fuel consumption issue.

[Ligfishing]

Very strange situation. I finally got some time to do some adjustments. Armed with having read Cliff's book cover to cover twice, I decided to see if my idle mix could be improved. I have a good functioning engine but I thought i could do better since it seems  to burn twice as much fuel at idle as the other engine in my boat. fuel management gauge says starboard engine is burning 1.7-2.0 gallons an hour at idle. Port engine starts out at nearly 4 and gets down to 3.5 once warmed.

So i moved both idle screw all the way in and started to turn them out. once i got to three complete turns and no effect what so ever. I'm assuming i massive air leak somewhere. I could not get the fuel use down and the idle is smooth but maxed out at 650 rpm, the lowest it would go. so throttle closed 100% and screws closed 650 was the limit.

Is my assessment correct about the air leak? I should have been able to starve the engine to a stall by cutting off the air?

as to why shes gulping fuel? nozzle drip?

as soon as i go to an off idle throttle position the fuel use comes down dramatically. which is strange to run harder and use LESS fuel...

once off idle the numbers match the other engine.

I'm about to reassemble a fresh carb to swap out.
(using Cliffs parts) I will report back.

Greg-

[Cliff Ruggles]

Idle tubes may be plugged up, common problem with Marine carburetors......Cliff

[Ligfishing]

Thank Cliff, I did the re assembly of the first rebuild and I will mount it up and test tonight.

would clogged idle tubes cause the high fuel burn ?

Also Whats your recommendation as to a cleaning( soaking) solution?

Greg-

[Ligfishing]

OK fresh rebuilt carb installed no problems, but  now idle wont come below 1000 rpm. air leak or float set too high....
took air horn off everything looks good. throttle plates 100% closed.

[Ligfishing]

Forgot to add the engine has a 13 degree tilt ( common for marine application, however it has a wedge plate to compensate, still has about 8 degrees back tilt? do it adjust for this in the float level?

issue is definitely limited to the carb, as re installing the old the  idle returns to normal. ( no external vacuum leaks)

gotta be the float height?

[Cliff Ruggles]

Float level would not cause high idle speed.  If the float is incorrectly set, it can effect metering or calibration.  Too high for sure and it will cause flooding, but set anywhere in between completely on the bottom of the bowl to just below the gasket surface it will NOT effect idle speed dramatically. 

Loss of control of idle speed means that you are getting air into the engine from someplace at or below the carburetor throttle plates (vacuum leak). 

I see this a lot.  It can come from using the incorrect gaskets under the carb, or between the main casting and baseplate.

Usually it is a problem with the throttle plates not being fully seated when closed, or incorrectly centered and indexed.  I see this problem with twisted throttle shafts as well, especially the primary shaft when folks remove the fast idle parts without holding them and twist the shaft in the process of removing the retaining screw.......Cliff

[Ligfishing]

Cliff:
Rebuilt second carb and no problems with idle speed.
Idle mix screws now have an effect and all seems good. However my original issue still presents itself.

Using 4.0 gal per hour at idle 650 rpm and as soon as I touch throttles drops to 2.0 at 900 rpm

And if I shift into gear it also drops to less than 2.0 gph

My starboard doesn't do any such thing.

Ideas???

[Cliff Ruggles]

Are  both carburetors exact "blueprints" of each other in every respect?  What I mean is same part number, and everything from idle tubes, airbleeds, jets, metering rods, and internal parts the same with the same settings?

If they are, swap them from one engine to the other and see if the problem follows the carburetor in question.....Cliff

[Ligfishing]

Yes they are but I think I've found the issue.

Seems I trusted the flow sensors too much.
I switched the flow sensors and the problem seemed to follow the sensor.
I've ordered a replacement and  will install later this week.

I should have  tried to remove the senors from the diagnostic flow.

Plus a mechanic buddy said  if one motor was processing that much more fuel at idle you would not have a very good running engine, there would be some other by-product. Both engines run equally smooth at exactly the same rpm( verified  with laser tach).