Handed a CB750k, Need help

Henchai

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Hello All! I was recently handed a CB750k from a friend, a friend who bought it from a previous owner who lived in another state. The bike was purchased pre-covid and ever since has sat in his garage. He then decided he didn't want it anymore and gave it to me, except its been sitting in his garage and I have no idea what type of work has been done to it. I'm no mechanic, but I'd like to think I have a decent head on my shoulders when it comes to mechanical stuff. I've recently started to work on it and there was been a handfull of issues that I need help with. Keep in mind, this is my first bike I've ever tried to fix, the last one I rode was pretty new so it didn't need much mechanical care.

First thing I did was an oil change, I used 10w-40 full synthetic motorcycle oil that I bought from Oreillys, 4 quarts. I also replaced the battery and fired it up. It was running soundly, but when I checked the cylinders by spraying some water on it, cylinder 2 was cold.
I then replaced the spark plugs, the old ones I took out were D7RES but the new ones I put in were D8EA as per the specs I found online.
I then did a compression test and found that each cylinder was only pushing out a womping 80-85psi.
I put the whole thing together again and fired up the bike, with cylinder 2 now hot but cylinder 3 cold this time.
The bike was also cutting out while idle, which it didn't do before, and when I rolled the throttle, it seemed unstable, I would roll it a certain amount and wait, and the rpm would slowly rise then all of a sudden shoot up real fast.
I started to test the coils to see if each spark plug was sparking up, and during that time the starter starting dying out.
It sounded like it was out of battery, but I thought this was weird since it is a new battery, but I have been turning on the starter to test for a while, maybe that killed the battery?
I tested the battery with a multimeter and it had ~16v, which I thought was good. I forgot to test it while trying to kick the bike on.
I put the battery on a tender and tried to kick the bike on, but the lights just dim and fade out and the solenoid clicks once.
Thats all the work I've done so far, its still in his garage and I can only check on it one weekends, if I'm not busy.

I really want to try and work on this myself, at least as much as I can as a personal project without it taking too much of my cost.
I can get a knockoff leakdown test from amazon and test it, but if a major issue with the engine comes up, I might not be able to handle that myself,
even though I'd be more than glad to try and take it on, I just don't have the space to work on an engine, nor the right tools.
Alkso, is my battery really dead? I admit I should have left the battery on a tender over night, but I didn't want it to be plugged in without knowing when the next
time I can check on it.
Also, what do you guys think of the low psi? 80 sounds super low, and quick tips on how to check for big leaks like that?
If all else fails, I just need to get it up and running so I can take it to a shop.

Also, my friend who bought it from previous out of state owner never even transfered the title, so I have an out of state title with the previous registered owner, and I
went to the dmv to transfer the title and was told they need to physically see it in order to verify the VIN and issue out a license plate, as there is currently no license plate.

This is my first post, so I am sorry if I am breaking any rules in posting for help, and I thank anyone for stopping by and reading.
Have a blessed day!
 

BAHNST

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Welcome......A few thinks to consider.
Many use a compression tester to test between cylinders rather than "absolute readings".
Was 80psi cold cranking?? or hot engine with throttle wide open?
Many believe a compression tester with the check valve away from the spark plug hole (at the end of a hose near the gauge) will read low....

If your battery is a "wet/flooded type" then most smart chargers will also diagnose its health and have a recovery function.

Do you have a shop manual for the bike??
 

dirtdigger

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The compression tester cant be accurate. It wouldnt start on 80psi. These engines need minimum 150 to be considered healthy. If it is indeed that low then you could have some stuck rings but more common on this engine is the valve clearance is too tight. Need to be .004 to ,006" or valves will be held open and burn.
 

Henchai

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Thanks for the reply.

The 80psi compression test was after I warmed up the bike a little. I did it with the throttle wide open.
I don't think my compression tester had a check valve, its just a cheap 20$ one I got off amazon.
As far as the battery goes, I have an AGM battery, not sure if thats considered wet.

If I were to do a leakdown test, would I need to pull the engine? Or would I be able to do it with everything still attatched?
 

dirtdigger

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Leakdown can be done in frame. you test each cylinder at top dead center with valves closed,easily done in frame.
 

Andyf66

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Regarding the cheap compression tester. I borrowed one and it gave a reading of 75 psi on all four cylinders. I then borrowed a good one and i got 168psi on all four cylinders. So i would suggest you borrow or get a better one
 

Henchai

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Hmmm, is there a way for me to rent a good quality one? Also, anybody got an idea of why it would be cutting off while idle? It started doing that after I changed into new spark plugs. I didn't gap it, would that make such a big difference?

I was able to put it on a tender today, going back on Friday to try and start it up and test starter solenoid and battery crank amps.
 

Henchai

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Update that nobody asked for:
I went back to start the bike after leaving it on a battery tender. It started up perfectly fine, but it's still dying out while idling. When I went in to try and see if all spark plugs were working, I found out that the second ignition coils were missing ferrules which made the wires real loose. I hope that's why my cylinders were running cold, because they'll fix one of my issues. I ordered a cheap leak down tester from Amazon, waiting on it right now. My next step is to try and run that and hopefully get another step closer to seeing why my compression is so low. Also, when I was running the bike, I was leaking fuel badly from my fuel hose, I may need a c clamp of some sort to hold it.
 
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