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Fuel Starvation

Posted: Wed Sep 06, 2017 7:38 pm
by TinChips

We recently noticed some fuel starvation under load on the Starboard engine. We recently went on a long trip (273 nm in 7 days) and cleaned both fuel filters beforehand (also did the little strainer in the carburetor) I'm going to check them again but if anyone could throw out ideas on what it could be I would appreciate it. 318 Chryslers. Thank you.

Re: Fuel Starvation

Posted: Wed Sep 06, 2017 10:01 pm
by BlueSkye
Could you explain the symptoms of this starvation in more detail?

Re: Fuel Starvation

Posted: Thu Sep 07, 2017 6:06 am
by Fastjeff
Also take a look at the anti-siphon valves at the tank. They can stick/ get plugged and cause this problem.


Re: Fuel Starvation

Posted: Thu Sep 07, 2017 6:38 am
by Tuggle
Not only the anti-siphon valve but make sure the pickup tube in the fitting has not separated and sucking air. Replacing both of mine as we speak, had some carburetor problems that I now suspect may have been the pickup tube all along.

Re: Fuel Starvation

Posted: Thu Sep 07, 2017 9:05 am
by bcassedy
Other potential causes (not allowing sufficient gas to be available at the carb):
- Float in carb sticking
- Float valve in carb partially blocked by ??
- air leakage somewhere in fuel delivery line/system (as part of Tuggle's reply above)
- blockage somewhere in fuel delivery line/system (as part of Jeff's reply above).
- faulty fuel pump
If cause not readily apparent, you're gonna need to go indepth...
* a lot of the above can be diag'd with the use of a 12v (Marine rated) fuel pump. (I got one from NAPA)
a. Get a couple of barbed fittings for each end.
** DON'T use teflon tape to ensure tight fit - teflon can "break off" into the line and block small orifices / passages). If you're concerned about an air tight fit, you can use plumber's putty (Lowes) on the male threads of the barb fitting. Just ensure the putty starts a thread or 2 away from the end.
b. Get 4' feet of marine rated fuel line and cut a - 6" piece, 2 - 1' sections, 1 - 1.5' section.
- you'll need 2 golf tees as well. Take the 6" section of fuel line and plug one end with the tee. (comes into use later).
c. Get 6 SS clamps - 2 each on each barb and 2 each on the line that you'll be testing.
d. Get a glass jar w/ lid. Drill hole in lid which would allow 1.5' section to fit snugly thru it and down into jar. Feed down into glass jar TO BOTTOM OF JAR.
** Drill 2nd hole into lid. This will allow for venting BUT be aware that the venting will be gas fumes and you should be running the blowers when doing your testing. ALLOW 5 MINUTES BETWEEN TESTS MINIMUM TO ENSURE ANY FUMES HAVE BEEN EVACUATED. (you could also get some clear vinyl tubing to put down into jar <just inside of lid - NOT TO BOTTOM!> and run away from batteries/test points, but still wait between tests!)
- 1 - 1' section goes on the inlet side of the pump. Double clamp.
- 1 - 1.5' section goes on the outlet side of the pump. Double clamp.
* You could "marry" the pump to the jar using tape. Makes it less cumbersome.
e. Get sufficient wire to reach all of your test points, with 1 end at the battery (I'd recommend tie wrapping the end at the battery to something < vertical support?> which will provide you with a strain relief and prevent the cable from pulling off as you move between test points [ or arc'ing - bad thing]).
- I'd recommend low voltage landscaping cable. It comes in 14 gauge and 12 gauge - I'd use 12 gauge (overkill but you'll have it in the future) The "married" condition of the 2 wires ensures you won't have to deal with 2 single wire catching on this or that. One of the wires has a flat side which you can designate your " - " side.
f. Get a toggle switch and place inline with one of the 2 wires. (using this will mitigate any arc'ing when you go to activate the pump). Don't place too close to jar to prevent any chance of fire). Connect the other wire (non-switched) to the pump. Connect "switched" wire to pump.
*** I'd recommend a "permanent" (ie. not just stripped and bare wire attached to pump's connections) attachment of the cable's other end leads to the pump. Your toggle switch will allow you to turn it on/off. Have in off position.
** TAPE OVER ANY/ALL CONNECTION POINTS - to prevent accidental arc'ing if the "hot (+) connection should it come into contact with bare metal of the engine/boat.
g. Get 2 alligator (1 black - 1 red) clamps to connect to battery. Connect 1 each to the 2 leads that will be connecting to a battery.
--> Ensure you're using the appropriate wire that corresponds to the switch's wiring. Ie. if you used the flat sided wire to the switch with the intent of making that wire "ground" < - > , then the (normally) black alligator clamp will be used for the "ground <flat>" side wire. The red clamp goes to the other wire.
h. Connect alligator clamps to battery. Pump is now (switched) live.
Diags -
-> you can now loosen/remove (one at a time) different test points and check for sufficient fuel delivery / air leaks.
- loosen/remove fuel line from test point (I'd recommend starting at carbs and working backwards towards tanks).
- place the short fuel line w/ golf tee on section NOT being tested (except carb if there's no barb at inlet. You'd need to plug the carb's inlet in some way.) This will minimize any back flow/dripping. There's no pressure so, with a snug fit, there should be no dripping.
- place the 1' "inlet" line (with <2> un-tightened clamps) on the side to be tested. Tighten clamps.
- turn on switch.
- you should see a steady flow of gas into the jar.
** Any bubbles (after an initial "fill the " hose/pump" period) would mean an air leak.
** a minimal flow could mean a blockage somewhere in the line.
- stop pump, loosen test section clamps, pull off line and plug the 1' inlet section with the 2nd golf tee.
- pull 'drip golf tee" from section not being tested and reconnect line.
- record results (if desired)
- wait 5 mins for blower to clear any fumes.
- repeat process "up the line". At some point you should find the cause**.
** if you don't -
- your normal fuel pump may be going bad (you'll need to check it's fuel pressure) - or -
- the carb may have an issue itself.


Re: Fuel Starvation

Posted: Tue Sep 12, 2017 8:02 pm
by TinChips
Thanks for the detailed troubleshooting tips. We ran the boat last weekend and it was somewhat better. (I still think something's wrong but will start looking more).

The tach was dropping a couple hundred RPMs while under load (3000 -3800 RPM).

I don't think it was a distributor problem because I just put a rebuilt distributor in the Starboard engine and timed it.

Thanks - I will post back with a status soon.

Re: Fuel Starvation

Posted: Tue Sep 12, 2017 8:25 pm
by BlueSkye
Not sure what it means to drop rpm at high rpm. You mean it doesn't reach the same rpm?

New distributor. Timed at idle I assume. Are you timing the advance at 3000 rpm also? Mechanical advance?

Re: Fuel Starvation

Posted: Wed Sep 13, 2017 8:46 am
by ericinga
Have you tested the fuel pressure? It's a simple test with a vacuum / pressure gauge attached to the carb fuel line? Should be in the neighborhood of 5 PSI or higher.

If pressure is low, work back in the system toward the tank. If pressure is ok, work toward the engine.

Bill's diagnostics above cover all of the possible problems in both directions.

Re: Fuel Starvation

Posted: Wed Sep 13, 2017 9:37 am
by bcassedy
>> "Bill's diagnostics above cover all of the possible problems in both directions."

Thx! Hey, my kids gave me the nickname "Overkill Bill".....

... seems to work...

... except when I'm trying to explain to the Admiral why we need this or that for da boat! ;-)