Missfiring cylinder, Is it my carbs??

88alltrac wrote:

#4 plug was fuel fouled. i pulled it out and cleaned it then switched it with #1. bike ran beautifully. after i left it for a while it started to not run as well. my petcock isnt working so i think #4 carb is flooding because the float needles isnt sealing perfectly. thus fouling out the plug.
sigh. i guess i should look for a manual petcock. i just put new gaskets and stuff in the petcock not long ago and it still doesnt shut off. it is the vacuum type

For discussion, the "float valve" consists of the float needle itself plus the float needle seat which has the small hole (orifice) that the pointed needle tip fits into.

Imperfect float valve sealing would likely allow the floatbowl fuel level to rise higher than spec, and perhaps keep rising to point where the excess fuel starts escaping through the floatbowl overflow circuit.

As this condition may occur while riding (i.e., imperfect float valve operation), a perfect petcock (while desirable) won't cure the problem.

Often, the float valve just needs a good cleaning to prevent further leaking, especially where there's grit or crud preventing full closure between the pointy needle tip and seat orifice.

Would examine the tip for any obvious wear or damage (magnification under strong light is helpful), and use tb's "fingernail" test. Also polish the orifice (Q-tip in drill may work pretty well). Metal to metal is fine where so designed, altho some valves use rubber tipped needles.

Imo it is best to keep the same needle with the same seat in the same carb with the same float, and don't mix and match these parts.

If replacing the float needle, it's a good idea to also replace the seat (where the seat is a separately removeable part), and wish them both a long and happy marriage to each other.

Good Luck!

i have pulled out the needles and seats and cleaned them. upon close inspection under adequate light they look like brand new. no signs of any wear or problems. i also did not mix and match them. if i tap on the side of the carbs they seem to stop leaking. at least for a while.

i am getting so sick of tracking down these problems. as soon as i fix one thing the next thing pops up. if i didnt love these old bikes so much i would have lit it on fire already.

88alltrac wrote:

i have pulled out the needles and seats and cleaned them. upon close inspection under adequate light they look like brand new. no signs of any wear or problems. i also did not mix and match them. if i tap on the side of the carbs they seem to stop leaking. at least for a while....

Perhaps float is binding for whatever reason, such as slightly bent float pin, or binding on the pin.

With some carb designs, it's possible to assemble with float installed upside down.

Not unusual to need a love tap against the float bowl to achieve full seating of float valve after reassembly, but typically only needed the one time if at all.

Are the tiny springs in position on blunt end of float needle?

The image below is included for reference. Hope it's the applicable carb.

Good Luck!

[Click on image for enhanced view]

Now that i think about it i have had the carbs on and off so many times lately it could be that i only have the problem with them overfilling after the carbs have been tipped upside down.
basically when i put the carbs back on i should give them a good tap instead of just walking away and then coming back to find gas dripping out of my airbox.
i think i will run it for a few days and make sure that the carbs needles are shutting off the fuel. then do a fresh plug change i guess. it was running beautifully today for a while after cleaning the plugs but the bowls may have been flooded before cleaning the plugs and they just ran for a while before crapping out again.
i didnt realize that plugs would foul up that quickly and easily.

i am still pretty new to all this stuff compared to alot of people on here so sorry for all the noobish questions and mistakes. i am entirely self taught on this stuff and only learn by making mistakes i guess.
thanks for helping me out for sure.

88alltrac wrote:

...gas dripping out of my airbox....

That's a RED FLAG for possibilty of fuel contamination in crankcase oil.

Often results from impaired or non-functioning floatbowl overflow circuit.

Typical situation is where leaking float valve allows excessive fuel into floatbowl.

The excess fuel is supposed to escape via the floatbowl overflow circuit (out the drain nipples and through the drain tubes to exit underneath the bike).

When floatbowl overflow circuit is blocked (often by crust built up inside carb or a kinked or obstructed drain hose), the excess fuel has no escape but to rise up into the carb throat.

From inside the carb throat, the fuel flows both ways, back toward the airbox, and forward toward the engine combustion chamber.

Fuel inside the airbox may drain directly into the crankcase through the rubber hose connection between airbox and crankcase breather.

