prior to storage, added stabilizer, and have 1\2 tank of fresh fuel now. I checked for water in fuel when I had the old carb apart, would have shown in the float bowl. Choke is auto and working.

I really suspect a bad ground to the ignition module. Have you ever heard of this srewing up the voltage output from the ignition module? Hard to tell if spark is 35,000 volts or 10,000 volts by sight.

Terry"

"Terry;
Did it run like this

"Terry;
Did it run like this before the carb changeout?
Is it the 4MV quadrajet you put on? If so, that may be a bit much of a carb for the 4.3; which only needs about 380 CFM at 5000 RPM.
In my mind it's not likely a spark problem. If the spark voltage is weak the spark will have trouble jumping the gap and you will get missing and stumble under load.
Have you got the correct plugs and the correct gap?
Have you possibly got some big restriction in the exhaust system? Plugged risers or elbows maybe?
Rod"

"Hey Rod,

Yea it did. The n

"Hey Rod,

Yea it did. The new carb is a factory rebuild of the old, so the same 4MV. The plugs are new and from a dealer. NGK's. I'm going to double check gap to be safe. Mechanic gaped them for me. Should be 35 thou.

When I did the intake manifold pressure, would an exhaust restriction not create backpressure and reduce manifold readings?? I am suspicious of them as they seem to be the single biggest general issue with I|O.

Thanks,


Terry"

"Check the plug reach also. Th

"Check the plug reach also. There was an issue in another recent post where the specced plug did not have long enough reach for the head. This is not good as then the spark is pocketted and the flame front will be slow expanding, so the same effect as late timing - low power.
Is your timing set correctly?
Rod"

"Hey Rod,

Checked the plugs

"Hey Rod,

Checked the plugs against the NGK website and they are the right ones for the 4.3LX, I assume they have the right reach??? How do you measure this??. Will buy gap tool tomorrow. NGK says 40 thou, Seloch manual says 35 thou. Who's right?

I don't have a timing lite but will buy one tomorrow. Any difference reading timing under load or just on the muffs? I would not think it would matter. Does Thunderbolt IV system require any grounding from module to set timing?

I am still suspicious of exhaust. When I did manifolfd vacum test, it seemed fine. Is this a good test for exaust restriction, do I need to do it under load??


Terry"

"Terry;
The plugs are likely

"Terry;
The plugs are likely correct and 35 thou is fine. 5 thou diff one way or the other won't matter anyway. To check the reach take a short piece of wire with 1/8" of the end bent at 90 degrees. Insert this into the plughole and measure the dist from inside the threaded part to the outside face where the plug seats. Hold this up against a plug and compare. Ideally you should have about one free thread of the plug shank inside the comb. chamber when seated on the outside.

Timing is set under no load at idle RPM. I don't think you ground anything with T'bolt IV; only T'bolt V. I'll check my book tonight.

You are right about the vaccuum readings; they look about right.
So what the H**L is the matter with your friggin engine???? Kind of running out of ideas. My 4.3 LX ran like a top, never missed a beat or needed any work at all.

Something else I just thought of: You shud check the fuel supply pressure at the carb inlet fitting by temporarily T'ing in a pressure gauge. If your elec fuel pump is putting out too much pressure, it will be flooding the carb constantly. It shud be about 4-5 psi only.

If all above checks out, I would take the top off the Q'jet and lower the float level about 1/4" just for fun. You really can't hurt anything and it should solve the rich problem. It would not be the first time that a rebuilt carb had the wrong float level, or even a leaky float.
As a very last resort, I would take the risers and manifolds off and visually see what you find. This is pretty easy to do and does not cost much.
You didn't upset the Boating God in another life, did you??
Rod"

"Hi Rod,

Thanks for the ti

"Hi Rod,

Thanks for the tips. I bought this boat 5 years ago and no problems at all. I have done just about every type of engine repair possible over the years and I think your right, I pissed off the boating gods somewhere.

By the way, the fuel pump is mechanical and brand new!!!

Thanks Again,

Terry"

"Terry:

I know that NGK is

"Terry:

I know that NGK is a quality plug. Only used them in Nissan. For some reason unknown to me, I have always had trouble over the years with Chevy truck engines when not using AC Delco plugs. They seem to foul out easier so I quit using Bosch platinum, Champion and a host of others. I guess it is a matter of choice.
Guy"

Did you do a cylinder compress

Did you do a cylinder compression check? Gasket failure can get cyclinder cross firing or not firing consistently>

"Did you actually check the co

"Did you actually check the coil with an ohm meter? I know it is not a perfect test, but it is a start. Also, check your timing chain by pulling the dist cap and rotating crank back and forth to see if there is any play. Dist rotor should move with crankshaft."

