79 CB750L plan of attack

MrMojoRisin

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OK, after getting a grounding issue and buying a decent set of jumper cables, I was able to do the compression test on my newly acquired 79 CB750L. Figured I'd move this over to the engine forum since electrical issues are sorted.

After seeing the compression test results, I'm wondering if I should pitch my compression tester in the trash beside my crappy jumper cables that I was using. Wasn't sure if this thing should even fire with the following readings.

Compression test
Cyl #1- 122
Cyl #2- 66
Cyl #3 - 81
Cyl #4- 92

It will fire on starting fluid video link here https://youtu.be/X6m_6KP7AXw

So, I hooked up a small aux fuel source and tried to get it to fire on regular fuel. Wouldn't fire. I took the carbs off and looked at the fuel side and see that fuel was present at carb 3 & 4 insulators, but carb 1 & 2 insulators were dry.

So, plan of attack is as follows. (probably not the right order, but gotta start somewhere)

1. Pull carb bank and tear down for cleaning.
2. Re-install and see if bike will fire on aux fuel tank.
3. Pull head and check valve clearance. (.005" is minimum, correct?)
4. Purchase higher quality compression tester and leak down tester if low readings are confirmed.

Starting to regret purchasing a basket case that was abandoned vs starting the build from scratch with a running donor bike. Realize that I still need to do all the maintenance tasks, but would be more fun if I wasn't chasing the known status of this motor. Lesson learned...
 

amc49

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Any cylinder under 100 psi is a dead one for all intents and purposes/ New engine compression around 170 psi, good running from 130 and higher.

.005" on valves is perfect. You can use .004"-.006" as a window.
 

MrMojoRisin

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Thanks amc49. Is it likely for a bike to fire up (even on starting fluid) with compression that low on 3 out of 4 cylinders? I was shocked when I saw #’s that low after hearing it fire up. I’m hoping I just have a bad tester.
 

amc49

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Your lawn mower starts and runs at maybe 5/1 compression, that should tell you something. Just don't expect bike to run long. The low compression will not burn the plug clean and it fouls out pretty quick. Just for comparison the bike has 9/1 compression.

Sounds like 2 and likely 3 cylinders at least are hitting. If the compression tester is one that does not screw into plug hole the numbers will be crap. You'll get a little more result withj opening the throttles to spray and then let them shut to trap more starting fluid. Set up gas to carbs and start it using ether, then let it go to gasoline.

Engine running if it sat will likely increase compression some.
 
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