#4 Cylinder Dead - Can't find Cause

[tkent02]

The plug wires are long enough, you don't have to take anything apart. Just pull it through to the other side, just wire #1 to the #4 plug and wire #4 going to the #1 spark plug. This takes less than a minute, and will tell you once and for all if it is an ignition or a fuel problem. If the problem goes over to #1 it's ignition. If it stays in #4 it's not.

No one is going to believe that this works, but I had a similar problem. Carbs spotless. Could not get #4 to fire. So I knew I had spark. I knew I had air. Here comes the part no one likes... Pour 2 teaspoons of raw gas down the spark plug of #4 and put plug in and connect wire. Start the bike and see if it runs a little using #4. I must of missed a little piece of something in that carb, because as soon as I got it to run on all four, it started pulling fuel in. Bike is still in use in my race bike 2 years later...
Curt

Curt,

I'm considering doing something similar, but perhaps a bit safer.

Thought I'd squirt some starter fluid into the vacuum port on the #4 intake boot and see if she fires.

***

[Brendan W]

I have seen plugs that fired in the open but wouldn't in the cylinder but it wasn't the plugs fault. Generally it's down to a weak spark.You could try pulling #1 and comparing the strenght of the spark - it should be 'fat'. I'd try running it in the dark with the tank off looking for escaping sparks along the ht lead. I don't know if it's possible to swap the ht leads on that coil but you should be able to get the longer one to the short side and see if it still runs on three if you get my drift.

[bellucci]

No one is going to believe that this works, but I had a similar problem. Carbs spotless. Could not get #4 to fire. So I knew I had spark. I knew I had air. Here comes the part no one likes... Pour 2 teaspoons of raw gas down the spark plug of #4 and put plug in and connect wire. Start the bike and see if it runs a little using #4. I must of missed a little piece of something in that carb, because as soon as I got it to run on all four, it started pulling fuel in. Bike is still in use in my race bike 2 years later...
Curt

[Nessism]

How about switching no. 1 & 4 plug wires? They fire in pairs so no worry about firing order. If the problem switches to the no. 1 cylinder the issue is ignition related. If not, it's either a carb or compression issue.

Just been reading back though the inputs from the other members, and tell me if I'm wrong, but have you actually tried pulling out number four plug and holding it against the motor to see if it's arcing?.....

Yes, I have done that, with three different spark plugs just to make sure I didn't have a bad plug.

All three had good spark when held against the side of the engine.

Still stumped...

Ed,

I actually used the carb rebuild tutorial article on BassCliff's web site to do the carb rebuild. Between removing the carb bank, doing the rebuild, and re-installing them I put 20 hours into it.

I was painstakingly slow and thorough to make sure I didn't miss anything. Cleaned every tiny passageway and checked them with air to see if they were open, used all new o-rings, the works.

Without checking my notes at home in my shop, I think I set the pilot screws at 2 & 1/2 turns out. I'll have to check that when I get home.

***

Just been reading back though the inputs from the other members, and tell me if I'm wrong, but have you atually tried pulling out number four plug and holding it against the motor to see if it's arcing? Usually best to it in a darkened room.
This may be a simple test to eliminate the spark as a problem before ripping off carbies again.
As frustrating as it may be, eliminate the obvious things before moving onto the complex problems.

Sounds like that no. 4 carb is still clogged up. You might want to go though the carb rebuild tutorial linked on Basscliff's website or my signature and see if there is anything you missed. Plugged pilot jets, choke tube, choke passage in float bowl, something. Where are the pilot screws set ?

Ed,

I actually used the carb rebuild tutorial article on BassCliff's web site to do the carb rebuild. Between removing the carb bank, doing the rebuild, and re-installing them I put 20 hours into it.

I was painstakingly slow and thorough to make sure I didn't miss anything. Cleaned every tiny passageway and checked them with air to see if they were open, used all new o-rings, the works.

Without checking my notes at home in my shop, I think I set the pilot screws at 2 & 1/2 turns out. I'll have to check that when I get home.

