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Mercruiser 4.3L Cranks, Has Sparks & Fuel, But Will Not Fire Up

J

_jp

Contributing Member
Bit of Background
Early this year (2025) I bought a second hand Maxum 1800 SR3 with 4.3L Mercruiser in visibly reasonable condition, some service history, lots of documentation and many of the larger items recently done - invoices for bellows, risers, etc. It took a couple of attempts to start and ran on a water hook-up ok when purchased, but I was unable to fully water test it. My family has always been in to boats, although this is our first inboard. My skillset is fairly competent DIY level.


Engine Specs
Mercruiser Inboard
MCM 4.3L V6 (190M C/Al)
Carb Type
Thunderbolt V Ignition
1996 (approx, I can find if important)


The Issue
After purchase, I had the engine running again on a water hook-up at home and then began spending time servicing the trailer. Two months later, with no changes made to the engine, I thought I would give it one final check (on the water hook up again), before taking it for a full water test and I am glad I did.

It had power at the ignition, but wouldn’t crank. Just clicked and the dash went dead, repeated this several times. At first I thought it was the 50 amp breaker, as that looked old and was mentioned in other posts i'd read (£70 now replaced). Long story short, it was the battery isolator (£5 now replaced) corroded at the back not allowing enough battery +power (I had cleaned the negative side a lot).

Now she cranks, has good sparks (at all 6 plugs), fuel at the carb, but wont start up.


Professional Works Completed on the Ignition side Prior to my Ownership
  • New Carb
  • Distributor Cap
  • HT Leads
  • Spark Plugs

As per invoices, all the above was done in 2023, as was other servicing - filters, oils, etc, without the boat having much use since.


Tests / Working
  • Cleaned All terminals (Relays, Starter, Battery, main Earths, etc)
  • Throttle Deadman in RUN position
  • Fuzes (Checked All I Can Find and OK)
  • Battery has 12.9V (& my Test Only Spare has 12.8V)
  • 12V at the + on Coil
  • Spark Plugs All 6 had good sharp flashes
  • HT leads have ever only been removed one-by-one since last running
  • Carb - Fuel at the Carb (possibly enough to flood it, but repeated over several days with no progress. Carb Float was in down position as I believe is correct)
  • Fuel Tank is 1/4 full, shouldn’t be too old and would have been winterised (according to paperwork).

My Thoughts
I have read timing could be an issue, but I can’t see how the timing could be affected with no mechanical alterations to the engine?

With all spark plugs firing OK I would think this should rule out the following being issues: throttle deadman switch, ignition sensor in the distributor, or the ICM; suggested in other posts (but please correct me on this if my thinking is wrong here)?

Spark plugs were slightly wet with fuel when checking them one by one (I would expect this as repeatedly turning over without being run up in order to view the spark).

I did look up the model number of the Starter motor, which said it was for a 3.0L, but some are suitable for the 4.3L and seeing as it started before, i’m keeping an open mind on that?

I’ve read about compression issues, but can’t see how this could change in two months. However, if this is a real possibility then I can look to do a compression test. I probably should do one anyway for peace of mind.

All i can think of next is to replace the Water Separator and add some fresh Fuel to the tank. I was tempted to drain the old fuel, but all the pipework is bonded/sealed as well as clamped - if there's a cheat way to do this I'm all ears lol.




Any ideas would be greatly appreciated. I was hoping to water test it and winterise it before winter gets here.


Thanks

JP
 
as far as verbiage goes, MCM are mercruiser stern drives...MIE are true inboards.

Per your description, I'd lean towards fuel...if the top of the carb smells like good fuel (vs stale) it should start unless it is flodding out (or the choke was never adjusted...So check the choke adjustment first...If thats ok and the fuel is off, the easiest way to make progress is to use an outboard tank as a temporary test source...be sure to plug up the existing hose and ventilate properly before turning the key.

A serial number would be beneficial to all of us - that timeframe saw many configuration changes and the serial number will narrow that down considerably..
 
Hi Makomark, thanks for the quick reply.

