Bonneville America Forums Forums Technical Forums Performance Mods Backfire!

Got a pretty consistent and powerful backfire or 'pop' on choked cold startup. No other time but starting. Currently running pod filters, 160 mains, stock pilots, 1 shim, 3 turns out and Specialty Spares short slashes. Runs like a bat outta you-know-where, but the backfire's a bit annoying. I've checked the couplers and condoms...any other suggestions?

Just a guess - the 160 jets are causing this? It's quite a leap from what happens in a carb designed for a stock 120 or 122 jet in choke mode on up to 160.

I wonder if you start at 1/2 or 1/4 choke would it help, although it's likely you've tried that.

I was kinda wondering about that myself Martin. I used the jet calculator for my pipe/air setup and came up with like 158 so I rounded up to 160. Maybe that's a bit much. I'll give it a shot with less choke and see what happens.

I had 160s in ours and 50 pilots at one point and am now running 155 and 48 , the other were just too big.

I know this is obvious, but do NOT touch the throttle while starting, you will get really loud explosions out the backside of the carbs....(Don't ask how I know, but I have a freak kit now cuz I blowed up my airbox)!!!

I started up yesterday afetr work with no choke at all. Struggled just a tad to take off, but no backfire. Maybe less is better in this case. Probably won't work when it's very cold out, but we'll see.

Must be the ticket. A bit less choke = no backfire. I'll see how it goes when the temps drop.

I never use full choke. I pull it all the way out and rap the end with my knuckle to close it to about 3/4.

Phil,

I do the same with the choke. Also when I shut the engine off I turn the gas off at least a minute before hand. When I go to start the bike, warm or cold, I don't turn the gas on until after the bike has started. My intent being that I have enough gas to start the bike, but not enough to cause a backfire or explosion.

Tom

I turn my gas off just before I kill the engine too. Always have it's good practice don't have to worry about a possible crank full of gas or gas all over. I turn mine on before start though.

Phil and Tom,

I don't imagine that in florida it gets cold enough temperature-wise to even need the choke. the heat from the temperature should keep the motor relatively warm, no? i'm in LA and never need the choke to start unless early in the morning.

thorn

Probably not but it takes less cranks to fire. Kinda rum-rum, fire!

Thorn,

Technically what we are calling a Choke is really more like a throttle advance. A true choke would kill the engine after it has warmed up, which ours does not do. So, by using the "choke" to me is very similar to just turning the throttle about 1/16 of a turn and starting the bike, and then holding the throttle to run the engine rpm at a level higher than idle. I use the choke so I don't have to hold the throttle, thus allowing the engine and carbs to warm up with minimal supervision. So even in warm weather I use the choke and as the engine warms up I slowly throttle the choke back.

Tom

It's a good idea to proportion the amount of choke to the temperature. When it's really cold, so cold you really start to wonder if you want to ride, full choke and switch left on for about 60 seconds to allow the carb heaters to warm up is good. Since cranking speed doesn't generate enough air flow to open the slides, the main jet size won't matter while starting up.
With the Epco pipes, mine will sometimes fire up with a bang, not so much a backfire as just a really hard initial combustion.