I have a few questions:-

Will there be any significant electrical implications with EFI removed (keypad start for example)?

What are the options for getting back to the equivalent horsepower of the EFI system?

Any advice on the best carb and intake to use.

Any other relevant advice would be greatly appreciated.

I understand this is quite a wide ranging question but any advice, especially from someone who has gone this route, would be greatly appreciated.

An electric fuel pump should be on the shopping list too.

My little Ski Nautique with the GT40 engine is running very well.  No complaints.  As long as the EFI and ECM continue to perform reasonably, then everything remains in the OEM configuration.  HOWEVER, if the EFI and ECM start giving me problems, then I'm just itching for a Winter Project.  If that day ever comes, this is what I would likely do:

- Block is "roller ready" so a roller camshaft is likely

- New roller valve train with roller lifters/rockers, etc.

- Aluminum heads (increase flow, compression ratio and drop weight)

- Aluminum intake manifold (Edelbrock Performer RPM)

- QuickFuel M-650 carb

- Carter electric fuel pump (keep the FCC?)

- DUI distributor with new plugs and wires

- K&N fancy-schmancy air cleaner/flame arrestor

- Audible horn with light for low oil pressure / high temp

JQ

It puts out a carburetor friendly 5 to 7 psi

My plan is to try a set of injectors this week and if that cures it great, if not the carb it is. I always had a carb and some form of performance tune in my head as a perfect project for the 196 but the tuning bit is  probably not an option for this season. I am happy I can do the carb conversion at short notice if the injectors don't work out. 

KENO, I believe you recommended a set of injectors from Summit Racing in a previous post, is this still my best option?

Many thanks again for all your help guys.

Quite a few people have used the Ford Performance  injectors like in the link below available from numerous places

http://lmr.com/item/M9593LU24A-K/Mustang-Ford-Racing-24lb-Fuel-Injector-Kit-w-Adapters-86-04" rel="nofollow - link

And here's a link to a thread talking about them and the adapters that come with them.

You need the adapters because the later injectors have a different style plug on the injector.

http://www.correctcraftfan.com/forum/forum_posts.asp?TID=50482&PID=608112&title=gt40-hard-start#608112" rel="nofollow - link

JQ

Neil, in your first post you mentioned it has idle issues, then later said it ran truly awfully.  Is this just at idle or also at mid range and high speeds?

Before you tear the efi out, I'm wondering how much you've looked at the ignition side of things.  You said plugs were replaced, how about plug wires?  Distributor cap and rotor?  These relatively cheap components can really make an engine run badly.

You asked about getting back to efi HP once you have a carb on it.  I'm curious what the answer to this is, I don't remember any discussion about it.  I'm wondering if there is any HP loss with a well done carb setup over the efi, I don't really expect there would be.  More fuel consumption yes, but loss of power no.  Curious for anyone's thoughts on this.

I searched the forums and did many of the simple things like IAC, plugs, leads, distributor cap, fuel tank breathers etc. but none of the larger items like injectors.

The original fault felt quite minor, most probably to my mind a sensor. 

I decided during the winter to do a fairly substantial refurbishment to the boat and hopefully at the same time replace the offending component.

Engine removed, new carpet, lots of powder coating, all looks great but when I started it for the first time last week it sounded poorly, far worse than when I last ran it in 2021. Not off a cylinder but just very lumpy.  A little rev will clean it up slightly but it does remind me of a fuel rather than ignition fault. 

Ordered injectors this morning, I really don't believe it is the ECU (perhaps that is more hope than belief lol). I also have a new distributor to fit.

I am keeping the carb as my back up plan for next week but fingers crossed the injectors cure it.

Jonny Quest and Keno have probably done enough to convince me carb is a good option but it has always been my understanding that fuel injection is more effecient therefore better performing than a carb. Over in Europe we have had very few carb engines for many years now, we just do not have cars with 5.8L engines.

Very much enjoying the input from you guys and I will keep you posted once injectors arrive.

Thanks again, Neil.

Sounds like a solid plan.

I don't think many people will say that a carb setup is better than a GT-40 efi.  Better than the earlier years Protec, yes, but the GT-40 is an amazingly good setup for easy starts, idle, skiing, etc.  The general problems have been diagnosed and can be overcome.  The lack of ECM replacements have led people to do a carb conversion and I've wondered if I would do that if/when mine fails, but seems like these days you can get a replacement ECM, though they are expensive.

