Rear Cylinder not Firing

First of I ride a 2002 SV650S.

On Friday I was riding and all of a sudden lost most of my power. I had the bike towed home as I was scared of doing any more damage to it by running it.

Since then I've narrowed it down to the rear cylinder not firing.

- I checked the plugs and the rear one is a little bit black and wet with gas (after running the engine)
- I swapped the front and rear plug and the front still runs with the rear plug meaning the plugs work fine.

Now I am out of ideas and experience. What could I try next? What controls the sparkplug timing? maybe it is firing at the wrong time?

Again this happened instantly during a ride. There was no signs or cutouts before it happened.

[embee]

You need to decide whether the rear cylinder ignition will actually produce a spark or not.

Easiest way to do this is to preferably use a spare spark plug (anything will do) and put it into the rear cyl plug cap, place the spark plug body onto something bare metal of the frame/engine (not painted), hold in place with a cable tie, elastic band, or use some wire to attach it securely. Make sure the plug body is in good electrical contact. Leave the proper spark plug in the rear cylinder, it's fine for a few seconds, it stops stuff going in where it shouldn't.

Fire it up briefly and see if you get a spark.

It is possible for it to spark in free air but not under compression, but at least it's a guide. Could be the coil has died, but they tend not to go suddenly, more usually they get sick first.

Of course first you ought to just check the connectors onto the rear coil to make sure it's getting power.

Come back with what you find.

Okay here is what I did. I connected the spark plug to the plug cap and had a friend hold it close to a screw on the frame. There is a spark when I try to start the engine.

What I was thinking is that the plug is not sparking when the cylinder is on the compression stroke. And I wanted to know which part controls the timing of the spark.

I get backfiring every 5 seconds so I think the fuel/air is getting into the cylinder but it is not being ignited.

I just took off the airbox and attempted to start it again. The rear carb is shooting gasoline straight out the top (air filter side). I don't know if it is doing this because the gas is not being burnt in the cylinder or because there is something wrong with the carb. What I do know is that I stink like gasoline now =)

[jambo]

Silly question, have you checked the front is definatly sparking?

The front spark plugs are known to get drenched in the wet and cut out. Sometimes it's easy to miss the obvious things, I'd check using the same method that the front plug is sparking and check it's not wet in the recess where the plug cap goes. It's quite unlikely to be a spark timing issue.

Jambo

[Biker Biggles]

Another common one is sticking chokes.Check the choke cables are free and the choke plungers where they go into the carbs.These can stick and cause a very rich mixture on one (or both) carbs causing the symptoms you describe.
The other thing that springs to mind is that your rear carb is just overfilling with fuel and causing a similar issue.

Jambo: I am positive it is the rear cylinder and have tested to make sure.

Biggles: thanks for the advice I will check as I too thought the same but ran out of time last night.

Another thing I'll try today is to replace the spark plug with a brand new one for what it is worth. Although my front cylinder works fine using the same plug that is in the rear.

[yorkie_chris]

Originally Posted by stnd

What I was thinking is that the plug is not sparking when the cylinder is on the compression stroke. And I wanted to know which part controls the timing of the spark.

I get backfiring every 5 seconds so I think the fuel/air is getting into the cylinder but it is not being ignited.

The SV ignitor box doesn't know when a cylinder is on compression, all it knows is a signal a few degrees before TDC iirc, it fires a spark at x degrees BTDC on compression AND at x degrees BTDC on exhaust stroke too.

That could just be the fuel being ignited by the gas from the front pot.

Try another spark plug, and also try just hold the rear carb open slightly by taking out the slack in the linkage, it's quite possible they're seriously out of balance.

Maybe time to try a compression test too, this squirting of PETROL out of the carb isn't normal, you can usually see a bit of PETROL but it squirting out isn't good.

So I've tried new spark plugs and as predicted that didn't do anything.

Checked the compression roughly with the palm of my hands and the front and rear feel very similar so I doubt it's a compression issue.

Some more funny carb behavior. So when I start the bike with the air box off and apply a bit of throttle the rear carb sprays PETROL straight out the top of the carb (picture a misty spray). Then when I pull the choke at the carb I can see sparks through the intake inside teh carb. Strange strange stuff... So this weekend I'll go to town on the carbs and clean everything...

If that doesn't solve it I think i'm going to have to give in and take it to a shop.

did you take your cams out when doing valve clearance?
if yes check where your cam timing is.