Uneven fuel useage

J

Jimmy B

Member
Joined
Nov 5, 2004
RO Number
15667
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8
I have a 1984 Carver with twin CH270 crusaders..I always take great care of them...They start right up, idle and run great...Recently added two rebuilt carbs (Q-jet) to help cure a higher fuel useage in one engine.I figured it was dirty...I still use more fuel in one engine then the other. I might use 3 or 4 gal / hr more at 3200 rpm.(Twin flowscan system)also shows when I fill the tanks. Could my seconderies be opening too soon?
Thank you
 
More likely one engine is loaded more than the other. Do they both hit the same WOT unsynched? Have you calibrated the tachs?? Have the props been scanned??
 
With both engines shut off, remove the spark arrestor. With someone else at the throttles have them move the throttle one at a time very slowly while you look down the throat of the carb. As soon as the secondary throttle plate (the big butterfly plate) begins to open, stop and mark the throttle with some tape or a marker so whenever the throttle lines up with this mark you KNOW you are begining to get into the secondaries. Any throttle position before the marks line up is only the primary circuit providing fuel. Do this to both throttles and you'll always know when you're getting into the secondaries and suckin extra gas.....gotta love those quadrajet carbs.
 
I have the same engines on mine and one engine always uses more gas than the the other. I run about the same RPM's at cruise as well. We put electronic ignitions in this year and that has made it closer now, but when I put a half tank in on each side, my port enginge always needs a couple gallons more that the other side. For comparison, last weekend I put 89 gallons in, 47 one side and 42 the other side. Not a huge difference, but it is different.
 
Props are not pitched exactly the same. As Bruce mentioned, when you put the boat on the hard at the end of the season pull the wheels and take them to a PropScan shop. I'll bet the free scan they do before they quote you a price for tuning will show the difference. That said, one engine will still likely show a little more fuel burn than the other because, besides one being counter-rotating, there will still be other minor differences between them.
 
My experience with my '86 Carver Crusader 270/350 showed a 10% difference between the port and starboard engine-the starboard engine being the more thirsty engine. The fuel transducers I installed midway through my ownership of her confirmed that difference. I've heard lots of theories as to why, but the difference in rotation seemed to me to make the most sense.
 
The 1st thing I suggest checking whenever I hear "uneven fuel usage" is whether there are 2 tanks and the crossover valve has been left open. Fuel is drawn most from the path of least flow resistance and there is always some difference. IMO , it's always best to run one tank to one engine, 2nd tank to 2nd engine, with any crossover closed except in emergency use.

As suggested by others above , there are lots of other causes of uneven usage including engine component condition and related tune ups, generator on one of the tanks, prop effective pitch difference , P/S trim , tank vent (or other ) slight restriction , Suction air leaks in fuel line, sometimes opposite rotation engine and transmission has been reported to make a difference -though it doesn't at all on my boat, whether or not you employ active RPM sync(for much better average sync) and it and your tachs are accurate , difference in fuel quality including octane rating, etc..
 
the counter rotating engine (not the trans) tends to use slightly more fuel.
 
My '87 270's do the same thing and it is the counter rotating side. I have had the props redone and there wasn't any change. I check my sync on flat/no wind lake by turning off auto and watching the GPS to see if it wanders off course, still burns more.

Niles
 
Looking at others similar experiences it looks like at 3200rpm you are within the differential between CW and CCW gas motors. Add in a generator if on the same side as the thirstier engine and I can see where you could have 3 to 4 gallons an hour difference.
Do you have a generator?
If so which engines fuel tank is it on?
Bill
 
Though it may depend on the particular opposite rotating engine and -output transmission, I believe it's a bit of a myth about "counters" generally burning noticeably more fuel. I suspect it's more likely the difference usually due to other factors which may be fixable or improvable.

The dedicated tanks for both my last set of opposite-from-each-other rotating gas (carbed) inboard engines and VD 71C transmissions and the replacement new (MPI injected) engines and new VD 72C transmissions on average did, and do, normally take within less than 1 gallon to maybe 2-3 gallons max difference ( w/ either side greater) at full fill (of typically 50-60 gallons each tank since I like to keep them full & ready). Rarely the difference will be greater, but the difference will change between the 2 tanks depending on which engine was used a lot more for maneuvering .

I'm sure if I didn't have a very-much-valued active rpm sync system the difference between burn totals would always be much greater, and depend a whole lot more on tachometer accuracy. (but all 4 of mine are more accurate digital tachs anyway and they match my ECM diagnostic device RPM readout.) Analog tachs are often a bit off and generally have a calibration screw as I recall.

But the 3 or 4 more gallons per hour flow rate difference you are reporting is a LOT. Is your FloScan recently calibrated for reality against actual average full fills of 2 dedicated tanks? If not, note the flow rates alone can be off a lot if not properly calibrated , and even if it has been calibrated that can be thrown off later if the flow sensor gets a little air trapped in it such as when a fuel filter is changed without prefilling, and/or if the downstream fuel line from the sensor is not pitched upward to aid air purging from the paddlewheel chamber, or there is just a suction air leak in a connection. .

I do see you mentioned a difference also shows up at the fill-up so the F/S data may be fine. Does that difference seem to pretty accurately reflect the difference in F/S flow rate, i.e. average per hour rate difference x hours used?
 
What type of Carburetor(s) are on these engines? Our RV of that vintage had a 460 Ford with a Motorcraft 4-barrel that was essentially a Holley. A backfire could cause a rupture in the Power Valve. With a blown PV, it ran about the same but used about 30% more gas. Speed shops sold a Check Valve Kit, Rebuilders incorporated a modification into their overhauls, and Holley eventually made it a standard feature.
 
quote:
Originally posted by j-d

What type of Carburetor(s) are on these engines? Our RV of that vintage had a 460 Ford with a Motorcraft 4-barrel that was essentially a Holley. A backfire could cause a rupture in the Power Valve. With a blown PV, it ran about the same but used about 30% more gas. Speed shops sold a Check Valve Kit, Rebuilders incorporated a modification into their overhauls, and Holley eventually made it a standard feature.

Click to expand...

JD he said QJets in his 1st post.
Bill
 
Thanks Bill, I thought I'd looked. I just looked, probably missed it, but if somebody hasn't suggested... Switch Carbs and see if the problem follows one of them?
Speaking of Rochester carbs, there was a model (and pretty sure it was not QJet) that leaked from the bottom of the fuel bowl. The shops would put epoxy. I don't think it was over the screws that held it together. Rather over some plugs or plugged passages.
If both carbs "squirt" the same and start without the bowls having to fill, I'd guess the gas isn't leaking down.
 
I bought a new Quadrajet for a 400 cubic inch Pontiac motor about 20 years ago and those plugs had to be epoxied right out of the box. JB weld. All these years later, still no drip.
 
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