New to me CB700SC with maybe low compression in one cylinder

[leftfield6]

Hi all, I picked up a fantastic looking and great running 85 Nighthawk S today.  It only has 5120 miles!!   Now I know sometimes low mileage is more a curse than a blessing, but this bike seems to be in great shape.  I test drove it, and it ran perfect.  Started from cold with no drama, pulled strong, idled smooth, etc.  All rubber bits like carb boots are nice and supple.

So, anyway, I'm super stoked, I've lusted after this particular model of bike since college, and I couldn't pass this one up when it popped up on Craigslist.  Like I said low miles, perfect cosmetic condition, new Avon tires, etc, etc. 

I know, pics, or it never happened, right???


So, I'm tinkering and caressing it tonight, and I found the original owners manual in the little rear storage pocket.   Start flipping through it, and I found a repair receipt from 1999.  It was a little hard to read, it was a carbon write-through thing.  Looked like mostly routine stuff, new plugs, fluids, adjustments, etc.  Okay, no worries so far.

Then I see a line that says "Compression check"  Now remember I'm not 100% sure I'm reading these numbers right, it's really faded and old, but it looks like:  #1-140, #2-150, #3-50, #4-140.   

Holy crap, if that #3 is really 50, what does that mean?  I hope there's a 1 there that is just faded away, it is kinda right in a fold of the receipt. 

So lets say it is 50,  How would I feel this?  Would the engine exhibit any vibe, stumble, burble, fart, anything? 

The bike seems to run great.  Should I have the compression checked for piece of mind??

[ariwhiteboy]

If one cylinder was running at what is essentially 30% compression the bike would run like dog$h!t.  I'm willing to bet that if the bike is running as good as you say it is then you hve nothing to worry about.

[Munkey]

Low compression would cause the bike to be low on power. If the compression in one cylinder was low enough that the fuel/air mixture does not burn then that cylinder would miss, again detectable mainly with being down on power.

Only way to be sure is to test the compression yourself or have someone do it. From your description of the way it runs, it doesn't sound like there's any problems.

[noahnsteph]

http://cgi.ebay.com/ebaymotors/COMPRESSION-TESTER-FLEX-DRIVE-ADAPTORS-NEW-NR-_W0QQcmdZViewItemQQitemZ320570740368QQptZMotorsQ5fAutomotiveQ5fTools

Cheap and easy test for peace of mind.

[gammer]

I agree with Ari and Munkey...If one cyl was running that low of compression then you would notice it. I'm sure it should have said 150, not 50.

But, you probably wont be comfortable until you test it yourself.

BTW, 140 and 150 compression sounds about right. The manual states the compression should be 171, +/- 31 PSI.

When you do your compression test, do it with all of the plugs removed and the throttle held wide open.

[fishmeister]

http://cgi.ebay.com/ebaymotors/COMPRESSION-TESTER-FLEX-DRIVE-ADAPTORS-NEW-NR-_W0QQcmdZViewItemQQitemZ320570740368QQptZMotorsQ5fAutomotiveQ5fTools

Cheap and easy test for peace of mind.

Thanks for the link, I had to modify mine because of the lack of room with it's quick connecter. This one's fittings look less bulky.

A tool every used bike buyer should bring with.

BTW, 140 and 150 compression sounds about right. The manual states the compression should be 171, +/- 31 PSI.

When you do your compression test, do it with all of the plugs removed and the throttle held wide open.

[leftfield6]

Wow, thanks for all the quick replies, this is a great forum.

Took the bike for another ride last night, a longer one, don't have tags yet, so I'm staying off the main roads.  It runs fine, I can't detect any miss, stumble, or lack of smooth goodness.   I'l get it insured today, and then tags on Monday (GA's computer system apparently needs to to "think about it" for at least 24 hours after you get insurance)

I'll also order the compression tester, i agree, a good tool to have.

This bike is so mint, I can't believe the owners manual is still perfect, as is the tool kit in the side compartment.  Even still has the little feeler gauge in the outer pocket.  I'll try to get some pics of it, and post over in the Introductions forum.

[Brittles]

Good to hear your machine is running good.

That tester being sold on ebay won't work on a 700s. Ya' need a 12mm, and that's a 18mm and 14mm.