Topic: 550 backfire

[550Resto]

Hello all, I have a 78 550 that on hard deceleration backfires. I've changed the mains to 100 from 90, I have a delkevic 4into1 with stock airbox. Carbs were rebuilt by harisuluv. I'm wondering if I should go up one more size on the mains or not? I can hammer down and get through the gears with no hiccups or backfire on hard acceleration. Any help is appreciated.

[BomberMann650]

The simplest thing to check first are your airbox and carb connections.

Decel pops were first explained to me as result of too much air.  Even if not a Lean condition.

550's were built to run on 90 mains.  Try a different needle clip position maybe?

If you had an old forties harley, that pop would almost be considered normal 

[b52bombardier1]

Do you have a hole in your exhaust pipe or a loose pipe at the manifold end?   That would allow air in to complete combustion that will make the backfire sounds.

Rick

[DaveBarbier]

Just because it's so easy to test on PD46C carbs, I'd clear tube them. Were they rebuilt recently? Fuel system above the carbs clean?

Ever done plug chops to see where your mixture is at various throttle openings?

Might also want to check for air leaks before the carbs, at the carbs and after the carbs including between the manifolds and the head with some flammable liquid spray (carb cleaner works but eats paint so I tend to avoid that) or propane.

[Lostboy Steve]

The simplest thing to check first are your airbox and carb connections.

Decel pops were first explained to me as result of too much air.  Even if not a Lean condition.

550's were built to run on 90 mains.  Try a different needle clip position maybe?

If you had an old forties harley, that pop would almost be considered normal 

How loud of a pop are we talking? My 77 has always had a bit of a gurgle on decel. Not superloud, but it's there. I should add that my plug chops are just about perfect.