Mercruiser 4.3 woes - help please

[saskboat]

We have a 1992 SeaRay 4.3 litre that we bought new. Only used in fresh water, garage kept, and low hours, it always ran great.

This year the engine began to develop issues at higher rpms, missing or even backfiring at speeds above 30 mph. So I kept it below that. Now trying to fix it.

Installed new spark plugs; old ones looked fine. Installed new distributor cap and rotor; old parts looked good. Tried it in the lake, and it did not run any better.

Next, checked compression. Thought it might have a bad cylinder. All cylinders have good compression at about 150 psi.

Then I took the screen-type fuel filter at the carburetor inlet out and cleaned it, even though it was clean. No obstruction there. This 4.3 also has a stone-type filter in a sediment bowl pointing upwards on the fuel pump. I had never touched that before, so I took that fuel filter out. It did not look very dirty, but thought it could be partially clogged with very fine particles that one cannot see.

Then I left out the stone filter and spring and put the bowl back on the fuel pump. Tried the boat and it now will not even build up to ten MPH or get up on plane. It starts great, but when the throttle is opened it runs a lot worse than before removing the stone filter. There also may be a gas smell (maybe engine flooding) although no gas leaks. I understand that the stone filter are NLA, and I am planning to put a new paper filter and spring in the fuel pump when I can get one next week.

Here's one question: Can someone tell me what the effect of running the engine without the stone filter (or modern replacement) would have on the operation of the engine? Do that filter and spring also help to regulate the fuel flow? Or should the engine run normally without a filter and spring at the fuel pump (other than the possibility of dirt getting to the carburetor)? Of course, I would not run it that way for long, but just for test purposes.

It also might be time to rebuild the carburetor, but that would come after I sort out the fuel filter/fuel pump issue. Any other ideas will be appreciated.

 

[Rick Stephens]

I'd probably start with a carb rebuild. Check and double check float height.

When you smell gas, it might indicate you have a ruptured diaphragm in your fuel pump. Is the clear plastic tube running from it to your carb got fuel in it?

If a carb rebuild does not get it, time to check fuel pressure. You can stick a gauge between pump and carb - should see 4-6 PSI. Or you can put a vacuum gauge (most vac gauges do both vac and fuel pressure) between tank and pump and see if you are pulling a vacuum, indicating something is plugged up, like the anti-syphon valve.

 

[CaptnKingfisher]

I'd start easy, water in your fuel tank can cause exactly what you describe. Take a sample from the tank and put it in a clear glass jar to check for water. If that's good then I see two possibilities (given compression is good). 1: carb rebuild required. 2: ignition circuit failing at high rpms. Given what you've done so far is suggest doing a resistance test on all your spark plug leads to make sure one of them isn't damaged, as well as the lead from coil to distributer. Also check the resistance across primary and secondary coil circuits. If these things are good, I suspect a carb rebuild is in order.

I don't know anything about these stone type filters and have no idea why removing a filter would cause your engine to run lousy. .. I'm sure the flow is slightly inhibited with the filter on... Perhaps your mixture is set incorrectly.. if it runs better with filter on then presumably it runs better with slightly less gas in the mixture, then I'm assuming your mix is too rich.. no that doesn't add up because the symptoms your describe (backfiring, missing) sounds like your lean or having ignition circuit problems. Ignore everything in this paragraph, just thinking out loud

 

[CaptnKingfisher]

Was trying to sleep and puzzling over why removing the fuel filter has caused such lousy performance.. thinking about the stones and the spring and what impact they'd have on fuel flow... Then it hit me I'm way overthinking this. You've got bad fuel. New charteristic developed of poor top end performance, then you bypassed the fuel FILTER (which prevents crud from getting to the carb) and experienced a severe degradation in performance. I wouldn't continue to run with the filter off, you may end up needing to rebuild the carb if you get too much bad fuel running through it. Let me know if I'm right cuz it doesn't happen that often and I feel pretty good about this hypothesis lol

 

[Bondo]

Ayuh,.... Welcome Aboard,.... Now would be a great time to add a canister type fuel filter,....

