Topic: Compression Readings

[Steven.Burns]

I'm using a Harbor Freight compression gauge.  My bike is running really #$%*ty and I know I need a new ignition but I wanted to check my compression too.  I checked my compression and #$%*!  I dont know much about this stuff but I'm pretty sure it's definitely not good.  I did a dry test and wet test on each cylinder.  For the wet test, I put about 1 teaspoon of oil in each cylinder using a cooking syringe and cranked it slowly 3 times for each cylinder.  I had my bike run for a few minutes to warm her up.  Here's what I got:
#1: Dry- 76,  Wet- 83
#2: Dry- 75,  Wet- 95
#3: Dry- 70,  Wet- 115
#4: Dry- 78,  Wet- 112

Doesn't look too good does it?  Let me know what you guys think
Also I need a new ignition.  My stock one is all the way to Retard and it's still too advanced.  I plan on getting Hondaman's Ignition but do I need to get a new points ignition too?

Let me know what ya think

[MoMo]

How are you checking compression?  Engine cold with throttle and choke open?...Larry

[TwoTired]

Let me know what ya think

I think you don't know the fundamentals of compression testing.  I wrote a short schpeil that got placed in the FAQ.

In addition, you haven't calibrated the test gauge, have you?  Do we assume you had the choke off and the throttle fully open during the test?  If throttle was closed, a carb slide imbalance can effect the numbers.

Without a calibration and knowing just how much volume your tester adds to the combustion chamber, the only thing your tester gives you is comparative data among cylinders.
Yours are all near 10% of each other in the dry test, which is good.  If you were accurate when adding oil volume to the chamber (I'd guess not), then your wet readings look like a bit of nonsense.

But, assuming they are real:
#1 and #2 has bad valves
#3 and #4 have bad rings.

Does the bike idle/run on all 4 cylinders?

"Running #$%*ty" tells me more about your capability than than the bike's condition, I'm afraid.  There are some significant details missing.

It's rather unusual for a Honda in regular service to have bad rings, if it's had any sort of regular maintenance.  Sabotage, abuse, and neglect can change that, of course.
The 77 CB750F did have a history of quick wear valve guides.  But, this was represented by oil smoke from the exhaust.  Perhaps that's a part of your "running #$%*ty" descriptor?

A compression test can be significantly altered by not having the tappets adjusted equally among cylinders, FYI.

I wouldn't condemn the engine or take it apart with only the information given.

Cheers,

[Steven.Burns]

I never calibrated the Guage and I don't know how to do that either. I did however test the compression with the choke off and throttle all the way open. As for the oil, I got pretty damn close to the same in each cylinder with the syringe. I'll test everything again tomorrow if I have enough time.
What I mean by running #$%*ty is that it shutters and revs very eratic. It starts perfectly fine with the choke on but it doesn't want to run with the choke off. When I get on the throttle it has major loss of power and it boggs out then revs up erratically. I don't see any black smoke so I guess it's not rings. Either way I need to get new points. Since I want to get the hondaman ignition, will I need to get new points ignition too?

Thanks, Steven

[MoMo]

If compression were actually those readings, your bike would not run-so I would discount those readings and assume that you have adequate compression  because all cylinders are within 10%.  Your description of lousy running leads me to midrange jets.  Were the carbs cleaned?  If so, were the midrange jets removed and all orifices checked for blockage as well as making sure the slide needles are not loose?

Are the points set between .012 to .016 and fire at the  proper time, the F mark, and advance between the 2 lines? ....Larry

[TwoTired]

What I mean by running #$%*ty is that it shutters and revs very eratic. It starts perfectly fine with the choke on but it doesn't want to run with the choke off. When I get on the throttle it has major loss of power and it boggs out then revs up erratically.

Sounds like pilot jet clogging and or accelerator pump system problems.
You know, these are supposed to have a complete tune-up every 3000 miles.
How old are spark plugs?
What do the deposits on the tips look like?
How old is the air filter?  Honda states to replace them yearly.  (Some think they know better than Honda.)
You can't expect it to run well if you can't get the points and timing correct.  Check the FAQ for out-of-range solutions.

I don't see any black smoke so I guess it's not rings.

If it were rings or valve guide issues, the smoke would be blue from the exhaust.

Either way I need to get new points. Since I want to get the hondaman ignition, will I need to get new points ignition too?

New, clean points are a good idea, given I have no idea what condition your existing points are.  I know how to recondition them, so they work for many thousands of miles on my bikes.  But, most don't seem to care to make that effort, or acquire the tools to do so.
If you are asking about new coils for the HM ignition, it's not a requirement, unless you can say for certain something is wrong with what you have.

Cheers,

[dave500]

hey steve,,a few psi out isnt here nor there,hose length isnt an issue either,any piston pump will pump up any length hose.


your readings are low,but an engine will run with lower pressures even,i wouldnt worry about the compressions untill the rest is sorted,,ignition first of all i think is your sluggish running,im not clear when you said it was cranked three times,,like as in three compression strokes??keep cranking untill the guage stops rising,,maybe five /six or seven"pumps".

[TwoTired]

hose length isnt an issue either,any piston pump will pump up any length hose.

The hose length is a lesser issue than where the check valve is placed, as any length added before the valve effectively adds to the combustion chamber volume, which changes the compression ratio of swept piston volume to the final compressed volume.

I did explain this in the FAQ, btw.

Cheers,

[w1sa]

My compression test gauge (bought from an auto-shop) came with one flexible line connection and one hard stand tube connection.......both of these connection apparatus have  'one-way check valves'  in the ends that connect to the spark plug hole, as well as the check/release valve at the gauge, all of which (accoding to my leak tests) hold introduced line pressure until released at the pressure release valve at the gauge...............I am confident the pressures I measure with this gauge are accurate to within the allowed instrument error of the gauge, with no measurable concern for errors due to line volumes.

I think every-one should check the function of their gauges/connections and re-think this procedure, before they assume the readings they get are flawed due to line volumes of the connection tubes etc.