No spark to cylinder 3?

I am confused about cylinder #3. As I was adjusting the air fuel mixture screw in an out on all of my cylinders, I noticed that the adjusting the #3 screw did not effect the way the bike ran. I felt the exhaust pipe. It was hot, but I could still grab it without it hurting. The others were too hot to touch, which makes me believe I am getting spark some of the time but not consistently. I am not sure though...

So, I pulled the plug and noticed it was a little black, and had a wet look. The others all looked "normal". I started the bike up with the plug out and it definately has compression. I put the plug back but did not connect the wire to it. I started the bike and it ran the same. I switched plugs with one of the other cylinders but that didn't seem to change anything. So, I have compression, and a good spark plug, but the cylinder does not seem to be functioning properly.



If the coil is bad, wouldn't it affect both cylinders on that coil?

I just rebuilt the carbs so I am pretty sure it is not a fuel issue.

Any ideas? Bad plug wire? Bad coil?

Post edited by: dannyh, at: 2006/04/20 21:36

Post edited by: dannyh, at: 2006/04/20 21:39

I have the same thing happening on cylinder #4 of my 77 kz650. Bike worked perfectly before carb adjustment (swap) and after that the cylinder works sometime but mostly not. I would love to hear if that is a common issue with an easy fix...

Can it be a clog in the jet perhaps?

dannyh,
Regarding the exhaust pipes - unless the pipe is sizzling hot - spray on some water or spit on them - then its not firing.

One of your coils fires cylinders 2 & 3. The other fires 1 & 4. Have you tried swapping the plug leads from 2 ->3 and 3->2 ? If you do this and the problem moves to cylinder #2 then the problem is with the plug lead or plug cap. If the problem remains on #3 then more than likely a carb/fuel issue.

I might have misunderstood but unless you use a compression tester you cant really tell if a cylinder has sufficient compression or not.

Check for actual spark by turning the bike over with the plug out of the engine, but still wired and laying on the engine. (Use a new plug for the test, the old one is fouled.)

I switched the # 2 and 3 plug wires and started it up. No change. I then pulled the wire off the #3 plug while the bike was running and it didn't affect anything. So I am pretty sure the wire and plug are fine. That probably means there is no compression or no fuel, right?
I guess I will do a compression test next.
Any other suggestions?
It accelerates great. Could the cylinder work while acceletaring and then crap out when cruising or idling?
Has anyone ever heard of that? Just a thought.

When you swapped the plug leads did the #3 exhaust remain colder than the others ??

You should try what loudhvx suggested. Get a new plug.

I didn't leave it running long enough to check. I just swapped wires, started it up, then pulled the wire while it was running and there was no change. I figured that was a pretty good indicator.

If I let it run for a while, then pull the wire and there is no difference but the exhaust is hot like the others, what does that mean?

Start up the bike, let it run for a minute and check the exhaust for heat. With the engine running spray some water drops on each pipe and if the water sizzles off immeadiately then that cylinder is running. If you pulled the plug and it made no difference then it's probably an indicator but personally I would prefer the hot exhaust test. On a non running cylinder the exhaust will get warm - conduction from the rest of the engine - but nothing like the heat of a running exhaust.

Next thing to do is what loudhvx suggested using a brand new plug. The old one is probably fouled and also unburned gas on a plug can sometime cause failure. Stick it on the plug lead, lay it on the side of the engine and turn the starter. Look for a spark. If you see a spark then its time to look elsewhere - perhaps carbs and fuel. Nost of the time these things are simple fixes.

I will have to get new spark plugs. However, I tried the spark test with existing plug. I pulled the plug, attached it to the wire, rested it agaist the block, and cranked the engine over. It sparks just like normal. The spark was a blue color. Does that mean anything?

I will have to get a new plug and see if that makes a difference.

Ok, just to make sure I followed it all: The #3 exhaust pipe is cooler, but swapping the plug leads on the #2 & #3 cylinders doesn't move the problem to #2. Also you checked and there's good spark (fat & blue is good) on #3. So this suggests the electrical side of things is fine.

You need compression and fuel checked next. Compression is probably the least likely, but its an easy check. Just get a gauge and check all the cylinders. Remember to check with engine warm and the throttle held open. If its good (or least passable) then fuel is the likely culprit.

If you're happy its a fuel issue then clean all four carbs while your at it.

hope this helps...Sam

dannyh wrote:

I will have to get new spark plugs. However, I tried the spark test with existing plug. I pulled the plug, attached it to the wire, rested it agaist the block, and cranked the engine over. It sparks just like normal. The spark was a blue color. Does that mean anything?

I will have to get a new plug and see if that makes a difference.

I just rebuilt the carbs 2 weeks ago. The only thing I didn't replace on any of them are the main needle, (the really long one). I couldn't get any all of the screws off the shaft to pull the shaft out so as to get to all of the needles. But everything else has been replaced and is clean. The float bowl height is a little low, but that wouldn't cause the entire cylinder to stop working, would it?
I will check the compressing when I get an adapter for my compression guage that will fit my bike.
Any other suggestions?

If you pulled the plug wire off the #3 cylinder while running, then # 2 should have stopped sparking also. If # 2 kept sparking, that means a continuity has been created between the spark circuit and ground. This means insulation has failed somewhere. It may only be a high-voltage failure, so you won't be able to find it with an ohm-meter. My point is, something is not right.

Your spark tests should include high-voltage tests. That means you need to make sure the coil can jump a spark across a 3/4 inch, open-air gap. If not, then the coil has a weak spot or the wires may have a weak spot where electricity is leaking to ground.

I'm not saying you don't have a carb problem. You may have more than one problem.

Post edited by: loudhvx, at: 2006/04/24 09:51