Strange Misfire

[dwjohnson]

   Working on an '84 Shovel, pretty much stock with points and drag pipes. Bike cranks, idles and runs good for about 20 miles or so at 50 mph, then seems to start to loose a cyl. backfires out the exhaust a bit, almost like it's running outta fuel or even trying to seize up one cyl.  Slow down or sometimes even stop, and it seems to snap out of it for maybe another 20-25 miles or so. Seems that whacking on the throttle helps it to recover while stopped but pulling choke out does not.
Let's see what all I have addressed so far...
   new coil, plugs, points, condenser, fuel line, double checked timing w/ light,
rebuilt carb, richened up intermediate circuit two jet sizes so far, checked thoroughly for air leaks, pulled and cleaned petcock and screen, checked lifter oil screen, checked compression.
Problem did improve with the bigger jets, new fuel line and removal of customer added in-line fuel filter, but the new coil was also added at same time as well so...
   I have raised float level but have not ridden yet to verify any change. Also have not inspected the flyweights. I did crack open gas cap for venting while riding.
   I'm leaning towards fuel, either delivery or running lean but applying choke doesn't seem to help while it's happening. I have good fuel flow out of needle and seat with bowl off. Could I be overlooking an air leak? Could a valve be sticking or piston be expanding?
    Any ideas or questions? Thanks for any help.

[Ohio HD]

Leak down test, and listen in the intake and exhaust.

[Burnout]

Sounds like it might be a sticking valve or an ignition that is crapping out (heat related?) Might even be a circuit breaker?
But I can't hear it from here! 
What kind of ignition?

[crock]

Had one go 25 or 30 miles and start acting very similar. Pull over and try to figure it out get back on and ride another 25 or 30 miles. Ended up finding a bad hydraulic unit in an exhaust lifter

[a_disalvo]

Pull the pipes and look for oil in the exhaust ports in the heads. Rear exhaust guides have a problem on shovels. Frank

[Lew]

Besides a possible bad breaker, might have a good look at the ignition switch and the engine stop switch on the handle bars.  Check for pinched or rubbing wires also.

[harpwrench]

Gas cap might not be venting

[fbn ent]

^^^ was going to mention that ^^^^

[roadkingdresser]

could be the filter on the petcock is plugged.

[roadkingdresser]

sorry I missed that in your lines.

[david lee]

the result will be interesting

[JW113]

Folks, before making suggestions, perhaps go back and carefully read what has already been replaced or tried. For example, he said:

" I did crack open gas cap for venting while riding."

It's not the gas cap!

I'm with Crock, sounds like a tappet is slowly bleeding down over time, slow down and it pumps back up for a while. Seems like a possible cause to eliminate.

-JW

[Lew]

Backfiring out the exhaust is usually associated with an ignition/electrical or timing issue.

[fbn ent]

Folks, before making suggestions, perhaps go back and carefully read what has already been replaced or tried. For example, he said:

" I did crack open gas cap for venting while riding."

It's not the gas cap!

I'm with Crock, sounds like a tappet is slowly bleeding down over time, slow down and it pumps back up for a while. Seems like a possible cause to eliminate.

-JW

Dammit...I have to quit speed reading! 

[dwjohnson]

     Yea I have tried a few of the ideas already but no worries, I love all the interest and ideas.
     I agree with the comment about the backfiring exhaust being electrical, that's mostly been my experience too. but this seems different, maybe not so much the loud typical backfire but not an intake cough either??
     I'm liking the lifter idea because, now thinking back about, it seems to be a fairly quick progressive thing. I felt I could hear it start if that makes sense, then it gets worse and worse until one cyl. quits. (guessing and not sure on which cyl yet either)
  Having trouble tying a breaker or ign switch to this perticular problem in the way it's happening, in my mind. Ign system is points and condenser which have been replaced. Have not inspected the flyweights yet. Problem seems to improve with richer slow jetting too. My reason for that was the mixture screw is out about 2 1/2 turns and plugs were white to me. Also why I thought I would find an air leak too but didn't seem to.
  Is the way to check the lifters still to compress them in your hand for a few seconds?? Thanks again

[fbn ent]

You would more than likely have heard a lifter back off. 

