Low compression from sitting

[Qaxe]

I have heard and read that when a motorcycle has sat for years then you can have low compression.
My CB750K6 sat for at least a decade before I got it. Honestly, it's taken me longer than expected to get it back to being rideable, so I haven't ridden it all that much myself. Naturally I keep the gas and oil fresh, my battery is new this summer, but it's not like I'm going touring or even using it for transportation.

My question is, once a bike has low compression from sitting, is that it? To fix it do I need to get into changing rings? Will any amount of compression come back by riding regularly? Or is it like teeth...once they're bad, they're bad? Mine 125-120-120-125

Seems like a stupid question, but I only know what I know. lol

 

[brettp]

I'm no expert on cylinders but I do know car engines that sit can be brought back around. The rings can sometimes be stuck, or stubborn, but letting them idle and driving them brings them back to life.

 

[Biebs]

Good even compression across the cylinders. Run some Transmission fluid in the gas will help clean up the rings / cylinders.

How many miles on the engine???

 

[brettp]

I used marvel mystery oil in my fuel when I first got it. Then Berrymans fuel system cleaner.

 

[Qaxe]

Hey guys. Hope you're having a good one.
BrettP: Did you wait one tank between the MMO and BFSC or are we talkin' the following season?
Biebs: Tranny fluid? Wow. I need to get out more. Mileage is a big fat "maybe" 11k.

 

[dirtdigger]

it may get better it may not. The engine will run with the compression you have....is it ideal.....no but it is good enough that it will take a tune up and run decent. I would guess your engine is tired though

 

[brettp]

Hey guys. Hope you're having a good one.
BrettP: Did you wait one tank between the MMO and BFSC or are we talkin' the following season?
Biebs: Tranny fluid? Wow. I need to get out more. Mileage is a big fat "maybe" 11k.

Just 1 tank

 

[Qaxe]

Sometimes getting an old bike feels like adopting an abused pet... what the hell did they do to this poor thing?

 

[brettp]

Like dirtdigger said, you could have a tired engine. But if you don't ride it like a daily commuter it week be a while before you need to worry about rings.

 

[Qaxe]

... seems like the bike and I both need a gym membership...

 

[Travis]

How many miles have you put on it since it sat for so long? You could experiment with some chemicals and a few hard rides to see if does anything. I burn seafoam in my forklift straight through the throttle body to get compression back because it never runs long enough or hot enough and carbon builds up on the valves. If you want to fix it, you'll need to determine if the low compression is from the rings or valves with a leak-down test.

 

[Qaxe]

250miles?
It's been mostly wrenching thus far, with test rides that end in more wrenching. I'd ride a bit...oh crap, the fork seals are blown. Ride a little more... oh crap, the brakes are squishy. Oh crap, why is it idling at 3k suddenly? Oh crap, why is cylinder #1 sputtering? I've basically been shooting craps all summer long.

It's my fault for praying for a motorcycle project instead of a motorcycle to ride.

The new one is, "Oh, you have a K7 frame with a K6 engine? Bro, your sprockets are out of alignment..." F's sakes...

There have been so many, many, many things wrong with this poor bike. I've forgotten which thread I'm even on... (looks up)... oh yeah. Compression. I'm going to bookmark this page and will come back to compression after dealing with this ridiculous sprocket catastrophe.

You guys have been really helpful. Thanks.

 

[brettp]

I hear you on project bikes. In hate to tell you mine hasn't been that bad, but I did the same thing. Ride, find a problem, ride, find another problem, repeat. If I was more knowledgeable on these bikes in the beginning, I would have done a valve job at the same time I was cleaning the carbs, and then I would have looked specifically at the front forks, rear shocks, tested the charging system, etc. All of that would have been easier at one time. But I wanted to ride the damn thing!

 

[Old_Crow]

I hear you on project bikes. In hate to tell you mine hasn't been that bad, but I did the same thing. Ride, find a problem, ride, find another problem, repeat. If I was more knowledgeable on these bikes in the beginning, I would have done a valve job at the same time I was cleaning the carbs, and then I would have looked specifically at the front forks, rear shocks, tested the charging system, etc. All of that would have been easier at one time. But I wanted to ride the damn thing!

Last time I tore a project bike down to every little nut and bolt, it took so long to put it back together that I ended up buying another bike to ride...then I had two projects sitting in the shop.

 

[brettp]

Last time I tore a project bike down to every little nut and bolt, it took so long to put it back together that I ended up buying another bike to ride...then I had two projects sitting in the shop.

I hadn't thought of that. I suppose it really could take a while if you can't dedicate large chunks of time to it.

 

[Qaxe]

That's how I caught motorcycle madness in the first place. I just had one bike (K7), and the most I did was oil changes. Then a crash left me with a bent frame, but lots of good parts. While looking for a frame/title, I ended up with an awesome K4. Great, strong bike. Still needed a frame though. I was regularly looking through the newspaper and the Cycle Trader (no Internet yet) and ended up with another K7, but no title. Then found a K2 and a K5 at the same time, both cheap. Their problems were different than what I needed, so those 2 were merged into one bike, and I sold it.

Finally I found a K6 with straight frame and a title. It was gross. Literally had a stove-pipe where the muffler should have been, held on with bailing wire, moss for a seatcover, and a 2nd front tire on the back, held on with a lag-bolt. I had lots of spare parts by then, and my first K7 found a new body. Sold that too. Somewhere in there was a different K7...which I ended up with again 12-13 years later.

Seemed like back then, most of the time an old bike was considered "dead" because some guy left it too long, and the carbs were full of gunk, tires were rotten, and it was rusty. They just didn't want it, so I'd buy it and fix it up. By 2001 I'd something like 13 bikes. I still wasn't a proper mechanic, I just loved Hondas. I never learned to do rings, or anything that required actual precision, and I still suck at tuning.
... good at rambling on though. I'll make a good old man one day.

 

[Old_Crow]

I hadn't thought of that. I suppose it really could take a while if you can't dedicate large chunks of time to it.

That was a perfectly running bike that I tore down over the winter because I wanted to fix a base gasket leak and replace the swing arm bushings. Due to circumstances beyond my control (I may or may not be slightly OCD) by the time I got done wrenching the bike was a bare frame sitting on a stand. Took me 4 years and over $4k to get her back together, but every nut, bolt, and tiny little part was either new or polished or painted when I got done. Bike was an '82 Shovelhead that looked and ran like brand new in the end.

 

[Qaxe]

Old Crow, as for myself, I prefer the word, "Eccentric".

$4k is a lot on the one hand, on the other hand you really know your bike now, and doing all that is fun (for me anyway). Plus, you got a great story.

I didn't really have much choice other than to go through this entire bike. Too much random unknown heinousness. I've spent more on this motorcycle than any other motorcycle I've ever owned. It's a bit of a money bit, on the other hand, I get to relive the joy of CB750 wrenching. Man's gotta have a hobby.

 

[Qaxe]

Both Dirtdigger and Travis have brought up the Leak Down Test. From the name, I assumed that it involved putting oil or something in the cylinders, but I'm looking at a video on Y.T. and it appears that I need a special leak-down tester... is that correct? Might have to order something like that, I sure don't have one.

If I keep buying specialty bike tools, Ima hafta get another project bike after this...

 

[brettp]

The leak down test uses oil because it won't go past the rings easily (if at all). Fuel and air will, or would. If the compression tester you have shows better numbers with oil than fuel, that is an indication you have bad rings. But you can't use too much oil or it will give you a false reading. I think a tablespoon is the recommended amount, but don't quote me.