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low compression FLH

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Posts: 219
Joined: Fri Mar 21, 2008 5:56 pm
Bikes: HD FLH'59
Location: Holland

low compression FLH


Post by hjans » Tue Feb 24, 2009 11:36 am

Hi there

I have the following problem :
Checked the compression on my '59 FLH wich was to low ( about 6 ), with as a reult bad starting.
After dismantling the engine i checked everything i can imagine that can cause low compression : pistons-rings-valves-everything seemes to be in quiet good order, except a crack in the heads ( causing no leakage to the outside), switched valves in one head ( causing no leakage).
Now i was told that there could be other causes : low compression pistons and a low compression cam.
What would be your opinion on this ?
Can imagine something with the pistons, but a low compression cam ??


Posts: 6911
Joined: Mon Aug 26, 2002 9:09 am
Location: Central Illinois

Re: low compression FLH


Post by Cotten » Tue Feb 24, 2009 1:27 pm


Many 'performance' cams have wide lobes for longer duration of valve opening. This means the time that both valves are open at the same time is longer as well.

This means that you may feel remarkably little compression when kicking, as the motor needs to turn much faster for the pulses of gases to block each other and create compression when running.

So please determine what kind of cam you are dealing with.
The pistons used with such cams are usually moderate to higher compression.

No matter what you find, it should not be entirely blamed for hard starting, as a motor should still fire at 4.5 to 1 compression.


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