Urgent help please!

Hi Guys

Sorry for the urgency, but I'm off to a trike ralley tomorrow and really need to sort out a problem. My no4 cylinder doesn't seem to be firing. When the engine is ticking over, it sounds really sweet and maintains 1000rpm all day without a hitch. The reason I found this was, I was checking no4 with a colour tune and there was a good spark but no flame. Took the colour tune out, replaced the plug, started the engine and as normal it ticked over lovely, I pulled no 4 plug lead and no change to the engine at all, it just kept running??? I replaced the plug cap and repeated this with the other three cylinders and, as expected, once the cap was pulled on each one the engine cut out. Does anyone have any ideas?

Cheers in advance
Lea

PS, I've changed coils, plug caps and checked voltage going into the coils all are fine.

 

Holy crap!

I was about to post the exact same problem with my 550!

I was tinkering around and found that pulling the #4 cyl plug while running did not affect the RPM at all. Not a bit. The other 3 make the RPM's drop and almost stall out. It definetely has spark, because it throws a nice blue arc to the engine body.

weeeeeird.....

 

Sounds like a fuel related issue. Have you turned out the pilot mixture screw and seen if that makes a difference?

 

Hi Samsr

Yes, I turned it all the way in, no difference to rpm, and then reset it to 3 1/2 turns out as it was before. I've replaced the carbs this week after a rebuild, all are spotlessly clean, balanced them all and colourtuned. this was when I found the problem with no 4

Lea

 

Is the exhaust pipe to #4 "cold" then?
If you are running on only 3 cylinders, you will definately know it. It will be down considerably on power and feel rough.
I had a plug quit on me last week. I pulled it and it looked good, but the exhaust pipe was "cold" on that cyl. I swapped the plug from another cyl into that cyl, and it ran fine. Easy way to check the spark out. Plugs may spark sometimes but won't fire under compression.

 

Hi Dan
The down pipe for no 4 is "cooler" than the others, but when I went out on it this evening, I got back and the down pipe was as hot as the other 3. There was some "lag" in it, when I took it out. I'll try new plugs in it tomorrow before I set off.
Thanks
Lea

 

I was told by a mechanic buddy of mine that resistor type plug have a tendency to fail when they get hold. I will admit it was a problem with a 2-stroke in a snowmobile but it may hold true on the high performance motorbikes as well,

 

The well looked-after motorbike owner cannot answer this question.

The Spark Plugs in a well looked-after motorbike are not allowed to get old.
They are replaced.
Frequently.
With new ones.
As you should do; too.

Therefore this is a Moot Point.

 

Point taken Rick, ,

 

You can try this ...
It's a "Cheap Trick" ... but, it has worked wonders for me ... on some "Difficult Cases" before.

Strip the end off some Ignition Wire
Twist-up tight segments of 4 or 5 strands.
Tightly twisted. (Like me!)
Cut 2.5cm lengths of the twisted-up strands.
"Stuff" these twist-up's into the end of the spark plug wire not wanting to fire as far as you can get them to go in.
Clip the end clean.
Screw-in the Plug Cap as tight as you can make it.
See what happens.
Good luck with it!

 

I thought just came over me. When was the last time the valves were adjusted? Could #4 be tight enough on the exhaust side so as not to build compression? Just allowing the mixture to pass right on through unti the revs ar up and then it burns some of the mixture. A quick way to check would be to pull the plug and hold your finger over the hole. If a sssst sound is heard then all is well. If not then it is time to worry. Just a thought and a quick way to hopefully confirm this theory. Worth a look.

 

My bike started to do the same thing.

She would run on 3 cylinders (and run suprisingly well) when cold. But once you got up to speed and opened the throttle a couple times all 4 pipes would heat up. After a while the dead cylinder started moving around and the problem SEEMED to be plugs failing... I.E. new plugs always "fixed" the problem.

Not the biggest problem in the world as long as it's getting oil BUT to fix the problem I added new spark plug boots (you should do this if yours are the metal kind - like mine were when I got it) AND unfortunately had to have the carbs gone through (cleaned and synced). Still needed new spark plugs, but I now don't fel the need to carry 4 with me whenever I ride... even when it's cold 8) ).

There is honestly nothing better than getting on an old bike and having it fire up first click and run the way it's supposed to (except maybe if it had a kickstart).

BTW the Plug Boots and Carb Job cost me 175 bucks - I got a mechanic at the yamaha dealership to do it in his off time.

 

At our carb clinic yesterday we had one bike that was doing this. Colortune plug showed steady spark but when running the bike, the cylinder was erratic, and pipe stayed pretty cold. After looking, talking, laughing, eating, etc..... we pulled the carb rack and opened the 4th carb. The problem?-------one of the jets had completed fallen out and was just rolling around in the bottom of the bowl. (presumably rolling around......since we had it upside down to take the bowl off, the jet was actually tucked up behind the float hinge). So, we put the jet back in, reinstalled carbs, then went for a nice ride.