[dwjohnson]

 I do agree with being able to hear the lifters or pushrods, as well as hearing them with a valve sticking closed or either the valve hitting a piston sticking open??         
   Unless the spring in the tappet could hold enough tension to keep that noise down some how.

[JW113]

Sure, the issue being a bad tappet may or may not be a long shot, but you have one reported case of a bike with similar behavior, and it was a bad tappet. Since you've replaced almost everything ignition electrical (didn't see spark plug wires listed), and no improvement, likely not that. These kind of problems can be a bit aggravating, sure, but after you've checked off the obvious, it's down to process of elimination.

My gut tells me it's not a valve that's hitting a piston, as if it did, it would bend the head, and there would be no more sealing with that valve and it would run like crap always.

Can you describe a bit more what happens when the problem starts to show up, and why you believe that it starts running on one cylinder? Besides the pops out the exhaust, does it sound like it's running on one? Does it start vibrating oddly?

You don't happen to have one of those laser temperature guns do you? Very valuable tool to have to detect any difference in the way the front and rear cylinder is running. In your case, if rode it until it started to drop a cylinder, try to keep it going for another few minutes, then hit the kill switch, coast to the side of the road. Then pull out the laser gun and measure the exhaust pipe temperature near the head. If they're not pretty close to the same, the cooler one is the one that is dropping out.

Get out your magnifying glass Sherlock, and solve the mystery! You have a few dozen Watson's here to cheer you on.


-JW

[guppymech]

Seeing as the bike is a '84 like mine, you may want to check out the intake manifold spigots on the head.  Mine left the factory with the spigot on both heads partially milled off when they finished the heads.  I had a minor intake leak that was more of a nuisance than anything.  I tried a few different styles of intake clamps and finally cured it by using 4 thin worm screw clamps.  I'm sure this isn't the cause of your problems but it may be a contributor.  Here's a pic of one of my heads.

[dwjohnson]

 I believe the lifter theory is quite possible and I will be looking into that. I also will be doing a leak down, cold and hot. I do believe the compression test would have shown any valve to piston troubles. And I do have a temp gun, thanks for that idea.
Thanks for the intake flange tip, I still believe an air leak could be causing these issues even though I can't detect one. Did you have any trouble spotting or confirming your air leak??
Does anyone suspect worn out flyweights could be an issue?
 

[guppymech]

Thanks for the intake flange tip, I still believe an air leak could be causing these issues even though I can't detect one. Did you have any trouble spotting or confirming your air leak??
Does anyone suspect worn out flyweights could be an issue?

It was more of a feeling that there was a intake leak that had developed, random coughing during warm up that didn't used to be there and minor seepage from the intake rubber bands.  I tried spraying the intake bands with the engine running but that didn't show a intake leak.  Using 4 individual clamps cured all that.

If you suspect the advance unit, pull it out and have a look at it.  Also do the drag pipes have a lollypop in them to break up the reversion wave in the pipe?

[crock]

Folks, before making suggestions, perhaps go back and carefully read what has already been replaced or tried. For example, he said:

" I did crack open gas cap for venting while riding."

It's not the gas cap!

I'm with Crock, sounds like a tappet is slowly bleeding down over time, slow down and it pumps back up for a while. Seems like a possible cause to eliminate.

-JW

It took quit a while to figure out as well. Riding around town it would run all day. At 55 mph you could go 25 or 30 miles but at highway speeds (70) in 10 miles you were on the side of the road. Talk about a head scratch-er as I was chasing a fuel system problem

[Trouble]

Joining the party kind of late, yes, the flyweights could be an issue. If not tight like new and lubricated, replace. Removal of customer-installed in line fuel filter or any kind of in line fuel filter is highly recommended.

[JW113]

Am curious how the flyweights/advancer would cause the described symptoms. As in, runs fine for 30 minutes, then starts to lose a cylinder. If the flyweights are stuck retracted, it will always not have power above idle as ignition is retarded. If stuck extended, it will fight the starter, ping at very low rpm, and run fine above 2000 rpm as ignition is always advanced. At least that's what I would imagine.

-JW

[Trouble]

Well, the part about 'whacking on the throttle while stopped helps it recover' was a clue something else besides all the other repair work might have something to do with overlooked advance unit. I know what you mean in your description of how the advance unit might behave if stuck weights were the problem. But it seems the OP has tried just about everything without success. I'd like to know the outcome.

