# Mercruiser 4.3L engine making a popping noise sometimes



## Live_bait (Sep 25, 2014)

On my trip to Lake Erie last week the engine made loud popping sound twice in the 3 day trip. It happened when I was getting on plane. It runs good at idle and when on plane. I think it is backfiring through the carb. Sure gets your attention. Both times I ran at idle for a few minutes then it would get up on plane. So far I have replaced the fuel/water separator and cleaned the distributor cap. There was no water in the fuel filter. Plan to pull the plugs and do a compression test next.

Has anyone had this problem with a V-6 or V-8 engine?


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## fastwater (Apr 1, 2014)

When's the last time a complete tuneup including plugs,cap, rotor, wires and fuel filters was done?
If your compression test shows normal and it hasn't been tuned in awhile...may be a good idea to go ahead and do one.
Backfiring through the carb could be a timing issue but could also result from a plug wire or ignition components(including coil) breaking down under load.


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## Southernsaug (May 23, 2019)

I would add, change your ignition wires


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## Nauti cat (Nov 30, 2009)

I had a Volvo Penta 4.3 made a popping noise like you said yours dose it was a burnt valve. I had maybe 200hrs on the engine.


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## joekacz (Sep 11, 2013)

When you pull the plugs you’ll know which cylinder is loading up and then you can analyze from that point.Least ways that’s where I would start.Plug inspection can help you pinpoint an area at least of where it’s loading up. IMO


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## wannaeye (Aug 29, 2013)

Live_bait said:


> On my trip to Lake Erie last week the engine made loud popping sound twice in the 3 day trip. It happened when I was getting on plane. It runs good at idle and when on plane. I think it is backfiring through the carb. Sure gets your attention. Both times I ran at idle for a few minutes then it would get up on plane. So far I have replaced the fuel/water separator and cleaned the distributor cap. There was no water in the fuel filter. Plan to pull the plugs and do a compression test next.
> 
> Has anyone had this problem with a V-6 or V-8 engine?


Yes I had the same problem, the cause turned out to be the power valve , it opens during high load on the engine to add more fuel,popping probably from running lean, on a mercruiser carb take the top of carb off, valve is located in the top plate its the small piston thing, remove and clean it and the hole, vaccum drop is what opens it so it must slide smoothly


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## FISHIN 2 (Jun 22, 2005)

I had the same problem with a 4.3. At a slow go, it run ok, throttle up and would backfire and eventually stall. It was a pugged fuel filter for me. Might b a cheap fix to start..


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## Misdirection (Jul 16, 2012)

I had a similar popping sound last weekend. It was a loose spark plug wire on the distributor cap (my fault). Check your distributor cap and make sure its not cross firing inside, check all of the wires incuding the one to the coil.

I was down there burping my fresh water wash down and knew i bumped it, and sure enough, a wire was almost off. 

Sent from my SM-A505U using Tapatalk


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## 0utwest (Mar 21, 2017)

Whats your location ? Helped a friend last year with his after he replaced a lot of stuff ! His ended up being the Distributor shaft bushing was wore and he could barely get 3000 rpms on the water . I have some V-6 parts including electronic ignition and other stuff and all send you a PM with my phone no. if you need help .


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## stekzus (Mar 26, 2008)

My boat did that last year until it finally just died..it kept coughing up through the carb....got towed in and found out it was a corroded electrical connection on the plug on back side of distributor ....cleaned up with fingernail file put a little lithium grease on it and never had another problem....really makes you appreciate marine assist when your broke down in the water


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## firemanmike2127 (Mar 17, 2013)

A couple of questions. Age of your rig & service history would be helpful. Previous repairs can sometimes point to a cause of a new running quality issue. Did you start with a spark & compression test to get a baseline ? Has the engine been maintained in a good state of tune ? Fuel treatment added on a regular basis & filters serviced ? Cap, plugs, & rotor in good condition ? Base & WOT timing adjusted correctly (& checked since the problem started) ? What carb is on your engine ? If it's a 2bbl Mercarb (1 low speed mixture screw) it's possible that the accelerator pump is 'weak' since you mentioned that it happens while you were getting up on plane. That's a common cause of an engine wanting to hesitate or sneeze with a 2bbl Rochester or Mercarb (copy)...especially if the rig is older & the carb has never been apart. Any evidence thet the carb has ever been off disassembled ? I'd make sure all of the first items I mentioned are covered & correct BEFORE disassembling a carb. Basic diagnostic checks give you a reasons on how to proceed. BTW...the power valve that Wannaeye mentioned sits in the base section of a Mercarb. It's appx 7/16" in diameter, brass, & has a very small spring loaded pin protruding up in the center of the screwdriver slot. There are a couple of other carb options that 4.3's were equipped with. My initial comments are limited to the Mercarb (2bbl Rochester copy). Mike


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## Live_bait (Sep 25, 2014)

firemanmike2127 said:


