Compression test DOHC

[FastCletus]

just did a compression test
1=78
2=80
3=75
4=78

Is that good or bad.

I had choke all the way out, throttle wide open.

[TOOLS1]

Those numbers are low, however since they are, so close to each other I would say that the gauge you used is off. Find another gauge, and do another test.
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[shinyribs]

I'm with 100%. Those numbers are way low. Under 100 psi is usually considered a non-runner period. 100-125 psi is about as low as you can go and an engine fire properly.

I'd definitely try and get my hands on another gauge.

Are the valves adjusted within spec? Did you try for wet numbers?

[FastCletus]

Just redid this test.  compared my gauge to my air compressor.  90 on the compressor was 80 on the gauge.  1-85
2-92
3-75
4-72

The bike runs and is ok to drive, starts easy, plugs look normal, but the bike just seems slow.  Engine loses pull at around 5K rpms.

BAsed on what I have read these numbers are way low and probably dumbfound alot of you that it runs.  

What do you think?  Head gasket?  Rings?  Valves? or all of the above.  

[TOOLS1]

Main jetting. Tr increasing the size of the main jet before doing major engine work.
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[seestheday]

I still say that your gauge is wrong.  Bike shouldn't run with numbers that low.

That said, I would guess it's carbon build up on your valves before rings going bad.

[Hoosier Daddy]

FastCletus wrote

just did a compression test


I had choke all the way out, ...throttle wide open.

Did anybody catch this?
FC you do not want the choke pulled out, you want it off, as in no choke, knob pushed in.
The idea is to get as much air as possible, unobstructed, into the engine for the pistons to compress. So the best reading would be obtained if the carbs were completely removed.
Hope that was the solution.

[FastCletus]

I thought the technique in doing this was the Choke out?  meaning pulled out like I was going to start the bike?

I really hope you are all right and I am wrong.  I dont want to park this bike for the summer and be fixing engine,  (was going to do that in winter with paint etc.)

Think I will try the jet kit.  I have heard a few people say that after adding a 4 into 1 pipe a jet kit is necessary.  Just havent felt any carb issues, so was thiking something more complex.

If the valves have a carbon build up, any way to remedy it without removing heads?

Havent checked valves yet, but engine sound doesnt indicate a valve issue

[seestheday]

Yep, the idea is to let as much air in as possible. Pulling the choke closes the butterfly valve to let the mixture get richer. You could also pull the carbs and take the measurement. Same result. You're using the starter motor to turn the engine over about 5 times right?

[Hoosier Daddy]

FastCletus wrote

I thought the technique in doing this was the Choke out?  meaning pulled out like I was going to start the bike?

I really hope you are all right and I am wrong.  I dont want to park this bike for the summer and be fixing engine,  (was going to do that in winter with paint etc.)

Trust me on this, if you pulled the choke, that is the issue with your compression readings.
Do it again, this time with the choke off... knob pushed in... fully open... however you want to say it. Let her inhale and squeeze that big breath of air. That will get you an accurate reading.

FastCletus wrote

Think I will try the jet kit.  I have heard a few people say that after adding a 4 into 1 pipe a jet kit is necessary.  Just havent felt any carb issues, so was thiking something more complex.

If it ain't broke, don't fix it. Adding a 4 into 1 if it has a deicent street silencer (muffler) shouldn't require a jet change. Now if you are running an open header or a exhaust without baffles, that's a different story.

FastCletus wrote

If the valves have a carbon build up, any way to remedy it without removing heads?

Havent checked valves yet, but engine sound doesnt indicate a valve issue

Yeah, there are a couple old school shade tree mechanic remodies,  But I bet once you do the compression test properly, your engine will be in better shape than you thought.

[shinyribs]

Good eye Hoosier!

[FastCletus]

Fabricated my own leak down tester using my air compressor (yes i am proud of myself).  I set the compressor on 60psi.
I inserted the hose that goes into the head and then connected the compressor hose to it.  Boom now I have 60psi going to the cylinder.  Heres what happened
Cylinder 1 -spun the engine
cylinder 2 leaked
cylinder 3 spun the engine
cylinder 4 leaked

What do you think?  I think this might coincide with my valve issue?

[TOOLS1]

Did you cycle the engine to the compression stroke before putting the air to each cylinder? It only works if the cylinder is on compression stroke.
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[FastCletus]

I set each cylinder to TDC prior to adding compression.  One at a time

[TOOLS1]

TDC on compression, or exhaust?
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[FastCletus]

hmm  How does one determine which is which?

[shinyribs]

The reason for TDC on the compression stroke is just to make sure the valves are closed. I'm not familiar with the DOHC valvetrain,but on a SOHC you can just slack the adjusters off to ensure they are closed. Not sure if that is an option for you or not. If not,your best bet would be to pull the valve cover and visually inspect that the valves are closed. JMO,of course.

Did you ever recheck the cranking compression with the choke and all open? If it reads good there may not be any need for a leak down test.

[seestheday]

On the DOHC the cam lobes open and close the valves.  No adjusters to slack.  It'll be easiest to just take the valve cover off and look at the lobes.  

[FastCletus]

Havent rechecked compression with choke open.  Working on valves now, and I have cover off.  Just trying to figure out now how to check the valve lash.  LOBE up or on its side.  Keep hearing contrasting sides

On 5/7/2013 8:28 PM, shinyribs [via Honda CB750'S] wrote:

The reason for TDC on the compression stroke is just to make sure the valves are closed. I'm not familiar with the DOHC valvetrain,but on a SOHC you can just slack the adjusters off to ensure they are closed. Not sure if that is an option for you or not. If not,your best bet would be to pull the valve cover and visually inspect that the valves are closed. JMO,of course.

Did you ever recheck the cranking compression with the choke and all open? If it reads good there may not be any need for a leak down test.

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[seestheday]

Lobes on their side facing out (away from eachother) will give you the best results.

This is TDC during compression and is the most critical time that the valves must be sealed.  It is for this reason that you need the clearance.

Lobes facing up may be "good enough" but if you're trying to get every bit of performance out of your engine you want to do it when they're facing out.

This is also what the FSM says (although it's not explained well).

Article on the "F" site that explains it: http://www.cb1100f.net/modules.php?name=Forums&file=viewtopic&t=70625&highlight=cam+lobes