Backfiring through the carb HELP!

So I've had this ongoing issue during the last half dozen outings or so. After running for about 45 mins the engine will start backfiring through the carb. If I keep the RPMs below 2600 it won't do it but if I try to run at my normal cruising speed (3000 RPM) it will start to spit, sputter and back fire.

Yesterday I was able to have someone take a look in the engine compartment while it was happening and we noticed that there was a little bit of water seeping out the port side exhaust tower where two pieces come together as if there is a bad seal. It only leaks at the higher RPM's.

We have swapped the ICM, fuel pump, and plugs. Oil is clean, oil pressure runs normal, and temp runs normal. Looking for any information that may help solving this issue.

Enging is an 86 Mercruiser 4.3LX.

Re: Backfiring through the carb HELP!

Scootimus said:

So I've had this ongoing issue during the last half dozen outings or so. After running for about 45 mins the engine will start backfiring through the carb. If I keep the RPMs below 2600 it won't do it but if I try to run at my normal cruising speed (3000 RPM) it will start to spit, sputter and back fire.

Yesterday I was able to have someone take a look in the engine compartment while it was happening and we noticed that there was a little bit of water seeping out the port side exhaust tower where two pieces come together as if there is a bad seal. It only leaks at the higher RPM's.

We have swapped the ICM, fuel pump, and plugs. Oil is clean, oil pressure runs normal, and temp runs normal. Looking for any information that may help solving this issue.

Enging is an 86 Mercruiser 4.3LX.

Click to expand...

Ayuh,... Welcome Aboard,... The exhaust manifold/ riser leakin' is a Serious issue, 'n needs to be addressed, Pronto,...

The backfirin' is probably from a Lean condition,...
What's the Contents of the fuel filter look like,..??

Re: Backfiring through the carb HELP!

Yeah figured the leak should be addressed, just a little nervous with some of the rusty bolts and there's no telling when those puppies have been broken loose. I've had the boat just over a year.

I swapped the fuel filter towards the end of last season when it first started acting up. It wasn't clogged up and there didn't appear to be any water in it. My father thought it might be a lean condition but the part that baffles us is the fact that it has to run at the higher RPMs for a pretty good bit before she starts acting up.

Still open to more suggestions.

Re: Backfiring through the carb HELP!

it is a lean condition. When you are getting backfires or hesitation it is due to being too lean. if I had to guess I would say that you have a fuel restriction somewhere. You have more than 1 fuel filter. There is one in the fuel pump, and there is 1 right behind the fuel inlet to the carb. it is a little mesh screen.

It sounds like your carb below 2600 RPM is getting enough fuel to fill the bowl. But when you take it above 2600, the engine is needing more fuel than the carb can supply. it will run fine for a bit because your fuel bowl is full when you first go to above 2600 RPM. between the fuel line pumping some gas in, and your full fuel bowl your engine will run fine at 2600+ RPM for a undetermined amount of time. But when you stay above 2600 RPM, your fuel bowl is becoming depleted until it is to low to supply the needed fuel. The fuel line is still pumping in fuel to the carb but not as fast as the engine is using it and that is where it starts to backfire.

I could be wrong, but I am guessing that is why it takes a bit to show the issue.

Check both fuel filters again, if you have a fuel pressure gauge check it again. what was the fuel pressure reading you got when you did the test? It could have something to do with the anti-siphon valve on the tank. At the bottom of the line connected to the anti-siphon valve in the tank there is a screen. Sometimes that can get clogged restricting fuel flow. Last thing I can think off the top of my head if the float in your carb might need to be adjusted.

Re: Backfiring through the carb HELP!

I will check for more filters. The boat has a filter/separator but I haven't done much more digging. The fuel pump had a small screen that didn't have any trash in it.

Since I know the PO didn't do much for maintenance, other than draining the oil through the bilge, it probably wouldn't hurt to do a little carb rebuild and clean up.

Re: Backfiring through the carb HELP!

pull the fuel line off the carb, (small nut first) then remove the large nut. A small screen is inside it. Personally mine was totally junked up. you can just clean it with some brake cleaner or something similar.