> A couple of questions. Age of your rig & service history would be helpful. Previous repairs can sometimes point to a cause of a new running quality issue. Did you start with a spark & compression test to get a baseline ? Has the engine been maintained in a good state of tune ? Fuel treatment added on a regular basis & filters serviced ? Cap, plugs, & rotor in good condition ? Base & WOT timing adjusted correctly (& checked since the problem started) ? What carb is on your engine ? If it's a 2bbl Mercarb (1 low speed mixture screw) it's possible that the accelerator pump is 'weak' since you mentioned that it happens while you were getting up on plane. That's a common cause of an engine wanting to hesitate or sneeze with a 2bbl Rochester or Mercarb (copy)...especially if the rig is older & the carb has never been apart. Any evidence thet the carb has ever been off disassembled ? I'd make sure all of the first items I mentioned are covered & correct BEFORE disassembling a carb. Basic diagnostic checks give you a reasons on how to proceed. BTW...the power valve that Wannaeye mentioned sits in the base section of a Mercarb. It's appx 7/16" in diameter, brass, & has a very small spring loaded pin protruding up in the center of the screwdriver slot. There are a couple of other carb options that 4.3's were equipped with. My initial comments are limited to the Mercarb (2bbl Rochester copy). Mike


Thanks for your reply. My rig is a 1987 Sportcraft 205 Fisherman with the 175HP V-6 2-bbl carb inboard/outboard. Have not done a tuneup for about 10 years, its been running very good. I changed the fuel/water seperator and found no water in the old filter. I always add Sta-bil Marine to gas year round. And fog the engine and cylinders heavy at layup. I cleaned the distributor cap and the terminals, they were not very dirty. I don't use the boat much, last year only 3 trips to Lake Erie and 10 hours on engine. Most years I put 15-20 hours on the engine. Total hours is 880 hours. I haven't done the compression check yet, just starting to work on it, it's been cold and raining the last few days.
One thing I should mention, it only pops when slowly getting on plane. If I slam the throttle to get on plane it never pops. And it runs good at idle speed and while on plane. So I am thinking you are right, the carb is not delivering enough fuel under load. I overhauled the carb about 14 years ago after I bought the boat.
I ordered a set of plugs and will do the compression check. Then look at the carb.


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## Live_bait (Sep 25, 2014)

stekzus said:


> My boat did that last year until it finally just died..it kept coughing up through the carb....got towed in and found out it was a corroded electrical connection on the plug on back side of distributor ....cleaned up with fingernail file put a little lithium grease on it and never had another problem....really makes you appreciate marine assist when your broke down in the water


I checked the distributor terminals, I think they are in good shape.
Thanks for your reply.


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## Live_bait (Sep 25, 2014)

0utwest said:


> Whats your location ? Helped a friend last year with his after he replaced a lot of stuff ! His ended up being the Distributor shaft bushing was wore and he could barely get 3000 rpms on the water . I have some V-6 parts including electronic ignition and other stuff and all send you a PM with my phone no. if you need help .


Thanks for the reply. I'll check the distributor shaft for movement. I am located in Columbus, will keep you in mind for parts.


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## Live_bait (Sep 25, 2014)

Misdirection said:


> I had a similar popping sound last weekend. It was a loose spark plug wire on the distributor cap (my fault). Check your distributor cap and make sure its not cross firing inside, check all of the wires incuding the one to the coil.
> 
> I was down there burping my fresh water wash down and knew i bumped it, and sure enough, a wire was almost off.
> 
> Sent from my SM-A505U using Tapatalk


Thanks for your reply, I have checked the distributor and coil and they look good. May replace the distributor cap anyway.


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## Live_bait (Sep 25, 2014)

FISHIN 2 said:


> I had the same problem with a 4.3. At a slow go, it run ok, throttle up and would backfire and eventually stall. It was a pugged fuel filter for me. Might b a cheap fix to start..


Thanks for your reply. Are you talking about the filter that mounts on the fuel pump? I was thinking of cleaning/replacing that filter.


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## Live_bait (Sep 25, 2014)

wannaeye said:


> Yes I had the same problem, the cause turned out to be the power valve , it opens during high load on the engine to add more fuel,popping probably from running lean, on a mercruiser carb take the top of carb off, valve is located in the top plate its the small piston thing, remove and clean it and the hole, vaccum drop is what opens it so it must slide smoothly


I am thinking of rebuilding the carb but will clean the power valve first. Thanks for your reply.


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## Live_bait (Sep 25, 2014)

joekacz said:


> When you pull the plugs you’ll know which cylinder is loading up and then you can analyze from that point.Least ways that’s where I would start.Plug inspection can help you pinpoint an area at least of where it’s loading up. IMO


When I do the compression test and replace the plugs, I will read the plugs for problems. Thanks.


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## Live_bait (Sep 25, 2014)

Nauti cat said:


> I had a Volvo Penta 4.3 made a popping noise like you said yours dose it was a burnt valve. I had maybe 200hrs on the engine.


Thanks for the reply. I will do a compression check to check the valves. Usually, a burnt valve will make the engine run rough at idle all the time.


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## Live_bait (Sep 25, 2014)

Southernsaug said:


> I would add, change your ignition wires


Yes, I probably should. Thanks for the reply.