Fuel inside the combustion chamber drains down around the pistons and into the crankcase.

I would caution against starting the engine until first assuring no fuel contamination of crankcase oil. This may be tested by smelling the oil (the "sniff" test) to determine any gasoline odor. Should also physically examine the oil for any abnormality beyond usual viscosity and ordinary good condition.

Fuel intrusion into crankcase may also be evidenced by a "rise" in the apparent oil level.

Where such contamination exists, both the oil and filter should be changed, and fuel supply to carbs discontinued (disconnect the fuel line) until the problems are resolved.

This is an especially critical consideration (i.e., having perfectly functioning float valves) when running Mikuni smoothbores or other carbs which don't have floatbowl overflows.

Good Luck!

i hadnt thought of that. i forgot about that little hose into the crankcase. but yea my carbs dont have an overflow.
thanks for the warning. i will check it out.
like i said i think after i install the carbs and tap on them they seal up just fine. i just forget to do that when reinstalling them. i think.

ok so i have some new info.
my float valves seal up fine now.
fuel levels are correct.
i rechecked the valve clearances and they are good and compression is good.


so today i moved all of the internal pieces from #4 carb into the other 3 carbs.
i put the main and pilot jets in #1 as well as the float needle and seat and float.
the diaphragm and needle are in #2
and the pilot air screw is in #3

and... #4 still isnt firing.
if i swap #1 and #4 plug wires it still stays on #4.
so i am stumped.
i tested my ic ignitor and it was out of spec. i am not exactly sure what the symptoms of a bad ignitor are.
but i dont think that is my problem because #1 fires fine and #4 doesnt

i have never been more puzzled in my life. i need some kind of great idea here. i might try to take it to the local community college to see if they can look at it for their motorcycle mechanic program. but i am not sure if they even need projects.

88alltrac wrote:

i have never been more puzzled in my life. i need some kind of great idea here. i might try to take it to the local community college to see if they can look at it for their motorcycle mechanic program. but i am not sure if they even need projects.

With a brand new sparkplug in #4, is combustion okay in #4 for a short time until the plug fuel fouls?

If so, where fuel level is correct and float valves are properly functioning, and other components are up to specs as previously reported, the persistent problem with excessive richness in #4 may result from clogged passage in primary circuit. :pinch:

Good Luck!

i think the new spark plug and it running well for a while was just a coincidence because now I will swap a new one in and it wont work.
it is not fouling the plug out anymore. at least #4 isnt wet when i pull it.
i have never gotten it to idle on all 4 but once in a while it will run fine on all 4 cylinders for a short period of time while i am driving. it seems to run better when it is colder outside but that may just be all in my head.

i cant believe i cant figure this one out. i have good compression.
swapping the plug wires and plugs doesnt change it so i assume ignition is ruled out at this point.
and swapping out all of the internals of the carb for ones that where know to work in another carb didnt move the problem to another carb.
what else could there be. this bike only has 5k miles on it so it may be possible that this bike has never run right. could there be a defect in the actual carb body? i know it sounds crazy but i am desperate for ideas at this point.

i wonder if i could find someone to loan me an extra rack of carbs to see if they would fix the problem. i am on a tight budget so i cant afford to buy another set of carbs without knowing that they will fix the problem.

exhaust obstruction of some sort? i dont really see how but maybe that would do this. externally the exhaust looks good except for a little surface rust.

88alltrac wrote:

i think the new spark plug and it running well for a while was just a coincidence because now I will swap a new one in and it wont work.
it is not fouling the plug out anymore. at least #4 isnt wet when i pull it.
i have never gotten it to idle on all 4 but once in a while it will run fine on all 4 cylinders for a short period of time while i am driving. it seems to run better when it is colder outside but that may just be all in my head....

When removed, does #4 plug appear still brand new and completely DRY or is there at least some evidence of fuel mixture in the combustion chamber noted by some dampness on the plug?

Am still suspicious of impaired carb primary circuit, either clogged fuel passage, or clogged air passage, or clogged pilot jet, or defective pilot jet, or some combination thereof.

Good Luck!