Uneven Fuel Consumption

[rdelie]

I have a 1990 310 Sundancer with 454 Mercruiser V-drives. The starboard engine consumes a lot more fuel than the port engine. After starting out with full tanks, when the starboard tank gets down to 1/4, the port tank is still at 1/2. It's not a gauge issue, because the starboard tank will take 20 more gallons to fill up than the port. Also, when I pull the spark plugs from both engines, the starboard engine's plugs are black and the port engine's plugs are white. The boat doesn't have a generator, so it's not an issue of the generator pulling fuel from the starboard tank.

My thought is that this is a carburetor issue. Is there any other possibilities?

 

[BonBini]

Your starboard carb certainly could be over fueling. I would first check the ignition timing and also see that the timing is advancing at the appropriate rpm. What carbs do you have and how well do your engines idle? ... Ron

 

[Spookeay]

Yes.
Start with the carbs.
Are they the same?
When did this start?
First 2 questions.

Carbon ya know, can be fouling attributed to a rich fuel mixture Maybe a sticking choke or a clogged air cleaner,
Worn points can also throw the timing off.
A worn timing chain or low compression can also do it but often will show signs of bad idle and runing.

 

[Quint4]

Also check your fuel pumps. I discovered fuel in the clear line that runs from the pump to the carb on my 454 while winterizing. This indicates a ruptured diaphram in the fuel pump dumping fuel into the carb. I will change my pump this Spring and hope it improves my fuel consumption. Welcome to the site.

 

[Spookeay]

Cool, A place to look also. but it would idle like crap and smoke. Since we don't know how it's idling it could be that.
Good catch there Quint4,
did not think about that one.

 

[rdelie]

The engines both idle and run great. The port engine is slightly harder starting than the starboard. I have to pump the throttle a couple times after it fires to keep it running, but then it smooths out and idles fine. The carbs are Rochester Quadrajets. They are the same on both engines. I checked the numbers on the body of the carbs and they match. The timing is right at 8 degrees on both engines. The engines have the Thunderbolt IV ignition, so there's no points. This has been an ongoing problem since I bought the boat three years ago. It's slowly been getting worse each season. I replace the spark plugs, cap/rotor and fuel filters every season. I also check the timing and clean the spark arresters every season. The spark plug wires were replaced last season.

I pulled the carbs off when I put the boat in winter storage this year. I'm planning to rebuild them over the winter. I was just wondering if I was on the right track or if I was overlooking something.

Could the props being out of pitch cause this problem? The guy I bought the boat from was the original owner and he told me that he had never pulled the props to have them adjusted. They appear in good condition and there's no vibration. The rpm is equal between the engines at WOT. Although the engines do run a couple hundred rpm higher than recommended at WOT (both at 4600).

 

[Spookeay]

If they are both at the same rpm and no yaw I dont think this is the problem. Unless there is a gear ratio problem in a drive and lets not go that far out on a limb yet.

Here are some photos in a link. Tell me what plugs best look like the ones you pulled on the port and starboard engines.

http://www.spark-plugs.co.uk/pages/technical/diagnosis.htm

I say this becasue I am not so sure if your starboard is running rich but perhaps your port is really running lean.

 

[rdelie]

The plugs in the port engine look most like the top picture (normal). The starboard plugs look like the second picture down (dry fouling). I think it's a combination of the starboard engine running rich and port engine running lean. The port engine plugs are completely white (no tan color at all). It appears to be running lean. Just not lean enough to melt the ground strap.

One thing I didn't mention. I have checked the chokes after the engines are warmed up and they are fully opened.

 

[Spookeay]

The plugs in the port engine look most like the top picture (normal). The starboard plugs look like the second picture down (dry fouling). I think it's a combination of the starboard engine running rich and port engine running lean. The port engine plugs are completely white (no tan color at all). It appears to be running lean. Just not lean enough to melt the ground strap.

One thing I didn't mention. I have checked the chokes after the engines are warmed up and they are fully opened.

Well if noting jumps out at you soon, rebuild the carbs... "CLEAN THEM PROPERLY" and see if it continues. Then go on from there. I think it is a carb problem. Plugged jets on one side maybe,,,, bad neddel seal on the other?

Let others throw some more ideas at you then bag them all up for spring testing........

Fule hard to do over the computer and give a definitive answer but Good luck,,,,,

But go on and clean the carbs.

 

[rdelie]

I'll rebuild the carbs and go from there. Thanks for the help.

 

[BonBini]

Since you have the carbs off you may as well rebuild them. As Spook mentioned carbon could be your problem. Quadrajets have vacuum operated fuel enrichment. Vacuum is needed to close down the main jets. This port plugs and you'll run rich. Fuel enrichment for The secondary’s are mechanical.

I mentioned timing earlier because of the advance aspect. Thunderbolt ignition will bring in around another 24* of dynamic adv on top of the initial static setting. You should see something like 32* at 3700. There will be a specific spec for your module. If your not running enough adv at cruise rpm, this could lead to carbon build up issues. An easy check that is often overlooked.

With the oxygenerated fuels of today you may not see as much tan color on your plugs as we did years ago. Having to pump the throttle a few times at first start is pretty much the norm. I personally think these older engines can be retuned to run a little better on today’s fuels ... Ron

 

[88 AmberRay]

I was using much more fuel on my port engine. When I removed the spark arrestor on the carb. I saw that the choke valve wasn't opening all the way.

After making the adjustment everything was fine.

 

[sprink56]

Q-Jets also are notorious for having a plug leak in the bottom of the fuel bowl. This will drive you nuts trying to figure out where the gasoline is going. When you remove the "rich" engine carb...look into the intake and see if the base is moist with gasoline...that leads to leaking bowl plugs. You can easily repair this with epoxy.

My guess is that you have a float sticking or you have a bad needle seat. All of this will be corrected with a rebuild kit.

Good Luck

John F