No rear cylinder fire
HDF Community Team
Joined: Dec 2005
Posts: 5,426
Likes: 2,623
From: NC USA
Despite my rant about intake leaks (which I stand by, generally speaking) since it's totally dead on the rear, I'm leaning toward a failed ignition module now.
Without getting into the weeds too far my logic is this - possible one of the pickups has given up and though it's firing on both plugs, could be it's only firing at the correct time for the front and what shows as the rear plug firing is actually the wasted spark for the front. Meaning, it's not firing at the correct time for the rear and the front isn't getting a wasted spark. The "wasted" spark for the front is the compression spark for the rear and vise-versa with a dual fire coil.
This is the nose cone version, right? What setting is the "Mode" switch" on? Did you install it or was it already on there when you got it? (I'm trying to come up with an easy "check" that might verify my guess on the module half dead)
Without getting into the weeds too far my logic is this - possible one of the pickups has given up and though it's firing on both plugs, could be it's only firing at the correct time for the front and what shows as the rear plug firing is actually the wasted spark for the front. Meaning, it's not firing at the correct time for the rear and the front isn't getting a wasted spark. The "wasted" spark for the front is the compression spark for the rear and vise-versa with a dual fire coil.
This is the nose cone version, right? What setting is the "Mode" switch" on? Did you install it or was it already on there when you got it? (I'm trying to come up with an easy "check" that might verify my guess on the module half dead)
Club Member
Joined: Apr 2011
Posts: 5,817
Likes: 7,850
From: SE Louisiana
You can test spark strength with a screwdriver, insert screwdriver in plug boot, hold plastic handle of coarse, then hold screwdriver shaft about 1/4" away from a cooling fin and crank engine, adjust gap to see how wide of a gap it'll jump, compare to other cylinders plug lead. **ground the other plug lead, or put the plug in it and lay it on the head, not good on the ignition module to fire with no resistance on the lead**
it takes a lot less voltage to fire a plug in open air than it does under compression.
Being it has a brand new DTT module I doubt it's bad, but maybe some sort of timing issue, I would make sure the timing cup for the cam sensor didn't come loose or something, or a mode coil issue, That link I post in post #11 talks about what ohm coil to use, if it was mine, I would get a single fire coil and go single fire for sure.
I remember reading something long ago about two different timing cups on certain years when replacing ignitions? I not sure if that's even relevant here.
it takes a lot less voltage to fire a plug in open air than it does under compression.
Being it has a brand new DTT module I doubt it's bad, but maybe some sort of timing issue, I would make sure the timing cup for the cam sensor didn't come loose or something, or a mode coil issue, That link I post in post #11 talks about what ohm coil to use, if it was mine, I would get a single fire coil and go single fire for sure.
I remember reading something long ago about two different timing cups on certain years when replacing ignitions? I not sure if that's even relevant here.
Last edited by Schex; Feb 20, 2022 at 02:36 PM.
HDF Community Team
Joined: Dec 2005
Posts: 5,426
Likes: 2,623
From: NC USA
You can test spark strength with a screwdriver, insert screwdriver in plug boot, hold plastic handle of coarse, then hold screwdriver shaft about 1/4" away from a cooling fin and crank engine, adjust gap to see how wide of a gap it'll jump, compare to other cylinders plug lead. **ground the other plug lead, or put the plug in it and lay it on the head, not good on the ignition module to fire with no resistance on the lead**
it takes a lot less voltage to fire a plug in open air than it does under compression.
Being it has a brand new DTT module I doubt it's bad, but maybe some sort of timing issue, I would make sure the timing cup for the cam sensor didn't come loose or something, or a mode coil issue, That link I post in post #11 talks about what ohm coil to use, if it was mine, I would get a single fire coil and go single fire for sure.
I remember reading something long ago about two different timing cups on certain years when replacing ignitions? I not sure if that's even relevant here.
it takes a lot less voltage to fire a plug in open air than it does under compression.
Being it has a brand new DTT module I doubt it's bad, but maybe some sort of timing issue, I would make sure the timing cup for the cam sensor didn't come loose or something, or a mode coil issue, That link I post in post #11 talks about what ohm coil to use, if it was mine, I would get a single fire coil and go single fire for sure.
I remember reading something long ago about two different timing cups on certain years when replacing ignitions? I not sure if that's even relevant here.
A plug will fire in open air much easier much easier than under pressure. These modern ignition should throw a bright white/blue spark at least a 1/4"
Good suggestion checking the cup and mode setting. Should be on 0,1,4 or 5 with the pink wire to the stock type coil.
The later Shovels used a single window cup that were silver (zinc) and they changed to a 2 window cup that is gold (yellow zinc) in mid - '83 and up.
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6th Gear
Joined: Feb 2022
Posts: 11
Likes: 1
From: Virginia Beach
Modes switches are definitely correct and the cup is brand new as well as front fires no problem so the signal is getting to the module correctly. Just opened the pushrod tubes and it does have screaming eagle adjustable pushrods. They are tight and the jam nuts are set. I have bike on a jack and will go through the adjustment process to ensure they are correctly adjusted. Hoping this is the issue.
Seasoned HDF Member
Joined: Mar 2010
Posts: 21,116
Likes: 7,640
From: Poolville
Pull the timing plug on the left side of the case and Bring your front cylinder up to TDC. . Then take the cover of the ignition in the nose cone as well as the ignition module.. take a picture of the rotor cup and post it up.. The front cylinder must be at TDC for this..
Seasoned HDF Member
Joined: Aug 2008
Posts: 5,081
Likes: 2,159
From: Ohio
2 different rotor cups available, carb and efi, they will not interchange, not saying that's your problem but this one is strange.
You have a wet rear plug on dual fire, crossing plug wires doesn't make a difference, it won't fire, compression is good. Since this is such a strange one, if you can't find anything wrong with it, make sure a mouse didn't build a nest in the rear muffler, the cylinder will not fire if it cannot get spent gases out, just a back up plan.
You have a wet rear plug on dual fire, crossing plug wires doesn't make a difference, it won't fire, compression is good. Since this is such a strange one, if you can't find anything wrong with it, make sure a mouse didn't build a nest in the rear muffler, the cylinder will not fire if it cannot get spent gases out, just a back up plan.
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Club Member
Joined: Apr 2011
Posts: 5,817
Likes: 7,850
From: SE Louisiana
Modes switches are definitely correct and the cup is brand new as well as front fires no problem so the signal is getting to the module correctly. Just opened the pushrod tubes and it does have screaming eagle adjustable pushrods. They are tight and the jam nuts are set. I have bike on a jack and will go through the adjustment process to ensure they are correctly adjusted. Hoping this is the issue.
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6th Gear
Joined: Feb 2022
Posts: 11
Likes: 1
From: Virginia Beach
unfortunately I won’t have a chance to work on this again until Friday late day at the earliest. Also going out of town Saturday and Sunday so may be the following weekend before I really do much more.
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