HONDA CB750 Custom 1980 REPAIR GUIDE

Posted: June 16, 2012 in Helping Hands Events, Secrets, Table Of Contents, TECH
Tags: , , , , ,

This guide is only focused on solving engine noise so far. As I have more time, I will add other data. I have owned and repaired 9 similar bikes over 20 years so there will be lots more added as I get to it.


QUESTIONS OFTEN ASKED: 

?1. IN STEP #1 … why remove the AC generator cover IF using IDLE method to tighten instead of hand crank?

ANSWER: I find no reason yet, plus this is usually hard and corroded and could break if being removed.

?2. IN STEP #2 …. how tight do you tighten the tensioner bolt while rotating the crankshaft? ? IF the bolt is tight, is it possible it is stuck and the tensioner is not working correctly? I have read you can take the bolt all the way out and thread a tail light bolt down on the tensioner itself to wiggle it loose. The manual says DO NOT DO THIS however…. thoughts???

ANSWER: Use the 2 finger rule. Once you feel tension on the bolt, stop!! Do not go tighter then you can easily turn with two fingers. COMMENTs needed on how to know if it is stuck?

?3. IN STEP #3 …. how crucial is it that you tighten BOTH nuts AT THE SAME TIME on the rear while rotating the CRANKSHAFT? (trying to avoid needing extra hands)

ANSWER: No problems have been reported by those who have turned both REAR tighteners separately.

?4. AT the end of the manual directions … WHY DOES it state “WHEN THE front tensioner is loosened, the tensioners will provide the right tension”???????  Is in implying that you repeat the process again in the front for the final step????

NO ANSWER or Feedback yet!

?5. WILL starting the engine to reach idle WHILE HAVING THE TENSIONER LOOSE, cause dangerous torque on a possible slack in the CAM chain? (IS IT recommended to have the engine in IDLE BEFORE loosening the tightener?)

ANSWER: no one says for sure but it is widely recommended to start while tight.

1980 CB 750 HONDA MOTORCYCLE

REPAIR MANUAL ONLINE: (message me if you just want 1 section and I will email it to you!)

http://creativeconceptcomputers.com/CB750/dohc7983fck

Diagram Schematics:

http://www.cmsnl.com/honda-cb750c-750-custom-1980-usa_model1019/partslist/

FORUMS:

http://honda-cb750-s.456789.n3.nabble.com/

Problem: Engine Ticking

Solution: (after putting bike on center stand with engine off, dip stick un screwed and touching the rim and making sure oil level is satisfactory?)

#1 CAM Chain Tighten

#2 CARB SYNC

#3 

#1. Tighten Cam Chain Tensioner p.374 section 25-20 of manual

http://www.youtube.com/watch?v=rBGKxgH7q9o&feature=related

The tensioners automatically adjust after the bolts are loose so you want tension on the cam chains to ensure they’re tight. Spin it by hand OR adjust while in IDLE to maintain that tension.

TOOLS NEEDED:

10mm with ratchet wrench on one end to turn upper rear tensioner. 12mm to turn front lock nut.

STEP 1.

Level bike on center stand! Consider removing AC generator cover with arrow in picture below OR the side points cover to turn crankshaft by hand. The AC generator could be corroded and break so I recommend just idling the engine instead. START and idle the engine.

(I was wondering WHY the AC generator cover was to be removed… Some say that this AC GENERATOR bolt will turn the crankshaft. I had not read this anywhere else.. I had read you would turn the crankshaft by removing the points cover on the right side for SOHC and assumed it was the same. This is a helpful point. Hope this helps clarify for future readers.) 🙂

STEP 2.
Loosen FRONT CAM Chain lock nut and bolt. Then tighten the bolt while rotating the crankshaft clockwise and then tighten the nut. (THIS CAN BE DONE WHILE IDLEING instead of turning by hand) In my experience the bolt was already tight with tension.

? IF the bolt is tight, is it possible it is stuck and the tensioner is not working correctly? I have read you can take the bolt all the way out and thread a tail light bolt down on the tensioner itself to wiggle it loose. The manual says DO NOT DO THIS however…. thoughts???