 

[saskboat]

Thanks for the ideas and the welcome. CaptKingfisher - sorry to hear that you lost sleep over this, but thank you. I also thought about it during the night.

Since I haven't had a chance to pick up a new filter for the fuel pump yet, I cleaned off the tiny bit of visible dirt on the original stone filter, blew air through it, and reinstalled it. I also rechecked the screen filter at the carburetor, thinking that dirt may have moved through the fuel pump without the stone filter in there. Nothing on the screen filter.

I also took the clear hose from the fuel pump off from the carburetor and put it into a container and then cranked the engine a little; no fuel in the line. And I checked for fuel flow from the metal fuel line at the carb. Fuel came out well, although I don't have a fuel pressure gauge to check the actual fuel pressure. So, I suspect that the fuel pump is fine.

I also drained the fuel tank as best I could by syphoning it (made up a small electric siphon pump - beats sucking on a long hose). A little fuel likely remained in the tank, but the fuel gauge read empty. Fresh ethanol-free premium fuel added. The boat has never had regular gas or any gas with ethanol in the tank. Always ethanol-free premium fuel, which is not hard to find here.

It's raining a little here today, so trying it on the lake again may have to wait for another day. If the issues continue, then it may be time for a carburetor rebuild.

 

[isaacs]

If you have water rolling around in the very bottom of your fuel tank it's likely that when you siphoned out the tank you may have left some or all of the water behind. When you use non-ethanol fuel it will not bind with this water and it will remain in the bottom of your tank.

I know that as boaters we are supposed to use alcohol in our drinks, not in our boats, but in your case some dry gas might be just the thing to soak up any water you may have.

 

[saskboat]

Unfortunately, no good news to report. With new premium fuel in the tank, water absorbing (non ethanol) additive added to fuel, stone filter reinstalled in the fuel pump, tried it in the lake. No improvement. It's still not running even as well as before I took that fuel filter out of the fuel pump. Before that, the boat would get up on plane and at least reach 30 mph before running poorly and refusing to rev further.

The engine builds up rpms when revved in neutral. Ran it up close to 4500 rpm in neutral after launching the boat into the lake, and it sounded good. But when in gear, it does not have the power to get the boat up on plane. And there does appear to be a smell of gas, although no fuel leak visible.

It might be time to install a new fuel pump. I probably will do that before tearing the carb apart for a rebuild. Any opinions on the relatively inexpensive fuel pumps available on line (about $120 Canadian)? A fuel pump from the dealer is about $350, so I would rather not go that route. The dealer also says rebuild kits for fuel pumps are NLA.

 

[tank1949]

Did you replace the OLD plug wires??? Gas leak could be going into oil pan or carb, as it is supposed to do per CG regulations, from leaky fuel pump. Examine oil and smell for gas. Look down carb air-horn and rev motor and watch for excessive gas. Been there too!

 

[saskboat]

Plug wires crossed my mind. Although they look perfect, I realize that there could be internal damage.
Since the performance worsened a lot after messing with the fuel pump filter, I do suspect that the issue is fuel related. My approach is to change one thing at a time; right now I'm waiting for a fuel pump to arrive in the mail. It might be a week or two before I can install that though.

 

[Lou C]

Don’t forget to check the anti siphon valve on the gas tank. It’s a spring loaded ball valve that is supposed to shut off the fuel flow if the fuel line from the tank to the filter leaks. I had this same problem on my boat years back when we first got it. The engine would start and rev fine in neutral but it would not plane out the boat.

 

[QBhoy]

So in my experience…7/10 times you’ve had any amount of bad water contaminated fuel through the carb…ultimately at some point you need to take the carb off and properly clean it out. Sometimes you get lucky.
mother than that…cleaning out the fuel tank the best you can won’t be good enough. Needs to be completely cleaned out dry. The water will be at the bottom. If you have a gallon of water contaminated fuel in the tank…then add 20 gallons of fresh fuel…what you end up with is just 25 gallon of contaminated fuel.
only other reason for what you have might be sticking valves, a vacuum leak on the intake side somewhere or perhaps even timing or rotten manifolds.

