First Project - 76 CB750F !
- Thread starter goodgam
- Start date
surffly
Over 1,000 Posts
There are two types of checks that are called compression.
One is spinning the engine with a pressure gauge in place of the spark plugs.
There is a set PSI you need to reach. To much or to little is bad. Make sure you are not using a cheap gauge. Also the test is not really that helpful on a cold motor that has been stored for years. Will it tell you something? Sure. But often the reading will be massive low and after the engine is run a bit things go back to normal.
Other test is a leak down.
This is when you lock the motor and pump air into the cylinders, you measure the amount that leaks out. There is a percentage of loss that is expectable. Also where any leaks form you will hear the air. This will tell you what's wrong.
Again a hot engine is much better.
One is spinning the engine with a pressure gauge in place of the spark plugs.
There is a set PSI you need to reach. To much or to little is bad. Make sure you are not using a cheap gauge. Also the test is not really that helpful on a cold motor that has been stored for years. Will it tell you something? Sure. But often the reading will be massive low and after the engine is run a bit things go back to normal.
Other test is a leak down.
This is when you lock the motor and pump air into the cylinders, you measure the amount that leaks out. There is a percentage of loss that is expectable. Also where any leaks form you will hear the air. This will tell you what's wrong.
Again a hot engine is much better.
crawdadman
New Member
aesthetics are important, you want to be proud of what you are ridding! But i just realized recently, it is no fun ridding a turd covered in sprinkles...
goodgam
Active Member
Well.. I sort of made progress today.
But..ugh :-\ compression test didn't go as well as I had hoped. 50, 45, 45, 32. The cylinder with 32 had a cap over the carb that had worn off and I saw some sediment and am guessing that water leaked through a bit around where the intake boot was, didn't see it go very far back though.
Found some more problems though, the throttle wasn't working at all, loosened it up a bit but still wasn't getting anywhere, so now I'm going to try and work on an electric start on Sunday.
Couple of questions..the intake boots are SO HARD.. I realize they are thick plastic but it was near impossible to get the carbs detached. Is there a way to salvage those?
Good news is though.. the carbs look pretty good considering the condition of the rest of the bike. Still going to clean them up though.
But..ugh :-\ compression test didn't go as well as I had hoped. 50, 45, 45, 32. The cylinder with 32 had a cap over the carb that had worn off and I saw some sediment and am guessing that water leaked through a bit around where the intake boot was, didn't see it go very far back though.
Found some more problems though, the throttle wasn't working at all, loosened it up a bit but still wasn't getting anywhere, so now I'm going to try and work on an electric start on Sunday.
Couple of questions..the intake boots are SO HARD.. I realize they are thick plastic but it was near impossible to get the carbs detached. Is there a way to salvage those?
Good news is though.. the carbs look pretty good considering the condition of the rest of the bike. Still going to clean them up though.
You can soak the boots overnight in xylene - available with the paint strippers at your friendly hardware store. If they are hard but not cracked that should do the trick.
As to your compression test: did you do so before taking the carbs off? Should be done with the throttle wide open, which would have been difficult if the carbs were still on since the throttle doesn't work
As to your compression test: did you do so before taking the carbs off? Should be done with the throttle wide open, which would have been difficult if the carbs were still on since the throttle doesn't work
goodgam
Active Member
Rich Ard said:You can soak the boots overnight in xylene - available with the paint strippers at your friendly hardware store. If they are hard but not cracked that should do the trick.
As to your compression test: did you do so before taking the carbs off? Should be done with the throttle wide open, which would have been difficult if the carbs were still on since the throttle doesn't work
Yeah, trying to figure out the throttle stuff is what lead me to take the carbs off and then I re-tested. Thanks for the tip about the Xylene, I'll try that out!
surffly
Over 1,000 Posts
Should do a leak down to know if its the rings or valves that are he issue.
Normally I would say start the engine and see if things improve, I still maintain cold compression numbers tell you little, but I don't think you could start the motor with numbers that low.
Are you referring to the yellow stuff on the carbs as sediment? That's marking paint for the bench sync done at the factory.