STEP 3.
KILL ENGINE AND Loosen BOTH the top and bottom REAR tensioner lock nuts. START and IDLE engine. TIGHTEN them WHILE ROTATING THE CRANKSHAFT clockwise or while IDLEING the engine.


STEP 4.
**ONLY NEEDED IF manually turning the carnkshaft. Put the AC Generator cap back on and/or the points/crankshaft cover.


ENGINE STILL NOISY?

SOLUTION #2. SYNC the CARBs

(on my way to do this soon and will report back)

While trouble shooting carb issues, these following videos will help.

 

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Here are notes I found ON A pre-1979 Single Over Head Cam Engine (SOHC 750)

MANUAL: (I have individual sections I can email you upon request)

http://data.sohc4.net/SB750/750_19.pdf

FORUMS:

http://forums.sohc4.net/

YOU NEED TO KNOW:

The coil on the left (clutch lever side) is for cylinders 1 & 4 which is the two outer cylinders. The coil on the right is for 2 & 3

If you reverse the wires on the coils is doesnt matter.Like if you revers 2-3 or 3-2. They fire together since they are hanging out of the same coil.

STEP 1.

Level bike on center stand! Run the bike for 1 minute to warm up and stretch the cam chain. Remove the tappet covers for cylinder 1 (free’s up engine valve for turning!) THIS IS THE OUTTER CLUTCH LEVER SIDE CYLINDER. Your left if sitting on the bike.

STEP 2. Remove the points cover and rotate the engine to “T” mark.

STEP 3. Check BOTH valves on #1. IF tight rotate 360 degrees. I am not sure how to decide this. Someone please comment.

STEP 4. Rotate crankshaft clockwise until it matches the diagram below. This is the 15deg ATDC 1.4 position.

STEP 5. Loosen the cam chain tensioner lock nut AND back out the setting screw  until you hear the tensioner arm move/release,
*IF nothing happens and you are certain the arm spring is stuck, you can take the screw out of the tail light cover and thread it in to move the arm free.

STEP 6: Re-tighten setting screw, then lock nut then point and tappet covers.

Problem: CAM

Solution:

#1.Read this http://forums.sohc4.net/index.php?topic=92541.0

http://data.sohc4.net/SB750/750_19.pdf

Here is a video showing how it works.  http://www.xs11.com/forum/showthread.php?t=35129

Problem: Stripped Thread

Solution:

#1. You can go to a good auto store or machine shop and get the stud with the next size larger threads

on the end that goes into the head and the other end is still 6mm. Just retap and use red Locktite. Time serts are used on bolts that are going to be taken in and out on a regular basis. Like spark plugs.

One more thing.. The size of tap and drill problem. When you buy the TAP many times the package tells you the drill size, and sometimes the tap and drill are sold together. IF all of that fails just go to Google and type in “TAP drill chart”. If the next size bolt is not holding, get a heli-coil.

MORE TO COME….

Problem: Brake lights and dash out but fuses are good?!

Solution:

#1

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Comments
  1. jake says:

    hey man thnx for this post! verey informative. im hoping u can email me the section covering valve adjustements for a cb750c 1980. i tried the cam chain adjusting and the ticking is still there. my guess now is a valve adjustement?? thnx for ur time n help man much appriciated. boogabooka26@gmail.com

    • I have my friend shim the valves but I didn’t take pictures or take notes on the details. If you find a good guide and picts on this let me know so I can add it to mine. After shimming, my bike runs like new, leaks less, idles/starts perfect and the ticking is minimal.

  2. bastien h says:

    hi i have a cb750c 1982… i took the engine apart to redo valves and rings etc.. now i m trying to assemble it back together.. i need lots of info.. torque data,.. timing chain for cam shape and crank case,.. engine head.. etc etc.. i been looking online for a clymer book or anything else with all the know hows i need .. can u help.. do you have any online manual ??? tx for your help.

  3. J.R says:

    There is something very wrong with my 79 cb 750k. There are four holes in the engine wall were the timing chain runs. Have you seen this before?

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