cylinder #4 does not fire? Cold exhaust.

So today I've been trying to find out why exhaust no. 4 stays pretty much cold. I recently noticed it after I accidentally touched my exhaust pipe, calling my self stupid for doing so after a trip, but then wondering why I did not get burned badly.

While riding, the engine still runs smooth after it's warmed up. Fuel mileage has also not changed.

The spark plug is OK, I've switched the plug with #1 and #1 still gets hot.
Internal resistances is a nice 2.8 ohm (measured on the wiring connector).
Resistance of the plastic tubes (the ones that go on the plug) are all in the 10.5k - 10.9k range, which is OK if I'm correct.

The main thing that puzzels me, is why only #4 is failing and not also #1, as they share the same coil.
The bike's still got the original ignition coils.

Last time I had the carbs of the bike (3 weeks ago), they all seemed fine and had no vacuum leaks. They were cleaned about a year ago and have run pretty well since then.

Hoping for some insight from you guys and not having to replace the coil. Although that's the only solution I'm currently seeing.

 

no spark or no fuel. i would start with spark only cause it can be easier to troubleshoot and then make sure the carb in working.

 

Pull off plug wires 1,2 and 3.

Pull #4 sparkplug, and insert it into the #4 plug wire, and position so that it grounds to the head.

Hit the starter and see if you get spark, from that plug. It should spark twice, if the coil is good.

Now just because you see a spark, doesn't mean it is a good, strong spark. You need about 40K volt charge to cross that gap, AND fire the fuel mixture. So, if you barely see a light purple spark, instead of a bright purple spark, your coil may be failing.

Are the coils original?

And make sure the gap is correct. .030 or .76mm, I think... and make sure the plug is clean.

Finally, just for esses and gees, run a compression test, on all four cylinders.

Report back on all of these suggestions and we can help narrow it down.

 

More likely to be fuel related, but check plug caps are fitted correctly and not cracked /damaged.

HT lead shorting out / tracking, examine in low light level and watch for spark.

 

i'm not going to jump on the compression wagon yet.....let's check the usual stuff first........if everything checks out, and that cyl STILL won't fire, then we'll look at compression, or valve clearances again.........

If you're getting a good spark on 1, then the coil is ok. If the spark is still good on 1 when you swap the plug, the plug is ok.

SO:

The coil is good
The plug is good

That leaves:
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a bad plug wire, or
a bad plug cap
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To check that, pull the caps off plug 1,2,3. Put a known-to-be-good plug in the wire cap, let the tip of the plug (or the threads) lay on top of the engine where there is bare aluminum, and you can see the tip. Hit the start button and see if there is spark.

If there is, the wire and cap are ok.
If there is not, unscrew the cap, look at the wire end. Are the wires clean? Try swapping a different wire cap on. Are the wires dirty/corroded? then snip off about 1/4" so you can see fresh copper. Screw the cap back on. Try again. Should have spark now. If you don't, try the other cap again, too.

If you have spark now, put the caps all back on and take if for a spin. Should be all set.

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or lack of fuel.
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I you had good spark to begin with, and the pipe is staying cold, then you're not getting fuel. That's a carb issue.

Chances are a float is either hanging up, or the fuel inlet needle is stuck shut. Time for carb cleaning!! Now here's something a lot of people don't account for:

While the jets can all be clean and clear, it is very possible that the fuel-rail tube from carb 3 to carb 4 is clogged. I've seen many carbs where those tubes are plugged shut but nobody pulled the rack apart to check them.

Keep us posted on what you try, and what you find.................

dave

 

You forgot: Look inside the plug cap, and ensure that the resistor core hasn't come loose. There are slots on either side of the hole the tip of the plug goes in, you can gently snug them up.

 

Couple points for discussion...

First, don't test anything by cranking the engine without plugs attached.

You can damage your TCI by telling it to spark and then making it very difficult for it to do so (like by disconnecting the plug wires). There is energy in the coil when it comes time to spark, and if that energy can't go through the spark plugs, then physics dictates that it MUST go somewhere else. You don't want that somewhere else to be back into the TCI.

Put plugs in the caps and lay them against the fins. Don't crank it with completely disconnected wires and caps laying open. As long as they're attached, it's OK. They don't need to be screwed into the block, but they have to be in the caps and making contact with the block. If you've got spares, you could even leave the plugs in the head and use spare plugs in the caps laying against the head.

Second, the plugs are in series and use a waste spark system. 1 & 4 are in series as are 2 & 3.

Why does this matter? Because if #1 is open circuited so badly that it can't spark (like if the plug wire is pulled off the plug) then #4 won't be able to spark either. The corollary to this is that if #1 IS sparking well, then #4 probably is as well. It's a big loop... Through the coil and down the wire into plug 4, through the head to plug 1, out plug 1 and back through the wire to the other side of the coil. Unless something is jumping out of the loop somewhere (like a carbon trace on the plug ceramic) then if it goes through #4, it goes through #1.

If the only cold pipe is #4 and plug #4 looks OK, then I'd spend my time looking for a fuel problem.

 

Thanks all for so many helpful responses. Great as always.

Because the spark plug and the high voltage cords were OK, I made carb #4 my primary suspect.

I took 'em off the bike 2 hours ago.
Float was not stuck to my surprise. The filter inside was clean as well.
Main passage and all the holes in that bronze-like pin were also clean.

The diaphragm however had two little holes. Replaced it with a spare one, and put everything back on the bike. Still nothing though.
Decided to give fuel mixture screw half a turn wider, kind of a shot in the dark, but hey who knows. After waiting a few seconds still no changes in engine sound so I was about to revert the change I made, when the rpm starts increasing!

Long story short, the exhaust now does heat up. Took her up for a spin around the neightborhood and yes, it gets hot. Unsure if it's getting as hot as the others, but it's definitly hot and no longer cold.

Throttle is more responsive and in general it's a smoother (and faster) ride now.
Still, that idle mixture screw behavior puzzles me. Guess I still have to clean that again soon.

 

The mixture screw effects the idle circuit, which was (and may still be partially) clogged. You may be able to sneak the float bowl for #4 off with the carb still on the bike (and with come creative use of tools). The holes in the diaphram would have kept the slide from lifting enough to bring the main fuel circuit into play. Good job