Fuel starvation conundrum

[Lou C]

 

[rolmops]

I just went into the maintenance guide for the 1988 4.3 liter omc and just fir you to double check, here are some numbers.
The sparkplug : AC (delco) MR43T with a gap of 0.035 inch
Breaker Points: Dwell (degrees)39+/-2 Gap 0.019
Fuel : RON 93
Timing : 6BTCD
The reason why I wrote this down, is the fuel.
The 93 0ctane may not be what you are getting with the ethanol mix that you use and maybe you have to take that into account with the timing and dwell.
As for the hoses holding perfect pressure, great,but there may crap inside that will not show up on any pressure gauge ,but impedes the fuel flow.
Fresh fuel and new hoses is what you should do so you no longer have that question lingering in the back of your mind, and when you run the new hoses , make sure that they not kinked anywhere and that they have the right diameter for your engine. Also maybe check for a float drop or throttle linkage for binding. The fuel lines must have an internal diameter of at least 3/8 inches.

 

[Lou C]

I just went into the maintenance guide for the 1988 4.3 liter omc and just fir you to double check, here are some numbers.
The sparkplug : AC (delco) MR43T with a gap of 0.035 inch
Breaker Points: Dwell (degrees)39+/-2 Gap 0.019
Fuel : RON 93
Timing : 6BTCD
The reason why I wrote this down, is the fuel.
The 93 0ctane may not be what you are getting with the ethanol mix that you use and maybe you have to take that into account with the timing and dwell.
As for the hoses holding perfect pressure, great,but there may crap inside that will not show up on any pressure gauge ,but impedes the fuel flow.

I've been running this engine on 89 pump octane fuel for 20+ years with no problems like this one, except back in 2004, and that was the last time the anti siphon valve was replaced. In fact it says to use 89 AKI octane right on the flame arrestor....and the #s on our road gas pumps, are AKI. When I buy it at the gas dock, they sell 93, but I'm not sure if that's RON, or AKI.
I thought of that with the hoses, I cut the hose from the anti siphon valve to the fuel filter in half, looked inside, didn't see any evidence of narrowing down, or delamination of the hose.
I thought of siphoning out a few gallons and run it in my pressure washer as a test. These run at wide open throttle under a pretty high load, so if it runs OK with the fuel that was in the tank, well, I don't know what to say!

 

[rolmops]

One final question. Is the fuel in the main tank and the fuel in the portable tank from the same gas station?

 

[Lou C]

No that's a big difference, the fuel in the main tank is mid range pump gas E-10 (89 AKI) the fuel in the portable tank is E-0 94 octane and was fresh, just bought it in Sept when I was doing the tests. Only one gas station around here sells it, and it's like $7.50 a gallon, I wanted to try it in the boat because I had good results with it in my yard engines. Since the boat ran bad from when I launched it, I never used up the fuel in the tank, which is now about 15 months old but was stabilized when new.
Now every year at the start of the season, I have always topped the boat off with fuel I stored for the generator, that at that time is maybe 6 months old but was stabilized when I bought it and all these years I have never had a problem like this. During the season when the boat's on the mooring, it gets fueled with 93 octane marine fuel because that's all the gas docks here sell.
BTW since OMC I/Os have not been sold for many years, just for the heck of it I went on BRP's website and looked up the last Evinrude (Etec) and Johnson (4 stroke) manuals to see what fuel is recommended, for both, it's 87 AKI.
As far as timing, I have always used 6*BTDC, and I checked the total timing advance as well, which as you probably know it much lower on the 4.3 V6 than it is for the V8s, it only gets 12* of centrifugal advance at 3200 rpm, so your total timing advance is only 18*.

 

[rolmops]

It seems to me that you have now pinpointed the crucial difference which is causing the engine problems.
The fuel in your tank has been in your possession for 15 months and because it was stabilized it probably did not stink. But you do not know how old it really is and gas only stinks when it gets to be very bad, while yours is not quite at that point yet
I strongly suggest that you start removing the bad gas from the boat fuel tank and use it up in your car a few gallons at a time until your tank is empty. And if you still can, you should mix some injector cleaner into the portable tank and run the engine for a while in order to make sure that all the residue of the old gas gets washed out of your fuel lines, quadrajet and manifold. Then winterize the engine and start next spring with fresh gas which you should get from a busy gas station.












it sounds to me that you have now pinpointe

 

[Lou C]

I think you’re right. The carb on the boat had been run on the E0 94 octane fuel for about 3 hrs and I had to winterize it due to the temps starting to drop into the 30s here at night. I have to figure out what to do with the fuel. Over the winter I want to replace the rest of the fuel lines & old fuel fill. It’s just odd, I’ve been boating here since 2002 & never had this but there’s always a first time I guess. The other thing I have to do is inspect the exhaust manifolds since they have been used in the salt for about 6 seasons. What I started doing is loosen the rubber hoses slide them down & remove the 90* down pipes . Then I use an inspection camera to look inside the elbows and you can see if any water has popped in the bottom of the manifolds. This engine is currently on its 4th exhaust system…a fact of life in the salt…

 

[Chris1956]

Lou, You might remove the antisiphon valve and replace it with a straight pipe for a test. Sometimes that can cause just a bit too much restriction.

I cannot run an A-S valve on my 150HP Johnny OB. At high RPM she starves of fuel. I tried replacing it, but no dice.

 

[Lou C]

OMC actually has a spec for anti-siphon valves, the original and replacement ones are both what they say you have to have. I tested the one that was in there and the new one (identical) that I replaced it with both test the same, the ball opens at 1"-2" Hg when tested with the vacuum pump. OMC's spec for a fuel pump on the vacuum test (inboards) is that they have to develop 9"-10" Hg to pass. Both the valves I have now are the part # they say to use in the shop manual. For 3/8ths" hose they say to use pn # 173274. They go on to say "Anti siphon valves must have a flow restriction pressure drop of no more than 35" of water at a fuel flow rate of 20 gallons per hr". Not sure if any of the aftermarket meet that, it's not an easy thing to test which is why I used the vacuum tester just to get some data on that. I do have a couple of aftermarket ones, that I can test also.

This is a new problem, that original anti siphon valve was replaced by my OMC mechanic back in 2004 and I never had a fuel starvation problem until this season, after testing everything and running it on a remote tank with different gas (boat ran perfect, hit 4900 rpm with the standard 15"x17" prop) the gas, is the only thing left. Unless someone can think of something else, LOL!
Lesson learned:
with a performance problem you think might be fuel starvation, first verify that the ignition system is up to standard. Did that. I knew engine compression was good, tested it the fall before this season.
Next before rebuilding carbs lol, run it on the outboard tank, that isolates the boat fuel system from the engine fuel system.
It would have saved time if I had done that, but now in the future I have the tank ready if I ever have to deal with that again on this boat or those of friends or relatives.