carb advice - 4.3l obscene fuel consumption

[H20Rat]

So let me start out by saying I've been boating my entire life, I'm on my 3rd 4.3l powered boat. Also owned a couple 2 stroke v6 powered jet boats. I know boats don't get great fuel economy, if you want that, buy a sailboat. (which I have also!)

With that said, I've had my current boat 2 years now (2002 19' Hurricane deck boat), and the fuel consumption is stupid, even compared to my sportjet's. It performs well, but seems to be running very, very rich. It starts instantly, but almost always has some smoke that smells of raw gas on startup. (goes away after a bit) The engine runs/sounds very healthy beyond that, I don't suspect any internal engine issues.


Any idea what could be wrong in the carb? (2 barrel mercarb) Since i'm still in the middle of winter here in the frozen north, I'm contemplating pulling the carb off and going through it myself. (disclaimer, not a carb guy) My worry is that if something is missing/messed up, just swapping in a carb kit won't necessarily fix it, as I might miss the root cause. Normally I'd say a carb kit is great for a plugged up carb, not one that is dumping fuel in. I'm ready to possibly buy a reman carb just to have piece of mind.


I haven't done much troubleshooting on it beyond verifying the choke bufferfly isn't sticking closed.

 

[achris]

First off, what IS the fuel consumption? How many nautical miles per litre are you getting? If you quote a litres/hour figure, what revs (and boat speed) are you doing at those revs.

I would first go though and check the 'usual suspects', ensure you have good even compression, the timing is set correctly and advancing as per spec, and the spark is healthy. Also, are you getting to the top recommended revs at WOT? I think for a 4.3TKS it should be 4200-4600... Then, and only after I have verified the compressions and spark timing and quality, would I pull the carb.

You called out the year of the boat. But what year (and serial number) is the engine?

Chris.......

 

[Scott Danforth]

Hey achris doesnt the TKS module require a diode to prevent it from over fueling after start? just a potential place to look. referencing old threads https://forums.iboats.com/forum/eng...tks-blown-ignition-fuse-and-shorted-tks-diode


H20Rat what fuel consumption are you at? the BSFC for an internal combustion 4-cycle boat motor is about 0.4# of fuel per HP per Hr. fuel is 6.82#/gallon

 

[alldodge]

The OP said he has a choke plate and its open
What do the plugs look like?
4400-4800 rpm

 

[achris]

If you could also post any numbers on the carb.... (Let's check it's the right one)

 

[H20Rat]

Boat is 60 miles away at the lake on a road that may or may not be open, so can't immediately access. going to rummage around on the phone pics, I probably have engine/carb #'s in there somewhere. Have not pulled plugs recently either.

As far as specific #'s, I unfortunately don't have a fuel flow gauge. I do know my boating habits haven't changed between boats, and I know based on fuel spending that I spent over 50% more on fuel with this one compared to my nearly previous identical 4.3l deck boat. (which was older and slightly heavier) (and yes, adjusted for fuel cost) If anything, I put on fewer hours the last two years.


WOT is 4800 RPM with a 21 3 blade SS, 1.81 drive, 45 mph.

 

[harringtondav]

H20Rat what fuel consumption are you at? the BSFC for an internal combustion 4-cycle boat motor is about 0.4# of fuel per HP per Hr. fuel is 6.82#/gallon

I'd say you calc is reasonably accurate, SD. My 180hp 4.3 4bbl got 4 mpg on a 400 mi Miss river round trip cruise. MPG was fairly accurate since I used USACE chart river miles, and gas dock gals.

I calculated 10.56 gal/hr. with your calc. At an avg speedo of 30 mph, I got 2.5 mpg. But I wasn't using the180 hp much. Using 120 hp ave. in your calc, I get 4 mpg. Seems reasonable to me. Hole shots coming out of the locks, off the docks, otherwise cruising.

H20Rat I don't know the TKS, but I know carbs. A usual suspect is a too high float setting, although that tends to cause flooding and hard starting. My guess is dirt, especially in the bleed air circuit(s) and air jet(s) causing a rich mixture. A dirty spark arrester can also act like a choke.

Cleaning a carb can be intimidating, until you do it once. A good soak in an immersion cleaner, passage blow out with an aerosol cleaner, using the straw, and a final blow out with compressed air. Follow the cook book instructions included in the kit, lay out the parts in disassembly order, and don't loose that tiny accelerator pump check ball.

 

[achris]

Here's what the performance (including fuel usage) looks like for my boat... (Yours may be different, but this will give you an idea of what it should be) Fuel usage is in 'litres per nautical mile'. I don't use gallons because there are 2 measures of a liquid called 'gallons'.. Besides, litres are SI units.:behindsofa:

 

[Scott Danforth]

Here's what the performance (including fuel usage) looks like for my boat... (Yours may be different, but this will give you an idea of what it should be) Fuel usage is in 'litres per nautical mile'. I don't use gallons because there are 2 measures of a liquid called 'gallons'.. Besides, litres are SI units.:behindsofa:

however the BSFC standard is SAE and written in # of fuel per hour. from there you have to convert to your measure of volume. Love the graph. you can see your holeshot and the best cruise RPM of 3250 (many are about 3400)

harringtondav , dont forget to use the flywheel HP value and not the prop value. your 180hp motor is actually putting out about 205 at the flywheel. BTW, most diesel engines are between 0.32 and 0.35 #/HP/Hr. and old 2-stroke outboards about 0.5#/HP/Hr

 

[achris]

... you can see your holeshot and the best cruise RPM of 3250 (many are about 3400)...

The other 'sweet spot' is around 1400rpm (6 knots), my trolling speed.

Chris......