Rear cylinder not cooperating

[Atti2ude]

Lots of threads relating to one cylinder not firing but this is different. Bike started running rough during a ride and I’ve pin pointed it to the rear cylinder not hitting all the time. The problem is consistent and does it from start-up on regardless if the bike is cold or warm. All electrical/ignition components check out so now I’m leaning towards fuel or mechanical. Here’s what I’ve done so far:

1. Replaced Thunderheart ignition to programmable type.
2. HD dealer did a continuity test on coil and all checked out. I retested and get 3.5 ohms from both blue and yellow to red. I get 10K ohms from spark plug connector to red. I even switched wires around on the coil so if it were bad the problem would move to the front cylinder.
3. Replaced plugs and also swapped plug wires around. Pulled plugs and they are both firing.
4. Ran a jumper from ignition module rear cylinder wire to the coil to eliminate any wire harness issues.
5. Pulling the rear plug wire makes very little difference in idle but bike will die if I pull the front wire. This tells me the rear cylinder is hitting part of the time.

What am I missing?? Is it possible something is going on in the valve train or heads to cause this? There isn’t any unusual or excessive noise from the motor.

 

[Brew]

Might want to perform a cylinder compression check to rule that out.

 

[ringo]

you might try compression checks on both cylinders, that should tell you something about the mechanical condition
sorry Brew, a little behind you

 

[woodbutcher]

could be carbon keeping a valve open or could be a broken valve spring. or could be a faulty compression release stuck open.

 

[killinTime]

My 06 K9 was doing the same thing. It was a bent exaust valve. Had to have the heads reworked and new valves. Hope this helps..

 

[JeffM]

I keep telling Todd it is our rough state roads but he won't believe me.

I have ridden the bike it is smoother than my K9, but i know how particular he is with his bike so if he says it has a vibration, he must have.

Hope you get it sorted by the time i get back, riddin time is here.

Can't have you hanging back.
Cheers
Jeff

 

[DRBarnhart]

Might want to perform a cylinder compression check to rule that out.

you might try compression checks on both cylinders, that should tell you something about the mechanical condition

:iagree:

Sounds like you've covered everything ignition wise...

Dennis

 

[Atti2ude]

Appreciate the input and will start with compression test before I tear into the head. Will also dbl check the compression release isnt stuck open. Strange thing is the heads were redone with new valves etc last year and I've only put about 600 miles on it since.

 

[MNDave]

Tell us what you find out I took my 05 chopper out yesterday and It's doing that exact same thing its only firing on the front cylinder and the rear is cold, only firing every once in awhile. I checked the spark it seems to be a little weak and intermittent, I'm ordering up a thunder heart tonight.

 

[Sven]

You can't go wrong hitting every post above that is running better than your rear cylinder. Pun aside, leakdown is more what you want. If it was intake, id pop thru the carb. Exhaust, who's going to hear that?

The deal is it runs. I would not tear it down just yet. Yes do the compression test to get you in the ball park. Yes do a video or cell phone the leak: Head work times 600 miles = A timing light on the troubled head.

The next deal of the cards or variables is to keep the camera rolling, tape a timing light to said cylinder, ride by the camera with the timing light aiming for the one holding and panning how that cylinder is receiving a constant spark, you never see the light go offandango = Head work!!! Watt was the last thing that was done to the bike?

And when I mean NOLTT, your words against my video better have proof you go pointing fingers is the assumption.

 

[Atti2ude]

Compression test results

Both cylinders tested at 250psi which was done by the HD dealer using a Snap On gauge while the bike was warm and no WOT. Do these numbers seem high? The compression releases were connected and I confirmed both are working.

John at Millar Machine did my head work last year and he is leaning towards an ignition problem...either not enough spark or its happening at the wrong time. Makes sense and with consistent results from the compression test, I can reasonably rule out valve train issues. Will try another coil if I can get my hands on one and also go through all the related wiring again.

 

[Little-Boo]

I don't think there is anything wrong mechanical. If the rear cylinder is not firing it has to do with Ignition. You replaced the Ignition module. Was the old one working good, if it was, put it back on there and see if the problem clears up, if it does than you know the problem is. Make sure you have the coil wires hooked up to the correct post like the red in the center, Yellow for the front cycled and Blue for the rear cylinder. Check the connector at the ignition module that it did not push it self out and it is not making a good connection. If nothing else works replace both plug wires both with stock wires. Some people have had issues with after market wires. I don't know what wires yo have but that could be an issue if they are not the stockers.

