Author Topic: Compression test readings  (Read 3853 times)

Offline Stu55Flash

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Compression test readings
« on: 17.02. 2010 10:00 »
I did a compression test on my Flash before I put the engine under the bench. The readings were both 120psi. As my old grey fergie tractor has readings around 100 psi and my Fanny B 2 stroke is 75 psi both from the same era I think this is ok. Does anyone have any other experience - what should the reading be?

Stu
"Keep a distance from lady "L" drivers in cars. Some are not mechanically minded, are slow to acquire road sense, an are apt to panic..." The Pitman Book of the BSA Twins.
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Online orabanda

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Re: Compression test readings
« Reply #1 on: 17.02. 2010 10:12 »
With 7.25:1 pistons, my bikes are from 150 psi - 155 psi

RGS replica = 180 - 190 psi

Regards,
Richard

Offline Josh Cox

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Re: Compression test readings
« Reply #2 on: 17.02. 2010 10:49 »
14.7 psi (1013.8 mb) = 1

Therefore Biggus,

155psi = 10.5 : 1

190 psi = 12.92 : 1

120 pisi - 8.16 : 1

100 psi = 6.8 : 1

75 psi = 5.1 : 1

If you want accurate readings the engine needs to be warm and the oil rings wet from running ( thats how it is done in the aircraft world ).
Black 1953 Golden Flash Plunger

Offline mike667

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Re: Compression test readings
« Reply #3 on: 17.02. 2010 11:51 »
14.7 psi (1013.8 mb) = 1

Therefore Biggus,

155psi = 10.5 : 1

190 psi = 12.92 : 1

120 pisi - 8.16 : 1

100 psi = 6.8 : 1

75 psi = 5.1 : 1


at sea  level right?

If you want accurate readings the engine needs to be warm and the oil rings wet from running ( thats how it is done in the aircraft world ).

Online RichardL

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Re: Compression test readings
« Reply #4 on: 17.02. 2010 12:19 »
I believe the additional pressure is due to adiabatic heating, which we talked about a while back. If interested, you can use the "Search" tab to find the word "adiabatic" and get to the two threads where the idea is supposed (coincidentally, by me).

Richard L.
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Online a10gf

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Re: Compression test readings
« Reply #5 on: 17.02. 2010 12:37 »
Some info here: compression topic

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Offline brackenfel

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Re: Compression test readings
« Reply #6 on: 17.02. 2010 14:03 »
Goodness !! I've just had a quick look at the "compression topic". I was going to check mine but now I'm not so sure, I never was any good at maths.. *smile*

Adrian
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Offline Josh Cox

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Re: Compression test readings
« Reply #7 on: 17.02. 2010 21:47 »
Perhaps it goes without saying, low compression can be from a little carbon under the exhaust seat etc etc.

Piston needs to be at top of stroke ( both valves shut ).

If you have low compression, is air is coming from:

1) carby - intake valve prob,
2) exhaust pipes - exh valve prob, or,
3) engine cases / rocker box - rings.

if 3), some have suggested running AJAX powder through the engine whilst running, not sure about that one myself.
Black 1953 Golden Flash Plunger

Offline A10rocket

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Re: Compression test readings
« Reply #8 on: 17.02. 2010 22:55 »
  120psi sounds OK to me for a flash (in standard form with cast iron head).  The fact that both readings are the same is important and if it's hardly using any oil leave alone.

The Franny Barnett however may need peping up a little.  New rings?

Ken.
1961 Super Rocket

Online Brian

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Re: Compression test readings
« Reply #9 on: 17.02. 2010 23:19 »
A couple of things here.

Firstly the AJAX, dont even think about it ! This was a thing that started back in the sixties specific to diesel engines. Back then it was common for diesels to glaze the bore, some genius came up with the idea of tipping ajax powder down the throat while the engine was running. It did have the effect of removing the glaze on the bores but halfed the life of the engine at the same time. There was also the idea of tipping water in the carby of a petrol engine while running to supposedly remove carbon, equally bad idea because if you tipped too much in it would bend conrods.

Now to the compression testing. There is a correct way to go about this. As Josh says the engine should be warm and have been running. You need a tester that screws into the plug thread, the ones with the rubber end that you hold are useless. In a twin cylinder bike remove the plug from the opposite cylinder, you dont have to do this but it makes it easier to kick over. Fit the tester to the cyl you are testing, hold the throttle wide open and kick the bike over as fast as you can about a dozen times. Repeat on the opposite side.

The actual reading can vary a lot but around 110 psi and above is usually fine, as Ken says its more important that both sides are close to the same. If both sides are the same but low then that indicates general wear, if there is a difference of around 15 or 20 psi then that indicates a specific problem with the low side, burnt valve etc.

At the end of the day if the motor is running well and using no or very little oil then leave well enough alone.

Offline muskrat

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Re: Compression test readings
« Reply #10 on: 17.02. 2010 23:34 »
G'day all,
             Just went down and checked the cafe. A10 with 10.5:1 +0.010" pistons, A7 head with big valves. Cold 115 psi, hot 170 psi. The gauge is correct.
Cheers
'51 A7 plunger, '57 A7SS now A10CR,  '83 CB1100F, .
Australia
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Offline BSA_54A10

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Re: Compression test readings
« Reply #11 on: 20.02. 2010 12:41 »
Quote
There was also the idea of tipping water in the carby of a petrol engine while running to supposedly remove carbon, equally bad idea because if you tipped too much in it would bend conrods.

Actually you are supposed to spray water mist into the throat of the engine running at about 1/2 to 2/3 throttle.
It was used (and still is by yours truely) to decoke 2 strokes and works very well when you get the hang of it.
It is of less use in 4 strokes as the carbon build up is substantially harder than the oily coking found in stokers and now days modern 2 stroke oils do not build up like the engine oils of old premixes.

You can introduce oxygen to the air flow to burn off the carbon but that runs the big risk of also burning the exhaust valves so like the Ajax it is more of a tart up bodge to sell a worn out motor than serious maintenence.
Bike Beesa
Trevor