4 spark plugs 1984 FLTHC
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Novice
Joined: Jul 2008
Posts: 22
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From:
As far as the compression check goes, I didn't open the throttle at all. Didn't know I was supposed to.(still learning) I'll check it again today if I get time. Thanks for all the input, knowledge is fairly easy to come by on this forum. Lot's of good people with good ideas! Dale - New compression check with throttle wide open, front 145#, rear 140#
Last edited by Learch; Nov 21, 2008 at 08:51 AM.
HDF Community Team
Joined: Dec 2005
Posts: 5,402
Likes: 2,605
From: NC USA
Yea, you're good!
Rear is most always less, unless the rear piston is really carboned up bad and/or there's a problem with the front. Anything less than 5% difference is real good - the book says 10% is acceptable.
Mine had about that first time I checked it at 28,000 miles, put another 27K on it before I did the top end, but it was piston rattle rather than low comp. or oil usage that made me want to do it.
And I ran my original plugs 45,000 so I wouldn't worry much about the 2 that are hard to get out - just leave them alone
Ride Safe and enjoy!
Rear is most always less, unless the rear piston is really carboned up bad and/or there's a problem with the front. Anything less than 5% difference is real good - the book says 10% is acceptable.
Mine had about that first time I checked it at 28,000 miles, put another 27K on it before I did the top end, but it was piston rattle rather than low comp. or oil usage that made me want to do it.
And I ran my original plugs 45,000 so I wouldn't worry much about the 2 that are hard to get out - just leave them alone
Ride Safe and enjoy!
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Novice
Joined: Jul 2008
Posts: 22
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From:
I got a book and it said compression is supposed to be 90#. There is a lot of difference between 90# & 145# so I called Harley. They said 90# too and the bike must have high compression pistons. Go figure!! Can't find the original owner to ask him.
Road Warrior
Joined: Dec 2007
Posts: 1,286
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From: SoCal
Learch,
If you ever get curious about what your pistons look like in there, just pull all four plugs (like the next plug change) and shine a light in one side while looking in the other. You ought to get a pretty good view.
If you ever get curious about what your pistons look like in there, just pull all four plugs (like the next plug change) and shine a light in one side while looking in the other. You ought to get a pretty good view.
HDF Community Team
Joined: Dec 2005
Posts: 5,402
Likes: 2,605
From: NC USA
Not to say you don't have domed, high compression pistons. I'm assuming with a domed cut head and domed pistons, one sorta offsets the other. But that arrangement would give more even combustion area in the entire chamber. Again, I'm not up on the "high-peform" stuff.
If you take out the plugs on the primary side and use a tiny light, like a rifle bore light, you could probably see well enough to tell. Main thing is that it runs great!
HDF Community Team
Joined: Dec 2005
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From: NC USA
Yea, that's be even better!!
Tourer
Joined: Nov 2008
Posts: 328
Likes: 72
From: SE TN
Learch-
My 45" flathead with only a 4.75:1 C/R had 90 lbs (before I stroked it)...
For a stock bore evo @ 8.5:1 C/R, 120-145 is good operating compression
Any over-boring will (of course) add additional psi readings.
I'd be glad it runs good, & ride when ya can...
My 45" flathead with only a 4.75:1 C/R had 90 lbs (before I stroked it)...
For a stock bore evo @ 8.5:1 C/R, 120-145 is good operating compression
Any over-boring will (of course) add additional psi readings.
I'd be glad it runs good, & ride when ya can...
Last edited by Buddy7394; Nov 26, 2008 at 01:21 AM.
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Road Warrior
Joined: Dec 2007
Posts: 1,854
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From: Eagle River WI
When I first got my knuck the compression was 48PSIG per cyl The wheels were 61 ci and the jugs were 74 ci At TDC the piston dome was still 1/4 in below the top of the cylinder I used to start it with my right arm Lots of $5 bets at the bars now my wrist is shot !! Thank you very much!
Tourer
Joined: Nov 2008
Posts: 328
Likes: 72
From: SE TN
George-
I can remember long ago losing a bet like that...
Ahhh, memories...
That 61" short stroke in 74 jugs will sure lower the C/R quick...
That .250 squish (term used loosely) is a bit excessive... LOL!!!
Bummer about yer wrist..
But I bet you could count the winnings at the time ....
I can remember long ago losing a bet like that...
Ahhh, memories...
That 61" short stroke in 74 jugs will sure lower the C/R quick...
That .250 squish (term used loosely) is a bit excessive... LOL!!!
Bummer about yer wrist..
But I bet you could count the winnings at the time ....