4 spark plugs 1984 FLTHC
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Joined: Jul 2008
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4 spark plugs 1984 FLTHC
I recently got a 1984 FLTHC w/evo motor. It has 4 spark plugs, 2 on each side. I have never seen this before and was wondering if all evos are like this. I bought a Clymer M422-3 service manual and can't find more than 1 spark plug per cylinder. Also how do I get the second set out? They are located between the pushrod tubes. This is my first Harley so I don't know to much about it yet. Any help would be appreciated. Thanks, Dale
Road Master
Joined: Feb 2005
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From: nc mountains
The 4 plug head was sometimes needed on shovels but not evos . It was still done years ago. The shovel had a chamber design that when comp was raised the second set of plugs was needed to control combustion and ping. The evos design does not need them. Now if you do a high HP motor you have holes that you could adapt comp releases to. To change you would have to change the ignition so if it works well just run it ,looks different .
Road Master
Joined: Nov 2007
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From: Peoria, AZ
The two plug design as Hardluk has noted may have been done because of a compression modification to the head raised to 10:1 or higher to eliminate detonation problems and improved high comp burn (similar to XL1200S design) or because the previous owner thought they'd give him "higher HP" (not true). Either way, unless you are having an ignition problem, it should be okay to leave it two-plugged. Not familiar with the "between the pushrod" location but if they are in the way, try removing the pushrod tube upper covers and lowering the upper tube to get better access to the plugs.
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From:
The spark plug wires go to all 4 plugs and are routed between the cylinders to the coil. It sounds to me like someone was experimenting with possible high compression motor and wanted to see if more spark would help. The bike seams to have a lot of power, but then again this is my first Harley. I took a compression check and it was 120# both cylinders. Not bad for 69,000 miles I guess.
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HDF Community Team
Joined: Dec 2005
Posts: 5,422
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From: NC USA
Uh, depending on how you did the check...
Supposed to:
remove both plugs (in your case, one from each head - remove wire from the others)
lock throttle in wide open position
turn it over at least 5 compression strokes
all this preferably with the engine warm
120 is acceptable, but a reasonably fresh engine (stock) will pump up 140 - 155 if done as above. Surely they freshened it up, even if the only thing done was remove/modify the heads. At the same time, if it runs well, doesn't smoke or use a lot of oil - just ride and enjoy it
Just an after thought - don't think you could put dual plugs like that in an Evo head that wasn't cut out. (domed) Possible they put domed / twin plug heads over flat top (stock) pistons?? Possible I reckon, and if that be the case, 120 compression is jammin!
I'm not that up on the "trick stuff" some of these guys do, so I'm totally open to being corrected on the last paragraph...
Supposed to:
remove both plugs (in your case, one from each head - remove wire from the others)
lock throttle in wide open position
turn it over at least 5 compression strokes
all this preferably with the engine warm
120 is acceptable, but a reasonably fresh engine (stock) will pump up 140 - 155 if done as above. Surely they freshened it up, even if the only thing done was remove/modify the heads. At the same time, if it runs well, doesn't smoke or use a lot of oil - just ride and enjoy it
Just an after thought - don't think you could put dual plugs like that in an Evo head that wasn't cut out. (domed) Possible they put domed / twin plug heads over flat top (stock) pistons?? Possible I reckon, and if that be the case, 120 compression is jammin!
I'm not that up on the "trick stuff" some of these guys do, so I'm totally open to being corrected on the last paragraph...
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Road Master
Joined: Sep 2006
Posts: 1,121
Likes: 6
From: Huntersville, NC
That's what is so cool about used machines...you never know what your are going to get. If it runs good don't worry about it. If they are the original 84 heads they are sand cast and can't be welded up anyway....enjoy the ride.
Tourer
Joined: Nov 2008
Posts: 328
Likes: 72
From: SE TN
compression readings & actual compression can be quite different, if the cam has a lot of overlap, then initail readings will be lower. But actually C/R is higher.
The 4 plug deal, I would think someone has added 'domed pistons' & perhaps a hot cam..
Ride that baby....
The 4 plug deal, I would think someone has added 'domed pistons' & perhaps a hot cam..
Ride that baby....