[Trouble]

Had a strange problem on a bike similar to the symptoms described here. Runs good for a while, then starts to cut out. After checking and double checking everything for half a day, found a strand of wire on the wire-covered oil lines touching the brake light switch connection which would cause the engine to cut out after riding a couple miles, then it would recover for a bit and repeat. The devil is in the details.

[76shuvlinoff]

I have definitely had a worn out dash ignition switch drive me nucking futs before. .

[dwjohnson]

 Customer has bike back right now, had to take a break on it and not enuff time to ride 20-30 miles after each change/check
  It's really not like a switch or wiring short or pinched wire, more like a progressive worsening  type of  issue once it starts. I can hear/feel it beginning ever so slightly and in a matter of 20-30-40 seconds it worsens to a point of what I describe running on one cyl. Almost like a piston is slowly seizing up so to speak.
It does have a braided oil line right up against the sending unit, I'll look into that. I'd also like to disassemble and inspect the tappets soon too, that really seems to fit.
  Thanks for all the replies and tips!!

[One4Tone]

...switch the plugs around see if that makes a difference...what plugs are you using?...even the Harley plugs are old stock for shovels....how is the auto advance?..nice you  don't have to deal with computers...

[a_disalvo]

Had a similar problem, mine would run 12 miles, than shut down. Turned out to be the coil, was a racing 3 ohm and was overheating. What coil did you change to? Frank

[guppymech]

I had a bad carbon core sparkplug wire on a new to me moped, it would run good and then die.  Riding around the block troubleshooting, it would die at about the same distance every time.  Tried a new plug wire, problem cured.  With points ignition you can use solid core wire or some spiral wound conductor wire.

[dwjohnson]

 Spk plugs are new Harley 5RL. Problem did not change any.
Coil was replaced by customer, not sure of the ohm but again no shift or change.
Have not tried different plug wires, didn't think it acted like that would it, but easy enuff to try now.
  So next chance I'm looking to check on the tappets, the plug wires, the flyweights, and I want to look closer at that braided oil line touching the sending unit.
Thanks again

[Burnout]

Better yet remove all the braided lines from that bike to prevent future blood loss and rub spots.

[JW113]

I'll second that. Braided SS lines look great in pictures. Beyond that.....

-JW

[76shuvlinoff]

I've got (Russell?) braided brake line from the front mc that's coated.

[JW113]

OK, I confess I use braided BRAKE lines as well. Bad Johnny. Bad. But have to admit, they sure make the brake lever less squishy.



-JW

[rageglide]

Does the bike still have the VOES?  They can present a vacuum leak and once heated up might be worse.  Is the ignition stock?  '84 shouldn't have mechanical advance.

[dwjohnson]

 Yes bike does have VOES on it but I know it's probably not in use being the customer has replaced ign. with a set of points. I did inspect vacuum lines and all seem to be good. I have had a bike with a bad voes and it acted similar except that bike just kinda scaled back on the "pull" and changed sound a bit while riding down the road when it would happen. I mean you could surely tell when it happened, it just wasn't as extreme as this one.and with the points ign. I don't think it is operational. 

[rageglide]

Ah, yeah VOES would be non-functional with points ignition.  If it's still connected it could be creating a vacuum leak.  The rubber plug dries up and can leak badly.  If it's connected to the carb/manifold still, I'd disconnect it and put a vacuum cap on the fitting.

[dwjohnson]

 Great idea and it is still plumbed so I will address that as well, Thanks

[Hillside Motorcycle]

Hold the phone.....check to insure your gas cap is venting.

[Hossamania]

Hold the phone.....check to insure your gas cap is venting.

Addressed in the op.

[One4Tone]

...how about trying a set of solid lifters...to eliminate the possibility of leaking lifter. I had a situation years ago on a 72 fl (that I still own)..the valve seat loosened up and it would ride up and down with the valve. It eventually bent the push rods once the seat rotated and did not slide back in, if you decide to switch lifters you may want to check push rods for straightness...with the motor running see if the push rod housings have any slight movement

[waltcentral]

Bring a volt/ohm meter with you and when it dies on a ride check to see if the coil is getting 12 volts. Probably help to remove the coil cover to make it easier.
This eliminate the wiring up to the coil.

[david lee]

 the outcome will be interesting