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I need a shower
55 Posts
Discussion Starter · #1 ·
Time to start molding this little ol' 1980 CB750F into something I haven't a clue about yet. I dig the tank shape, thus why I bought it. No cafe though. Thought about short-chop (bobber for you hip kids), but the tank just doesn't' suit that. Maybe tracker; maybe retro-mod or maybe just whatever my mind comes up with along the way. For me, it'll be low and mean.

I've had so many different directions circulating in my skull since last year when I picked her up I honestly don't know where to begin. Okay, that's a little lie. I started this past week with a few checks. Found first of all that the engine only had 2 quarts of oil in it. Hmm, previous owner error? Bad rings? Haven't removed the exhaust yet, but I did see some oil wetness at the muffler. Time will tell. The battery seems to be holding a charge after a few hours on the trickle charger; a plus for the tests.

The previous owner said he recalled cylinder 3 having lower compression as compared to the others. But he didn't know much and only had the bike for a year; barely rode it and sold it due to wifey demands, lol!

Now, since I took the carbs out middle of last year and determined that two floats where cracked and haven't ordered any parts for her yet, I could not run any hot engine tests. Cold it is.

Cylinder compression and leakdown test results:

Cylinder compression test cold/DRY
- #1 = 125psi
- #4 = 95
- #2 = 98
- #3 = 96

Cylinder compression test cold/WET
- #1 = 130psi
- #4 = 103
- #2 = 115
- #3 = 118

Leak down test cold = All about 5% or less

I'm going to do another wet test once the oil seats into the rings a little more, maybe adding a bit more. I'm a little concerned with #1 being a bit high; could be carbon build up. However, I imagine on a warmed up engine those compression numbers should jump up about 15%. On an engine with 35K miles and OEM specified 170psi, maybe that's not so bad.

Gotta get that 900 sold so I have some build funding. Might be a bit early here, but stay tuned.


I need a shower
55 Posts
Discussion Starter · #6 ·
Looks like cylinder 1 is kicking ass.The other 3 lazy buggers are just depending on him to get the job done.
New rings?
Good question; I don't know. It looks like the head has been off of this given a little wear on the bolts. How do those 'lazy' numbers look for a stone cold 750 engine with 35K on the clock?

I got around to measuring valve clearances today. Results as follows (MM = millimeters, I = inner valve, O = outer valve (from center of head accordingly)):

Cyl - In MM(I/O) - Ex MM(I/O)
#1 - .13/.08 - .08/.08
#4 - .03/.20 - .10/.10
#2 - .03/.05 - .06/.08
#3 - .05/.04 - .10/.13

Numbers I'm concerned with are notably on the intake side. One in particular being cylinder #4 intake side outer valve = .20mm (.008in). It's quite the difference compared to even the other out of spec ones. I don't have a correct valve shim tool for this, so it's either order one and wait, or offer me a creative suggestion? :rock:

Is my bike ok?
15,243 Posts
0.004" ( .010mm)intake lash. 0.005" (.013mm) exhaust lash. +/- 0.001" Here's a pic of the tool with the part number on it. You can get the shims local, rather than ordering then. They are 25mm O.D. shims and most auto parts stores will stock them in various sizes. My local O'Reilly's sells them for a little under $3 each. Way cheaper than any bike shop or eBay seller I've found. I think it's Toyota autos that also use the 25mm size. I'm sure there are others, too.

Factory spec for comp is 175psi. Too-tight valve lash will cause low comp, as I'm sure you know.

Also, the rings may be healthy, just frozen from extended sitting. You have enough compression to start and run. I'd probably run it through a few heat cycles and recheck the compression numbers then.

Stuck rings and light surface rust on the cylinder walls is normal in stored engines and will give you poor ring seal. I'd say you're probably alright, she just needs life breathed back in to her. :rock:

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