Re: Backfiring through the carb HELP!

there was a screen style filter in your fuel pump? I am not familiar with that type of filter. Mine has a paper looking filter under the fuel pump bell cover at the top of the fuel pump. I am wondering if they bought a cheaper filter and garbage bypassed that filter and either clogged your screen filter behind the fuel inlet, or clogged your carb and it needs to be cleaned out top to bottom!

from my understanding the filter in your fuel pump should look like this.
http://www.iboats.com/Fuel-Filter-Sierra/dm/*******.021094561--session_id.126940253--view_id.171739

Re: Backfiring through the carb HELP!

You are definetely starving the engine for fuel. Let it go and you'll start melting internal engine parts.
Especially exhaust valves.

I just went through a similar situation with a Volvo 3.0L. Mine would just run out of gas at higher RPM's.

I found an air leak at the line going from the external fuel filter to the carb.

I would start with fresh fuel filters. ALL.

Also, you have an anti siphon valve which IS the outlet barb of your fuel tank, those can stick and only allow partial fuel delivery. They are cheap and easy to change.

Re: Backfiring through the carb HELP!

STI, I have not seen that type of filter on the boat anywhere but then again I haven't traced the line all the way from the tank to the carb. I do know the line runs from the tank to the water separator, then to the pump and then up to the carb. The one place I haven't looked at too closely is from the pump to the carb. I'm not 100%, mainly because you all have gotten in my head, that there isn't a filter between the tank and the pump but I don't recall seeing one. I will take a closer look and possibly try to get a few pics as well.

Oh yeah, you guys are rocking on the responses. \m/

Re: Backfiring through the carb HELP!

There is not going to be a filter between the pump and the carb on the fuel lines. the fuel filter is inside the pump.

Here is a picture I made for you.

Blue Arrow- Loosen this nut
Red Arrow- Remove this retaining bar from the pump
Green Arrow- lift this bell cover off the pump



Under that bell cover there should be a fuel filter that looks like the filter in the link I sent you. If there is not, or there is a mesh looking filter, I would swap it out for one like the link I sent you. There should also be a black rubber gasket that the fuel filter sits on, and a spring on the top holding it in place. This is the first fuel filter in the system and it gets clogged faster than any other filter so it needs to be inspected at least every season.

The next filter in the system is the filter behind the fuel inlet of the carb. number 33 is the filter, and number 32 is a spring that holds it in place. My carb did not have the filter in it behind the inlet and I just left it out. If it is there, check and make sure it is clean. you can remove this filter from the carb and run it without a filter and see if this helps the issue.

Then there is a little mesh screen inside the accelerator well inside the carb. #23 on the list.
I doubt number 23 has anything to do with your issue, but that screen can clog up as well, but that is more of a hesitation issue.


If there is a fuel filter on the fuel line then it is after market and should not be there.

Re: Backfiring through the carb HELP!



This is the type of fuel filter I have. I will try and get a couple pictures tomorrow and post up.

Is a fuel/water separator not a fuel filter?

Re: Backfiring through the carb HELP!

that is your electric fuel pump, nothing else.
Check coil tower for cracks, carbon tracks(use a magnifying glass)
Check inside dist for the same lines/cracks/tracks
Make sure spark plug wire have some seperation ,strapped together for neatness cause cross-firing

Re: Backfiring through the carb HELP!

Sorry I just read what I posted, yeah that the pump NOT the filter.. DUH!

I know my dad checked the distributor once and he said it all looked normal. I haven't replaced the plug wires but was thinking since I've done all the other basics that getting new ones probably wouldn't be a bad idea.

Re: Backfiring through the carb HELP!

you could run the engine at 2500, then pull the plugs and look at the color.Then run untill it starts popping, pull the plugs and compare.
If their the same color, your not running lean and have to look elseware, if their white you are running lean and need to determine why.
fuel pressure, fuel delivery, vac leak, intake leak, etc.