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Compression
Posted: Sat Aug 05, 2023 7:13 am
by sky02
Hi,
I’ve recently checked compression on my bike. She pulls hard, start normally but she is 30yrs old so I wanted to know the condition of engine. Mileage is 47k but maybe 147k who knows.
Both cylinders are around 140psi. Completely cold and after a good warm up the reading is the same. According to manual it’s on the edge of rebuilding.
I have added 5ml of engine oil to the cylinders and it risen by about 10psi. So its not rings.
The gauge is not an expensive one but with valve at both ends of the tube so there is no tube error.
What do you guys think about those readings.
Re: Compression
Posted: Sat Aug 05, 2023 7:32 am
by Designer
Keep Ridin'
These motors are pretty much "bullet proof" and have a proven history of going over 160,000 miles without a rebuild.
Ask our resident Longevity Expert,...Herb.
He knows
Re: Compression
Posted: Sat Aug 05, 2023 7:48 am
by Herb
I ran my 87 for over 154K with no engine problems.
It is really hard to get a good compression reading on these engines because the slides don't allow a lot af air in.
I did a compression check at 100,609 and could only get a reading of 120, pulled the slides from the carbs and redid the test got 165.
Designer is correct, ride it and have fun.
BTW folks, except for the break in and initial dealer service it NEVER had anything but car oil. The lowest cost 10W40.
Re: Compression
Posted: Sat Aug 05, 2023 9:28 am
by Lechy
When doing a compression check the throttle needs to be wide open.
Re: Compression
Posted: Sat Aug 05, 2023 12:16 pm
by FallenAngel
sky02 wrote: ↑Sat Aug 05, 2023 7:13 am
Hi,
I’ve recently checked compression on my bike. She pulls hard, start normally but she is 30yrs old so I wanted to know the condition of engine. Mileage is 47k but maybe 147k who knows.
Both cylinders are around 140psi. Completely cold and after a good warm up the reading is the same. According to manual it’s on the edge of rebuilding.
I have added 5ml of engine oil to the cylinders and it risen by about 10psi. So its not rings.
The gauge is not an expensive one but with valve at both ends of the tube so there is no tube error.
What do you guys think about those readings.
For a 30 year old bike 140 psi cold and warm is pretty good
Engine should be running smooth and well balanced with those reading increase of 10 psi with oil is a good sign as well
Re: Compression
Posted: Sat Aug 05, 2023 2:40 pm
by sky02
It was WOT as it should be. I was just wondering what are real life readings. Service manual specify that it should be tested on warm engine with WOT and it should be 142-199 psi. Nothing about taking the carbs off the bike or the slides to make it easier to pump.
Re: Compression
Posted: Sat Aug 05, 2023 2:45 pm
by Herb
Even though you do wide open throttle the slides that actually control airflow don't move from the idle position.
The reading you are getting is as good as you are going to get wit the carbs in place and it is a great reading for the way you are doing it.
Re: Compression
Posted: Sat Aug 05, 2023 2:48 pm
by Designer
Herb wrote: ↑Sat Aug 05, 2023 2:45 pm
Even though you do wide open throttle the slides that actually control airflow don't move from the idle position.
The reading you are getting is as good as you are going to get wit the carbs in place and it is a great reading for the way you are doing it.
RIDE ON! .......er,........or,.....
....... should I have said,...RIGHT ON!?
Re: Compression
Posted: Sat Aug 05, 2023 2:58 pm
by hillsy v2
The more important thing is that the cylinders read within 10psi of each other. Sounds like you're good.
Re: Compression
Posted: Sat Aug 05, 2023 3:08 pm
by sky02
Thanks guys, maybe I will check it again without the carbs when i will be redoing the sync next time.
Re: Compression
Posted: Sat Aug 05, 2023 4:08 pm
by Designer
sky02 wrote: ↑Sat Aug 05, 2023 3:08 pm
Thanks guys, maybe I will check it again without the carbs when i will be redoing the sync next time.
I sincerely doubt you will get a much different reading if you take the carbs off,...as opposed to what both Herb said (about the slides removed)... and Lechy's observation (the throttle held open) .
Even if it was a bit better,........the readings to have reported are more than adequate to just keep riding your Bad Boy without any need for a rebuild.
Re: Compression
Posted: Sat Aug 05, 2023 8:57 pm
by Herb
sky02 wrote: ↑Sat Aug 05, 2023 3:08 pm
Thanks guys, maybe I will check it again without the carbs when i will be redoing the sync next time.
No need to pull the carbs. pull the 4 screws that hold the slide in, lift the slide out, do the compression check. Just be sure you keep the front and rear parts separated and the slide diaphram back in correctly, note the way the diaphram is oriented when you pull them out.
While you have them out check the diaphrams for holes.
Re: Compression
Posted: Sun Aug 06, 2023 1:05 am
by sky02
I have rebuild carbs and tuned the jets recently co everything is perfect.
I know you can just take the slide out. But you have to lift the tank and mess the sync.
Re: Compression
Posted: Sun Aug 06, 2023 1:25 am
by hillsy v2
Why bother?
I mean - you pull the carbs and don't get a "sufficient" reading. What do you do then? Get a better gauge? Rebuild a sweet running motor?
You got both cylinders putting out close to 140 on a dubious cheap gauge - just call it good.
Re: Compression
Posted: Sun Aug 06, 2023 7:40 am
by sky02
Just to satisfy my curiosity
and test if there is difference with and without the restriction of slides. But I will do it during the next carb sync. Im not that curious.
Re: Compression
Posted: Sun Aug 06, 2023 8:55 am
by Lechy
A heads up on the diaphragm covers, the rear is no problem but the front cover has a small detent on the inner circumference which aligns with the small orifice on the carb body. Not sure if they all have this or what purpose it serves but it is another thing to check.
Re: Compression
Posted: Sun Aug 06, 2023 1:08 pm
by sky02
Lechy wrote: ↑Sun Aug 06, 2023 8:55 am
A heads up on the diaphragm covers, the rear is no problem but the front cover has a small detent on the inner circumference which aligns with the small orifice on the carb body. Not sure if they all have this or what purpose it serves but it is another thing to check.
I already rebuild them and tuned the needles, even changed one slide but thanks