Four Stroke Fuel Usage Mystery

**Fastjeff57** · 09-03-2013

I'm not talking about outboards, but this one has me (and a bunch of other smart folks) totally baffled.

Background: Inboard boat with a Mopar 360 V-8. Has a Flo Scan device that monitors fuel consumption. Going along in rough seas last week, at 2,500 rpms cruise speed, the fuel usage rate increased from 9 gph to 10 as the boat slowed down (from climbing a wave). After the wave passed, however, the rpms came back up to 2,500 and the fuel usage returned to 9 gph. And it did this consistently for hours.

This is a carb motor with the throttle fixed in position at that rpm--it wasn't moving on its own. To me, the fuel consumption should have dropped as the rpms dropped, for less intake strokes per minute were occurring. But it went UP!

Anyone have an explanation?

Jeff

**Yellowjacket** · 09-03-2013

Since its carbs and floats are involved, my guess is that the pitching of the boat is causing it to run rich when the boat pitched up, and then went lean on the downslide... Could also be the G's involved on the upside vs downside, but that seems to me to be less likely. Typical old style American carbs never did well on sports cars since they tended to flood or starve going around corners (which is why Chevrolet developed their fuel injection system for the Corvettes way back in the 50's). I'd suspect something similar is happening in this case. Back in the old days you never would have been able to accurately determine what was happening and ignorance was bliss..

**Fastjeff57** · 09-03-2013

Thanks for your input.

I can assure you flooding did not occur, for the motors (that are kinda loud) never lost their correct tone. They just dug in a bit harder, slowing a bit and burning more fuel for some reason; and it was a consistent reaction for over an hour.

Good one, huh!

Jeff

**Yellowjacket** · 09-03-2013

They don't have to flood to go from rich to lean, just a change in mixture. BTW what was the change in fuel flow? From what to what? If it wasn't that large (as a percentage) then it is most likely just the effects of the floats. Remember that you are reading the change if fuel admitted to the float bowls, not the fuel consumed by the engine. Going down the wave the floats could be prematurely cutting off the flow and on the upstroke they are admitting more fuel to make up for what they didn't flow into the bowls when the boat was tipped forward. Doesn't mean the cylinders got more fuel, just means that the flow into the float bowls wasn't constant. Since the boat was pitching a good bit this makes sense.

**Fastjeff57** · 09-03-2013

The fuel flow increased from 9 gph to about 10, then went back to 9 as the load decreased. It was NOT flooding--I know what that sounds/ feels like--nor was the climb up the waves violent--kinda like encountering a smooth upgrade while in your car.

The carb is a new Edelbrock 1409 that runs really nice.

Jeff

**Yellowjacket** · 09-03-2013

It's just the effect of the float opening and closing. Here's a link to owners manual for that carb.

http://www.edelbrock.com/automotive_...409_manual.pdf

The float pivot point is aft of the float. When the boat pitches nose up, the float opens as the fluid level in the bowl flows aft. When the boat pitches nose down, the fluid goes forward in the bowl and the float rises. The flow rate isn't changing that much, just just +/- 5%, the average flow is 9.5, and the min is 9 and the max 10. The fuel flow into the engine probably isn't changing that much and the engine isn't going to flood or run lean if the mixture is fluctuating +/- 5% which it probably really isn't because that's the flow into the bowls and not the actual flow delivered to the cylinders.

Not magic...