Carbon Fouled Plugs - Backfiring out of Carb

[ninskrillz]

Hey gang,

Quick question about these symptoms, as they appear to be hypocritical. The bike has been backfiring out of the carbs and kinda boggy when idling, and have been trying to fix that. I through new plugs in this morning to see how they looked after running a bit, and they came out with dry black soot. This indicates a rich fuel mixture, correct? I am confused because I have read/been told backfiring out of the carb is an indicator of a lean mixture? Any insight much obliged!

 

[TwoManyXS1Bs]

Yep, sooty black is too rich.

Here's a likely scenario when running a single-coil wasted spark.

During idle, ignition timing is in the 15° BTDC neighborhood. Both plugs fire together, one for the compression/combustion event for one cylinder, the other occurring during initial intake valve opening for the other cylinder. If that other cylinder has a lot of unburned fuel (like from a skipped previous ignition event), it could ignite and blow back out the intake.

That skipped ignition event could be caused by a fouled/fouling sparkplug, or by simply being way off in the air/fuel ratio.

Not an uncommon condition on ol' V-twin Harleys.
The fire-belch outta that Linkert carb is a real crown pleaser...

 

[ninskrillz]

Yep, sooty black is too rich.

Here's a likely scenario when running a single-coil wasted spark.

During idle, ignition timing is in the 15° BTDC neighborhood. Both plugs fire together, one for the compression/combustion event for one cylinder, the other occurring during initial intake valve opening for the other cylinder. If that other cylinder has a lot of unburned fuel (like from a skipped previous ignition event), it could ignite and blow back out the intake.

That skipped ignition event could be caused by a fouled/fouling sparkplug, or by simply being way off in the air/fuel ratio.

Not an uncommon condition on ol' V-twin Harleys.
The fire-belch outta that Linkert carb is a real crown pleaser...

Great info thanks! It has fresh plugs so it might be the mixture. I'll tweak it tonight to see if there's any improvement.

 

[ninskrillz]

Great info thanks! It has fresh plugs so it might be the mixture. I'll tweak it tonight to see if there's any improvement.

leaned out the mixture screw 3/4 of a turn making it 1.5 turns out. no help, still backfiring out of the carbs. let it out another half a turn and still no change. could this be a timing issue? i also noticed the right side petcock leaks when on reserve, could that be a part of the problem?

 

[ninskrillz]

leaned out the mixture screw 3/4 of a turn making it 1.5 turns out. no help, still backfiring out of the carbs. let it out another half a turn and still no change. could this be a timing issue? i also noticed the right side petcock leaks when on reserve, could that be a part of the problem?

Also this problem isn't there when the choke and enricher plunger are out, only after they are shut off does it happen.

 

[TwoManyXS1Bs]

leaned out the mixture screw 3/4 of a turn making it 1.5 turns out. no help, still backfiring out of the carbs. let it out another half a turn and still no change...

What kinda carb leans out thataway?

 

[ninskrillz]

What kinda carb leans out thataway?

Vm34s in is rich out is lean.

 

[hooser]

It's possible to be lean on idle circuit and fat on everything else.. what are you running from pj to main. what plugs.. for the street you should be at least 7's to much idle time..can you bilp the throttle ? what does it do?

 

[TwoManyXS1Bs]

Anything else on the bike that's non-stock?

 

[hooser]

Just a not to as to setting you mixture..once warmed up and idling. turn one side all the way in and them back out until you hear it stop increasing in rpm...not stop the motor and see where you are as far as turns out..as a general rule if more them 2-2.5 turns out go up on the pilot. and do it again..

 

[ninskrillz]

PJ-25
MJ-185
Slides-2.5
NeedleJet-P6
Needle-6F9
Spark Plugs- NGK BP7ES

I'm using a Pamco and PMA, as well as a sealed battery. Battery voltage is good. I just noticed tonight that both petcocks are leaking pretty severely, and I'm about to order rebuild kits for both of them.

I blocked the advance mechanism from advancing the bike, because it was causing a hanging idle, so when I blip the throttle it revs but coughs more and sometimes pops out of the exhaust. If I dont keep peppering the throttle though the bike will eventually cough and pop till it dies.

edit: I only blocked the advance from operating while I was stiffening the springs, put both of them back on before I finished tinkering today.

 

[hooser]

I would say your 185 are too fat...a 700 with #1 cam I ran 185's.. I run 40 pj but I also run p5 and 6dp1 needle..

 

[ninskrillz]

does the main affect the idle circuit at all? the bike is solid at 3k+

 

[hooser]

No but the needle and needle jet does.. put a 165 mj in, what does it do when you blip the throttle ?

 

[ninskrillz]

No but the needle and needle jet does.. put a 165 mj in, what does it do when you blip the throttle ?

when I blip the throttle it revs but coughs more and sometimes pops out of the exhaust. If I dont keep peppering the throttle though the bike will eventually cough and pop till it dies.

 

[hooser]

Start by putting 35 pj in there and set the mixing screw 1.5 turns out..It should idle , bring the idle screws up to idle at 1200, turn one mixing screw in until you hear a change, then back till you hear the rpms go up. do the same on the other side. now adjust idel back to 1200s. start there let me know what it does from there.

 

[ninskrillz]

Start by putting 35 pj in there and set the mixing screw 1.5 turns out..It should idle , bring the idle screws up to idle at 1200, turn one mixing screw in until you hear a change, then back till you hear the rpms go up. do the same on the other side. now adjust idel back to 1200s. start there let me know what it does from there.

35 pj? Isn't that like 5 steps up?

 

[hooser]

I run 40 in my stock 73 650.. you said you had 25 in there.. when something is not working in little changes I like to make big change to see results.. then fine tune later.

 

[ninskrillz]

I run 40 in my stock 73 650.. you said you had 25 in there.. when something is not working in little changes I like to make big change to see results.. then fine tune later.

did you see i was running vm34s? ive got 27.5s ill try those first and go up again if that doesnt help.

 

[hooser]

I run vm34 also. I am old school.. 25,30 35,40..try it then get back with me. got work to do so later>