Compression test question

My 1982 Seca 750 has been experiencing low power issues when trying to accelerate. I currently have the carbs removed to inspect, clean, etc. Did a full rebuild and clean a few years ago on the carbs. Bike was running good last year. Can I perform a compression test with the carbs removed? I was going to pull the two connectors to the TCI so I don't have to deal with grounding the plugs. If I am remembering correctly, the cylinder pressures have to be within 10% of one another? If you drop some oil in each cylinder and test again and see the pressure increase then rings may be the issue? If it stays at the same pressure value then maybe valve issues? If you see the pressure start to bleed off once you hit the max value when cranking the engine is that a sign of ring problems? Engine has about 17,000 miles on it.I have spark on all the cylinders. checked the resistance on the coils. They were within spec. Valves are in spec. Before I pulled the carbs I did a propane check around the inlet boots and around the carbs. No change in RPM so no leaks. Carbs were balanced and color tuned last year. Balancing was a bit of a pain. That's why I checked the valve clearances. Any other diagnostic procedures I should perform on the carbs before I drop them back in?

 

have you checked your air cleaner and replaced the spark plugs.
did you ohm out the plug caps and pickup coils too?

you can do it with out the carbs but it will be different than with carbs on
pulling tci is good too
The Information Overload Hour covers the how and what your numbers mean
XJ650 and XJ750 air-cooled engines:
Minimum: 128 psi
Standard: 156 psi
Maximum: 171 psi
Max. variance between lowest and highest: 14 psi

 

Pull all 4 plugs so engine has no resistence ( sure you know this already) if all readings are low then yes drop some oil and test again ... if still low try a different gauge.

 

Thanks guys for the info. Looked at the Overload information area on this website on compression testing. Going to try and get some time tomorrow to perform the test.

 

Jetfixer, i did a resistance check on both the coils and plug caps. I removed the plug caps when i tested the coils. All within spec.

 

he is not talking electrical resistance of spark plugs he is talking about cylinder compression resistance slowing the spin of the motor

 

Well, I got some time to perform the compression test. Below are my results. I did pull all 4 plugs when I did the test. There were also no carbs installed.
DRY:
Cylinder #1 - 145 PSI
Cylinder #2 - 145 PSI
Cylinder #3 - 150 PSI
Cylinder #4 - 145 PSI
OIL ADDED:
Cylinder #1 - 185 PSI
Cylinder #2 - 185 PSI
Cylinder #3 - 180 PSI
Cylinder #4 - 190 PSI
My pressure meter is a 300 PSI meter in increments of 5 PSI.
Dry values are below the standard 156 PSI spec but above the 128 PSI low value.

So reading some of the "Information Overload Hour" documentation is making me think I may have some ring/piston issues. The document did state trying to introduce 20-30 PSI of compressed air directly into the cylinder via the sparkplug hole and then try to determine if there is air introduced into the crankcase or through the valves into the exhaust or intake. What type of adapter would I need to connect a compressor into the sparkplug hole? Once you connect the compressor to the cylinder, set you maximum output on the compressor to 20-30 PSI, just pressurize the cylinder and listen for escaping air? Is this kind of a leak-down test?

 

Say what ...those are good readings for a 3 year old bike ...I personally would not mess with it. Just rebuild the carbs and ride.

 

only a 5 psi variance is great,

 

I will start working on the carbs. See if I can find any issues there. I may check valve clearances again. Really had problems trying to synch carbs when I put the new carb rack in two years ago. Thanks for everyone's advice.

 

What are you using when balancing/ synching carbs (Carbtune, 2-bottle, etc.)?
The Morgan Carbtune makes life easy.

I am broke, so made a homemade 2-bottle manometer.
I had some issues with too small of diameter hosing and couldn't synch for poop . . . went to a larger diameter and synching was easy.

 

I have a carbtune. Followed procedure in Yamaha manual. It was real time consuming. I would get one cylinder synched correctly with another and start to work on another cylinder and it would mess up a prevous cylinder. One thing that did happen was some of the rubber seals on the YICS shutoff tool melted on me. I thought I got all the bits out of the cross-tube. Hoping nothing got up in the little openings in the cross-tube. Used a makeshift pick and tried to probe those openings. Nothing came out.

One question I forgot to ask regarding the compression test. After I reached the max pressure value cranking the engine two out of the four cylinders I watched the pressure decrease pretty rapidly once I stopped cranking the engine. I made sure the hose assembly used with the pressure meter was seated good and tight in the spark plug hole. Should this be a concern?

 

The easy way to do a running synch is to not use the YICS blockoff tool. It's not needed unless you want a level of perfection that won't last but a few months.

The melting seals likely contributed to the trouble that you had while synching.

 

Know the order of the sync screws. Set the idle with the idle knob. Adjust 1&2 with the screw between carbs 1-2. Then adjust 3&4 with the screw between 3&4. Finally, sync 1-2 to 3-4 (left side to right side) with the sync screw in the middle.

If at any point in your sync your idle changes, adjust it back to normal idle (to make sure you are using the correct fuel circuits). The direction you need to turn the screws is different for one, make a big adjustment and draw a diagram so you know which way you want to go. From there the adjustments are small (1/16 of a turn or less). If using a YICS tool, oil it up and make sure to remove it after each sync.

That's your gauge leaking. The gauge has a one-way valve, so that each time the cylinder builds pressure, it's "saved" in the gauge. This is necessary because the gauge adds "volume" to the cylinder, and is the reason why you have to crank it over several times to build pressure up to the spec. That one-way valve is slowly leaking, such that you are getting a reading (and a good one that that with those numbers!) but it doesn't hold the reading for very long.

 

Guys, thanks again for your input. When I get some time I'll go through the carbs again, get them back on the bike, and try to synch and colortune the carbs. See if she runs better.