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Compression test results: what is normal?

N

Noeatnosleep

Bicycle
I did a compression test tonight with the carbs off the bike, in preparation to sync my freshly steve'd carbs.

I got 147, 125, 144, 145. (1,2,3,4)

The 125 is bothering me.

Any ideas? Also, does anyone know what normal compression on these bikes is?

It's a '95 and it has 10k miles on it. Before I mailed the carbs to Steve and did the 7th gear mod, it ran great. Haven't done anything else besides cosmetics and some wiring for plugs and aux lights.
 
No idea. This is a new to me bike. I put a few hundred on it and then tore it down, did some farkles, 7th gear, and mailed El Steve my carbs.

I'll do a valve adjustment. Would that account for one cylinder being 20lbs off?
 
That won't do any good when the carbs are off the bike.

And I wouldn't really consider one cylinder being 20lbs off the rest of them 'really close', would you?
 
why don't you do the piston height test now and report back. you need to wait for the carbs anyway.

your numbers seem low. if I remember correctly, others had reported compression ratings above 180. I am sure someone will chime in with their numbers.
 
What, testing for a bent rod with the carbs off? I was thinking that ruling out a bent rod is causing one cyl to be lower compression. The bike can run well even having a bent rod.

I know when doing a compression check the throttle has to be opened but never did it with carbs off. Stupid question but you don't have the intake blocked off while the carbs are out when you did the test.
 
Nope, intakes are open.

I'm in the middle of taking my tip over bars and fairing off. Going to do a piston height check and valve adjustment tonight and tomorrow. Will report back.

If I have a bent #2 rod... What next? Button it back up and sell it and find another one? Haha.

(My reply about 'that won't do any good with the carbs off of the bike' was in response to someone who said to make sure I was at WOT, but looks like they deleted their message.)
 
I guess I could keep running it. I hate to have paid half a grand, plus some for a wicked carb job and not end up being able to have them synced up. Haha.

Anyway, back to work on the bike. I'll update it after valves and piston check.
 
good point. i understand.

that is why when i bought my bike before i did anything i sent my carbs off to steve since they were plugged and needed fixing. but stopped there before spending any more money on anything for the bike. had my carbs not been plugged up i would have done a piston height test before spending any money on it.

since you have done all the work and your bike is running well, just run it out. and with such low miles i bet the valves have been ignored. i know mine were, and extremely tight. i bought my bike with 6800 miles and the previous owners never cared to do the valves becasue they hardly rode it 500mi/yr!

 
If the piston height checks good, fingers crossed, you might try a teaspoon of oil in the cylinder and doing the compression check again, may be a stuck ring.

You may have a build up on a valve seat causing a slight leak.

You are using the starter to turn the engine over and not turning the crank by hand, right?

 
DC Concours said:
why don't you do the piston height test now and report back. you need to wait for the carbs anyway.

your numbers seem low. if I remember correctly, others had reported compression ratings above 180. I am sure someone will chime in with their numbers.
Click to expand...

The low numbers might be due to a cold engine? Evenness is the important part. Height test and set the valves. If the valves were tight, you may have to adjust the tight ones a few times as the deposits get worked out. Hopefully its the valves, but having a bent rod on #2 is quite a bit easier to deal with than #'s 1 or 4.

Good luck.

:beerchug:
 
Isaac - you didn't tell me that you had not done the valve adjustment yet. What you have now makes no difference. Ignore it. do the valve adjustment. Read the article on compression I wrote in the Concourier. you may have to adjust each set a few times, as per the article. you should be at 180 +. Steve
 
That's right. Listen to Steve. He seems to have seen it all.

Your bike is probably just fine. It runs well so why worry too much. So what if it has a little bend in the piston. It adds character. And chicks love battle scars.

 
I've seen compression numbers vary so much from various gauges that I don't look so much at the actual numbers arrived at as the variance among them. You have one (#2, obviously) that's way out of whack. Sometimes there is more to the story than a mechanical issue.
Were they all done in order (1-2-3-4) or did you jump around and then write them down in that order? If #2 was done last it could even be by then the battery was not spinning it over near as well, etc.
Do the piston height test. Hope for the best. Go from there. Listen to the experts.
 
Haven't forgotten to report back. Saturday my shop was full of someone else's truck, and Sunday I was sick. I'll do the tests and report back sometime this week.
 
Ok. Piston height test is A-OK. This is a relief.

Gonna do valve adjustment tomorrow evening and update after that. Thursday night I plan to build an oil tube carb sync tool and do a sync job, and then I have a full weekend of motocamping planned. Wish me luck!
 
I know this is an old thread, but wanted to mention something that may not have been considered already (and which happened to me around compression readings): an inaccurate compression tester. I bought a cheap tester off Amazon and (after valve adjustments, bent-rod test with no bent rods and warming the engine) got compression readings of 150, 150, 140, 150 (cylinders 1-4). These seemed low to me for a bike with only 8k miles (98 Connie, hardly ridden by PO). I went to my local auto-store and rented their compression tester (OEMTools brand) and retested the bike's compression....200, 195, 185, 190 respectively!

Bike is smooth as glass and responsive as a startled cat!
 
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