View Full Version : Bad Rear Cylinder

11-17-2004, 05:35 AM
Okay, this is really just continued from "My Exhaust Sucks".

It was too good to be true. But I did in fact find my float off it's rocker, so I'll just start a new thread.

Rear cylinder misses BAD at idle. Does better at cruise RPM, but not great.

I've just about eliminated all causes known to me. Swapped the plugs. Swapped and reversed the plug wires. Readjusted my pushrods. Cleaned my lifters and supply screen. Regardless of what I do, the rear cylinder refuses to fire at idle. It will every now and then... but most of the time, it's just pushing air. The only thing I haven't done is slap a timing light on the rear plug to see if it's firing. I'm pretty sure it is since I already reversed the plug wires with no change.

All I can think that I'm left with is a bad valve spring. And I don't know how to test for it before I pull the heads. Could it be anything else? A cruddy valve guide? I could run up the RPMs and spray some water down the carb throat to clean 'em up a bit but I think I'm grasping at straws there.

I'm really not looking forward to pulling the head off - especially the rear one since it is so close to the frame and basically just a bitch to remove. My tools aren't really up to the job either - I've no socket wrench that will allow me to use a socket on the head bolts. And a box end is... well... just hard to use and fairly difficult to torque putting the head back on. I've done it before, but I had more patience back then.

Any thoughts appreciated... I got to get to work, and having to cage it in with the wife aint cutting it.

11-17-2004, 06:25 AM
You have to do a compression check along with seeing if it's got spark before tearing it down.


11-17-2004, 07:31 AM
you check the spigot (intake) seals for a vacuum leak yet?


11-17-2004, 11:01 PM
slow down a tat here before you tear into it , do a wet and dry compresion test , also you can use the tester house and with the cilender in top dead center put air into the cilerder and see if you can here it coming out say at the exsaust pipe or through the carb, also you might want to get one of those cheapy spark testers and make sure it is fireing , and your water idea is not that far off base eather , the only differnce is i mix in some ajax scrubbing powder with the water when i spray it in helps to clean it out better , if you get no results from all of this then you will have to tear it down

11-17-2004, 11:12 PM
This has the making of a great thread, great answers from all angles

11-18-2004, 02:54 AM
All of the above suggestions are good ones. As was suggested, did you check for vacuum leaks between the intake and the head?

What kind of ignition are you using?

11-18-2004, 03:22 AM

Thanks for taking the time to respond.

First - I did hook up the timing light.

No difference between front and rear cylinder fire. I don't know what a fire-tester would tell me that a timing light wouldn't. Never heard of one, so just have to guess. :-)

I have not sprayed water into my carb yet. I know glazed valvestems are a possibility, so I've not given up on it. Just forgot to try it.

Next - To answer some questions.

The ignition is pretty much stock. The coil is an Accell, but the ignition itself came on the bike. I've never had a lick of trouble out of it... that is if I'm not now.

As for vacuum leaks, I don't believe I have any. Used up a can of WD-40 spraying it on the intakes trying to convince myself that's all it was.

As always, I'll never leave a thread hanging - especially for "the next guy" who has similar symptoms. I'll post my findings as they happen.

Thanks again HCG!

11-18-2004, 03:50 AM
What kind of plug wires are you using? Suppression core or steel core? My little grey cells just remembered that the early HD ignition modules were extremely radio frequency sensitive. Solid core wires would drive the modules crazy...even to the point of destroying the module. Also the early modules, the later will do it too to a lesser degree, would give erratic firing problems....ie...wouldn't idle but run better at a higher rpm and vice versa.

11-18-2004, 04:04 AM

Your making my "grey matter melt" AAAHHH! Where is my medication. :lol: Waaaay tooo much fun! :P


11-19-2004, 12:05 AM
there is another cheap way to eliminate your ignition from being part of the problem. I use to carry it all the time, not trusting an electronic ignition that much, they sell a kit with points, condensor, backing plate, and weights, for about 35 bucks, put that in and that would eliminate your brain box, pick up, and so on...just leaving your coil and I haven't had to many problems with coils other then cracking. and if everything checks out ok you have a spare ignition that can be carried in your saddle bag. I'm just tossing this at you, but it's cheaper then buying solid state componants.

