3 and 4 not firing???

well... last year i had a problem with #4 firing.... even after new plugs... it seemed like #4 wasn't working all the time... but.... it would either work when i starte the bike or not work.... i relieved the problem by putting in one of my old plugs.... just started the bike last week (SOME HOW IN THIS COLD ASS WEATHER)... and #4 AND #3 weren't firing..... i was told by my buddy's dad (a harley rider) that he used to have to carry extra plugs in case one stopped firing.... do you think it might be my plugs??? or.... what else could it be??? #3 and #4 run on different coils... sooo.. it shouldn't be the wiring..... (thats my thought anyway).... i have never had a problem with #1 or #2... i've had the bike for about a year now... curious.... help me out guys!... you're the pro's.... i just like to ride... but i have had to learn how to do a lot of maintence (oil, filter, clutch cable, throttle cable, running new power source to tail light, plugs, headlight unit replacement, turn signal replacement, carb cleaning, air filter cleaning (i have pods), chain tightening, etc... not bad for a novice having never done vehicle maintence before).... i am usually pretty good at problem solving and mechanics... but.... i'm stumped with this #3 and #4 not firing deal....

thoughts, comments, and suggestions are appreciated!

Check for dirty carbs.When you have the bike running block off # 3or 4 with your hand and feel the pipe if it starts getting hot you probably got dirty carbs.Check for stuck floats run the bike then pull the plugs on 3 or 4,are they wet they wont fire if there wet.Check for spark just lay a new plug on the engine (plugged in of course)and try and start.Should have a nice fat blue spark.

check the resistace of your plug caps they should 5kohm.

And assure good battery, fully charged, with clean snug terminal connections, including the negative cable attachment at ground to engine.

Then measure voltage reaching coils. Should ideally equate to battery voltage.

Are spark plug wires old and hard, and perhaps leaking voltage??

Good Luck!

If not already done, would assure all valve clearances being to specs.

And also perform compression test.

Good Luck!

Start with the compression test if you can.
The numbers should be within about 15% of eachother - cold. -less than 10% hot.

Check each of the plugs by laying them on, or holding them against head and trying to start. As mentioned, should be nice blue snappy spark for each one.

Remove carburetor rack.
dissasemble carbs 1 at a time, using a single small strand of stainless steel brush thread, push it through all of the jets and pilot passages carefully. Were not trying to enlarge the holes, just make sure they pass some fuel.

Re-assemble everything. AIR TIGHT!

Turn your idle way up to about 2000rpm.

Get the bike running, off choke, and check the header by quickly tapping each pipe to see if its getting hot.
If you rev the bike up and those cylinders start firing, then your pilot passages are still clogged.

If you turn down the idle and a cylinder (or 2) stop firing, the sync may be way off, or, the pilot circuits are still clogged some.

Turn idle up to point that all cylinders fire.

Remove the gas tank, start the bike, and then pour seafoam directly into the gas line (straight into the carbs). Rev the bike up to 4000rpm and hold it there for a minute to get the seafoam into everything.
Turn off, and let sit overnight.

Next day, just put tank back on.
It will likely run MUCH better after this and idle much lower.

Good luck, let us know!



Likely need vacuum sync afterwards...

Theres way more and other ways of going thru everything, but hey, you just want to ride the darn thing, right!

I solved an intermitent firing problem on #1 & #2 by simply disconnecting then reconnecting the spark plug wires at all points from the coils to the plugs. The wires unscrew from the coils. The connections become poor from oxidation over time. Some dielectric grease wouldn't hurt.

IF tips of spark plugs #3 and #4 appear black and sooty, thereby evidencing a too rich running condition in spite of decent compression, correct valve clearances, and a good properly timed spark - - -

Would also assure correct fuel level in float bowls via the clear tube test (bike in normal running mode with carbs fully assembled).

Because, where the float bowl fuel level is too high, even otherwise perfect carbs will run too rich. And this is true regardless of perfect compression and ignition.

Point being, the other possible suspects underlying poor combustion should be checked and cleared before addressing the carb internals. And if not done, tearing into the carbs may likely prove a futile exercise, because even perfect carbs can't compensate for deficiencies in the other systems.

So sayeth the peanut gallery. :laugh:

Good Luck!

Checklist:

1. Spark - blue and fat? Check plugs and resistance of spark plug caps. May need to replace a bad plug and or trim spark plug wires.

2. Check the timing and the wires and connections from points/ condensors to the coils. Check all ground wires and check for oxidization. Is the advance stuck? Take the advance off and clean and lube it. Ensure timing is bang on. If required lightly file the point contacts to ensure there is no rust preventing the points contacting one another.

3. After diagnosing electrical ....and still not working...check carbs. Clean and rebuild if required.

4. Check valve clearances.

Fingers crossed that it is only a simple fix like a loose wire ect.

YUKABODOS wrote:

...
3. After diagnosing electrical ....and still not working...check carbs. Clean and rebuild if required.

4. Check valve clearances....

Good points, but imo valve clearances should be checked and assured correct before addressing carbs.

Even perfect carbs can't compensate for poor combustion due to poor compression resulting from a too tight valve, especially at normal engine operating temperature.

Good Luck!

Patton wrote:

YUKABODOS wrote:

...
3. After diagnosing electrical ....and still not working...check carbs. Clean and rebuild if required.

4. Check valve clearances....

Good points, but imo valve clearances should be checked and assured correct before addressing carbs.

Even perfect carbs can't compensate for poor combustion due to poor compression resulting from a too tight valve, especially at normal engine operating temperature.

Good Luck!

I have to agree. I should have said this before... :blush:

According to the manuals, this is the first item to be checked as part of regualar maintenence.
It is not very difficult to do, and re-assures you that the engine is not mimicking a carburetor problem, which so often happens.

Always perform all regular maintenence before tackling other problems, it really elliminates alot.

my self personally i like to check the easiest things first what could be easier then checking spark and plug caps they are right out in the open