No compression

jmspen2

New Member
I’m hoping someone on here has a bright idea because I’m going insane trying to figure this out. So here’s the lowdown. Rebuilt the top end of a 1982 cb900c. New piston rings, gaskets, repainted and shined up real nice. Didn’t pull the valves. Replaced the tappets to spec in each valve, put it together and it won’t turn over. Shooting flames out the carbs and backfiring. Check the compression and no cylinder is even reading on my gauge (starts at 15psi). Checked ignition and cam timing and it’s all correct. Dropped oil down the cylinder and no change. At the 1,4 position the #1 lobes are pointing toward the spark plug, the marks on the sprocket are horizontal with the case, I’ve checked the manual 100 times to make sure the timing is right and it is. I’ve even gone through and made sure the valves open (they do) and that they are on the correct stroke with the valve positions (they are). I’m completely stumped. As are my father and brother in law (who actually build diesel and race engines for a living). Any bright ideas??

Thanks,

Jason


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cxman

Active Member
DTT SUPPORTER
do a leak down test bet your valves are not closing properly either from the tappets or the b chain timing or something

wrong head gasket ect

a leak down test will show you
 

Nebr_Rex

Member
Why did you 'rebuild' the top end? And if you didn't pull the valves and recondition the face of the valves and the seats you didn't do a complete job. An engine with rings and valves that seal and cam and ignition timing that are correct will have compression and will not backfire thru the carbs.


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SONICJK

Reminds me of...me No, I'm sure of it. I hate him
I second a leak down test.

Quick and ghetto, put a cylinder at tdc on compression stroke and hold it there. Add compressed air to the sparkplug hole and listen.

If it's bad enough to not register any compression then you'll hear air surfing out the valves.

Pull it back apart :)
 

jmspen2

New Member
Even if the valves aren’t seating properly, or I have an incorrect head gasket, or hell if I even left off a gasket (I didn’t) should I at least be getting some kind of compression?? I’m not getting any, whatsoever.


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jmspen2

New Member
I shouldn’t say any, I’m getting puffs of air out the spark plug holes but not enough to register...


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teazer

Well-Known Member
DTT BOTM WINNER
SONIC. said:
I second a leak down test.

Quick and ghetto, put a cylinder at tdc on compression stroke and hold it there. Add compressed air to the sparkplug hole and listen.

If it's bad enough to not register any compression then you'll hear air surfing out the valves.

Pull it back apart :)
As long as you can hear over the noise of your compressor, that's a great way to determine where the air is leaking out. If you are using a Harbor Freight or ebay cheap compression tester, they are not very reliable and often read below 30psi.
 

SONICJK

Reminds me of...me No, I'm sure of it. I hate him
Put your finger over the spark plug hole and crank it over.
If it blows your finger off the hole you're probably ok and something else is off.
If you can hold your finger over it while cranking over you're forked
 

cxman

Active Member
DTT SUPPORTER
Ichiban Moto said:
should I make a video on leak-down testing ?
yes but clearly show how the cans of compression are added when the test is over

to many get that mixed up
 

teazer

Well-Known Member
DTT BOTM WINNER
. +1. I never know how many cans to add after that test. And all bikes are different in the plumbing department, so where is the low pressure valve?

And be sure to focus on techniques to maximize that leakdown that is being measured........
 

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