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Message started by J2 on 05/07/11 at 06:42:00

Title: backfire on deceleration
Post by J2 on 05/07/11 at 06:42:00

I have had my 2006 S40 for over a year, and it does wake up the traffic when I clutch coming to a stop. Sometimes it is a pop-pop. Sometimes a fairly loud bang. Doesn't happen every time. Someone who owned the bike before me (It had 1200 miles on it when I bought it) drilled the plug out of the idle screw, so I figure they turned it richer. The bike gets exactly 50 mpg, doesn't seem to matter at what speed. I think it should get a little better than that, but I am not real concerned ... runs great! I have torqued the exhaust header bolts and the muffler clamp, trying to keep the air out.

Now to my question: I have read opinions on this site that the backfiring is normal and does not harm the bike. Just being careful about this ... Does everyone agree with that? If I go richer on the idle screw, how much MPG do I lose?

Title: Re: backfire on deceleration
Post by Routy on 05/07/11 at 07:12:58

Its normal because they are set up from the factory a little on the lean side.
It won't harm anything, and w/ a little experience you can control the backfire.
If the plug is already out, adjust it a little richer may help limit the backfire, and will NOT hurt MPG one bit.

For myself, I can control the backfire w/ throttle position, and I have a perfect idle, and would never hack the carb to try to elliminate backfire.
But all this is my opinion only !

Title: Re: backfire on deceleration
Post by verslagen1 on 05/07/11 at 08:13:45

I use the idle mixture screw to control the shutdown bang. Any richer than to get it to poof is a waste and may cause it to idle roughly.

What you have is an abnormal bang, a roll off may cause grumble pops on decel, but should not cause KERPOWs at shifts or stops. In my experience this is caused by air leaks, which you've already addressed. But you might check with a lit candle, as you might need a new gasket.

Being all that checked and you still got it there still is another solution. Your air shut off valve may need cleaning. It's the one with the triangle cover plate. I think you can pull it off w/o removing the carb. Brush off the dirt, use a rubber safe cleaner (seafoam) and put back together.

Title: Re: backfire on deceleration
Post by J2 on 05/07/11 at 12:54:47

OK. Thanks. I will open up the idle mixture just a tad and see how it reacts.
As to the possibility of a blown gasket, do you mean the exhaust header gasket?

Title: Re: backfire on deceleration
Post by verslagen1 on 05/07/11 at 13:25:42

5F27150 wrote:

OK. Thanks. I will open up the idle mixture just a tad and see how it reacts.
As to the possibility of a blown gasket, do you mean the exhaust header gasket?

yes that and the muffler seal.

Title: Re: backfire on deceleration
Post by J2 on 05/08/11 at 07:53:47

OK. I intend to have service mechs at the dealership go over the engine soon ... check valves, new plug, check head bolt torque. If they see any blow back around the exhaust header, I may have them put on a new gasket (depending upon the price).

I did turn the idle screw about an eighth of a turn richer, and I took it out for a ride yesterday. It is indeed backfiring less and less violently. It still does it every now and then, when the clutch is engaged, coasting to a stop. Since I do not hear a lot of concern among experienced riders on this forum, I assume that a few back fires is NOT a sign that the engine will eventually be damaged. Since the gas is firing in the exhaust pipe, I assume that it represents no immediate threat to the valves or other engine parts.

When I have the bike serviced, I am going to ask the techs to try to install a new turn signal flasher than I purchased. I have been looking for a flasher that emits either a beep or at least a loud click to audibly indicate when the turn signal is engaged. I have never had a bike that had silent turn signals such as this one does. With the LEDs being mounted on the tank, I do not see them easily, since I ride with my head up, not down staring at the tank. I am not sure that the flasher I bought will work. It is a normal, 12-volt flasher, and its description says it is LOUD. It is, unfortunately, round and not square like the one under the tank. So, the tech will have to design alternate mounting. I have seen all the plans for inserting a beeper into the bike's wiring, and I think that simply replacing the flasher might be a better solution, rather than doing surgery on the wiring. I will report on the outcome of this experiment.

Thanks for responding to my questions.