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What is the propper procedure for compression check?
- Thread starter cybrhick
- Start date
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- Tags
- engine question
littlebill31
Smells of Raw Fuel
From the Maintenance and Troubleshooting thread in the "Tech" section.
"Compression tests will tell you if your engine has good compression...an engine is essentially a self-powered air pump, and needs good compression to run efficiently, cleanly and for easy starting
- in general, most engines should have 140 to 160 lbs. cranking compression with no more than 10% difference between the cylinders
- low compression in one cylinder usually indicates a bad exhaust valve
- low compression in two adjacent cylinders usually means a blown head gasket
- low compression in all cylinders indicates worn rings and cylinders - engine needs to be overhauled
- check compression manually with a gauge
- all the spark plugs must be removed
- disable the coil or ground the high tension lead
- electronic ignition, disabl the coils to prevent them from firing
- hold the throttle open
- crank the engine for a few seconds holding a compression gauge in a spark plug hole
- note the maximum compression reading, repeat the process for the other cylinder
- compare the individual cylinder readings to see if the results are within specifications, 10% difference is ok...150lbs is great, under 100 is marginal...be aware that xsive carbon build up will distort readings upwards".
I leave the ignition off and kick the engine instead of using the starter to prevent frying the coil by accident.
"Compression tests will tell you if your engine has good compression...an engine is essentially a self-powered air pump, and needs good compression to run efficiently, cleanly and for easy starting
- in general, most engines should have 140 to 160 lbs. cranking compression with no more than 10% difference between the cylinders
- low compression in one cylinder usually indicates a bad exhaust valve
- low compression in two adjacent cylinders usually means a blown head gasket
- low compression in all cylinders indicates worn rings and cylinders - engine needs to be overhauled
- check compression manually with a gauge
- all the spark plugs must be removed
- disable the coil or ground the high tension lead
- electronic ignition, disabl the coils to prevent them from firing
- hold the throttle open
- crank the engine for a few seconds holding a compression gauge in a spark plug hole
- note the maximum compression reading, repeat the process for the other cylinder
- compare the individual cylinder readings to see if the results are within specifications, 10% difference is ok...150lbs is great, under 100 is marginal...be aware that xsive carbon build up will distort readings upwards".
I leave the ignition off and kick the engine instead of using the starter to prevent frying the coil by accident.
TeeCat
One-Mik Wonder
cybrhick -
Bill and I cross-posted (thanks, Bill!), but anyway...
Per Leo:
Do perhaps three tests per cylinder and take the highest reading, iirc. Make sure you have the fire off and the throttle wide open when you kick.
TC
Bill and I cross-posted (thanks, Bill!), but anyway...
Per Leo:
Do perhaps three tests per cylinder and take the highest reading, iirc. Make sure you have the fire off and the throttle wide open when you kick.
TC
jayel
#9 Guru 74 TX650A
first check that valves are set correctly then remove both sparkplugs,(makes it easier to test each side) prop the slides open, throttle butterflies full open (or just remove entire carbs), put on compression guage 3 to 4 kicks to build compression reading or use E-start, anything below 100psi is poor probably won't start 100/120 marginial 120/140 useable 140+ good 150/160 excellent, if low compression add a small amount of oil to the cylinder if compression goes up rings are suspect
Attachments
I am Carbon
shade tree mechanic
to get all the air in as possible. if the butterfly is closed not much air
cant get any wider open than no carbs
A compression gauge that has a disconnect is nice, if compression is low turn your compressor air pressure all the way down. Turn the engine to TDC on the compression stroke. hook your air hose to the tester stem and slowly turn up the air listen at carbs exhaust and oil filler hole to hear where the air is going. Repeat for the other side. Keep the bike out of gear or on the center stand, the air pressure may spin the crank.
HooliganCycles
XS650 Addict
That is a leak down test not compression check
"That is a leak down test not compression check" Hooligan
Other posters answered the compression test question, but if you do a compression test and it's not great, locating the leak is next. This is not a true leak down test which requires yet another fancy $$$ gauge but a simple DIY test that requires no additional investment. I kinda thought a guy that posted asking how to do a compression test might be interested in this procedure?
Other posters answered the compression test question, but if you do a compression test and it's not great, locating the leak is next. This is not a true leak down test which requires yet another fancy $$$ gauge but a simple DIY test that requires no additional investment. I kinda thought a guy that posted asking how to do a compression test might be interested in this procedure?
HooliganCycles
XS650 Addict
gggGary, I totally agree, I wasn't trying to argue with your post, just letting him know what you described was doing a leak down not compression, sorry for the confusion
Old thread but good info here. I'm assuming it's best to have exhaust off as well? I have my engine in an engine stand. No carbs, no pipes or crankcase breather. I obviously can't do this with my engine warm, but am assuming I can get a good reading none the less. Still lots to learn but curiosity is getting the best of me. Kinda want good readings, kinda want bad so I can do my first rebuild.
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angus67
Welder's penetrate deeper!!
everyone wants good readings, but if you find you have to rebiuld, these engines are easy. All the bearings are cage rollers, not alot can go wrong. The most sensitive part for a first timer is rebiuld of the cylinders/pistons. Buying a upper end kit, and taking the new cylinders and pistons to a good shop, and haveing it all inspected is best. I took my kit to a shop, didnt cost much for them to mic everything so it would all play nice together.
I needed a good crank assembly, found one for 80$ had good rods, low miles, (supposedly)bearings spun smooth. Didnt have to pull my trans apart, so basically, I just replaced rebiult the rotating assembly, and did a 1.5 over rebore. Rebiult the head with new valves, giudes were still good, fifty for the head rebiuld, and the piston kit was i think 500$.
I needed a good crank assembly, found one for 80$ had good rods, low miles, (supposedly)bearings spun smooth. Didnt have to pull my trans apart, so basically, I just replaced rebiult the rotating assembly, and did a 1.5 over rebore. Rebiult the head with new valves, giudes were still good, fifty for the head rebiuld, and the piston kit was i think 500$.
XS650D
XS650 Junkie
Don,t get to caught up in the numbers,i have 2 compression gages,1 reads 134 on both sides hot and the Snap on Gage reads 154 both sides hot,both gages r fairly new,they do fluctaute some also certain years had higher or lower cpmpression numbers,early 650,s had 150 lbs were the middle 70,s units had 136 as per my manual,mine is a 77 with a rebuild (new rings and valves seals ect) Had a 1980 G which had 150 but it would ping on hot days regardless off fuel type or carb jetting,my 77 never pings regardless off how lean i run it or fuel type.something to be said for lower comp,less running issues,yes there is is a small power loss with less comp all things being equal but i dont like to here pinging,engines don,t like it!
