changing the carroborators

[captainjohn]

My wife and I have just purchased a "1990 "406 Carver Aft Cabin" The engines has two 454 cc gas engines. The fuel economy is a concern for us, as it is for many folks. We were wondering if we could start to investigate the idea of putting a new corroborator with an up to date fuel distribution on this vessel. We were thinking that in 15 years there may be some technological advances. We would be willing to reduce the top speed to get fuel consumption benefits.

We are new to the forum and are very excited about our new boat.

[JediJD]

I'll throw in my $.02, and please take it for what its worth (not much). If you aren't getting the mileage you expect, it is certainly possible that carburetion is a factor, but it usually is a little way down the checklist and hopefully this explains why. The way carburetors work is simply to mix air and fuel (most of the time, to reach the precise proportions of 14.7 parts air to 1 part fuel). If you have a larger percentage of fuel to air than 14.7:1 ...they call that a "rich" mixture. More air as a percentage...that's a lean mixture. Rich mixtures burn a little cooler, lean mixtures burn a little hotter...neither burns as cleanly or completely or as powerfully as 14.7:1 (ok...fuel management rocket scientists...I agree there are some small differences and circumstances, but I want to start with the big picture). Whether you do this through a new carb, an old carb, or fuel injection, it all works pretty much the same way. Carbs can be "set", mixture needles can (in most cases) be changed and at the very least they can be adjusted to provide this magic ratio. However, it would make sense that, for the mixtures to make any difference whatsoever, the air and ignition are pretty important too (wouldn't matter if you had perfect air/fuel if the plugs are firing or if they are firing at the wrong time or an air leak messes up the balance in a cylinder (vacuum leaks can crush mileage, make sure there are no vacuum leaks...there are electronic leak detectors which work great, but some people will use tiny propane tubes or sprays to spray hoses to see if the engine rpm is affected, but don't try that unless your area is really are well ventilated, you know what you are doing, and you don't have anything particular to live for anyway). One other thing to check is compression. You want to be sure that all the cylinders are working roughly proportionally to each other, and the way to do that (for me anyway) is to pull the plugs, put a compression gauge on each cylinder and have somebody spin the motor over (or do it remotely). Write the numbers down. Then do the same thing again, but first put a squirt of light oil in each cylinder. Look at the numbers...the dry numbers should be within 10 percent of each other (highest to lowest) but no more than 20%. The wet numbers shouldn't be ten percent more than the dry numbers. If you think about it, this makes sense, too. If you want a boat to row smoothly, the four oars on the right should be moving with about the same strength as the four oars on the left, otherwise, you'll be out of whack (technical term).

Sooooo...the process usually goes something like this...first, check air filters. Make absolutely sure they are allowing the most/cleanest air possible with the fewest obstructions. Then, with air absolutely perfect, look at the plugs, plug wires, cap/rotor...anything associated with the primary or secondary ignition system and make sure all of that is spot on. Then, with the air right, and the ignition right, check the timing and advance mechanisms (that's the whole timing light thingy). The idea here is that the piston is coming up in the cylinder and compressing this air fuel mixture and you want most of the explosion when the compression is precisely the highest (think about the difference between gunpower exploding loose on a driveway versus being compressed in a firecracker case) and it would make sense that if you try and explode the mixture too early...or too late...you won't get the maximum benefit. Making this even more interesting...the mixture doesn't all explode at once. The spark actually lights a small spot which spreads across the mixture (its called flame front propagation and that is part of the theory behind multiple node spark plugs) so not only is the timing important for full compression, but also takes into account the amount of time it takes for the mixture to burn. Meaning...get the timing right. There are actually two parts of timing...base timing (where the engine is at idle) and something called "advance". Advance has to do with the idea that, as your engine rotates faster, the piston spends even less time in the compression stroke and thus you have to start that spark even earlier...sort of like leading a receiver with a football. OK...now that you have air, ignition, and timing its a good time to double check fuel filters. Clogged fuel filters can affect the volume of fuel available to a carburetor...especially at wide open throttle.

Now, with air perfect, ignition perfect, timing and advance perfect and plenty of clear fuel, delivered in sufficient volumes, you can take a look at the carb. You could just change it, but most of the time, the carb is matched to the intake and the exhaust (makes sense that if you want the perfect mixture, you want to know exactly the volumetric efficiency of the intake, but believe it or not, getting the spent exhaust out of the engine is equally important (think headers on an old Pontiac). Won't do you a bit of good to burn more air better if it doesn't have anywhere to go. Assuming you aren't changing it, and insist on fooling with it, most carbs have adjustment screws at the base and a pretty standard process is to rotate the adjustment screw out a full turn or two, then bring it back in, a quarter turn at a time, waiting for RPM's to drop about 50 or so, then back it back out an eighth. However, I hope it makes sense to you that if you start with the last part, without doing the earlier stuff, its sort of like washing your hands before you clean a toilet...nothing wrong with it, but it doesn't accomplish much.

[captainjohn]

WOW!! Thank you for your efforts on our behalf. I feel that a well tuned engine is the best place to start. I will endeavor to start there and get the best of what we have. That being said, I believe that the semi displacement hull is designed to get up and out of the water. My wife and I plan to live on this boat so our need for speed is really only in emergencies and I plan real hard not to have those kind of days or nights. I am certain that that 4 barrel carb is designed to be able to be able to give us that top end. It however makes me think that we suffer from the big tubing syndrome at the crawl speed. It that true?

[Lyndon670]

Very well said Charles! My .02 won't be anywhere near as technical, but will be to the point. There is nothing that you can do to that engine setup to make it "fuel efficient". I would be willing to bet that once it is tuned up, there is nothing mechanical that you could do (new carbs, switch to fuel injection, etc) that would give you more that a 5% improvement to the fuel consumption. The only things that may help you are making sure your props are tuned, making sure your bottom is in good shape, and running as light as possible. Only take as much fuel as you need (remember the 1/3 rule), carry only the fresh water that you require and get to know your trim tabs.

If that doesn't work, you will get amazing fuel economy staying at the dock !!

[captainjohn]

What is the 1/3 rule for fuel. I have never heard of it. Please tell us.

[waybomb]

Figure 1/3 the fuel for the transit trip, 1/3 the fuel for the return trip, and the lay third reserve.