Topic: Compression question.

[Sheik Yerbouti]

Hey guys. I'm still looking for a bike, but talked to a local guy and cheap bike lined up. It's a '78 CB750K. I'll host and attach some pictures in a bit.

It doesn't run, but it turns over. I asked him to check compression and he said that 3 cylinders where measured at 95-105 and the last one is 80. What do you guys think? Just the rings need replaced? It also doesn't have an exhaust but I was planning on throwing on a MAC 4-1 anyway.

Here's some pictures:







He wants 550, I'm going to offer him less. I hope to get it for 450.

[Sporkfly]

Could be rings. Could be valves. Dry vs. wet test would tell you.

Either way you're looking at top end work...

[Sheik Yerbouti]

I was planning on rebuilding the top end anyway. What do you think of it as a deal?

[pelermon]

Mac plus engine rebuild = $1k +$450

You should be able to find a nice 80's 650 Nighthawk for about that.

[Sheik Yerbouti]

Mac exhaust is 250.

http://www.macperformance.com/store/product1.cfm?SID=3&Product_ID=612

Is a rebuild really that much? Gasket kits are like 50 bucks and how much are new rings?

Think I should try to get the price lower due to the compression?

[Sheik Yerbouti]

Oh god, I've been reading about how you guys treat K8 owners around here. Now I'm scared.

[754]

I was going to mention..(not that you will necessarily heed the advice) to go for the 75 or whatever that was..

 Unless,...you want to have less availability or choices of parts and dont plan on changing a lot of stuff.. ie like to keep it stock.. then go for a 77/78..

..looking like this one (latest in a series you have brought to our attention) needs work... and is not that great..


..as Frank Z says... some of these things were owned by flakes...& eaten by snakes..

..you can stab & shoot & spit,
but they wont be fixing it..

theyre lying and lazy..

theyre driving you crazy..................................

[OakBehringer]

I might be wrong, but I think that compression is low across the board, even for cold engine. You deff have problems with the 80. You should first do a wet test to determine if it's rings and, if so, ask yourself if you're really up for that big of a job. You could spend twice as much, which is still not a lot of money for a bike, and get one that won't have these problems.

Is that Tom Waits in your avatar?

[Sheik Yerbouti]

It is none other than Tom Waits.

How does one go through the wet test? I'm just going to pass if it's really that low across the board, I don't want to get myself into a money pit. I wouldn't mind rebuilding the top end at all, I kind of want to do it regardless, but what other stuff would I be getting into?

[Sheik Yerbouti]

Sorry to bump it, but I want to know whether or not to tell this guy to shove it. I can definitely wiggle down the price, especially considering it might need a top end rebuild. But I am treating this bike as a learning experience, not a money pit.

CB750s for a reasonable price are damn near impossible to find around here.

[Sheik Yerbouti]

I was going to mention..(not that you will necessarily heed the advice) to go for the 75 or whatever that was..

 Unless,...you want to have less availability or choices of parts and dont plan on changing a lot of stuff.. ie like to keep it stock.. then go for a 77/78..

..looking like this one (latest in a series you have brought to our attention) needs work... and is not that great..


..as Frank Z says... some of these things were owned by flakes...& eaten by snakes..

..you can stab & shoot & spit,
but they wont be fixing it..

theyre lying and lazy..

theyre driving you crazy..................................

I lost the 75, he sold it before I was off work. Bummer....

[KingCustomCycles.com]

OK guys, those compressions are fine for a bike that has been sitting.  I have purchased them with seized engines for $250.  Buy it for $350, run it, and if it blows smoke or uses more oil than you are willing to dump into it, swap the entire engine.  The old engine can then be refreshed with a head job at your leisure at at bargain prices.  Touch up the frame with the engine out and repaint those tins.  Perspective? a good used 50cc moped is now $600.  A used but good running CB750 motor will be $250.00 a set of new 4-1 macs will add value and have you looking sharp.  Do you want to have fun and work on the old iron or do you need a warranty?  bottom line. New paint, running good, new Mac, $1800-$2400 dollars on eBay all day.   

[Sheik Yerbouti]

OK guys, those compressions are fine for a bike that has been sitting.  I have purchased them with seized engines for $250.  Buy it for $350, run it, and if it blows smoke or uses more oil than you are willing to dump into it, swap the entire engine.  The old engine can then be refreshed with a head job at your leisure at at bargain prices.  Touch up the frame with the engine out and repaint those tins.  Perspective? a good used 50cc moped is now $600.  A used but good running CB750 motor will be $250.00 a set of new 4-1 macs will add value and have you looking sharp.  Do you want to have fun and work on the old iron or do you need a warranty?  bottom line. New paint, running good, new Mac, $1800-$2400 dollars on eBay all day.   

That's the kind of perspective I need. I'm not the kind of guy who is going to want to turn around bikes though. I realize that parts for the '78 are a bit harder to find and all that, and the carbs are kind of wonky, but if I can get it running and keep it running strong (it has about 25,000 on the clock), I'll ride it until the engine turns to dust. I'm going to start at 300 and if he doesn't yield the most I'll do is 350. I realize that even though the engines on 77-78 are a bit heavier, they're also known to be a bit more powerful? And some people say they hate the carbs and some claim they're better than sliced bread.