Yes those manifolds get hard over time. Often they crack. Can be soaked in something to soften, but better to replace them.
Picture of the outside of the carb tells nothing as its the inside that does the work.
Hate to rain on the parade, but it might be time to pull the chute and see about getting a different bike that more fits your skill set and goals.
While defiantly not impossible for you to revive this bike the time and expense might not be worth it to you.
Normally I would say start the engine and see if things improve, I still maintain cold compression numbers tell you little, but I don't think you could start the motor with numbers that low.
Are you referring to the yellow stuff on the carbs as sediment? That's marking paint for the bench sync done at the factory.
Yes those manifolds get hard over time. Often they crack. Can be soaked in something to soften, but better to replace them.
Picture of the outside of the carb tells nothing as its the inside that does the work.
Hate to rain on the parade, but it might be time to pull the chute and see about getting a different bike that more fits your skill set and goals.
While defiantly not impossible for you to revive this bike the time and expense might not be worth it to you.
620dark
You can always get it running with time or money.
Another thing to consider is that the rings may be stuck from sitting, leading to low compression readings. Even with some oil in the plug holes, it sometimes takes things a while to loosen up. Sometimes automatic transmission fluid or PB Blaster down the plug holes can help loosen things up, but make sure when you turn it over the plugs are out, as if there is too much oil in the cylinders you might hydrolock the motor if there is nowhere for the excess oil to go.
Also, you were using the kick start with the carbs off to get your compression readings, right? Did you take all the plugs out to make it easier to kick over? I know on my 650 (using the electric start) it can take turning the motor over for a few cycles to get max compression. Do you remember about how many times you kicked the bike over while testing compression?
You might get a higher reading using the electric start - I'm going to be the optimist and say not all is lost
Also, how did the oil look when you drained the crankcase? Was there water in it?
Also, you were using the kick start with the carbs off to get your compression readings, right? Did you take all the plugs out to make it easier to kick over? I know on my 650 (using the electric start) it can take turning the motor over for a few cycles to get max compression. Do you remember about how many times you kicked the bike over while testing compression?
You might get a higher reading using the electric start - I'm going to be the optimist and say not all is lost
Also, how did the oil look when you drained the crankcase? Was there water in it?
goodgam
Active Member
Ok drained the sump this time. The Oil can was dry as a bone (not surprising) I guess that should have been a no brainer. :-X
Used the electric start with the compression test and got readings in the mid 60s.. sort of, I noticed there was some inaccuracy in the last cylinder and took off the valve covers and saw thought that they looked sealed. Tried using a leak test to see if it could open the valves. Turns out the rocker assembly is off (basically just moved loosly back and forth) and Ill need to replace that I think. The compression in the cylinders is good though as far as I can tell. So that's encouraging. I wonder if the old owner was riding it around with only 3 cylinders.
My neutral light and headlight didn't seem to be working but the oil light was reading..
I think I'm going to start by replacing the rocker arm assembly, cleaning out the carbs, getting new spark plugs that seal, that might keep me from panicking. I may have to sell it before I get in too deep, but at this point I still want to try get it going.
QUESTION: The bike started when the bar switch was in the on and in the off position... this seems problematic. Any suggestions as to why?
Used the electric start with the compression test and got readings in the mid 60s.. sort of, I noticed there was some inaccuracy in the last cylinder and took off the valve covers and saw thought that they looked sealed. Tried using a leak test to see if it could open the valves. Turns out the rocker assembly is off (basically just moved loosly back and forth) and Ill need to replace that I think. The compression in the cylinders is good though as far as I can tell. So that's encouraging. I wonder if the old owner was riding it around with only 3 cylinders.
My neutral light and headlight didn't seem to be working but the oil light was reading..
I think I'm going to start by replacing the rocker arm assembly, cleaning out the carbs, getting new spark plugs that seal, that might keep me from panicking. I may have to sell it before I get in too deep, but at this point I still want to try get it going.
QUESTION: The bike started when the bar switch was in the on and in the off position... this seems problematic. Any suggestions as to why?
michaelmcgowan_
New Member
Save that poor Hondas life! the aesthetics look sick, love the baby blue.
classic but café!
classic but café!
