backfiring on deceleration

[Daggett]

I need your help fellows.  I just purchased my first Concours, a 2002 with 48k on it. The seller told me that he got it from a honda dealer who had drilled out the jets.  He says to 24mm, but I think that is the part no. because that seems quite large for a jet.  Anyhow, I have noticed when riding it that when I decelerate it makes a pft... pft.. pft... sound.  I cant tell if it is coming from the motor or the exhaust, although it sounds like it is coming from the exhaust.  I have also noticed that when idling on startup I can smell fuel sometimes and it is definitely running rich.

Any suggestions.  I have only had the bike for a week and I have put seafoam in a fresh tank of gas.  A mechanic told me to just run that through the carbs and make sure I decelerate a lot to force the fuel through the idle jets.

I dont understand how clogged carbs can make it run rich, but I am by no means a mechanic.

Any help or guidance would be appreciated.

I live in a northern suburb of Cincinnati so if anyone is local and can call me feel free.  305-6247...

[timsatx]

Who the freaking hell would drill out the jets when you can just buy some? If I was you I would buy some new jets properly sized and put them in and see what happens. Besides that, 24mm is almost an inch in diameter so that doesn't sound right in the first place.

[Daytona_Mike]

A mechanic told me to just run that through the carbs and make sure I decelerate a lot to force the fuel through the idle jets.

Wow, that is a new one. You can also do an 'Italian Tune Up too'  Put it in neutral and hold the throttle wide open. that should clean any cob-webs out.

Just kidding please do not do that!!

[datsaxman@hotmail.com]

Check the carbs to see if everything is connected, and that the little rubber caps that cover the fittings you use to synch the carbs are all there.  Mine did what you describe, and one of the caps was missing.  Better now.

saxman

[Stasch]

Also make sure the carb manifold and carb boots and carb boot clamps are properly in place and snugged up.  Vacuum leaks here can cause spitting and huffing like you describe.

[Daggett]

I appreciate the input.  When I got home tonite I started the bike and it is now making the noise when idling.  Having a little time to look at it I figured out that it definitely is backfiring throught the carburetors, not the exhaust.  I'm hoping there is a stopper missing or maybe a loose carb boot or manifold.  Once again, thanks

[AZBiker]

A mechanic told me to just run that through the carbs and make sure I decelerate a lot to force the fuel through the idle jets.

I dont understand how clogged carbs can make it run rich, but I am by no means a mechanic.

There are air passages in your carbs in addition to fuel passages.  Some of the air bleeds on my carbs were clogged.

The person that told you to "decelerate a lot" may be a lot of things, but a mechanic they ain't. 
Protip:  Do not ever take anything you own to that person to be fixed.  The universe was being generous by dropping you a hint.

[VodkaAndPickles]

Mine also backfires really loudly on decel, like a gunshot.  It comes from the right muffler.  Bad gasket maybe?

[MAN OF BLUES]

Mine also backfires really loudly on decel, like a gunshot.  It comes from the right muffler.  Bad gasket maybe?

Stuck "air cut" valve on carb, or even sticky reed valve on valve cover....will cause this.

[VodkaAndPickles]

Stuck "air cut" valve on carb, or even sticky reed valve on valve cover....will cause this.

I have block-off plates, does this affect it?

Is the backfire harmful to the bike?

[snarf]

I have block-off plates, does this affect it?

 
I dont see how it would.  The reeds allow air not fuel to enter into the exhaust. If they were stuck open I would think you would actually be reducing the fuel to air ratio.  If there is to much fuel in the exhaust; it got there way before the reed valves. I may be way off on my thinking and I am always ready to be taken to school.

[snarf]

Stuck "air cut" valve on carb, or even sticky reed valve on valve cover....will cause this.

Can you please explain this to me one more time

[VodkaAndPickles]

I dont see how it would.  The reeds allow air not fuel to enter into the exhaust. If they were stuck open I would think you would actually be reducing the fuel to air ratio.  If there is to much fuel in the exhaust; it got there way before the reed valves. I may be way off on my thinking and I am always ready to be taken to school.

What would cause too much fuel in the exhuast?

[snarf]

What would cause too much fuel in the exhuast?

The fuel/air ratio can only come from one place...the carb.  I am by know means an expert on carbs; hopefully SISF or MOB will explain a little more in-depth.

[VodkaAndPickles]

The fuel/air ratio can only come from one place...the carb.  I am by know means an expert on carbs; hopefully SISF or MOB will explain a little more in-depth.

It only happens in the right exhaust, so it's either carbs 3, 4, or both.  What would cause only one or two carbs to run rich and the other two to be fine?

[snarf]

As MOB mentioned a stuck "air-cut" valve.  I sent SISF a pm.  Im sure he will be by shortly.

[VodkaAndPickles]

So is this damaging the bike, or no big deal?

[snarf]

I'm by no means an expert; a large enough back-fire 'could' damage the exhaust. While highly unlikely, it could still happen. Backfire is generally from to much unburt fuel in the exhaust gases. They enter the exhaust and detonate after the exhaust stroke. Are you getting good spark on all four cylinders? If one or two cylinders aren't burning all the fuel during the compression stroke this could be where your backfire is coming from. Like I said this is just speculation.

[VodkaAndPickles]

I'm by no means an expert; a large enough back-fire 'could' damage the exhaust. While highly unlikely, it could still happen. Backfire is generally from to much unburt fuel in the exhaust gases. They enter the exhaust and detonate after the exhaust stroke. Are you getting good spark on all four cylinders? If one or two cylinders aren't burning all the fuel during the compression stroke this could be where your backfire is coming from. Like I said this is just speculation.

Don't know about spark; I've been doing so much work to this bike lately that bad spark would just be one more thing wrong with it.  How would I go about testing it?  I replaced the spark plugs 10k miles ago or so.

[snarf]

I know you have B-O plates, but do you have stick coils. New spark plugs does not mean good plug wires.