View Full Version : Mercruiser Random backfire. Help boat Doc!
deepsix
08-29-2009, 10:05 AM
I have a 4.3l mercruiser with a factory weber 4 barrel carb. This boat has the thunderbolt IV ignition system. The engine runs good but will have a random backfire at around 1500 rpm under load. The back fire might happen 2 times in a day of running. It sounds like it is through the carb. It only happens while accelerating and the engine seems to stumble a bit at theat RPM. The stumble clears up as the RPM increases.
I have changed plugs, cap, And adjusted the idle mixture settings on the carb. Ran fresh fuel and changed the external fuel filter. I also ran the engine from another fuel tank to eleiminate fuel issues. I still have the problem.
This is a stock engine with about 450hrs. It runs with good oil pressure charging system and the temp looks normal.
Any clues on where to look next?
fishkisser
08-31-2009, 07:48 AM
Check for an intake leak either at the carb to manifold gasket , or some of the capped off vacuum lines if there are any ...
A propane torch (not lit) waved around will tell you if one is present by raising your RPM's ...:twocents:
micropeterous
08-31-2009, 02:39 PM
I have a 4.3l mercruiser with a factory weber 4 barrel carb. This boat has the thunderbolt IV ignition system. The engine runs good but will have a random backfire at around 1500 rpm under load. The back fire might happen 2 times in a day of running. It sounds like it is through the carb. It only happens while accelerating and the engine seems to stumble a bit at theat RPM. The stumble clears up as the RPM increases.
I have changed plugs, cap, And adjusted the idle mixture settings on the carb. Ran fresh fuel and changed the external fuel filter. I also ran the engine from another fuel tank to eleiminate fuel issues. I still have the problem.
This is a stock engine with about 450hrs. It runs with good oil pressure charging system and the temp looks normal.
Any clues on where to look next?
I'd pull the spark plugs and see if one looks different. If one does you may have a sticking valve. If you have a smog system on the engine make sure your pcv valve is clean and working.
deepsix
09-01-2009, 09:49 PM
Checked the plugs today. 5 of them looked the same tannish color.#1 was darker with more carbon buildup but not fouled. This has me now wondering about valves. Perhaps it's time for a compression check.
There is no smog stuff on this engine except for the valve cover return to the carb intake. Not to much in the way of vaccum lines except the
vac-choke. I haven't checked the carb gasket yet.
I have never used a propane torch around a running engine before. I can see how it might work though. I'm wondering how my eyebrows might look if it decided to backfire! Seriously, does that method work?
fishkisser
09-04-2009, 08:21 AM
I have never used a propane torch around a running engine before. I can see how it might work though. I'm wondering how my eyebrows might look if it decided to backfire! Seriously, does that method work?
Yea it works just fine ... Never had one blow up on me at all ...
You can always spray some carb cleaner around places also
and listen for the RPM change ...
fishin"G"man
09-05-2009, 10:55 PM
Checked the plugs today. 5 of them looked the same tannish color.#1 was darker with more carbon buildup but not fouled. This has me now wondering about valves. Perhaps it's time for a compression check.
There is no smog stuff on this engine except for the valve cover return to the carb intake. Not to much in the way of vaccum lines except the
vac-choke. I haven't checked the carb gasket yet.
I have never used a propane torch around a running engine before. I can see how it might work though. I'm wondering how my eyebrows might look if it decided to backfire! Seriously, does that method work?
We always looked for and found air leaks with engine oil. If the engine ran better when the oil was in a specific location on the manifold we knew there was a leak there. Same result with propane or engine cleaner. If your engine is a computer tuned engine look into that prospect as well.
With one carboned up plug you already have an answer. Look at the compression on that cylinder, do a dry test then a wet one to assure your result.
baitsauce
09-06-2009, 01:02 AM
Plug wires. Boat motors by their very nature viberate a lot. So they tend to wear plug wires out fast. If I'm wrong it won't cost anything to find out. Just fire it up at night and take a look at them. If you see any spark then there you have your problem.
Oh and "reading" spark plugs isn't what it used to be when lead was added to gasoline. Just ask the NASCAR boys about that.
:twocents:
AlseaAssassin
09-09-2009, 03:24 PM
Replacing the plug wires did the trick on my boat (which had close to same symptoms, just at higher RPM's.)
Backfire usually indicates lean condition. The accelerator pump is used off idle to richen the mixture and avoid that stumble when accelerating. I suspect your accelerator pump on the carb is not working or not adjusted correctly.
With engine stopped, remove the air filter, look in the carb, and give a run up of the throttle (med speed). You should see a good squirt of fuel from a couple nozzles in the primary bores. If no squirt, then get a new accelerator pump from the auto parts store. If you have a squirt, then move the pump linkagae up 1 hole (furthere away from the carb) on the pump lever, and test that.
deepsix
09-11-2009, 09:06 AM
I moved the accelerator pump adjustment from max lean where it was set to the middle position. It didn't seem to make any difference for me.
I haven't swapped the plug wires or ran at night yet. I guess that will be the next thing I try along with the vac-leak check. I'm trying to hold off tearing the carb down until this winter. I have the manual but no experience with the procedure and don't want to put the boat to bed for the rest of the year.
This engine is also very cold blooded at cold startup. I have wired a bypass switch on the oil pressure sensor to manually get the electric fuel pump started earlier so I don't have to crank the engine so long. It does start easier but runs rough for awhile. A internet search found others having the same issues. I guess I just need to keep eliminating things until
I find the problem. Thanks for the suggestions!
deepsix
09-18-2009, 01:45 PM
update on repairs.
The engine started acting like it was locking up while starting. I got it into the shop and pulled the coil/distributer cable and cranked it over. I was getting a large arc out of the coil through the wire boot to the + and - connectors on the coil. I have another wire set on the way. I have my fingers crossed that this is the fix for all of the problems.
Might want to check the timing, possable it may have jumped a tooth on the timing gear?