11-19-2004, 12:18 AM

the only thing i've found in regards to that choice is that points coils are 5-6 ohm impedance, while electronic ignition coils are 2-3 ohms. you can check it that way, but i wouldn't leave it in....

once i did exactly as you describe, but i left the points in there. after a day or so of riding it that way i was treated to the unmistakeable odor of melting bakelite as i headed away from home. i fried the coil becuase of the mismatched primary impedance.



11-19-2004, 12:25 AM
Junior, you are absolutly correct I use this only for elimination and diagnostic or on the road break downs. It has saved me a few times, and is handy to keep around. 90% of the time I end up repairing it the proper way once we figured out what was bad, I look at it as just a tool, but I should have mentioned it in my post, thanks for catching that. B)

11-19-2004, 12:34 AM
i figured that's whatya meant, bud.....

that day when i fried the coil wwasn't a total loss tho- i crapped out by the hanapepe canyon lookut, so at least i had a view until i thumbed a ride....lol


11-19-2004, 04:37 PM

Soretail hit on something. I just replaced my plug wires not long ago... Wish I had a points set up as Wrecker suggested. I also wish I hadn't tossed my old wires. Well, I always keep one as a spare, but would like to have both. But I honestly don't know what kind of wires they are. If they aren't the suppressor sort, would an AM radio pick them up? Is there any other way to tell?

The trend on this thread is this problem is thought to be electrical. I can't understand how it is possible since I've swapped EVERYTHING with the front cylinder. But I tend to agree with you all anyway. I'm going to check everything again, plus ohm out my coil and ignition (if that is possible - I don't recall seeing any tests in my shop manual). I'm also picking up a compression gauge on the way home today.

What are the "usual" symtoms of a weak valve spring? I've had sticky valves before - exhaust - and I had no problem diagnosing that!!! But a weak spring I've not experienced.

At least it's Friday and I'll have this weekend... Wish me luck!

11-19-2004, 11:09 PM
timbuk2 i think the only reason we are pointing you to igntion , is that it is more common plus we are trying to save you a tear down , if we elementate the igntion system first, then at least you know what you are going after as for testing for a week spring i dont know of a in bike test that you can do inless it is so far gone that it effects the compression , and that is why we have you doing all the test , try to elemate as many things as possable before a tear down B)

11-19-2004, 11:35 PM
not to nag- but the compliance fittings in shovels are famous for leaking...

i'd still check for an intake leak...


11-20-2004, 12:17 AM
I think too that there is a intake leak or second one is burnt exhaust valve.
If it sounds like shupshup on idle run the it's burnt exhaust valve...

11-20-2004, 01:29 AM
A points ignition with a primary resistance of 3 ohms will run just fine. Most Accel coils are 3.2 to 3.5. I run a three ohm Dyna coil with points on my 02 Sporty. I also ran 3 ohm coils on my strokers for the last 20 years with no problems. I, personally, have never had a problem running 3 ohm coils...can't recall any customers having problems either. The 5 ohm coil is what HD used from the time they went to a 12 volt system until the 80 model Magnavox unit.

Wrecker's suggestion of carrying a points setup when traveling or testing is a good one. Whenever I trip, a spare points plate, points ( I use Accel), condenser (also Accel) points cam, mechanical advance unit, the 10-32 advance bolt, and a 5" piece of #14 wire made up with the proper wire terminals go with me. I sometimes will carry a spare coil. You can put it all into a zip lock plastic bag slightly larger than a sandwich baggy. I have never failed to make it home.....at least from a dead ignition....can't say the same for my customers or riding buddies whose ignition module decided to die....they came home on a flat back. All the above components including a cheap coil can be had for about a hundred bucks.

Some of the electronic coils have resistance as low as 1.4 ohms. Be wary of using ANY HD coil that went with a stock electronic module no matter what the primary resistance.

