Rear cylinder not running right

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57pan
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Rear cylinder not running right

#1

Post by 57pan » Sat Oct 23, 2004 12:07 pm

On my 57 panhead with about 6000 miles on the top end - When I start it up and it's sitting at idle the front cyl. is running smooth but the rear seems to be running weak. It's hard to explain -the exhaust note just sounds quieter than the front, but then every once in a while it will fire good. It is most apparent when you are riding very slow in 1st gear, like just going to the end of the driveway or something; it will kind of lurch along. But it runs stong when I'm going thru the gears - smooth acceleration.
Maybe the problem has always been there and I was either ignoring it or just denying that it was a problem, or maybe it has just been gradually getting worse to the point where I can no longer ignore it or deny it - I'm really not sure.

Here's what I have done so far:

Checked, double checked and triple checked for the evil manifold leak by pressure testing.
New plugs - no change.
Swapped the plug wires in the coil - no change.
Tried a different wire on the rear plug - no change.
Removed the hydraulic units and replaced them with solid converters and adj. the valves - no change.
Measured the stroke of each lifter, thinking maybe there could be a problem with one of the lifters or cam lobes - the lifters all travel the same amount (within my crude measuring capabilities).
Checked compression - both cylinders are exactly the same: 105 psi after 5-6 kicks, motor cold.

I have been racking my brain trying to figure out what else to check. The only thing that comes to mind is the ignition timer. Maybe one of the lobes of the cam that opens the points is worn or the shaft itself is worn in such a way that it is changing the timing on the rear cylinder. I probably can't replace the timer without raising the front cylinder though and I kind of hate to do that since I have not had any problems with leakage around the base gaskets and don't want to create a leak by breaking the seal.
I now know lots of things that are not causing the problem, but I still don't know what IS causing it - I need the benefit of the collective wisdom of this forum.



Skip
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Re: Rear cylinder not running right

#2

Post by Skip » Sat Oct 23, 2004 4:17 pm

does it do it in any other gear...say 3rd when cruising about 40...when you say lurching is it like it needs down shifted?..mine is doing the same thing...I try to follow my wife on her sporty and it is a miserable ride for me...as long as I am cruising at 60 or so I'm great...my thought was in drive train somewhere...my compression in 120 in front after 3 cold kicks and 85 in rear after 15 cold...did all the same you did above and that is why I decided rear or primary chain...just rebuilt tranny so don't think there...someone told me to check the fuel, but I run hot fuel so not an issue for me...going to put new drive chain on tonight and I will let you know...Skip

fourthgear
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Re: Rear cylinder not running right

#3

Post by fourthgear » Sun Oct 24, 2004 3:23 am

57pan
First, its the head you would remove not the cyl . to remove the circuit breaker (distributor )and if you remove just the top you may get by with out removing the head . @ idle the rear always seems to stumble , it the nature of older HD's, thats not saying some thing is not wrong. What kind of carb are you running and how and where are you setting your timing ? You say its firing fine at speed , may be just a carb, adj. or a timing thing.
skip
What kind of rear brake are you running ? It could be dragging or binding at low speed , of course it will not be obvious when @ speed . Put tranny in neutral and spin back tire with it off the ground . With plugs removed and tranny in neutral , can you kick the motor over relatively easy with your hand ? These tests will show any binding or hard spots and look for the un-obvious.

57pan
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Re: Rear cylinder not running right

#4

Post by 57pan » Sun Oct 24, 2004 2:18 pm

Skip,

It is not nearly as noticable in other gears. I have lugged it down pretty good in 4th gear and when I twist the throttle it seems to accelerate fairly smoothly. I'll have to do some more testing along those lines. Because it may be, as Forthgear says, that's just the nature of the older H-Ds.

Fourthgear,

I am currently running an older Keihin (not CV) carb. I static time it by setting the points at .022 and put the timing mark just forward of the middle of the timing hole.

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Re: Rear cylinder not running right

#5

Post by Panacea » Mon Oct 25, 2004 4:33 am

57, the rear cyl fireing on every third opertunity while the front chugs along never skiping a beat has been a symptom i've seen on both my pans, I cant understand it either. I also thought the lobes on the breaker could be the cause but never proved it. If you figure it out let me know....Mike

Skip
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Re: Rear cylinder not running right

#6

Post by Skip » Mon Oct 25, 2004 10:04 pm

Hey 57: not that this will help you, but may give a direction, after several experiments this weekend I found that MY problem is in the carb/throttle somewhere...at the same spot on the throttle in every gear I get the "dead spot" it is like it is not getting any fuel for that fraction of a second...still trying to figure out why, but at least we now know the cause...good luck to you...Skip.

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