Excessive fuel usage?
 

I've got a Formula 292 SR1 with twin 330's. I'm spinning hydromotive Q IV 24 pitch props and I'm getting around .8 MPG at 3000 RPMs average. To me this seems like really poor mileage for a boat this size. I've rebuilt the carbs and the engines seem to run fine other than I'm having an issue with sediment in the tank. For comparison sake, what are other people getting for MPG with similar boats?

I never checked MPG..With boats its usually GPH

It was easier to calculate my mileage and fuel used than try and figure out how long I ran the engines. I know GPH is a more common number than MPG.

Are you sooting up the transom very bad? That is pretty poor mileage. You are either running very rich or your economy range is at a faster speed. Sometimes an increase in RPM will net a greater fuel economy, even though you are burning more GPH you are in turn running a faster speed so your mpg goes up. Plus, the faster you go the more hull you get out of the water, so the more efficient the whole package becomes (to a point).

I'm leaning toward running too rich. The silent thunder tips are quite sooty and I get a light gassy film at idle. I'm running the stock quadrajets. Any suggestions as too how much I should try and jet down?

Are you into your secondaries at 3000? You might have to much prop and you might have to give more throttle to achieve your desired cruise RPM. I mark my throttle right before the secondaries open and then check what RPMs I am running. I can also feel them opening with the more resistance on the throttle.

I'll need to check but I'm hitting 4500 RPMs at WOT so I would be hard pressed to believe that I'm already in the secondarys at 3000 but you never know until you look!

I get about 1 mile to the gallon in a 14,000 lb boat with 502's. I would think you should get about twice that.

Have you rebuilt the carbs? With all of the sediment problems you have had it would not be unheard of for a little bit to get past the filter and cause problems with the carbs. Q-jets usually run fairly efficiently. Also, the main power valve has a spring under it that lifts the main metering rods off the jets. This is counteracted by vacuum at idle to keep the power valve closed. If for some reason you are not making enough vacuum, then the rods will be lifted up too soon, causing it to run rich at idle and at light cruising speeds. You can frequently see this condition by a constant drip of fuel out of the primary boosters at idle. The fix is to either get your vacuum signal up where it is supposed to be (vacuum leak or clogged vacuum passage in the carb), or if you are running a larger cam, install a lighter power valve spring.

Also, the secondaries for Q-jets are vacuum actuated. While the actual linkage to the butterflies is mechanical, the air valve on top that lets the air flow is vacuum actuated. There is a spring adjustment on the underside that controls how easily the flap opens. If it opens too soon, you will get a bog when you hit the secondaries.

There is a guy named Cliff Ruggles up in Ohio who is one of the leading Q-jet gurus. Google his name and you will find his site. Lots of good information here, and he has a QA forum where he will answer your questions. He has an excellent book that takes a lot of the mystery out of rebuilding and tuning Q-jets. He also offers a rebuilding service, or he will sell you a kit with instructions to rebuild it yourself. Stand up guy, and I highly recommend him. The Q-jet is actually an excellent marine carb for a stock marine motor, and you would be amazed at the kind of power the Super Stock guys are getting out of these things with some tweaks.

Budman-
Thanks for the great info. I finally cleaned the tank as good as one can without ripping out the tank. I ended up buying an electronic scope so I could look in the tank. I then sucked all the "crap" out.

As to the carb, I'm going to take a look and see if anyhing is dripping at idle. There is currently no bog but I do have the soot issue. Do you think jetting down one size would be of any benifit? Either way thanks for the tips and for Cliffs info.

If it has not changed from stock, I can't see a reason to jet leaner. They may just need to be freshened.

Well I looked into when the secondaries were opening. I found that 2800 rpms is max on the primarys. I had been "cruising" around 3000-3500, at which point the secondarys were open even thought they are not open much I'm guessing its still enough to drop my economy a fair amount.

Yep, that's not uncommon on a big heavy boat. The flappers over the secondary butterflies are adjustable (these are actually called secondary air valves). You may be able to delay the secondary opening point a little without hurting performance.

Will it stay on plane at 2800? If so, try running it at that speed, and see if your mileage improves.

However, what it comes down to is that it's a boat, and it feeds on money. As the saying goes, buying the boat itself is the cheap part! :cool:

hi there can anyone help me I need info on fuel consumption for mercruiser engines : 5.7 , 7.4 502 magnum thanks in advance

i have a garmin fuel sensor(GFS10) on my baja 272 with a 496 HO.it show at 42 MPH GPS at 3500, 2.5 MPG US,3MPG canadian or 1.5 litter a mile,arond 60 liters by houres with a 23 rev 4,cant remember gallons houres i think it is accurate because the litters by hour is about the same as my smartcraft.

OK, tested my boat to get some real numbers. I went 7 miles and used around 8 gallons of gas. I cruised 85% of that at 2800 rpms, (max speed of primarys). A short WTO burst and 1/2 mile idling. Either way fuel consumption seems way excessive. Does anyone have any concrete numbers with carb engines on a similar sized boat?

I would suggest checking the ignition timing at idle and at high speed. Should be 8 degrees at an idle and around 30-32 degrees at high speed (above 3000 rpm). Do this with a dial back timing light or install timing tape on the balancer. You don't need to check the high speed timing with the boat at speed. Just check it by reving the engine in neutral on the flushing attachment. It should not have more than 34 degrees above 3000 rpm.
Also make sure you don't have a lot of water in the tank.
Dennis Moore

If you think the carburetor is running rich you should shut down the engine after a long drive at 3000 rpm. Pull the spark plugs and check the color. Down/inside at the very base of the plug you should have a nice brown color (like a paper bag). This won't work with new plugs.
I suggest replacing the spark plugs with RV15YC4 Champions, gapped at .045 inches (after the plug check).
Check the height of the flame arrestor. If you have a tight engine hatch the boat builder may have installed a low profile flame arrestor that is very restrictive. Install a free flowing K+N marine flame arrestor. Buy the tallest K+N arrestor that will fit. Also make sure that the engine compartment is capable of flowing all of the cool fresh air the engine needs. If nothing else, the addition of the K+N and the Champion RV15YC4 plugs will increase performance!
Dennis

my 502 mags in a 38 top gun would do 2 mpg at 60 mph... thats each... i would use 60 gals to go 60 miles in 1 hour...

and the question you should be asking is more like... " gee... the boat USED to do xxx... gal/ hr... now it does yyy " and what's changed to make it happen...

Very good info, I couldn't have said it better myself, Smitty

Yes, did it this year. I cruise between 3000-3400 rpm, running mid 33-38 mph at those revs. 26' Powerquest Legend BR with carbed 7.4 Bravo III drive. My measured use was 2.3 mpg. I used an outboard tank filled with 6 gals and measured distance travelled by gps. Was trying to get a good idea of range on one tank.

Dennis-
I've got a pair of HP 500 flame arrestors. They are not on the engine yet but it would be a easy swap. Are the K&N's much better than the HP 500 flame arrestors?

I am sure the HP 500 flame arrestors will flow very well and you should use them. I should also suggest that you make sure your chokes are operating fine.

With the flame arrestor change my fuel usage just jumped substantialy! I checked my fuel consumption using a 5 gallon gas can and going about 5 miles I used about 2 gallons per engine, which puts me around the 1.3 mpg range. It seems now that I have a problem with my fuel pickups. I think there cracked. My tank had 4 but two have broke off, I have a feeling the other two are cracked because when I open it up when hooked to the tank the engines surge, when running from the gas can they run great?