 

[saskboat]

A few more questions for those with experience with the mechanical fuel pump 4.3 engines. I now have a replacement fuel pump, but before installing it, I want to test the fuel pump pressure.

Where and how should I connect the fuel pressure gauge? It has a plastic hose and various fittings that don't look particularly impressive. I'm also assuming that it will be necessary to rev the engine beyond idle to determine the pressure?

Second question: can someone provide (or point to) good instructions on installing the fuel pump? I understand that keeping the push rod in place is the challenging part of this. To what point should the engine be rotated, how can one hold the push rod in place, etc, etc?

Thanks for any help.

 

[Scott Danforth]

check your flappers. if they drop, will choke off the motor preventing it from breathing

 

[saskboat]

Here's another update. We tried the boat in the water again today and discovered part of the problem. The automatic choke would not open all the way, which accounts for the gas smell and lack of power to get the boat up on plane. When I held the choke open (with someone else operating the boat), the boat would get up on plane. It ran reasonably well up to about 4500 rpm and then started missing/backfiring. I also checked whether the fuel pump was sending fuel up the small plastic line to the carburetor; it was not. So, the fuel pump diaphragm likely is good and no point in changing the fuel pump.
The automatic choke appears broken, so I'm going to replace that and the entire top section of the carburetor with available used parts. But that likely won't help with the missing near top speed. I may also clean the entire carburetor while I'm at it.
And I ordered a new set of spark plug wires in case that is causing the missing - the distributor cap, rotor, and spark plugs are all new.
Any other ideas?
Oh, and there is this loose brown wire with a blue stripe not attached to anything near the thermostat housing - see picture. Should that be attached somewhere, or was it likely never used?

 

[nola mike]

So, the fuel pump diaphragm likely is good and no point in changing the fuel pump.

Still might not be putting out enough presure at high rpms. Did you ever check the pressure?

Oh, and there is this loose brown wire with a blue stripe not attached to anything near the thermostat housing - see picture. Should that be attached somewhere, or was it likely never used?

That's for the alarm system, it was optional for a while. Think it became standard at some point.

 

[saskboat]

No, I was going to check the fuel pressure today, but by then the engine was warm and I did not want to start undoing fuel lines and risk spilling fuel on the engine. I did have a couple of fire extinguishers handy but would rather not have to use them.
Here's a video of the boat running on the lake - as you can hear, it starts missing at about 4500 rpm. Sound familiar?
Also, in the video (did not notice it when in the boat), a few times there appears to be something (gas or ?) splashing towards the top of the carburetor from below. Makes me wonder if it's not getting too much gas.
You also may notice that the bracket that holds the flame arrestor is missing; it broke off earlier this year and is another reason (along with the choke) why I'm going to replace the top end of the carburetor. No pieces from the bracket went into the engine; all accounted for. The liquid on top of the manifold is water, not gas, by the way. It has been there for a while. No water leaks anywhere that I can see.

 

[tank1949]

Don’t forget to check the anti siphon valve on the gas tank. It’s a spring loaded ball valve that is supposed to shut off the fuel flow if the fuel line from the tank to the filter leaks. I had this same problem on my boat years back when we first got it. The engine would start and rev fine in neutral but it would not plane out the boat.

Me too!!!!! I know they provide a great benefit but I have seen them corroded internally. A FP may not be able to overcome their resistance due to corrosion.

 

[nola mike]

Me too!!!!! I know they provide a great benefit but I have seen them corroded internally. A FP may not be able to overcome their resistance due to corrosion.

Pressure will be decreased if your anti siphon valve is bad.

 

[tank1949]

Pressure will be decreased if your anti siphon valve is bad.

I had one freeze up (aluminum body) and nothing would come through it. Somehow saltwater got into tank? I suspect spray through tank vent, but I cannot be certain. I had extras so I replaced and problem went away. I don't know if there is an efficient way to clean them or just best to replace them.