Like you, I'm doubting your problem is the ECM.  I hope the injectors fix it for you.

JQ

I haven't seen a reasonable solution come up yet

One item that I have not found a reference to is a little white unit connected to breather pipe under plenum chamber. What is this and does it require maintenance or replacement?

Thanks again, Neil.

If that little white thing has an arrow on it and the line runs to the MAP sensor, some people call it a filter and others say it's a check valve that doesn't seat real tight and it acts to dampen or smooth out the signal to the MAP sensor

I don't think they get much of any maintenance

https://ibb.co/chz24PR" rel="nofollow">

JQ

I think car guys may have a check valve associated with the vacuum lines in that area since there's a lot more vacuum plumbing on a car.

PCM has a part number for it and they call it a MAP fuel vapor filter. Part number is R077008

I think what you call the plenum and the upper intake are  the same thing.

Idle is much worse than when I last ran the boat 2 years ago and obvious even when cold. 2 years ago idle issue did not cause a problem until engine had warmed up.

Jonny, I did a leak test on fuel system and it started at 40 psi and remained above 38 after 5 minutes. I took the vacuum pipe off the FPR and there was good suction from it. Fuel pressure increased by about 5 psi when off, no sign of any fuel leaking past.

I will probably change the regulator if I can pick one up locally but doesn't feel like the issue.

Fuel pressure does fluctuate when the idle starts to wander. I think this is more a result than a cause but obviously not sure.

One final point for this post. When I did my list of what I had done up to now I did replace the alternator 3 years ago. Actually writing this down reminded me that I had not actually checked for charge. As it turns out the alternator is not charging!

Can a faulty alternator cause poor engine running?

I do have a new distributor but I have never fitted one before. I believe the distributor gives the crankshaft position information to the ECU. Is this correct? If the distributor is an easy fit does anyone have a link to a good step by step guide?

Yet again, many thanks.

Distributor also fitted today with no solution.

I did buy an Inova 3145 fault reader but it did not communicate with the diagnostic port. Perhaps it was not the correct choice.

I am going to proceed with a complete strip out and carburetor installation now. I may return to the fuel injected set up if I feel dissatisfied with the result but I do crave the simplicity and the greater availability of knowledge.

Part of me would be quite determined to actually resolve the issue, it does feel like it should be quite simple but summer is short so carb it is.

Jonny and Keno, as already mentioned I am over in Ireland so sometimes parts can be little slow for me. Summit were excellent with international shipping on the injectors so I will probably use them for the carb stuff. It would be great to organise everything I need in one order (gaskets, flame arrestor, etc) so would be much appreciated if you would take a little time to ponder a complete carb fitment.

I will obviously keep you guys posted on the results and my "feelings" on the performance and general difference after the change. 

Yet again many thanks.

An intake manifold, Edelbrock Performer, Performer RPM, Weiand Stealth

Carburetor mounting studs SUM G-1423 should be good

An adapter plate SUM G-1420 if you're using an Edelbrock performer. It provides a wider seating surface for the carburetor.

Intake manifold gaskets FelPro 1262 S3 work good 

A carburetor of your choice

An oil pressure safety switch for the fuel pump. Holley 12-810 is easy to wire and works good.

Your new LP fuel pump for the gt40 should work just fine supplying a carburetor since PCM used it for that purpose on some carbureted engines.

Some marine fuel line from pump to carnuretor 

You could shop at SkiDim or Nautique parts and pay way, way over list price for a DUI distributor if they have it in stock 

A flame arrestor, not knowing how much clearance you'll have between the engine box and your carburetor depending on manifold choice, the lower the profile, the better. What works on a Holley doesn't necessarily work on a Quick Fuel due to the external float adjustment screws on the Quick Fuel sticking up a bit. A little dimpling of the bottom of the arrestor sometimes helps.

It's getting a little pricey, but does what you want to do............ like save your summer and let you go skiing  

And very importantly..............you will be Jonny's hero for actually doing this but only if you use a Quick Fuel and a DUI with a RED cap 

Once removed I intend to have both the ECU and the loom tested, part of me would still like to diagnose the initial problem but at 22 years of age I would assume I will not refit the injection system.

Will keep you posted.

most don't seem to want to work on something over 10 years old. I suspect it's more like eats alot of time and not many have the experience or access to parts.