Low compression

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Bondough
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Low compression

#1

Post by Bondough » Sat Feb 04, 2012 4:08 am

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Bondough
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Re: Low compression

#2

Post by Bondough » Sat Feb 04, 2012 4:12 am

55harley fl. Hard to strt cold but is fine when hot. Compression is low. Front is 58 psi. Rear is 54 psi. What next?

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Re: Low compression

#3

Post by RUBONE » Sat Feb 04, 2012 5:20 am

Is the engine stock? Does it have a stock cam? A little more info would help.
Robbie

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Re: Low compression

#4

Post by 1964FLDUO » Sat Feb 04, 2012 5:01 pm

When was the motor last rebuilt? How many miles on it? Those are low compression numbers, what was your procedure to check compression? You should open the throttle all the way and kick, kick ,kick until the number won't go up anymore. You can get an idea if it is your rings by squirting oil in the spark plug hole and compression testing again, if numbers go up, you probably need a set of new rings at the very least.

Cotten
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Re: Low compression

#5

Post by Cotten » Sat Feb 04, 2012 5:15 pm

Folks,
As Robbie infers,

Those are not unusually low compression numbers when a cam with a great deal of overlap is installed. Compression increases with motor speed.

If this is the case,
and all other things are in order,
including timing, valve adjustment if solids, and above all zero vacuum leaks,
yet it is still hard to start,.. I might suggest lighter oil.

....Cotten

Hauula Pan
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Re: Low compression

#6

Post by Hauula Pan » Sat Feb 04, 2012 6:09 pm

My motor also has compression in the range yours is, I have low compression pistons and an Andrews #1 cam installed, supposed to help with our Calif. gas. So the low numbers aren't necessarily bad. The easiest way to check to see if your low compression is due to engine wear is a good compression check. 1st do it dry then squirt a small amount of oil into the cyl. & do it again. If it stays low its usually valve guides, if it goes up its usually rings. But the question remains, Does it run OK when its running. If it runs OK doesn't smoke & doesn't use oil the low compression probably isn't a problem. I would go back to Intake leak, & timing. Pans can be fussy about starting cold vs hot & you have to get to know what each one likes as far as priming kicks, throttle open or closed, timing retarded, etc. & just when you think you have it figured out they'll throw you a curve. Also what carb. are you running? 6 or 12 volt sys.? 6 volt systems are very fussy about wanting a good fully charged battery. Lots of other things to check before getting too concerned about the low compression.

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Re: Low compression

#7

Post by George Greer » Sun Feb 05, 2012 7:10 am

Hauula Pan wrote: just when you think you have it figured out they'll throw you a curve.
Just as I have been informed, and the above quote confirms......panheads are female, and can turn B!tch on ya in a second!! :D

George

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Re: Low compression

#8

Post by PanPal » Mon Feb 06, 2012 5:02 pm

Make sure your clutch is not slipping too.

Bondough
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Re: Low compression

#9

Post by Bondough » Sat May 05, 2012 5:46 pm

Thanks everyone for all the tips. I think a lot of trouble is me. I have 3 pans and they all act different.

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Re: Low compression

#10

Post by 58flh » Sun May 06, 2012 12:44 pm

Every pan has its routine for STARTING!----As long as there are no leaks, timing-on,& its got spark & fuel ,It gotta run, if it dont start in 3-4 kicks(hot).ig. on!---to me something is amiss! As Cotton said those comp. #s are more commen then you know!-- I go by the 8-10 rule!---If that much difference, then as was mentioned earlier, a shot of some 60wt. in the cyl. with the ol-Ladys turkeybaster & remeasure!---If it levels out & increases I would test further with some air in them jugs & make sure the valves are seating good! If they are good THEN a new set-of rings along with a little hone just to bring the crosshatch back Then meas. bore IF in specs. USE hastings rings & it will be good to go as soon as RINGS find thier home!--usually within a 100 miles!----RICHIE 8) ---PS---DO NOT use 1 of those press & hold COMP.-TESTERS!---USE the correct thread in head ones!

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Re: Low compression

#11

Post by kitabel » Sun May 06, 2012 2:33 pm

The compression should not be that low regardless of the cam.
Overlap has no effect on compression.

Cotten
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Re: Low compression

#12

Post by Cotten » Sun May 06, 2012 2:58 pm

kitabel wrote:The compression should not be that low regardless of the cam.
Overlap has no effect on compression.
Its hard to compress gases when the valves are open, Kitabel.

Cams with long duration and overlap depend upon RPM to "pack the cylinder" and build compression.

Nobody can kick that fast,
but a machine can start reliably with cranking compression in the fifties, if everything else is in order.

....Cotten

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Re: Low compression

#13

Post by kitabel » Sun May 06, 2012 6:04 pm

Please explain how overlap affects compression.

Cotten
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Re: Low compression

#14

Post by Cotten » Sun May 06, 2012 6:57 pm

kitabel wrote:Please explain how overlap affects compression.
Kitabel!

How you going to compress gases when the valves are open?

It takes the rapid motion of the motor, along with the forces of combustion, to make walls of turbulent gases become the real "valve".

Less efficient, but a whole lot more fun.

....Cotten

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Re: Low compression

#15

Post by 1962FLH » Mon May 07, 2012 12:45 am

Valve Overlap,
The amount of time, expressed in crankshaft degrees, that describes the window of time between the the Inlet Cam's opening point BTDC and the Exhaust Cam's closing point ATDC. This figure can vary between zero degrees on some stock cams to as much as 70 to 90 degrees on some race motors. In general most street engines will have 20 to 30 degrees of overlap and most performance cams will have 50 to 60 degrees of overlap. Increasing the degrees of overlap tends to move the powerband up the RPM band. Increasing the overlap can increase peak power, but only if the exhaust system is properly designed to scavenge the cylinder. Decreasing the overlap tends to boost lower rpm performance.

I prefer the power band in the rmp range I use the most 2500=mild cam.
just saying.

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