significance of same cylinder-pressure

The Force power

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What would be the long term effect of running 3 cylinder with 2 having 125 psi & 1 having 145 psi
 

pnwboat

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Depends on what is causing the difference in pressure. If it's a head gasket, then it may eventually fail compltetly. If it's cylinder wall/ring seal, then the motor may not run at peak performance, but will probably run for many years if maintained.
 

The Force power

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Depends on what is causing the difference in pressure. If it's a head gasket, then it may eventually fail completely. If it's cylinder wall/ring seal, then the motor may not run at peak performance, but will probably run for many years if maintained.
Nope it's a new head gasket
I'll explain; there's two used pistons/rings and one new in it, that's what's causing the different in pressures.
 

Nordin

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The force power I think it will act as pnwboat second explanation.
It will run with a bit lower performance but will last for many years if you maintain it well.
A rule of thumb is that no more then 10-15% difference between the cylinders.
If more, there will be problem to tune the engine and more then 20% it will run bad.
I have done an "easy" rebuild of an Chrysler 70Hp 1969.
Top cylinder had low compression compared to the two other.
About 100 PSI compared to 135 PSI at number two and three.
The top cylinder was a little bit scored, I honed it and put a used piston in and the compression increased to 120-125 PSI and it runs pretty well.
But I think it would perform better with higher compression in the top cylinder.
 
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jerryjerry05

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The low pressure really doesn't make any difference.
When the pressure gets way off past 10-15% then it can cause
starting problems.
The uneven pressure makes the motor real hard to turn over.
One of my customers has a 2005 200 Yam.
Top cyl. has low comp and lightly scored.
Yet runs like it has no problems at all???
But that's a V-6 The inline2= 3=4=5 cyl.is affected more by
low pressure.
Run it.
 

Redbarron%%

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You are measuring pressures from idle with no ignition. There is vastly higher pressure when running.
By the way the three cylinder has stock a different reading . From the service manual:

NOTE On 90 hp models, compression on the No. 1 cylinder is normally 5-10psi (34.5- 69.0 kPa) less than the No. 2 and No, 3 cylinders. Refer to Table 2.
A variation of more than 10-15 psi between 2 cylinders indicates a problem with the lower reading cylinder, such as worn or sticking piston rings and/or scored pistons or cylinders. In such cases, pour a tablespoon of engine oil into the suspect cylinder and repeat Steps 3 and 4. If the compression increases significantly (by approximately 10 psi on older engine), the rings are worn and should be replaced. Some 2-stroke engines are plagued by hard starting and generally poor running for which there seems to be no good cause. Carburetion and ignition check out satisfactorily and a compression test show that the pistons, rings and cylinders are in acceptable condition. What a compression test does not show is lack of primary compression. In a 2-stroke engine, the crankcase must be alternately under high pressure, then low pressure. After the piston closes the intake port, further downward movement of the piston causes the entrapped fuel and air mixture to be pressurized so that it can rush quickly into the cylinder when the transfer ports are opened. Upward piston movement creates a lower pressure in the crankcase, enabling the fuel-air mixture to be drawn into the crankcase from the carburetor.
 

The Force power

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When the pressure gets way off past 10-15% then it can cause
starting problems.
The uneven pressure makes the motor real hard to turn over.
There were my thoughts were going as well...the momentum would be thrown-off as there are un-even forces on the crank????
 

racerone

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So do motors with built in compression relief ( lower compression ) have starting problems ???
 

jerryjerry05

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Force made a 120 with uneven compression on 2
of the 4 cyl.
The top/bottom was 155 and the middle two were 145
It didn't make any difference in the way it turned over.
But if one cyl. gets beyond 25% it can make it really hard
to turn over, this affects most of the 3 and 4 and 5 cyl.
motors.
 
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