Quick Backfiring Question

[TNRabbit]

I have a Honda 5518 tractor w/ the 18 hp two cylinder gasoline engine. It runs great except when I drop off the throttle suddenly it ALWAYS backfires once or twice loudly. In the past, I've experienced this when there is dirt or other contaminants in carb....but I cleaned it out completely and found nothing. I have Champion -vs- NGK sparkplugs (recommended) and I don't think they are the correct temp. Gap is fine. I've found nothing else to point to the problem...

Could the plugs be causing the backfire???

:thanku: in advance for any suggestions/help!

[Mrwiggles2]

Its possible that you need the NGKs, I know I need them in my VW.

[aguysmiley]

My guess is you get a real rich mixture going into the cylinders when you drop the throttle real quick. There isn't enough air in the engine to burn the gas that's there so it ignites in the hot exhaust.

[TNRabbit]

My guess is you get a real rich mixture going into the cylinders when you drop the throttle real quick. There isn't enough air in the engine to burn the gas that's there so it ignites in the hot exhaust.

But if it's running correctly, this shouldn't happen. Still haven't figured it out...what could else could be causing it to backfire?

[lawnmowertech]

But if it's running correctly, this shouldn't happen. Still haven't figured it out...what could else could be causing it to backfire?

backfire can be numerous things wrong such as valves not seating properly valve guides worn sheared keyway too rich of fuel mixture air leak somewhere it can also be ignition problem such as a coil mis fireing can be spark plug issue i mean there are tons of possibilities that can cause a backfire

[TNRabbit]

Thanks for the ideas, guyz! I'll do some more troubleshooting...

[grapeknutz]

Or a leak at the exhaust system drawing in air reigniting the exhaust gas.

[TNRabbit]

Come to think about it, it does sound like it could use a new muffler...

[Mickey]

Haven't had to deal with something like this for MANY years but from some of the comments here it seems like I have a different defination of backfire than others do. What I call a backfire is when there is ign of the fuel in the intake system not the exhaust. Fire in the intake is backwards of intended path so the term, backfire.

Ignition in the intake is typ a timing issue. As dumb kids we used to retart the timming so we could get the engine to backfire, make noise and get attention. Igniting unburn fuel in the exhaust is another problem. Don't have a sure fire answer for that other than fuel mixture is likely rich when it happens. A lean mixture will cause the engine to run a lot hotter than it would with opt fuel/air ratio.

You might put the 2 things together. Normal lean mixture, running hotter that it should and then a quick let off of the throttle causing a rich mixture for a short period and that makes for the needed conditions to cause the problem.