1979 CB750K with problems

[SoonerRon]

Hello to all, I just signed up for this forum. I recently picked up an old 1979 Honda CB750K for a song that was not running. The story on the bike is (i am told) it was running in 2010 but very poorly due to a rusty gas tank and clogged carbs. After moving the bike to my garage, I removed the tank and cleaned with phosporic acid. I have not sealed the tank yet. I removed the carbs and cleaned in accordance with the procedure described in my copy of the bike's Clymer manual. I also had the starter rebuilt. Installed a new battery and, behold, it started right up! Oops, is has a nasty rattle in the top end though. It also smoked for a while, but then pretty much stopped as the engine warmed. I drove it around the block, rattle and all. It did not have any power. I pulled the plugs sequentially while running and determined it is hitting on #1, #2, and #4. #3 is dead. My buddy and I have speculated it may be either bad valves or a piston/rings issue. I have a compression tester I intend to try this weekend, though I don't expect #3 will hold significant compression. Obviously I'm looking to pull the motor and open it up in the near future. Any advise from the masters of this engine would be greatly appreciated!

[TOOLS1]

Welcome to the forum. I would definitely do the compression check before tearing it apart. I would also do a leak down test. That would tell, you exactly what to be looking, for.
TOOLS

[Piute]

         Welcome SoonerRon,
 That one carb may be plugged bad,As long as your getting fire at the plug,
 no master but my 1st thought

[SoonerRon]

Tools1,

Could you elaborate on the leak down test? I don't have that much experience diagnosing these engine problems and I'm not familiar with this test. Thanks,

[TOOLS1]

No Problem. What, you do is put compressed air in the cylinder, and listen, for it coming out. If it comes out the exhaust, it is the exhaust valve. If it comes out the carb, it is the intake valve. If it comes out the crankcase breather, it is the rings. You can buy a fitting from most auto parts stores that goes into the spark plug hole that an air hose connects to, but I made mine out of an old spark plug. I did it by breaking all the porcelain out of the plug. Then brazed an air hose fitting on, you could also JB Weld it on. Any way that's it.    
TOOLS

[LukeM]

Pre Step #1:  Get the Factory Service Manual from the home page of this forum.  Look in the Manuals section.  Read up on the maintenance section (tune ups and stuff), and then take it step by step, checking out all the systems.  

It's possible the carb for #3 is not good.  Check to see if there's any gas in the bowl by turning out the screw for that float bowl.  Look for color and signs of rust or dirt.  If yes, then it's time to pull the carb bank and clean them all.  Better to do all 4 while you have them off the bike.  If the gas looks good, then verify you have spark for that cylinder.  You have spark on #2, so that coil is probably good: possibly a connection problem between that coil wire for #3 and its spark plug cap/boot?

I agree with the TOOLS man: compression and leak down should add more data to the diagnosis.

Luke M

[SoonerRon]

Very slick. Thanks for the help. I will pursue that line of tesing.

[SoonerRon]

Thanks, LukeM. I appreciate all the help I can get. I hope to attempt further diagnosic tests this weekend. Just can't get past the nasty rattle in the top end, though. Somethings gotta be loose in there. Surely the noise could not be caused by a poorly adjusted cam chain?

[TOOLS1]

A dead cylinder will make the engine sound like it is flying apart. Does the rattle quit when, you rev the engine?
TOOLS

[sgtslag]

Yes, it can.  It is simple to adjust on these bikes, but refer to the Service Manual for the proper procedure:  loosening, then tightening, two bolts, in proper sequence, to allow the Cam Chain to auto-adjust its tension.  I'd get all cylinders firing, first, though.  Cheers!

[SoonerRon]

No, the rattle does not stop when the engine is revved up. It just gets louder.

[SoonerRon]

Many thanks. That will be one of my goals this weekend.

[SoonerRon]

I have the Clymer manual as well as the download honda shop manual available through the link on this forum (thanks!). Would not have bought the Clymer manual from amazon had I known about the download... isn't that the way it usually works?

[Lucky 1]

Just clip on a DC timing light to the cylinder in question and the timing light will show you if it is firing.
A timing light is NOT just for timing. It can be used as a dianostic tool.
See what I mean?

[Lucky 1]

But if you down load the manual online you only see it at 72 dpi.
If you buy the manual you get to see it at 300-600 dpi.

What I am telling you is that the PICTURES are clearer.

[Lucky 1]

Remove all of the tappet covers and make sure all of the rocker arms have adjusting nuts still in place.
A rattling sound can come from many different things.
Loose cam chain.
Broken piston rings in the combustion chamber.
Sometimes young guys just think the engine should have no noise at all like a modern engine. WRONG.
One time a guy told me he had just completely rebuilt his chevy six cylinder 1956 engine but it had noises.
He was really worried.
I went over to his house and it sounded like a very smooth 1956 chevy six cylinder engine.
I grew up during that time so I had a reference to the sound.

You would be surprised how many owners have adjusted the valves on their Honda but never took the slop out of the cam chain. Ever.
OR they attempted to take the slack out of the cam chain and did not follow directions in the shop manual, and made it worse than it would have been if they had just left it alone.
The cam chain MUST have the slack taken out of it BEFORE the valve adjustment.
That is WHY the cam chain adjustment comes before the valve adjustment in the shop manual, because it is supposed to be done first.

[Lucky 1]

One thing you forgot to mention.

One very important thing.
The cylinder being tested must have both valves closed.
The engine must be in the correct position.

IF you have a pressure gauge on the hose you would see the pressure leaking down. That is why it is called a leak down test.
Just make sure the air in not leaking out of one of the fittings on the air hose, or compressor or spark plug hole fitting. Lots of ways to be leaking.

[Piute]

        The Second time I fired my bike up was after a quick cleaning of the carb /Tank flush,
    (1st was just to know it starts(2 minutes)shutter down,)
  Had all the probs throttle sticking ,ect.ect.,sounded bad ,but ran,checked the points ,Spark could of been hotter (by eye),. Coil wires were backward "but working" no prob.ran for "O" 5min. still sounded bad (top end ?)touched the pipes #3 cold,fire at the plug ,
         Pull the carbs all the way off and good look at #3 prev.owner put float upside down ,took all them completely apart and back in,fired right up sounded better "sort of " due to taking completely apart the carbs were  now out sink, Bike ran fine ,
     
You can go get your Buddy's tools and some help N Or do the basic 1st is all I'm trying to say,I'm no 750 F2 Mechanic " But live with cycle / car mechanic "and at his shop.
   Have build many engines cycle and V8,but never anything with 4 carbs on it,or o.h.cam..

[SoonerRon]

I went through the carbs myself before attempting to start the beast, but I could've screwed up #3. I soaked it and blew everything clear so I figured it was all good. many a slip twixed cup and lip...

[TOOLS1]

Soaking, and blowing will not do it on these tiny carbs. You have to poke, and prod every hole with a thin wire, or drill bit.
TOOLS