Outboards, Inboards, Stern Drives... Got it! You learn something new every day (y)

Can't say I've ever smelt stale fuel before, I can only describe the current fuel smell as strong. I'm sure I certainly flooded it initially, as fuel was puddled in the carb and weeped out the gasket a little. TBH 'm not really sure what i'm looking at when it comes to Carbs, (normally spray a bit of cleaner in and that's about it), but happy to learn. I'll see if I can find a suitable Youtube video, or if you can point me in the direction of any existing setup tutorials/advice that would be great.

I do have spare outboard tanks kicking around that I could use to test, I'll try and find time this week to do that and connect up to the Water Separator unit as per this video:

I couldn't see how to update my first post with the engine serial number, but it's below for anyone's reference:

0W034153

Thanks
 
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You have about 5 minutes to edit a post...and then it is out there forever.

I would remove the hose at the inlet side of the fuel pump and then plug it...the outboard tank hose can go onto the hose barb that was exposed when the hose from the tank was removed.

I could be the carb's choke plate is stuck or even disconnected...I cant see or touch it...so can't tell...it may also have the TKS carb which is a different beast...

No youtube reference I have...FWIW, buy a used factory manual off ebay (or your other preferred source) or a new one outright...it will save you many hours off redo-s from bad information...

If the fuel smell was normal, that's not a bad thing...need to check to make sure the fuel pump runs (its electric from the diagram)...a test light helps there...they are wired such that the pump is fed thru an oil pressure switch...so if it goes bad, no fuel pump...
 
I'll focus on trying fresh fuel via an external tank first, following your info. The carb was brand new in 2023 and I would presume it was setup correctly by the servicing company that installed it. If it's not the fuel, then I'll have to get stuck in to the carb and will add some pics for reference. No idea if it's TKS.

If the fuel is reaching the carb already, wouldn't that indicate the fuel pump is working and therefore the oil pressure switch being activated?
 
Now she cranks, has good sparks (at all 6 plugs), fuel at the carb, but wont start up.
Click to expand...
If the fuel is reaching the carb already, wouldn't that indicate the fuel pump is working and therefore the oil pressure switch being activated?
Click to expand...
You have good spark
You have fuel reaching carb and can see fuel squirt down throat of carb

There is no reason something wouldn't fire, even back fire
 
AllDodge said:
You have good spark
You have fuel reaching carb and can see fuel squirt down throat of carb

There is no reason something wouldn't fire, even back fire
Click to expand...
That's why I am so stumped and have had to ask - my basic knowledge say's it should go. It's really got me puzzled. I will try fresh high-quality fuel in the week and then take some pics (or video it). Maybe I'm doing something stupid when trying to start it? :oops: I've tried the three pumps on the throttle, no pumps, etc (waiting in between).
 
I managed to crack on a bit today, with some success (all on the driveway with hose connected)...

1. Tried the Cold Start spray, got a tiny bit of life ("whimper"), as if it wanted to start. Still lots of fuel entering the carb and looked like possible flooding to me.
2. I then disconnected the fuel pump electrics, so that no more fuel was entering and cranked it to clear any flooding.
3. More Cold Start spray and she fired up on this round of cranking and then shut down, presumably on what fuel/mix was in there.
4. Repeated again, but this time rushed to reconnect the fuel pump, kept the revs a bit high, but then did the same and eventually shut down.
5. Discounted the fuel line again and connected up a jerry can with fresh premium fuel via the Water Separator. It did the same as step #4. I didn't have my tool to remove the Water Separator (would have like to have filled that with fresh fuel), so expecting it to take a few runs to pass through.
6. Repeated again with the same results, several times.
7. Decided just to put fresh fuel in the main tank and a canister of sea foam. Got her started and kept the revs above 1000 to avoid it stalling. Ran it for about 5 minutes ish, with the fuel pump still connected.

Was happy I had made some progress until i stepped out of the boat and there was a lot of oil on the floor, more than i've seen out of our two stroke outboards, which is very concerning!! :oops: . Checked the oil level, and seems excessively high on the dip stick - I missed that before. I don't remember seeing oil the last time I ran it up on the driveway, certainly not to the degree being seen today.

