How Do I Do A Compression Test?

[gasgasssser]

After eliminating all external possibilities for the cause of a splutter at very small throttle openings on my 290,i think the next step is to look inside the engine.Today i changed the trans oil,thinking maybe its escaping via a dodgy seal and affecting the compression/suction.The oil was pink/cloudy.This is my first oil change on this bike.I`m thinking water in the oil,but rad is full.

How do i do a compression test on my sherco 290?,I am now convinced that there is a loss of suction,which is in turn causing the idle screw to be wound right in to maintain tickover.I think beacause of this,the mixture is rich when the throttle is opened causing the splutter.Am i making sense??

Cheers

Steve

[how61]

Morning Steve,

the used ATF that you drain from the gearbox will be cloudy pink colour anyway. Have you checked that the jets are clean in the carb ? You will experience a similar sort of problem to that which you are describing if the pilot jet is blocked, may be worth a look!

regards

Howard

[gasgasssser]

Helieve me Howard,after extensive checks/double checks/tripple....all external parts/electrics are functioning as they should.

I need to test the crankcase pressure in my endless search for the cause of the fault.

Thanks

Steve

[alan bechard]

Do you want to check compression or crankcase seals?

Have you thought along the lines of having someone else look at the bike as well? Sometimes a second set of experienced eyes can save you a lot of hassle.

[gasgasssser]

My initial thought is to check the crank pressure.I have changed the seal at the ignition side,the clutch side is not as easy.A crank case seal test seems a good way of checking.If i cannot fix it my self,it will go to a specialist,but the are not cheap,so i am trying to eliminate as much as i can

I`m pretty sure ther is no loss of compression regarding the piston.My friend has the same bike.There isnt any difference when trying to kick them up.

Steve

[alan bechard]

You can make up your own set of block off ports, find an ultra low pressure gauge, use something like a hand bicycle pump, or a blood pressure cuff pump to pump up the pressure (I think it is like 1.5 lbs but not positive) and check the seal that way,

For me, I would just change the seal's and gaskets out, or have the pressure test done by the someone that does them regularly and has the kit all together to check them. I have a fair amount of shop stuff and would still take me a bit to assmeble the pieces and get it working.

Before I did anything like that though, I would again (I know you said you did it already) go back through all the basics and make sure I had not missed anything.

Pull Carb, clean carb, check fuel flow, check jets, check #s, check air filter, carb seated well, carb sealed well, reed cage good, reed boot look ok, clean new fuel, clean tank, fuel filter in tank and carb, new spark plug, check wiring / cap, fresh oil change, AF up to speed, just over the fins, timing correct, flywheel seated right, etc. etc.

I would spend an hour or two just going back over details to make sure I had not missed anything.

I would probably also get someone else to watch over my shoulder with me to make sure I had that second set of eyes on it.

If all that fails, and you want to check the seals, I would take it by a shop that does it regularly (sometimes Chainsaw shops do it more regularly then MC shops) and see what it would cost to have them do it.

[sw74gls]

Cylinder Compression Pressure Table Bob Spidel and Harlan Polk Per Wikipedia: If the nominal compression ratio of an engine is given, the pre-ignition cylinder pressure can be estimated using the following relationship: where is the cylinder pressure at bottom dead center (BDC) which is usually at 1 atm, is the compression ratio, and is the specific heat ratio for the working fluid, which is about 1.4 for air, and 1.3 for methane-air mixture. (But about 1.3 for air-gasoline after accounting for engine heat.) For example, if an engine running on gasoline has a compression ratio is 10:1, the cylinder pressure at top dead center (TDC) is This figure, however, will also depend on cam (i.e. valve) timing. Generally, cylinder pressure for common automotive designs should at least equal 10 bar, or, roughly estimated in pounds per square inch (psi) as between 15 and 20 times the compression ratio, or in this case between 150 psi and 200 psi, depending on cam timing. Purpose-built racing engines, stationary engines etc. will return figures outside this range. Factors including late intake valve closure (relatively speaking for camshaft profiles outside of typical production car range, but not necessarily into the realm of competition engines) can produce a misleadingly low figure from this test. Excessive connecting rod clearance, combined with extremely high oil pump output (rare but not impossible) can sling enough oil to coat the cylinder walls with enough oil to facilitate reasonable piston ring seal artificially give a misleadingly high figure, on engines with compromised ring seal. This can actually be used to some slight advantage. If a compression test does give a low figure, and it has been determined it is not due to intake valve closure/camshaft characteristics, then one can differentiate between the cause being valve/seat seal issues and ring seal by squirting engine oil into the spark plug orifice, in a quantity sufficient to disperse across the piston crown and the circumference of the top ring land, and thereby effect the mentioned seal. If a second compression test is performed shortly thereafter, and the new reading is much higher, it would be the ring seal that is problematic, whereas if the compression test pressure observed remains low, it is a valve sealing (or more rarely head gasket, or breakthrough piston or rarer still cylinder wall damage) issue. If there is a significant (> 10%) difference between cylinders, that may be an indication that valves or cylinder head gaskets are leaking, piston rings are worn or that the block is cracked. If a problem is suspected then a more comprehensive test using a leak-down tester can locate the leak. Compression Ratio Estimated Cylinder Pressure @ TDC given 60% volumetric efficiency at cranking speed Percent Change from 8:1 CR Nominal Dynamic* PTDC = 1atmosphere · volumetric efficiency · nominal CR1.3[corrected air-gasoline specific heat factor]= for 10:1 CR & 60% VE, then 14.7 psi · 0.6 · 25.119 = 176 psi – 14.7 psi = 161 psi gauge reading 7:1 4.2:1 96 psi gauge reading -18% 8:1 4.8:1 117 psi gauge reading 0% 9:1 5.4:1 139 psi gauge reading 19% 10:1 6.0:1 161 psi gauge reading 38% 11:1 6.6:1 184 psi gauge reading 58% 12:1 7.2:1 208 psi gauge reading 78% * at 60% volumetric efficiency at cranking speed, primarily due to valves open more than 180 degrees duration.

[tony27]

You say your friend has the same bike, see if you can substitute his carb for a quick test to see if it makes a difference, will help eliminate the carb as a possible cause & costs nothing

[jukitokek]

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