***

[Nessism]

Sounds like that no. 4 carb is still clogged up. You might want to go though the carb rebuild tutorial linked on Basscliff's website or my signature and see if there is anything you missed. Plugged pilot jets, choke tube, choke passage in float bowl, something. Where are the pilot screws set ?

Sounds right mostly. So it won't idle on #4, how about if you hop on and open the throttle? Hard to believe it wouldn't be getting gas and firing then, if there's any fuel in the carburetor.

You still haven't eliminated a bad plug wire or wire end, or have you swapped wire #4 over to plug #1 to test that?

I actually hopped on it and rode it up and down my street for about 10 minutes total. Got up through 3rd gear, no further, since just a small subdivision that ends in a cul-de-sac. Actually pulled pretty good on just three cylinders.

After riding it for 10 minutes I put it back up on the center stand still running and pulled the boot off of #4 cylinder. Still no drop in RPM at all, plus I could still lay my hand on the exhaust pipe coming out of the cylinder head and it was barely warm, while the other three were too hot to touch for even a fraction of a second.

I've put three different spark plugs in #4 now, a new one and two old ones that were in the engine when I got her. They all had spark when laid up against the side of the engine. I removed the boot from the #4 plug wire and inspected the end. Looks fine, no cracking, etc. I have not swapped the plug WIRES between #1 and #4 yet, but I could sure do that.

I'll get into the valves as soon as I get some time. Maybe the intake valve on #4 is not opening properly??? That would reduce the level of vacuum available to pull in fuel/air mixture.

I could test that I suppose as follows... If I remove the screw from the intake boot vacuum port on #4 and run the engine I should get some idea if the cylinder is sucking in air if there's a decent vacuum at that port.

I am also gonna see if I can drop the fuel bowl on #4 carb with it on the bike to see if I can get a look at the floats and the needle valve.

This one really has me baffled.

[tkent02]

Sounds right mostly. So it won't idle on #4, how about if you hop on and open the throttle? Hard to believe it wouldn't be getting gas and firing then, if there's any fuel in the carburetor.

You still haven't eliminated a bad plug wire or wire end, or have you swapped wire #4 over to plug #1 to test that?

More to Report

Well... here's the latest:

I did not have a proper sized fitting to connect my compression gauge to the #4 cylinder, so I just pulled the plug and tried to hold my thumb against the spark plug hole while cranking the engine.

No way I could hold my thumb on the spark plug hole! The compression in #4 literally blew my thumb off the hole with enough force to move my whole hand. So, I'm pretty much convinced that it's not a compression problem.

I also tried two different spark plugs with no change. Cylinder #4 is still dead. When I disconnect the plug boot from #4 I get absolutely no drop in RPMs. When I pull #1, #2, or #3 I get about a 500 RPM drop.

All that said, I'm leaning toward suspecting that no fuel is getting to #4, but not because of lack of compression to pull it in to the cylinder.

If I have to pull these carbs, I'll be pretty upset, since it's extremely difficult on this 650. No spare room at all to get those things out due to the small frame.

So, I thought I'd see if it's possible to drop the fuel bowl off the bottom of #4 carb with it still on the bike and see if the float is stuck or the needle valve stuck closed for some reason. It was clean and loose after the carb rebuild, so I don't know why it would be stuck now. I did get a little gas out of the drain plug when I loosened it a day or so ago to check, but I didn't drain the entire bowl, so I don't know how much was actually in the bowl.

YES, I know I need to check the valves. But my point is, I've got way more than enough compression in #4 for it to pull in air/fuel as she sits.d It should run if it's got spark and gas.

Help me noodle this out please. My brain is on overload.

Am I right or wrong about some of my above deductions?

Also look at the end of the wire. They often get corroded and need to have the last 1/4 inch or so trimmed off and the cap put back on. even in dry climates this often needs to be done.

You beat me to it

Had a weak spark on #4 on mine it was sorted by cutting back quarter of an inch of the lead and refitting the plug cap, the rest of the leads may appear supple but your best unscrewing the cap and cutting a little back on all of them, if it's been stood as long as you say, can only do to it some good.

[tkent02]

Also look at the end of the wire. They often get corroded and need to have the last 1/4 inch or so trimmed off and the cap put back on. even in dry climates this often needs to be done.