Weak/collapsed valve springs will often idle OK but will misfire above idle and valve float at increased rpm with tappet noise. When you go back to idle the noise will stop. Sometimes, in a shovel, pan, or iron head, the valves will collide and bend one slightly. This can be caused by a valve sticking momentarily in the guide, weak/collapsed or broken valve spring. This will cause poor idle quality, but like a burned exhaust valve will appear to go away above idle except in the case of valve spring problems. A broken spring, inner or outer can exhibit the same symptoms...sometimes with noise. I've had both of the above in the shop within the last year.

I believe Wrecker or Junior suggested a compression check and leakdown test. This will tell you right away if you have a bent valve, burned valve or a sticking valve. I have seen very few burned valves, but many that were not seating because of a big chunk of carbon lodged between the valve and its seat. If the guides are badly worn they will stick a valve as quickly as a guide that is too tight. Burned oil and other crap will develop a goo in the excessive clearances and when hot will grab a valve stem like a vise grip. Result is the valves will collide or the piston will contact the exhaust valve since it is the one being chased.
These are just some observations from rebuilding hundreds of heads.
Do a compression check and/or leakdown test!

11-23-2004, 02:34 AM
Initial compression results.

This is my first time compression testing, so I'll explain what I did just in case.

Both plugs out. Tested front and rear "dry". For both, I turned the engine over about 4 times. I used oil that is somewhat heavier than penetrating oil for the "wet" test. But it's not 30 weight by any means. I use it to sharpen knives. Wasn't sure about it... but if I should use something else I can do it again, eh? Plus I was doing it in a very poorly lit garage. Pisses me off every time I try and do anything out there at night. I'll address that some day. But, I digress...

I've not even looked at my shop manual, so I don't know what my readings should be. But I can already say, I don't like what I got. I'll post here, then go look up the bad news.

DRY 110 130
WET 110 140

I've not done a "leak-down" test. I can guess that what I'm after is to see how long the cylinder stays compressed. But I don't know much more than that, so let me know what I need to do and I'll do that as well.

There ya go guys. What's it look like to you so far?


11-23-2004, 04:14 AM
The figures you posted are not too bad. Although to do a proper compression check, the engine should be hot, so everything has expanded to running clearances, plugs out and throttle locked open. Spin the starter about 6 pumps on the cylinder. The pressure should take a big jump the first couple of pumps and continue to build to its max pressure. Even cold your figures are above 100 psi, and fairly equal. I still would do the check again as I have described above. If it comes out good, I would look again at the fuel system and air leaks.

11-23-2004, 04:55 AM
Thanks STC...

Yea, as I said I went back to look to see what my shop manual had to say, and it had a pretty good description of how a compression check *SHOULD* be done. It even described how much oil to use in a "wet" test... and I was way short on that. I didn't hold my throttle open either.

As you said, I should probably have turned her over a couple more times per test. But I will say the gauge stopped reading increased pressure by the second revolution. I took that to be a good sign. It was the difference in the front and rear that I didn't like. But I am reassured by your comments.

Both my plugs looked like they were running rich. The center conductor didn't look too bad, but the rest of the plug was pretty sooty. Though I have been running it a lot in the garage choked trying to keep it running. I would think if I had that significant an air leak on my rear cylinder, the plug would look like it was getting a leaner mixture. But then again, it fires so seldom at idle that I don't think I can go by that.

Anyway - I'll keep at it. I am honestly enjoying this. I'm learning a lot from you guys. I've wrenched my bike from the day I rode her home from the shop. The satisfaction of fixing her myself is worth far more than a phone call from the shop saying "Your bike is ready to be picked up". Unfortunately, my bike is my transportation - not a "recreational vehicle". Sure hope I get to the bottom of this soon!

11-23-2004, 05:12 AM
I hope you can get to the bottom of it soon too. The plugs in your shovel will usually look differently. The rear will generally be darker than the front. Believe it or not, the front cylinder runs hotter than the rear because it runs slightly leaner. It gets a different breath than the front and runs a little richer.
You might try new plugs.....it doesn't take much to foul a plug.

