I've been chasing this gremlin for some time now and am about to run out of patience. I have a '76 FXE Super Glide Shovel. It has a Super E carb, Dyna S ingnition, V & H drag pipes. The rear spark plug suddenlly turned up black, (rich), w/ lots of buid up on it. The front was fine, good color, no build up. I changed plugs and stated checking them every 100 miles, and the rear plug stayed black and wet. New plug wires, no help. It was suggested that the carb was way out of tune and that I probably had an air leak in the O rings in the front cylinder intake that essentially caused the front mixture to correct itsself. New O rigs and billet clamps, no help. I swapped locations with the coils to see if the problem moved to the front cylinder before I sprung for $150.00 for a new pair off colis - it didnt. When the bike has a fresh, or freshly cleaned plug in the back jug, it runs like a top, no cough, wheeze or sputter.

Any ideas anybody?

 

have you ran a compression test?
is the engine smoking at any time when running?

 

Nope, no smoke and good compression, (about 150). The motor has about 1500mi on a complete rebuild.

 

wow that an impressive compression reading!
I'm sure you've done all of this but:
check ignition terminals and plugs use dielectric grease where ever possible even on the plug wire boots.
check battery for charge clean terminals and inspect primary and secondary cables.
good charging system?
use a can of carburetor cleaner to confirm a good seal on the intake bands.
burning quality premium fuel.
what brand spark plugs are you using?

 

Motor's still tight I guess
I didn't use any dieelectric on the new plug wires, but I will
battery and cables are brand new with engine rebuild
The battery stays right around 12.4V
I've tried the carb cleaner around both intakes when it's at a high idle, and haven't found anything
Yep, Shell or BP 93
Lately, I've been alternating every 100 miles or so between Autolite and NGK. I can't tell any seat of the pants difference.

 

You say the rear cyl plug stays black and wet Wet gas or wet oil ?

 

fxe: That is a very high compression test.... I thought 125 on a new rebuild was great...was yours with or without the throttle WOT??? My leakdown is great, but must have some valve spring weakness as mine is 90psi front and back. My cam is an Andrews A, and I thought anything with more duration would yield a lower psi.... Can you all clear this up for me? Your wet plug does sound like a weak ignition...

Thanks,

Hogback

 

with the Andrews "A" grind in my 68 with 8.1 to 1 compression when cold using the electric start with WOT is around 90 psi.
FXE could be running some high compression pistons or the gauge is giving a false reading IMO.
12.4 volts may be an indication of a weak electrical system. should be more than that when running above 1000 rpm.

 

I'd have to say, the heads are 'decked' to get that kinda compression. My '78 is at 125/129, kickstarts like a beotch, and Farmers '75 88" pushes over 145, both holes.
Did you do a 'leakdown' check on the back hole?
And I, too, am unclear. Gas wet or oil wet?
By your post, you have done the carb cleaner thing for leaks, that's good to go. I'm thinkin' rings are not seated, or worse, upside down,(DON'T YOU DARE ASK ) but JMFUO based on posts so far. Leakdown would tell ya that.
Keep postin', tho. Always like to hear MORE solutions. Knowledge is our friend.

 

I bought this bike about 4 months ago andi I called the guy I bought it from this week-end, and he swears the cam, heads, and pistons are bone stock, and he held with his original story on the comp. check. I never checked it myself until this week end, (95 front, 95 rear). Light bulb should have gone off when it wasn't that hard to kick start.

No, I have not done a leak down.

definitely gas wet.