Author Topic: CB750 K4 Compression  (Read 157 times)

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Offline JeffK

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CB750 K4 Compression
« on: February 09, 2019, 02:17:26 pm »
I have my project at the point in the pic below. I think now would be the time to see if I have good enough compression to continue assembly. Is anyone interested in telling me or directing me to the very best way to determine compression in my garage? I know it should be easy enough but I have really got varying results in the past and I am not too confident.

History is the bike was running great a few years back and now I am rebuilding it. I know for certain that the carbs are going to be all refurbished (maybe professionally) but I want to know for sure if the engine needs top end work or not.

Thanks

BTW - Can I hook a battery up to two of the wires on the harness and get the starter to engage?

Offline BenelliSEI

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Re: CB750 K4 Compression
« Reply #1 on: February 09, 2019, 02:44:51 pm »
Jeff...... I’ll be interested to see what some of the most experienced guys have to say, but here’s my advice.......
You probably should have done the test when the engine was still in the frame and recently running. If it was running well, I’d put it back. It only take you (and a friend) a short time to relocate it in the frame, and then you can hook the oil lines up, fill the oil tank, and have oil circulating when you spin it. That will be harder to accomplish on the floor. Once it’s in, hook up the main ground lead and the power lead to the starter (from the solenoid, and crank away. Before you lift it back, I would definitely pull the cam cover, set your valve clearances, and install a new cam cover gasket. So easy now, and the cover cannot be removed when the engine is in place.
Since the engine has been sitting for a while, you may not get very high readings. The rings will be dry, there may be some internal bore and vale seat corrosion which don’t make for great readings. Since you have easy access right now, I might be tempted to spray some SeaFoam down the intake and exhaust runners, and down the spark plug holes while you are adjusting the valves. Rotate the engine by hand through several rotations to get some lubricant on the valve seats and rings.......
When you do your readings, look for consistency across all four cylinders. The Honda manuals suggest some pretty high readings. Yours has been sitting for a while, so i’d suggest anything over 110-120 psi would indicate enough health to proceed. Readings should be with 3-5% of each other. Let’s see what some others suggest........

Offline ekpent

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Re: CB750 K4 Compression
« Reply #2 on: February 09, 2019, 03:12:02 pm »
   How many miles do you think the engine has on it ? Is the engine leaking oil anywhere from the top end or bottom ? You mentioned it was running well when you parked it a few years back. Unless it was smoking,clunking,leaking or doing some other nasty stuff I would just reinstall it like Benelli said and get it running to see where you are at and then do your compression test in the frame if you feel the need. These engines are sturdy and simple.
  You also won't run into any carbs that are easier to work on either then K4 roundtop carbs. They are very easy to do. Best advice is to keep all your old brass and just clean everything well,no need for 'rebuild kits'. I have never even had to replace a stock float bowl gasket on all the ones I have done through the years. New o-rings on the fuel tees is a good idea.
  Good luck with it all and 'if it ain't broke don't fix it'  ;)
« Last Edit: February 09, 2019, 03:14:14 pm by ekpent »

Offline JeffK

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Re: CB750 K4 Compression
« Reply #3 on: February 09, 2019, 03:43:40 pm »
Guys, thanks for answering. Here's a question for you or anyone out there. I bought a cheap compression tester from Harbor Freight and I think it is quality enough to do the job but the spark plug access is so deep that I couldn't possibly do anything but finger tighten the rubber hose when inserting the check hose. It's like I need a solid extension to get out of the spark plug "hole".

Does anyone understand what I am going through?

Thanks

Offline BenelliSEI

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Re: CB750 K4 Compression
« Reply #4 on: February 09, 2019, 03:56:26 pm »
Yup! I have one with a bunch of adaptors that I screwed together to get a solid piece to screw in first. If you have an “O” ring at the top of the threads, it only has to be snug...... good luck.

Offline JeffK

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Re: CB750 K4 Compression
« Reply #5 on: February 09, 2019, 04:11:19 pm »
OK I can get it them snug. Thanks for your help

 

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