My 4th cylinder drops at high speeds

When trying to push past 65-70 mph it seems like my 4th cylinder drops. I can hear it when it happens because my engines goes from a nice smooth sound to a course buzz. Anything below that and it's fine and I can actually push past it if I open my throttle, however, it never seems like 4 kicks back in.

I pulled the plugs and 1-3 are nice and chocolate. 4 is black and sooty.

1) Colortuned it. 1 is 2.75 out, 2 is 3 out, 3 is 2.5 out, and 4 is 1 out.

2) Took the carbs off and cleaned them. All jets are in the right places and clean, though they do have some pock marks, I'm guessing from being 23 years old. Clunk test is good. No holes in the diaphragms. Emulsion tubes are good. Fuel enrichment circuits are clean. Floats are all set to 16mm(stock).

3) Bike was vacuumed synced 4 months ago.

4) New plugs, gaped correctly, do not make a difference.

5) I took the plug boots off and cleaned the connections. Took the ignition coils off and cleaned the connections.

The problem still continues. It idles very well and has plenty of power but up in the 7,000 - 9,000 rpm range it drops the 4th cylinder, no matter what gear I'm in.

The only other thing is that when riding the right side on the 4th header seems to always have a yellow-ish liquid on it. I always thought it was just burning off the gunk on the outside of the engine. I replaced the header gaskets but the yellow liquid also persists.

Any ideas?

 

Have you checked for spark? The next time it happens pull the plugs and see which is getting spark and which isn't.

 

I did a plug chop on it at 80 mph today. Just as I thought, cylinder 4 is getting dropped out. 1-3 are fine but 4 is still black. Switched plug 1 and 4 and after a ride the condition was the same. The old 4th plug(now in 1) was clean and 4 was black again.

I switched the cables on the ignition coil so 4 was now on 1 and visa versa. From the sound it made it seemed like 1 got dropped this time.

Could an ignition coil going bad show this kind of issue? Running well in low rpm's but dropping out at high rpm's?

 

This doesn't happen at lower speeds? It sounds like fuel is just getting dumped in there, not that your spark is disappearing.

I realize that high RPM's under load, and not under load are two different things, but can you recreate the problem by revving the engine while sitting still?

 

high end jet is to large?

have you checked your valves?

if its electrical it has to be between the coil and the plug

 

Take the plug cap off and ohm it out.

 

I got a reading off my ignition coils with help from chacal's instructions. They are within specs so it looks like my coils are out.

I've got the tank off right now because I'm trying to fix a dent but when I put it back on I'll see if I can get it to do it while idling. Is that safe for the engine as long as I have a fan blowing on it to red line it at idle?

 

This sounds nearly identical to the issue I was having - over 3 - 4000 rpm in 1st/2nd mine would have a nice "bwaaaaa" sound, and lose power. Eventually it would also stall on idle, and eventually it stalled below about 2-3000 rpm, and eventually I couldn't run it. I also had nice black plugs. You can check out the thread here:

http://www.xjbikes.com/Forums/viewtopic/t=15158.html

Turns out it was my float valve(s), or so we think; I cleaned the carbs, and it ran well for 1 day, but then went back to the same problem - so it was deemed to be mechanical failure rather than a cleaning issue. I have replaced them with Yamaha float valves (Action motorcycles said don't go aftermarket - they have lots of complaints of failures), and it seems to be working well after a few rides.

 

If you swapped the wires around and the one plugged into cyl 4's wire dropped, you have a problem with that wire/coil.

 

I know you ohm'd out the coil and it checked out ok, but you aren't able to check the condition of it "in operation". You may have a weak link at the clip where it attaches to the plug and under high current, it starts dropping out.

Take the clip off, and trim the wire back 1/3 of an inch, and reattach the clip. Then try again at high RPM's and see what happens.

 

Exactly, take the plug cap off the coil wire and ohm it out.
You may have a bad resistor.

I guess I dont understand your statement: I got a reading off my ignition coils with help from chacal's instructions. They are within specs so it looks like my coils are out.

Sounds like you tested the pickup coils maybe? And they checked out ?

By moving the coil wire from 4 to 1 and 1 starts having problems, sounds like that one wire or plug cap is bad.

 

What I mean is I took the plug boots off and put my ohm reader directly into the wire. The secondary wires were about 12,000 ohms and the primary wires were about 2.5 ohms. Both of which are within the 10% tolerance.

This would mean the coils themselves are correct and the problem may be within the wires or the boots? Correct?

Also since 4 seems to always be black, even after the coil wire swap, could it be possible that the float valve has a crack in it that's naked to the human eye that could be causing it to flood the cylinder?

 

My theory is that #4 plug is black because the spark fades or cuts out and then the plug fouls. Yes, I'm saying that the coil is fine but that the end of the wire where it connects to the clip that goes on the plug may be worn. That's why I suggested cutting it back a hair, and making a fresh connection.

 

While you have the plug cap off, ohm it out. What does it ohm out at?

 

I did ohm it outwith the plug cap off. It was 12,000, well within the factor spec range.

sushi, I took the cap off and trimmed back the wire. Still no dice. Had the same problem still.

 

Not the plug wire. OHM OUT THE PLUG CAP .

 

Oh. *palm to forehead*

I understand you now. I'll pull the caps off and compare all four of them first thing in the morning.

 

All the plug caps were the same. 9,500 ohms. Is that correct? I would think since the other 3 cylinders are just fine.

 

All hail the K&N air filter.

So after all this testing and annoyance I put a new K&N filter in and the problem went away. I just maxed out my speedometer, which ends at 135 mph! I still have a little bit of a dead space in the 9,000- 10,000 rpm range, but it seems like all my problem was was a bad air filter.

I feel like a moron.

 

It's odd that the air filter would cause problem on one cylinder and move with your plug wire.


I hope it is gone but I would be suspicious.

 

haha. we're all chasing spark and a messed up filter was causing air starvation at high rpms? thats awesome

Edit: Bill, i'm wondering if maybe it was just a coincidence that time, maybe a plug wire didn't get pressed back on completely or something. The air filter makes sense if cyl 4 was _always_ soot'd up, regardless of messing with the plug wires.

 

Bill, I am still going to be replacing parts. It does seem to me like an air filter shouldn't have caused that much of a problem.

I did however make new connections for all the boots and clean off the connections before I put in the new air filter. It seems like every time I repaired one thing the problem got a little better. For all I know I had a bunch of small problems that added up to one major PITA and the air filter was just the final fix. :lol:

Edited by Robert, Language please! Thanks!

 

That could be. I hope that is the end of the issues. Switch has a point too. Some issues are flaky at best.