So she starts, but anything under 1000 revs it starts to stutter and dies. Seems to need the Cold Start to help it fire up. Haven't fully resolved the fuel issue and could be the Carb, which is really beyond my current knowledge.

Once the engine is switched off, is it normal for fuel to continue into the Carb for a while? You can see it dripping in.
 
makomark said:
Where are you seeing the "dripping in"?
Click to expand...
I'll try and upload a video later, but you can see the golden colour in the screenshot of the video. Engine dies, I put it in neutral, switch the ignition off and fuel keeps entering the main body area (I need to learn the Carb components). It puddles on the top of the two "flaps" is the best I can describe it as at the moment lol. Dripping out the pipes (Venturis?).

IMG_2658.jpg
 
makomark said:
Where are you seeing the "dripping in"?
Click to expand...
Here's a small video of the fuel entering the Carb after the ignition is off...


The one thing I did note, was when moving the Choke flap by hand, sometimes it felt as if it was rubbing a little (metal on metal), but not always.
 
That appears to be flooding...

The flaps are called butterflies..

Pipes are called nozzles...

The venturi is the restriction in each bore or barrel..
 
The carb is flooding and needs to be taken apart, cleaned and adjusted correctly

The most likely case for the oil level to be to high is from gas. The carb is flooding the cylinders and fuel is leaking past rings into the oil pan. Pull the dip stick and smell, can even put a match to the dip stick(away from the boat) and it will probably burn
 
Thanks @makomark and @AllDodge, that knowledge is much appreciated. Do you think the Carb problem is most likely the issue with the poor starting too - Simply flooding itself from the start?

That's interesting about the fuel going past the cylinders to the oil pan, I would never have thought of that, but does make sense.

If the Carb is 99% likely to be the culprit to all the issues, then I may buy a new one and try and refurbish the current one for a spare. Seeing as it had a "new" carb three years ago, could there be anything else that would cause the problem - fuel pump running on (for example)? Or if the fuel was already old, I suppose it could have got blocked again quickly.

I'll have a look for some video tutorials 🤓 (...butterflies, nozzles, bores / barrels 👍)
 
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I would add that once you get the carb straight, you should also change the oil and the filter...

The fuel will dilute the oil and represents, its vapors are an explosion hazard...
 
I was planning to change oil and filters before winter and did cross my mind, especially as the level was well above the Max mark. But thanks for bringing it up.

What would cause the amount of oil slick on the floor, coming through the water cooling or exhaust, i would presume? Is that something else I should be concerned with? Gaskets, piston rings, other internal damage, etc? Or is that likely to resolve with the Carb as well and the oil being back to normal level?

As it had a new carb in 2023 (apparently), wondering (or paranoid) if its signs of other issues?
 
Your residue on the floor is unburnt fuel.
Carbs typically last many years trouble free. Especially if you are sure to source non ethanol fuel.
You say that carb was "new" in '23. New as in NEW or rebuilt?
I have never bought a new or rebuilt carb in my life & I bought my 1st boat in 1979. I have however rebuilt lots, both I/O and outboards over the years.
 
Droid said:
Your residue on the floor is unburnt fuel.
Carbs typically last many years trouble free. Especially if you are sure to source non ethanol fuel.
You say that carb was "new" in '23. New as in NEW or rebuilt?
I have never bought a new or rebuilt carb in my life & I bought my 1st boat in 1979. I have however rebuilt lots, both I/O and outboards over the years.
Click to expand...
Thanks for explaining, and it makes sense.

Well there is an invoice for a brand NEW carb and setup charges, along with other servicing items in 2023. However, whether the workshop actually fitted one, or just charged the previous owner for one after refurbishing the old. Or the previous owner swapped it out before sale could be possible, but both somewhat unlikely. I have no serial numbers to check. However, everything else that has said to have been done looks legit.

