Save Fuel With 2 Barrel Carb??

[DAFF]

Anyone ever tried to change the carb on their Supra and put on a 2 barrel. A friend of mine who owns a scrap yard is always running demolition derbys. He will allways pull the 4 barrel to run a 2 barrel. He says it give him better throttle responce out of the hole but not as good wot .....

All in all the top end of the performance might suffer a little bit but it would be nice to save some fuel for the fact in Ontario it is pushing $4.00 a gallon.

Very EZ conversion with a adapter plate and another carb.

[beast 496]

A two barrel carb will in no way save any fuel unless you run WOT all day. The way a 4 barrel carb works is: The front primary barrels are used at up to 75% throttle, the remaining two barrels are used at WOT. The Holley 600 cfm carb has 4 equal barrels, the primary barrels are 300 cfm. Most 2 barrel carbs are 500 cfm, thus allowing more air and fuel at lower rpms. If you wish to aquire better fuel economy try using a Marine quadrajet. The Rochester quadrajet is a great carb for economy and throttle responce. I would think you might recoupe the cost of fuel in a couple years based on running 100 hrs or more. Al

[DKJBama92Mariah]

Well first, it would have to be a marine two-barrel carb. Second, since you're going for fuel savings, to be most efficient you would really need to go to a two-barrel manifold instead of an adapter plate. This is getting expensive Third, you'd be severely hampering your engine's top end performance. I don't believe the PCM 351 was ever even offered in a two-barrel version; they had a 2V 302, but not a 351.

Beast496 offers an interesting alternative. I have not had personal experience with Q-jets so I'll leave the discussion of that option to those with experience with them.

My advise would be to slow down a bit and try and stay out of the secondaries. Stock marine Holleys have vacuum secondaries that are actuated by engine load. Mine start to crack open around 2900 rpm or so, but different engine models, transmission ratios. and boat sizes will react differently. On your boat, if you run (as in drive the boat across the lake, not revving in neutral) with the motor box open, you can hear when the secondaries open as a whistling sound from the air rushing through the barely cracked open throttle plates. Take note of the RPM at which this occurs and try and do the majority of your cruising below that RPM for best fuel economy. On two barrels your motor is just sipping gas.

Also, ask your skiers to fall less. :D Seriously though, repeatedly pulling up skiers or boarders burns much more fuel than simply cruising. Teaching a heavy guy to deepwater start on a slalom ski will absolutely DRINK fuel. I know this from when I learned myself behind my own boat.

Actually, just straight cruising for long distances at speed, the flat bottom of the rear half of our hulls gives these boats excellent cruising fuel mileage. For example, a few years ago, me and some friends took a daytrip downriver to see boat races at Pickwick Dam. The load was 6 people, a full tank of fuel, two coolers full of adult beverages for my passengers and soft drinks and bottled water for myself. I ran the entire 44 mile round trip at about 3200-3300 RPM at 35mph; at this speed in my boat the secondaries would be open, but far from wide-open. If filled the tank both before and after the trip and calculated my mileage at 3mpg. That may not sound good compared to mileage in our cars or even a bass boat, but IMO in a carbureted v8 powered boat our size, it actually is VERY good mileage.

Tell us more about your boat and how you use it. What year/model/length is it? Is the 351 a std or HO version? Do you pull skiers, cruise fast or slow, putt around at idle, lots of speeding up or slowing down, etc?

Is the motor in good tune? When was the last time it got a new set of plugs? Etc.

What is the condition of the running gear? Does it vibrate at all? Does the prop have any visible dings? As a check for engine alignment, does the driveshaft turn fairly freely by hand when the boat is out of the water?

Is the boat stored in the water? If so, is the hull and running gear free from marine growth?

Get back with us and we'll get this figured out.

DKJ

[jet]

The cheap and easy answer is................................................ ................
unhook your rear arm that opens the back 2-barrels. Its most likely a vacuum secondary which means it opens up under a load (fat sacs) or WOT and if you unhook it it cant open and then you have your 2-barrel. I have to worry about it when running weight and only have a 18 gal tank it doesn't last very long so I dont want it to open while pulling a boarder so I just unhooked it. DONE! Jet

[cadunkle]

Scrap the idea of running a 2 barrel. It's ridiculous and tpo do nicely will cost you more than you'll save. Boats aren't cheap to run, accept that. If you're running out of fuel, either figure a way to install a bigger tank, or bring extra in cans to avoid paying the premium for gas on the water.

Check your timing curve. Run as much initial and as quick a curve as you can on whatever octane you run until you get predetonation, then pull 2 degrees for a margin of safety. Be careful, as it'll be more likely to detonate when loaded down with people and sacks.

What type of carb are you running? Probably a Holley 4160 style. You could try going to a smaller accelerator pump discharge nozzle, or a slow opening accelerator pump cam. Try leaner jets on the primary side if it'll take it without surging or detonating. Run a hotter thermostat will help you to run leaner jetting, but is bad to run above 140* in brackish or salty water particularly is using raw cooling.

The next couple times you're out measure your fuel use in gallons to fill afterward and hours on the meter. Do this a couple times to get a baseline of what it burns now, then try a good tune up with the timing, see what change it makes, then go after accelerator pump, jetting, etc.

[DAFF]

Wow everyone sure put some thought into this one. I wasn't totally sold on the idea myself... This all came about for I was looking for another carb for the old Supra comp. Has a 351 which hasn't seen any fuel since 1996....

The boat will be mainly for pulling the kids around the lakes.. They are 6 and 8 and have a combined weight of 90lbs.

A few months back the boat came sooo close to being parted out. In fact had a buyer for the trailer and odds and ends but not the drive line. Could not bring myself to the death of the ol Supra.

As of now the floor has been replaced, refoamed and the interior work can begin very soon.