The benefit of going the route I'm going is that if I can get something that cheap, I can work on it slowly over time. I don't have a lot of cash on hand, so this project may literally take years to complete. I also have access to wonderful tools and spray booths at my college shop (where I now work). Hell, if I wanted to I could CNC some custom badges and vacuum form new plastic bits.

Would doing a ring job and rebuilding the top end really be that much money? I see gasket kits for 50 bucks and ring sets for 50. That's 100 dollars and a lot of my time, which I am willing to spend of course.

[Sheik Yerbouti]

Just found the lowdown on the wet compression check with the help of google.

The wet compression check is a follow up check useful to determine the condition of the piston rings. Some perform it every time they perform a compression check, others do not. I perform it when I see lower than expected readings, especially if it is only one cylinder. The common instructions are, "pour one teaspoon of oil into the cylinder and test". Frankly, I always spill oil all over when I try to pour it from a tea spoon into a spark plug hole and, then to compound the problem, always get caught trying to sneak the wife's teaspoon back into the house. Then, I learned a better way. Get one of the pump type oil cans and find out how many strokes of the plunger it takes to fill one, level teaspoon full. Then, you can insert the spout of the oil can and depress the plunger the required number of times without making a mess. Insert the oil into number one spark plug hole and repeat the compression test on that cylinder, note your reading under the "wet" column and repeat for the other cylinders.


That sound about right?

[MRieck]

Would doing a ring job and rebuilding the top end really be that much money? .

It depends on whether you just want it to run or want it to run well.

[Sheik Yerbouti]

Would doing a ring job and rebuilding the top end really be that much money? .

It depends on whether you just want it to run or want it to run well.

Elucidate.

What am I looking at in terms of materials besides new rings and a new gasket set?

[Sheik Yerbouti]

Bueller?

[martino1972]

Would doing a ring job and rebuilding the top end really be that much money? .

It depends on whether you just want it to run or want it to run well.

Elucidate.

What am I looking at in terms of materials besides new rings and a new gasket set?

low compression might even be inproper set valves,or a valve not sealing of proper due to long sitting with that valve open,rust etc..
i wouldnt be too worried about the low compression right now,its an awesome subject to get the price down for buying it...
you probably will have to do all the standard stuff to get it running,clean carb's(probably twice),set the valves,replace points/condensor etc.. but thats all labour of love more then money...
then like they said before,you can see if it blows blue smoke or what not...
if you have to take the top end apart,you need to realize that you might or will get some extra cost for maybe valve lapping,valve guide replacement ,piston rings,all the gaskets involved etc...
but the strong factor inall that is are you looking for a perfect rebuild or a basic"to get it running halve decent" rebuild...
are you willing to let it burn abit oil and just keep the oil topped up,or is it gonne irritate the hell out of you when she smokes abit....
at the end,its a honda,they will run fairly good,even if they are worn down.....

and if you see this bike as a learning curve,then it should be no problem to put some money in for the learning...and mostly you can spread the cost out,by doing one thing at the time,till the money is there to take on the next thing for the bike,and still be able to drive it in the meantime..

if it does turn out that you bought a lemmon,you can always sell it again and get most of your money out of it...

[LoopsAndLogic]

Don't forget, if he didn't have the throttle WOT, then you'll get those readings!!

Sorry to tell you brother

I even had a KZ550 A3, and when I went to roll it down my driveway to save battery power when cranking it,
it pumped just a measly 110psi.

But when I tried it again with the throttle WO, it shot all the way to 172psi. And this was all COLD and DRY.

Don't wallow over it too much, there will be many more chances down the road to find a Fixer upper

Cheers

LL

[KingCustomCycles.com]

I like your style Martino72, I believe we would get along. 

[martino1972]

I like your style Martino72, I believe we would get along. 

as long as your not asking me to marry you,we could get along just fine....

[Sheik Yerbouti]

"definitely not wide open or choked"

That's what he replied when I asked him if the bike was choked and the throttle was wide open. I don't think he opened the throttle enough to get a good reading.

I'm going to go there tonight to take a look. I'll keep you updated. I'm going with empty pockets so I don't get tempted to buy right away.

[Sheik Yerbouti]

Well, I just got back from the visit.

Brian, the owner of the bike is a really good guy. We retested all the cylinders with and without oil. Originally he had the throttle closed, thats why the readings were so low. But still, the center 2 cylinders (2 and 3) measured about 150 after oil. The outer ones were reading something like 205! Before oil they were reading 120 and 150 respectively. What could cause such a high reading though, in those 2 cylinders?

He also offered to throw in a set of clubmans he bought, another seat pan, and an old tank that leaks (possible to use for cafe hump)

I think I might snag it. I'm going to offer him 350 or 400, because he was nice enough to throw in the clubmans and the tank and seatpan. The top end is going to need to be rebuilt, and I want to do it anyway.

[Sheik Yerbouti]

[gsgleason]

Why does it compress less with the throttle closed?  Is it because it cannot suck in as much air to compress?