Mild backfires at low RPMs.

[falconbrother]

I suspect that this has been discussed here..  My 99 IS has started mild backfiring at lower RPMs.  It still runs like a scalded dog so, it isn't hurting the power.  I'm running a tank of Seafoam through it but, I suspect that I have an intake leak someplace.  I hate pulling that tank, let me tell ya.  With all the chrome covers it's a royal pain.  But, I need to I suppose. 

My question is, are there common issues in the rubber in that area that causes intake leaks?  Something I should go straight too as I search out this issue?  I think I probably have one carb that is sucking air in somewhere. 

[WintrSol]

The most likely parts to leak are the vacuum tubes. Easily replaced with the tank off.

[Hook#3287]

If your bike hasn't been desmogged, there are more opportunities for vacuum leaks.

A easy check that may cause your issue are the intake boost caps, (#6)

http://www.partzilla.com/parts/search/Honda/Motorcycle/1998/GL1500CT+A/CYLINDER+HEAD/parts.html

You should have at least 3.  Desmogged with OEM petcock, you'll have 5.

They get old and crack, leaking air into the intakes.

Other than that, if original, you should check and replace all the rubber tubing.

Have fun

[Jess from VA]

If it has been de-smogged, look at the nipple caps on the back of your intakes.

Mine cracked (and later blew off) and that caused backfires.  But mine were on hard decell (dropping the throttle) at middle and higher speeds. 

Several people thought I was shooting at them.  I didn't blame them, that's how loud my backfires were.

[Valkpilot]

I suspect that this has been discussed here..  My 99 IS has started mild backfiring at lower RPMs.  It still runs like a scalded dog so, it isn't hurting the power.  I'm running a tank of Seafoam through it but, I suspect that I have an intake leak someplace.  I hate pulling that tank, let me tell ya.  With all the chrome covers it's a royal pain.  But, I need to I suppose. 

My question is, are there common issues in the rubber in that area that causes intake leaks?  Something I should go straight too as I search out this issue?  I think I probably have one carb that is sucking air in somewhere. 

I suspect vacuum leaks too.  Recently covered: http://www.valkyrieforum.com/bbs/index.php/topic,95448.0.html

[falconbrother]

I really hate pulling that tank.  I have done it a few times and it just irritates me.  The aftermarket chrome really makes it a task.  Since it's a 1999 model I think I'll rebuild the petcock while I have it apart.  I have a big selection of O' rings, probably don't even need the kit.   

[old grouch]

Before you pull the tank, check the clamps (12) that hold the rubber tubes from the airbox to the chrome intakes.  They can loosen up over time.  Stan

[Valkpilot]

I really hate pulling that tank.  I have done it a few times and it just irritates me.  The aftermarket chrome really makes it a task.  Since it's a 1999 model I think I'll rebuild the petcock while I have it apart.  I have a big selection of O' rings, probably don't even need the kit.   

There are no replaceable O rings in the petcock.  The rebuild kit consists of diaphragms, covers, and a spring.  When you order the kit, make sure you get the fuel filter (screen) too.

[davidsalford]

I really hate pulling that tank.  I have done it a few times and it just irritates me.  The aftermarket chrome really makes it a task.  Since it's a 1999 model I think I'll rebuild the petcock while I have it apart.  I have a big selection of O' rings, probably don't even need the kit.   

There are no replaceable O rings in the petcock.  The rebuild kit consists of diaphragms, covers, and a spring.  When you order the kit, make sure you get the fuel filter (screen) too.

The rings are replaceable, you just need to chop the rivets and drill screwthreads.

I have just done a rebuild with pics on here.

[Valkpilot]

I really hate pulling that tank.  I have done it a few times and it just irritates me.  The aftermarket chrome really makes it a task.  Since it's a 1999 model I think I'll rebuild the petcock while I have it apart.  I have a big selection of O' rings, probably don't even need the kit.   

There are no replaceable O rings in the petcock.  The rebuild kit consists of diaphragms, covers, and a spring.  When you order the kit, make sure you get the fuel filter (screen) too.

The rings are replaceable, you just need to chop the rivets and drill screwthreads.

I have just done a rebuild with pics on here.

I understand, but what you did was not a typical rebuild and is rare, to say the least. 

If the OP has a leaking diaphragm, replacing those O-rings won't fix the problem.  He would only need to go that deep if the valve is leaking from the selector switch side of things.  Otherwise, what is required is the cover set rebuild kit.

[Buza]

I have the same problem, there's very slight popping at rpm's below ~1400. It's not vacuum leak, carbs were completely rebuilt last winter. Also every vacuum line was changed and caps are new.

I have Cobra 6-to-6 exhaust and it's rejetted to 38 & 105. Dyno run gave 92 hp from rear wheel and mechanic said that +13% to that gives approximate engine power which in this case would be ~104 hp.

So I'm pretty sure engine and carbs are ok.

Could slight popping at very low rpm's be fixed by adjusting mixture screws? They are currently at 2 1/4 (or was it 2 1/2) turns out, according to red eye's instructions.

[falconbrother]

If it's not a vacuum leak and the seafoam doesn't work I'll be stumped.  It was running great and then it started.  It hasn't been sitting and has fresh gas.  Regular rider..  I have changed nothing on it. 

[Hook#3287]

Maybe try the starter spray while idling test.