Backfire thru carb

[MaritimerBill]

I have an older Mercury 9.9, 4 stroke, ser. No. 0H002248. It is intermitendly backfiring thru the carb and or missing. I had the carb rebuilt ($70)& new plugs but no change. The motor was bought used and I put a big Merc. prop on it to push my 26' sailboat. This problem developed 2 years ago and I had the rebuild done. Still the same. The motor doesn't get a lot of use so any suggestions on the fix? Thanks

 

[Silvertip]

Re: Backfire thru carb

If its been doing this since a rebuild, I'd suspect a cam timing problem -- as if its one tooth off. Either that or ignition timing is not correct. Backfiring and missing are two separate problems and you need to determine which it is. You have new plugs so that's not the problem. Buy or build a spark tester and make sure spark is good on all cylinders. Check ignition timing. If it is way off and the engine fails to run correctly (or at all) when properly set -- look at cam timing.

 

[Texasmark]

Re: Backfire thru carb

I agree with smoke. Backfiring is an intake valve open during the combustion process (where it is supposed to be closed to seal the combustion chamber).....so the bang blows out it back to the air intake.<br /><br />If the spark arrives out of time that can happen.<br /><br />Intakes close right before the spark ignites the compressed gas in the cylinder. If your backfiring, you are firing before the cam closes the valve.<br /><br />So that says to me that you are firing too early.<br /><br />Don't know how you do it on a 4 stroke OB. On a car you just loosen and rotate the distributor in the direction it turns. That will force a delay in the time that the ignition gets to the firing point for that cylinder.<br /><br />For computer controlled engines where there is no distributor the computer has to do it so it either malfunctioned or the sensor that tells it what position the crank is in is misaligned.....I think. Never been there but it makes sense.<br /><br />Mark

 

[WillyBWright]

Re: Backfire thru carb

Just cuz you had that carb rebuilt 2 years ago don't mean it ain't gummed-up again. That can happen in a matter of weeks if not months. Run fuel stabilizer in the gas all the time. You don't use the gas fast enough to keep it from going stale. Don't leave the tank in the sun. It'll go stale MUCH faster. Close the vent and store the tank in a compartment not likely to get hot in the sun when you're not using the boat. At the end of the year, use the gas in something else. Do Not just add fresh fuel to stale fuel in the spring. I see a lot of kicker motors that see little use where the gas is never used up, only added to. Some of that gas can be several years old. Fresh gas doens't make stale gas fresh. The varnishing components are all still there, just diluted.

 

[MaritimerBill]

Re: Backfire thru carb

Thanks guy's,<br />The carb was rebuilt only. The motor is origional. I have been burning gas with stablizer for years. Our sailing season is only 4 months at best so I burn the left over gas in my snowblower. The tank is kept below deck so it doesn't get hot.<br />The ignition system and wiring will be my next check.<br />Thanks again.<br />PS: I have Advanced Piloting Qualifications Canadian Power Squadron.