Compression check

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Beach

Beach

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Checking the compression on 850 commando and after 3 kicks each side reads 130 and 135...I figure this is ok but first kick reads about 90 ,second kick 105 and third about130....Just wondering if this procedure is correct?
 
Yep WOT and a few fast kicks needed to pump up enough to get idea if sealing well enough as yours seems too. Its more a relative comparison test to monitor sealing effectiveness than absolute real compression pressure measure that happens on first firing. Supposed to be done with fully heated engine. Putting cold oil inside jug will give some idea of ring blow by sealing loss.
 
Beach said:
Checking the compression on 850 commando and after 3 kicks each side reads 130 and 135...I figure this is ok but first kick reads about 90 ,second kick 105 and third about130....Just wondering if this procedure is correct?
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I would kick it as many time as you can (maybe 10 to 12 time) and as fast as you can within reason. Also, the throttle must be held wide open.

Kicking it only three time won't give you a true reading. It takes 5 to 6 time just to get the pressure up.
 
Are you testing with the engine HOT with the throttle wide open.

Compression check


Testing cold is a little less reliable, especially if the engine hasn't been run for a fair while.
Tip a spoonful of oil down each plughole, and see if the readings increase much.
If they do, the rings may be worn a bit, or just simply dry...
 
Just to add, 130/135 are not bad numbers, and they way you get them says that you are better than those number indicate.
 
yes on all the advice about warm and repeated kicked w/ throttle open etc, but worry less about the reading (unless its incredibly low) and more that they are within about 10% of each other - i have 2 testers - one gives me about 145 on my commandos and the other about 175- both read about 8% from cylinder to cylinder
 
If you really want to find out what is going on and pinpoint an area, valve or ring seating a leak down tester is good also.
 
I am so hoping to see video of anyone [but me] doing a leak down test on a Commando, ESPECIALLY the part pistons are to be moved down off TDC a bit where maximum combustion pressure is supposed occur and rings to seal for best torque force into drive train. We should all hope to have such good pressures as reported in this case so rings and valve seating more than adequate. On the other hand leak down tests done enough times to average out all the errors, gives a percentage number that some time down the road can repeat to access life left.
 
ya don't need a video Hobot. Make a tool from an old spark plug that has an aro fitting brazed to it. remove air filter. bring engine to tdc on compression stroke, put in top gear. jam foot brake on and apply air hose from compressor. [ someone elses foot on the brake pedal is easier]. listen to exhaust, listen to carb with bit of plastic hose. listen to oil tank with oil cap removed. You Know The Rest.
 
Oh I got Trixie pressurized and so balanced on TDC with guages listening to leak out intake drain hole indicating rings shot, to not need much holding force and got an absolute TDC marked by this, then tried it slightly off TDC to suddenly find top gear and friend standing on rear brake wasn't enough. I'd only used 80 PSI. I don't feel like much a mechanic to learn any more about leak downs on Commandoa. My respects to those more capable.
 
If you havnt got a electric starter , coasting down a steep hill in 2nd gear , plugs out will do the trick . If its long enough you get both sides before you push it up again . :twisted: :twisted: :wink:

Hot Engine , so take it for a spin first .
 
Those numbers were with the engine cold and throttle wide open...
 
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