"Hey All,

Plugs are are gap

"Hey All,

Plugs are are gapped right, reach seems fine. Compression check was done. Coil is new.

I beleive the timing was set at TDC not 8 degrees but it still seems to run rough when I advance. I took the distributor cap off and cranked to #1 to find a timing mark. There was a indent in the balancer. This seemed to line up with largest pointer on the timing indicator. I assume that is TDC and each smaller one is 2deg of advance.

Note here that I had to remove the distributor last year when replacing rotor and cap. I had a mechanic reinstall as I was not comfortable finding TDC and getting initial timing right. Now I'm not sure he did it right. When I set to 8 degrees the engine starts to heat up like there is too much advance. I am now suspicious of the Distributor being out a tooth or the chain might have jumped.

I have a freind who is an auto mechanic who is coming over later in the week to make sure I have TDC and set timing.

Any thoughts would be appreciated.


Terry"

"Just to update on this.

Th

"Just to update on this.

The timing was set wrong by the mechanic that helped me last year. So that was contributing, but not the problem. There is definately low spark. I've checked all contact and ground point and are good.

Here is what appears to be the problem. The ignition sensor. The Mercruiser test flow chart for Thunderbolt 4 indicates that if you ground the main coil with a gap tester, take the gr\wt wire from the ignit. sensor and touch it to ground it should spark, no spark, replace ignition module. I couldn't get spark so replaced module at $300. Not the problem.

Here is what I think is happening. Found on another post that the sensor in distributor should read 100 ohms across the sensor. The reading is 10,000 ohms. This indicates possible corosion, break in wire, something. This sensor is the first pulse that fires the ignition. If its output is down, all output will be down, but its enough to still fire the main coil in time. So, I'm ordering the sensor, and if it fixes the problem, and someone says that the ignition module or sensor either fail, or work, they are wrong. My father and best friend are electronic technicians and both agree that things like this can happen.

Curious if anyone has ever seen this one before???????

I'll post back in a week when the new sensor is installed.

Thanks,

Terry"

Terry: I've only seen the

Terry: I've only seen the optic type ignition sensor go bad from heavy rust. The replacement is all plastic. Save your module if the sensor fixes it.

"Hi Guy,

Just wondering if

"Hi Guy,

Just wondering if you have seen the same symptoms as I have. It was rusted quite bad.

Terry"

mine was an intermittent probl

mine was an intermittent problem of poor idle and start. Another man posted a no start problem and I advised him to check for spark using the module test procedures. He found the rusted sensor and replaced it. New optic sensor started the engine perfectly.

"Hey Terry,

i am super inte

"Hey Terry,

i am super interested in your problem. My 4.3 with thunderbolt v ignition seems to be having similar problem. At first it would simply surge when on plane and then it seemed to lose power. Tonight I had it out. It does not go up on plane, it plows thru the water for a while and then falls on plane. Could only get 4100 rpm at full up trim. I have only tried a coil and plugs so far because cap, button and wires were done just last year. Let us know how it goes."

"Terry:

Even though you sai

"Terry:

Even though you said the cap, rotor and wires were replaced last year, did you look at the rotor, cap and electronic sensor?"

"Guy & Scott,

Just rep

"Guy & Scott,

Just replaced the ignition sensor. It now runs even worse!!!

I am curious about something. The R\Wt & Gr\Wt were reversed on the new sensor from the old sensor, have you seen this Guy. It was R\Wt on the left post of the old one. I reversed them but wouldn't start so I assume its some kind of design change.

Also, I checked polarity also and it looks like its reversed. I reversed the coil wires and it still tested as reversed?????? duh!!!! I'm confused, I have a good electronics background, but this seems impossible.

Terry"

Terry:
Reset the timing. W

Terry:
Reset the timing. When I replaced mine the timing was off and ran like crap.

"First thing I did, cause it r

"First thing I did, cause it ran like crap. I can only think now that its internal. I'm going to have a leak down done.

AAAAAAAAAAHHHHHHHHHHHHHHHHHH!!!!!!!!!!!!!!!!"

"have 1996 chapparal with thun

"have 1996 chapparal with thunderbolt ignition,will run good for a few hours then all of a sudden will not go over 1000 rpms"