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## Live_bait (Sep 25, 2014)

fastwater said:


> When's the last time a complete tuneup including plugs,cap, rotor, wires and fuel filters was done?
> If your compression test shows normal and it hasn't been tuned in awhile...may be a good idea to go ahead and do one.
> Backfiring through the carb could be a timing issue but could also result from a plug wire or ignition components(including coil) breaking down under load.


The previous owner had a complete tune up at a marina before I bought the boat in 2007. I changed the plugs and fuel filter (on the fuel pump) in 2010. At this point I think it's a fuel delivery problem rather than an ignition problem. Thanks for your reply.


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## joekacz (Sep 11, 2013)

Live_bait said:


> Thanks for your reply. My rig is a 1987 Sportcraft 205 Fisherman with the 175HP V-6 2-bbl carb inboard/outboard. Have not done a tuneup for about 10 years, its been running very good. I changed the fuel/water seperator and found no water in the old filter. I always add Sta-bil Marine to gas year round. And fog the engine and cylinders heavy at layup. I cleaned the distributor cap and the terminals, they were not very dirty. I don't use the boat much, last year only 3 trips to Lake Erie and 10 hours on engine. Most years I put 15-20 hours on the engine. Total hours is 880 hours. I haven't done the compression check yet, just starting to work on it, it's been cold and raining the last few days.
> One thing I should mention, it only pops when slowly getting on plane. If I slam the throttle to get on plane it never pops. And it runs good at idle speed and while on plane. So I am thinking you are right, the carb is not delivering enough fuel under load. I overhauled the carb about 14 years ago after I bought the boat.
> I ordered a set of plugs and will do the compression check. Then look at the carb.


I’m not being a smart ass about this but no tune-up for 10yrs I think you just answered your own question. IMO


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## Live_bait (Sep 25, 2014)

Live_bait said:


> Thanks for your reply. My rig is a 1987 Sportcraft 205 Fisherman with the 175HP V-6 2-bbl carb inboard/outboard. Have not done a tuneup for about 10 years, its been running very good. I changed the fuel/water seperator and found no water in the old filter. I always add Sta-bil Marine to gas year round. And fog the engine and cylinders heavy at layup. I cleaned the distributor cap and the terminals, they were not very dirty. I don't use the boat much, last year only 3 trips to Lake Erie and 10 hours on engine. Most years I put 15-20 hours on the engine. Total hours is 880 hours. I haven't done the compression check yet, just starting to work on it, it's been cold and raining the last few days.
> One thing I should mention, it only pops when slowly getting on plane. If I slam the throttle to get on plane it never pops. And it runs good at idle speed and while on plane. So I am thinking you are right, the carb is not delivering enough fuel under load. I overhauled the carb about 14 years ago after I bought the boat.
> I ordered a set of plugs and will do the compression check. Then look at the carb.


Pulled the plugs today and was suprised to see a broken plug. The tip had broken and was floating around. I am lucky that it didn't come loose and fall in the cylinder.
Installed new plugs and the engine runs much better. Next is to take the boat to Alum Creek and go fishing. Then do a Lake Erie trip.
My theory is the floating plug tip would get very hot under load and cause the backfire.

Here is what the broken plug looks like.


















The compression test did not find any problems, 192 to 200 psi.
The timing at idle was 6 degrees BTDC, I didn't change it, don't trust my Harbor freight adjustable timing light. And the timing advances nicely at higher RPM.

Thanks for all the replies and suggestions.

Happy boating and fishing.


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## FISHIN 2 (Jun 22, 2005)

Live_bait said:


> Thanks for your reply. Are you talking about the filter that mounts on the fuel pump? I was thinking of cleaning/replacing that filter.


The filter on the carbureator was my culprit.


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## jdkswhite (Jan 3, 2008)

fastwater said:


> When's the last time a complete tuneup including plugs,cap, rotor, wires and fuel filters was done?
> If your compression test shows normal and it hasn't been tuned in awhile...may be a good idea to go ahead and do one.
> Backfiring through the carb could be a timing issue but could also result from a plug wire or ignition components(including coil) breaking down under load.


Nailed it those are the same steps I would suggest. Might see if he recently fueled up or just took it out of storage . wouldn't hurt to run fuel additives especially a dry gas. could be water in fuel. People just blow off all the things you mentioned .


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## MKULTRA (Feb 14, 2021)

Live_bait said:


> On my trip to Lake Erie last week the engine made loud popping sound twice in the 3 day trip. It happened when I was getting on plane. It runs good at idle and when on plane. I think it is backfiring through the carb. Sure gets your attention. Both times I ran at idle for a few minutes then it would get up on plane. So far I have replaced the fuel/water separator and cleaned the distributor cap. There was no water in the fuel filter. Plan to pull the plugs and do a compression test next.
> 
> Has anyone had this problem with a V-6 or V-8 engine?


i was a marine mechanic 30 yrs ago.i had an engine driving me nuts.it was only 1 year old and had this exact thing.i did every thing to that engine .the last thing idid finally fixed it.CHANGE THE IGNITION COIL!


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