12-20-2004, 08:40 PM

I've been away for a while, and since returning home I am trying to get a fresh start on this problem.

I'm still where I was when I left. The bike runs okay as long as the RPMs are well above idle. Revving then engine up and letting her run down with my hands off the throttle, she dies like it would if I had the idle mixture set to nothing. If I ease her down, the rear cylinder misses a good 75% of the time. A credit card held to the exhaust is sucked up rather than blown away at least half the time.

The only additional test I've done is on the coil. I pulled it off and took ohm readings of the primary and secondary windings.

The secondary winding looks perfect. Seems like it was a 19K ohms, but I may be wrong.

The primary winding however is just a tad out of specs. The shop manual says between 3.3 and 3.7 ohms. I'm reading 4.1 ohms. Is .4 ohms enough to be of concern in your experience? Previous posts were talking some pretty close specifications, so...?

That's where I'm at.

Thanks in advance, and HAPPY HOLIDAYS everyone!!!


03-06-2005, 03:35 AM
Been reading through this thread, and like a good book with the last few pages missing, I am left wondering how this turned out. Anybody know?

03-06-2005, 10:51 PM
I'm with y'a Horse. So TMB2 what was the solution? :rolleyes:


03-26-2005, 08:57 AM
I`m a newbie on this site and have been reading some post and was also really looking forward to finding out what the problem was. I see in his sig. that he has a 1982 fxrs which is also what I have. if it were me, after checking all the electrical posibility`s, I would take the carb off and take a good look at the intake band seal, if that checked out okay I would prepare to remove the head and closely inspect the valves. I have had problems that had the same mo as what he is describing, its turned out to be everything from electrical to burnt valves, but if he already went thru the electrical, I would strongly lean towards the intake band seal or valve/s. just thought I would break the ice and throw in my 2cents worth. bikerinthenight

08-11-2005, 10:18 PM
Sorry I didn't close this out with my final findings.

Turned out I blew a head gasket. But blew it on the INSIDE.

Basically, the oil return port was feeding oil into the cylinder. Being real close to the cylinder wall, it was obvious that the oil was being drawn in on the intake stroke by the condition of the gasket.

I'd never heard of blowing a head gasket backwards. Always thought they blew out to where you can see/feel escaping gasses. But they also work the other way, and at idle there was enough oil being drawn in to foul my idle. She ran fine with a few RPMs on her.

So I installed another head gasket, and it ran great for several months. However, I guess I didn't get the head torqued right, and I'm in the middle of doing the job over. I started another thread asking how the Hell can I torque the heads while the engine is still on the frame.

Anyway, that was the story. Hope this helps somone who experiences the same symptoms some day.

Thanks for all the advice and suggestions.

08-12-2005, 06:26 PM
Glad its only a head gasket. I`ve always used the grunt method, and never had a problem, but given the choice, it would be nice to do it corretly. on my setup, the head bolts are 11mm, so I used a box end, and torque it until the wrench starts to curve. I`ve had my heads off more times than i care to remember, but only once did i blow a head gasket, and I`m convinced that it was because I tried using one of them copper gaskets. it was the first time I used one, because they did`nt have the other (blue) ones in stock. I`ll wait a month of sundays for one of the blue`s ones to come in, before I`ll ever use another one of them copper ones. the biggest pain in the ass on the rear head, is the bolt that is on the primary side, and at the back, you have to take the solenoid off, and still can barely get a 1/4 of a turn caged as I`m sure you already know.

08-14-2005, 05:01 PM

I started another thread "Torque them Head Bolts", and that left rear bitch of a head-bolt is indeed the biggest problem.

Check out that thread - a few good suggestion aside from what I found that works really well. I'll never sweat a head bolt agan!


08-14-2005, 05:41 PM
COOL, i seen it. Thanks!!

08-18-2005, 12:58 AM
Glad you got er figured out timbuk2, I too had a bad rear cylinder many years ago on my shovel and it turned out to be a cracked exhaust valve. Called for a top end rebuild :)


08-18-2005, 02:54 AM
Holy Shit! It's skeezix! Where y'a been hiding?