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

AllDodge said:
Electric fuel pumps don't suck very well, would suggest putting more in the filter

Next are you sure the 12V wiring is connected correctly. If connected in reverse it won't hurt the pump but it will also run in reverse
Click to expand...
He got it to supply the fuel by manually priming
 
Hi @kghost, yes I did adjust the float drop too, but I didn't take a photo of that as it was a pain trying to hold the phone, ruler and carb, etc. It was set to 24mm to the dot on the drop.

Maybe I didn't run the pump long enough, but the jerry can technique worked pretty much instantly. I'll remember to put more in the filter next time @AllDodge. But, yes, the 12v was correct, red and black wires to the right terminals and contacts. I made a rig for future use (heat shrink, switch, etc).
 
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And she's alive...

Fired up first time...🤟 Running a little high on the revs at the moment (1000 at idle on the dashboard gauge), but i think I'm going to book it in for a timing check and get them to make final adjustments to the carb (on the water).


Thanks to everyone who has contributed and helped me resolve this, it's massively appreciated 🙌
 
Hi all,

I wanted to post a quick update and a summary of the fix, as I hate posts that don't share their outcomes - hopefully it's helpful to others.

We managed to fit in a trip to the coast (Christchurch, Dorset, UK) for a full water test this week, which went great and we got very lucky with the weather too. All apart from a being a bit choppy out at sea (so only managed to get some harbour photos). The carb needed a little further adjustment as was expected (when underway), but she ran great 🙏...

maiden-test-oct-2025-min.jpg


Summary of the Fix
Despite having a new carb installed, it was most likely a refurbished carb, which had not been setup correctly and the main fault stemming from the float. This created excessive flooding and ultimately prevented it starting. I would have liked to have tested the fuel pump pressure (as advised on here), but struggled to find compatible parts in the UK (especially with limited time) - I ended up skipping that advice and proceeded with the carb only.

Carb Issues Found
  1. Float was old and heavier than it should be (10+ grams)
  2. Float height and drop were not correct
  3. Old style gasket used, which potentially could have blocked float from working (unproven). Refurb kit had a new gasket design.
  4. Electric Choke was heavily set to lean, miles off the factory setting of 2 to lean.
  5. Choke unloader was miles off the 2mm opening, approx 20mm+
Start Procedure
I also followed the suggested starting procedure to the letter.
  1. Neutral
  2. Crank engine for a few seconds to prime and then stop
  3. Throttle Twice to full throttle and stop
  4. Throttle part forward
  5. Start ignition
It fired up straight away, idle’d at about 1000 revs on default carb settings - ready for further adjustments of mixture and idle screws.


Additional Works
Although the previous owner had new spark plugs fitted, these were not gapped correctly. I put fresh plugs in and gapped them correctly. I had cleaned all electrical contacts, but I did go round them all again. Plus I replaced the 90amp fuse as that was a bit corroded. I also fitted a new coil just incase; to maximise spark.

I added 50% premium fresh fuel (ending with about half a tank in total) and added a whole can of Sea Foam (cleaning and stabiliser additive). An Oil Change, Filter and Fuel Separator were also completed.


Sea Trial (Some Initial Backfiring)
It fired up straight away after launch and sounded great. We completed a 30 minute run up the river (1000rpm) to harbour entrance, then headed out to sea.

Upon accelerating, there was some backfiring. As the boat was now warm, I adjusted the mixture screw and fast idle screw as per the guidance in the video posted previously. After, which she ran fine. We completed over one hour out at sea, 3000 - 4000 and some at 4500 rpm and 30 minutes back up the river to the slip - all was fine. Will be winterising this week.

My sincere thanks to all those on the forum that helped me resolve this 🙌
 
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