TOPIC:

working on the rebuild of my '84 GPZ 550. (not sure if this belongs in the engine section since it may cover carbs, and electrical)
I have rebuilt the carbs and re-assembled everything, new wires and plugs (same coils)
carbs are getting fuel, float bowl is filling, The cylinders are getting gas (because I can see some leaking out the exhaust pipe!)
I have checked spark, both plugs fire.
After running for a min the pipes for 1 and 2 are hot and 3 and 4 are cold to the touch

So... Why are #3 and 4 not firing??

fyi, different coils (coil 1 fires #1,#3)

One coil is for #1 and 4 spark plugs and the other coil is for #2 and 3 spark plugs.9
In your constellation #3 and 4 get the wrong ignition timing.

You are correct. I adjusted the coils to plugs and also set the vacuum tubes up to connect 1 &4 and 2&3. I now get 3 pistons firing - so 50% improvement!
#3 is still not firing - there is no heat in the exhaust pipe coming out of the engine.
I pulled the top of that carb off and everything SEEMS to be OK.
any suggestions?
(PS. a VW Sirocco was my very first car, still have a soft spot for them!)

Have you check spark plug #3 got a spark?
Could be fouled and wet by the unburnt fuel in the combustion chamber.

took me a while to followup and check.
spark plug 3 was not sparking. I trimmed the plug wire going into the coil re-connected and I got spark. The spark 'looked' good.
however #3 seems to be firing some, but not consistent. the #3 pipe is not as hot as the rest and at idle I can hear it miss (or at least it sounds like it)

so... can 1/2 coil fail...or fire intermittently? seems a bit odd.

is it possible I still have a bad connection somewhere?
Thanks,
Lee

Replying to myself : )

so, ruled out the coil. I can switch the wires from 2 to 3, swap spark plugs. either way, it is #3 that does not fire (much)

so... must be the carb?
Yuck, why couldn't it be an outside carb?? I will have to pull the whole bank out again to get to #3.

It could be the spark cable connector on the coil or on the spark plug connector side.
If #3 is weak, # 2 must also, if a coil is shot.
A ignition cable can internally burned or corroded by a bad connection you will never seen from the outside.

You didn't mention if your replaced the carb holders ("boots"). If #3 has a leak then that cylinder may not fire.

All new carb boots!
However, that does not mean I do not have a leak somewhere. It is so hard to see in there. I will pull them today

#2 seems to work fine - even when I swap the wires between #2 and #3 that should completely rule out spark/electrical issues..I think.

So that leaves air or fuel. The float bowl is full.

This is only on #3 now
I am baffled. The carb seems to be operating properly (butterfly valve opening., I see spark on the plug, but no heat develops in pipe#3.

I have swapped plug wires between 2&3 (same coil) .
I can pull the top off the carb, gently spray some starter fluid in there while running - no change.

Is it possible I am getting too much fuel in there?
Any help appreciated.

Just went through similar issues with my KZ650 on the fourth cylinder so speaking from that experience..

My cylinder would not fire up except on two cylinders originally which was fixed by a new IC Igniter. Then the fourth cylinder would fire but not until the entire bike heated up after riding for an hour +. Ended up being a lean condition due to leaking carb holders. Are you experiencing any issues with a hunting or hanging idle? Slow to return to idle? Does the cylinder fire after heating up after a while?

Also, how does the spark plug look....any color what so ever? If the plug is old try a new one. Also make sure the gap is correct. We don't know for sure if your spark is hot enough to ignite the gas (assuming you are getting enough gas).

Check your float level to make sure the level is correct, and that it isn't staying open and flooding the cylinder. It is possible that there is in fact enough gas. What kind of petcock do you have....vacuum or standard?

I have only had it sitting in the garage running so far. I will try to get the shifter installed today so I can actually ride it a bit and test.
all new plugs and wires.
I have swapped #2 and #3 plugs to verify I did not have a bad plug - so now they both have some color. I verified the gap is the same on both plugs.
Once it warms up it seems to idle OK, and return to idle fine.

I replaced the vacume petcock with a regular one and the vacume line is plugged that did go to the petcock