2001 43L 190hp Mercruiser backfires occasionally

"Hi. I have a 2001 (purcha

"Hi. I have a 2001 (purchased new in 02) 19.5ft Bayliner with 4.3L 190hp Alpha One. Since last year I have been having what can best be described as a knock or backfire from the engine that occurs occasionally. Of course I brought it back to the dealer and they couldn't duplicate the problem. I have changed the distributor cap, changed the oil, been through multiple tanks of gas, changed water/fuel seperator and cleaned the flame filter(sorry not sure of its correct name). Anyway I have driven this boat at various speeds and at various rpm's and I can not narrow it down to a specific speed or rpm. I guess aroud 20-25mph would be the closest although it has happened at higher speeds. Never has happened at idle speed only when moving. Last week I was out for a couple of hours and had no problem, but on my way back to the boat ramp it knocked/backfired 5 times in a 3 or 4 min span then it didnt knock again. Last year went camping/boating and for 4 straight days nothing then occasional knocking/backfiring on my 5th and last day. This is very fustrating because other then that the engine starts and runs very smooth. Kinda makes me think it is going to cause a serious problem and leave me straded out on the water. Still under an extended warranty. Any suggestios would be appreciated.
Thanks,
Rich"

"First of all, stop using the

"First of all, stop using the expression "knocks"--that is a bad bearing, and you don't want to go there! It's backfiring through the carb, not "knocking", and this is easier to fix (and won't ruin your day).

Reasons for this are:

1. Water in the fuel tank. (I assume you have a water separator and have changed it?)

2. Bad ignition wires that allow a 'cross over' from one wire to another.

3. Weak or broken valve spring.

4. Intermitent vacuum leak.

If I were a bettin' man I'd go with # 1 above.

Jeff"

"If it's under extended wa

"If it's under extended warranty, make sure the dealer looks at it and documents everything they do, how long it took and what their findings are. That way, if they don't fix it during the warranty period, you can usually get coverage specifically because it hasn't been fixed.

Is this a loud sound or more of a thud? Has it happened when you could look at the motor? Remove/open the motor cover and see if it's coming through the carb on hard acceleration. If it is, take a fuel sample- if that's clean, have the carb adjustments checked."

"Thanks for your responses. I

"Thanks for your responses. I have changed the water/fuel seperator last year, but seeing how it is not a big expense I will change it out again as well as the wires (they have not replaced yet).
I have had a friend try to watch the engine while I was driving and when it did backfire he did not see anything coming up thru the carb.
If it still persists I will bring it back to the dealership and advise them to look at valve spring and check for vacuum leaks.

Also because of the 10% ethenol being inserted into the gas these days is their any other precautions I should take as far as adding any additives to the gas or should 10% or less ethanol be fine in the motor?

Thanks again for your responses any other suggestions let me know.

Rich"

"Try some octane booster. Don&

"Try some octane booster. Don't know where you live, but with the ethenol, your octane rating is diminished"

"Another thing- how long does

"Another thing- how long does the boat sit between outings and has it been left to sit for any long periods with gas in the tank? If it has been sitting for a long time, take a fuel sample and look for a milky appearance. If it looks this way, you have a lot of water mixed with the alcohol in the gas. This is heavier than gas and will be the main part of the fuel delivered to the carb. At times, the clean gas will be drawn from the tank due to sloshing bit this will only last a short time. You'll need to pump it out if it's particularly bad. I know you have filled the tank multiple times but the water ethanol mix changes with each temperature inversion and fillup.

Gas that sits for a long time loses octane. Gas with ethanol attracts moisture from the air. This one-two punch causes a lot of problems for boats that sit for long periods.


I have seen several posts about not using gas with ethanol and when there are problems, adding "dry gas" instead. Well folks,"dry gas" and gas with 10% ethanol are the same thing. Google dry gas and see what the articles say about it. Also, see what is written about different fuels and their affects on motors."

"Again all thanks for your res

"Again all thanks for your response. As far as the gas goes I am filling up with the 10% ethanol this weekend for the FIRST time. Topped it off last year before the ethanol was introduced and used a fuel stabilizer for winter storage, but will keep that octane boost in mind.

Thnaks,
Rich"