Carlos :2thumbs:

 

[Atti2ude]

I don't think there is anything wrong mechanical. If the rear cylinder is not firing it has to do with Ignition. You replaced the Ignition module. Was the old one working good, if it was, put it back on there and see if the problem clears up, if it does than you know the problem is. Make sure you have the coil wires hooked up to the correct post like the red in the center, Yellow for the front cycled and Blue for the rear cylinder. Check the connector at the ignition module that it did not push it self out and it is not making a good connection. If nothing else works replace both plug wires both with stock wires. Some people have had issues with after market wires. I don't know what wires yo have but that could be an issue if they are not the stockers.

Carlos :2thumbs:

I rehooked my original ign module and no change and still have the original plug wires which were swapped around as well. I'm with you on the ignition being the problem just need to figure it out. Talked to S&S who suggested replacing the manifold gaskets to eliminate any possible leaks and to also look into the coil and ignition.

 

[bearman]

Did you replace the coil?
What do the plugs look like after you run it?
If the back one is black (or not as clean as the front) then it's probably electrical.
If the back plug looks like the front (or cleaner), then it's probably air/fuel related (could be intake gaskets).
If it is an air leak or other lean fuel issue it should make the back cylinder run hotter than the front, this is not good and you should avoid riding if this is happening or run the risk of burning a piston.

 

[Atti2ude]

Did you replace the coil?
What do the plugs look like after you run it?
If the back one is black (or not as clean as the front) then it's probably electrical.
If the back plug looks like the front (or cleaner), then it's probably air/fuel related (could be intake gaskets).
If it is an air leak or other lean fuel issue it should make the back cylinder run hotter than the front, this is not good and you should avoid riding if this is happening or run the risk of burning a piston.

The coil tested okay but can't be ruled out for sure. Won't know for sure until I hook it up to another bike...you listening JeffM?

You bring up a good point about the plugs and not something I thought too much about. Rear plug is black but not wet. I know it's not running lean so that could rule out intake leak.

Last week noticed a tranny mount bolt was almost out so I removed the shocks to check things over. Figured since it was tore down I should go ahead and tweak the tranny alignment so my belt tracks better on the rear pulley.

 

[bearman]

dry and black probably means firing sometimes.
the best way to test a coil is to swap it with a known good one. I know it's not always practical, but you can try to rule out everything else first (like you have). Being that a coil has thousands of volts, the spark can jump out all over the place but still test good with an ohm meter. You might can try running it in the dark and see if you can see any spark jumping.

The only other thing I can think of might be the crank sensor, although those usually stop fire to both cylinders.

 

[Atti2ude]

No progress

Broke down and bought a new coil but no improvement. If a new ignition module, coil and plugs didn't fix the problem then it has to be in the heads. Doesn't make sense because both cylinders tested with equal pressure. Also checked for vacuum leaks again using carb cleaner.

I'm not crazy about tearing off the head but looks like that is my next step.

 

[Atti2ude]

Video front cyl vs rear

If a picture is worth a thousand words, what's a video worth? Self explanatory but at idle, I pull the plug wires individually so you can hear the difference between cylinders. Front sounds clean and smooth whereas the rear is rough. Something's off and don't know how it can be the ignition since everything has been replaced.

Rear cyl issue - YouTube

 

[bruce]

you can get spark tester that look like a spark plug but are clear with a bulb in them so you can see the spark. maybe get one or a couple and install and ride with them. you will be able to look and see the spark. this will take away the guess work on that. I am sure you no, you need fuel, spark and compression to run. the bike didn,t sound to bad in the video, and the fact that is stays running with front or rear cyl. plug wire pulled, tells you both cyl. are firing. you may be looking for something that not there. If spark plugs are black that is a rich condition, may want to look into fuel mixture.

 

[Atti2ude]

you can get spark tester that look like a spark plug but are clear with a bulb in them so you can see the spark. maybe get one or a couple and install and ride with them. you will be able to look and see the spark. this will take away the guess work on that. I am sure you no, you need fuel, spark and compression to run. the bike didn,t sound to bad in the video, and the fact that is stays running with front or rear cyl. plug wire pulled, tells you both cyl. are firing. you may be looking for something that not there. If spark plugs are black that is a rich condition, may want to look into fuel mixture.

The spark tester is a good idea, just hope I can find one local tomorrow. Even though the symptoms point to ignition, I just can't see how since the major components have been replaced. Swaping around the ignition & plug wires would cause the problem to move to the front cyl if it was an ign mod/coil issue. I rejetted & adjusted the carb, replaced plugs twice and they have good color. Is it possible the rear cyl isnt getting enough fuel? How would i check? Hopefully someone will chime in on what could be going on in the head if something was off.

I've got this feeling that it's something so simple that I'm overlooking it!