Downshift backfire?

[blue giant]

After cleaning, resynching my carbs and replacing all the fuel lines I've noticed a slight backfire in the left exhaust pipe when I let off the throtte or when I down shift. It seems more noticable when I downshifting and is more of a rumble that a gunshot type. I hope I'm explaining this correctly.

Any ideas?

Thanks Paul

[egsols]

Are carbs running slight rich? Stock exhaust or after market?

[blue giant]

Originally posted by egsols
Are carbs running slight rich? Stock exhaust or after market?

Stock exhaust, I haven't pulled the plugs since I changed them. Maybe worth a peek eh?

[79XS11F]

Is it back firing or "popping" Back fire usually means rich ... popping usually means an air leak somewhere between the intake boots to carbs or at exhaust headers to exhaust ports.
Rob

[prometheus578]

(you just forgot to mention it)

Popping, 79XS11F, as you well know, can also mean a lean mixture.(wthout having a vacuum leak)

[79XS11F]

yes that is true and air leaks will lean it out somewhat. Some may have carbs leaned out in error as well.
Rob

[blue giant]

So would that be lean on the idle mixture settings then?

I have them set at 2.5 turns out.

[79XS11F]

Is it idling well both cold and hot Paul? Idle setting is about 2.5 out from lightly seated.
Is it a true back fire it is it downshifting "popping"?
Have you done any changes to your carb jetting of float heights? Is this a "new" issue?
Rob

[blue giant]

Hey Rob,

Ya, new issue. Tried to set the idle mixture screws by ear/rpm, didn't seen to get very far. Carbs have been cleaned and synched with a vacuum gauge. I did replace the old air filter before I started, it was pretty full.

It's not a real serious issue, just wasn't dooing it before. Idle mixture screws were set at 2.5 turns out before I cleaned the carbs. I'll double check the carb boots.

I think I'll try the propane method.

Paul

[79XS11F]

Yeah or quick start. Spray it around the boots and the butterfly valves and see what happens. you can always turn the idle screws out another 1/4 turn and see what you get. did you sync with a single gauge or a set?
Rob

[blue giant]

I synched with a set, I made it myself with all new gauges.

All told the bike runs great, better than when I first brought it home.

It won't idle cold without some choke until it is well warmed up. Should it?

[79XS11F]

That's normal Paul. Full chock for a few seconds followed by 1/2 for a few min should be pretty good unless rather cold out
Rob

[lsippell]

A backfire is firing back thru the carbs. Out of the exhaust is called an afterfire.

http://en.wikipedia.org/wiki/Back-fire

A Back-fire or backfire is an explosion in the intake manifold, carburetor/throttle body or air cleaner of an internal combustion engine. Although an explosion in the engine's exhaust manifold or exhaust system, is often referred to as a backfire, it is actually an afterfire.

Louis

[AlanB]

Hi BlueGiant,

It is worth checking the calibration on your gauges; it only takes a second!

Once you have finished the synch job, and all of your gauges read the same, turn off the motor, swap the gauge hoses onto different cylinders, and try again. If the gauges are all correctly calibrated, they should all still have the exact same readings; If the readings have changed, you have issues with some (or all!) of the gauges.
I don't think that I have EVER seen a set that are exactly correct, after thirty-some years in and around the trade.....

Most pro mechs I know either have little calibration notes marked on the gauges, or use Mercury sticks..

AlanB