Author Topic: missing cylinder (Read 3816 times)

sloow · « **on:** July 16, 2005, 01:13:52 AM »

I was wondering why I was getting a difficult warm up, but until the engine's warm, it seems I only have three cylinders running. I didn't actually find that the engine was running that rough, but noticed it when I touched what should have been a hot exhaust header coming out of the engine of the running machine. It was still cold.

What's the best way to pinpoint the problem? Seems there's some kind of spark when I remove the plug and hold it to the head, but looks a bit chaotic. I don't know how to evaluate that. Right now I'm waiting on a set of plugs to come from ebay.

The carb has gas in the float chamber it seems as well. Why no ignition in this one cylinder?

thanks

sloow
aka Paul

smooth operator · « **Reply #1 on:** July 16, 2005, 06:52:56 AM »

Start off with new plugs

ray nielsen · « **Reply #2 on:** July 16, 2005, 06:26:57 PM »

The spark plug caps contain a 5K ohm resistor that's actually a carbon coating over a ceramic form. They sometimes crack and cause problems.

Measure from cylinder 1 to 4 and from cylinder 2 to 3 with an ohmmeter set to the thousand (K) ohm scale. The total resistance should be the coil resistance plus the two spark plug caps, for a total of about 20 - 30 K ohms.

If one measurement is open consider replacing ALL the plug caps. They're not particularly expensive at less than $5 each. I think you need a type VD05F, but you might double check the NGK part number on the plug cap to be sure.

sloow · « **Reply #3 on:** July 16, 2005, 09:10:11 PM »

Hey,

Thanks for this! I'll get right on it!

Paul

Maniac · « **Reply #4 on:** July 16, 2005, 11:01:24 PM »

Quote from: "sloow"

it seems I only have three cylinders

Congradulations, your bike has evolved into a Triumph!

I'm with everyone else, new plugs are a great place to start, especially if what your seeing when checking them is somewhat wierd. Frankly if it's not a nice big fat blue spark, I trash the plugs no matter how old they are. A weak spark may translate into no spark under compression.

ray nielsen · « **Reply #5 on:** July 17, 2005, 01:39:06 PM »

On many modern four cylinder Jap bikes the system uses two ignition coils. One end of the coil provides a negative voltage to that plug and the other end is a positive voltage to that plug.

If you inspect the plug electrodes you can see this as a "clean" spot where fuel deposits are burned away -- on one plug of paired cylinder (1 and 4 or 2 and 3). The burned away portion is where the spark originates, jumping from negative to positive. It's more difficult to originate a spark from a side electrode than the sharp edged center electrode.

Any extra resistance or breaks like a bad spark plug cap can cause misfire on the cylinder receiving the positive voltage to the center electrode. There's not much you can do about this condition exeept to be sure plug caps and wiring are in good order.

It is possible to reverse the plugs on paired cylinders and sometimes this will minimize a high speed miss -- but only if the plugs have been in service a very long time.

I've found that Iridium tipped plugs (NGK or ND) last a lot longer than standard plugs (30-40K vs. 8-10K miles) and might be worth the extra expense.

My two current bikes, a W650 Kawasaki and a Suzuki Bandit 1200 have extended mileage on these plugs and are running fine. The center elecrode is about 1/4 mm vs. 1 mm for standard plugs and it seems to concentrate the spark. That and the fact that Iridium doesn't erode as fast as conventional center electrode materials.