I have no record of how old the fuel is in the tank before my ownership, so I presume that maybe the primary culprit (if clogged up); if the carb was indeed brand new in 2023.

In all my years of boating I have never had to touch a carb. But, I do have friends who have and their DIY works tended to make things worse resulting in their boats going in to a shop for professional diagnostics. That said, I marathoned through some youtube tutorials last night and they don't seem overly complicated. This guy has created a useful three-part guide on refurbishing a 2 barrel Mercarb, which i'll link to for others future reference below...

PART ONE - DISASSEMBLY

PART TWO - REASSEMBLY

PART THREE - ADJUSTMENTS


I'm currently planning to...

- Rebuild the Carb (possibly getting a new one first)
- Change Oil Sensor Switch (looks old)
- Change Oil & Filter
- Change Water Separator Filter
- Fill Fuel Tank with HQ 99 Octane Fuel
- Keep Fingers Crossed lol
 
makomark said:
If you chose to rebuild what you have, get a high quality kit...you don't want to go cheap on that...
Click to expand...

I will definitely have a go at refurbishing the carb. It doesn't look to difficult now I've seen the videos and the kit I have found is from a reputable UK company and states....

"Produced with Premium Viton Tipped Needles and Coated Gaskets... MADE IN USA" 🙏;)


Although, I am not convinced the carb is the sole issue when considering the previous work on one of the invoices...

IMG_2659_sm-min.jpg


Essentially the workshop rebuilt a/the carb before without success to resolve a similar problem. Then installed a new carb, later listing "oil showed signs of petrol ingress". It's still the same problem now.

I'm guessing either the fuel has quickly affected the second carb installed, or the problem maybe being caused by something else (possibly on the oil pressure side, or electrical side for example), or a combination of the two. The only way i'll know is to start with the carb, plus checking its inline filter when replacing. I'll also have a look at the contents in the old Water Separator when I change that.

A friend suggested it could be the anti-syphon valve. First i will need to check to see if it has one. It's a shame that on the Maxum 1800 SR3 the fuel tank is fully built-in under the main deck and there's no way of getting it out for cleaning, or even looking inside it to check.

I shall try and have a look over the engine tomorrow and will inspect some of the other components that have been mentioned before ordering parts - the list seems to be getting longer each time lol. It should be a nice boat once it's all sorted and I do thank you ALL for advice and info to date.

Something that may, or may not be related. When the engine was running the other day I did notice the trim tilt gauge sporadically jumped form Down to Up on occasion. Almost like power dropping... leading me to start thinking about the alternator, possible electrical shorting, or back o my very first thoughts of a bad earth again.
 
Don't see it being the antisiphon valve because your getting fuel, if it wasn't then it could be that. Three things can cause carb flooding, to much pressure (above 7 psi), float level stuck/misadjusted and won't let fuel be shut off, or needle valve or set damaged
 
AllDodge said:
Don't see it being the antisiphon valve because your getting fuel, if it wasn't then it could be that. Three things can cause carb flooding, to much pressure (above 7 psi), float level stuck/misadjusted and won't let fuel be shut off, or needle valve or set damaged
Click to expand...
Thanks for keeping me on point! When you say too much pressure, do you mean the fuel pump producing too much pressure? I wish I knew why the workshop couldn't resolve the issue with a carb rebuild and it needing a new carb, which also hasn't fixed the issue. Unless it did for a while 🤷‍♂️
 
I'll add 'bad' float to the list...

Could very well be too much fuel pressure...which would be consistent with the new carb having the same symptoms...
It doesn't take that long to check it...
 
^^^agree, and yes if fuel pump is producing to much pressure the float needle won't stop it

Someone might have put wrong pump on it sometime back
 
The fuel pump is one of the "shiny bits" on the engine, so I haven't given it much thought tbh. I'll see if I can view a model number on it and check the specs to see if that may well be the case.

The float isn't included in the Mercarb kit (mentioned above) for some reason, it's a separate item, but I will get one of those anyway. Even if it's not needed I always like to have a few spares kicking about.

Cheers @AllDodge and @makomark
 
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