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Originally Posted by endeavour32
(Post 3442341)
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. |
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.
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Originally Posted by endeavour32
(Post 3445826)
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.
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
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Originally Posted by ivanex mpimag502
(Post 3446419)
hi there can anyone help me I need info on fuel consumption for mercruiser engines : 5.7 , 7.4 502 magnum thanks in advance
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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?
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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 |
Originally Posted by ivanex mpimag502
(Post 3446419)
hi there can anyone help me I need info on fuel consumption for mercruiser engines : 5.7 , 7.4 502 magnum thanks in advance
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... |
Originally Posted by Budman II
(Post 3442321)
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. |
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