endeavour32

iTrader: (4)

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?

35fountain

I never checked MPG..With boats its usually GPH

endeavour32

iTrader: (4)

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.

Philm

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).

endeavour32

iTrader: (4)

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?

Rookie

iTrader: (4)

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.

endeavour32

iTrader: (4)

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!

Steve H

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.

Budman II

iTrader: (1)

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.

endeavour32

iTrader: (4)

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.