Friday, April 7, 2017

How many experts does it take to make a CB77 run right?

It has been a push-pull experience with the 1962 CB77 Type 2 café racer bike during these past few weeks. After the initial excitement of “re-owning” the bike again (after it was away for 25 years) the reality of what had been built and what it needed to run properly again has caused unending headaches.

First, the fuel system was contaminated with 25 year-old gasoline residue, which attacked the old KREEM tank liner. After flushing out the old gasoline deposits (I thought), fresh fuel was added as the bike was brought back alive with a new battery and carb cleanout. Apparently the Kreem leftovers contaminated the new gasoline to the extent that it created a gummy substance that caused the carb slides to gum up and hang partially open. After cleaning the carbs several times, the tank was finally treated to a full strip cleanout and coating with POR15 sealer. Of course the petcock and carbs all needed to be cleaned again to rid themselves of the gooey gum deposits. What also happened was that the old contaminated fuel that did go through the system coated the insides of the intake ports with slime, which kept getting spit back onto the back sides of the carb slides, even after the fuel system was cleansed and sealed.

The motor has been out four times now for:

1. Cam sprocket replacement with a Dream unit; falsely thought to be the cause of ignition system inaccuracies. The point cam was the actual culprit.
2. Removed for a bottom end teardown, initially to correct shifting issues and noises. One offset cotter was cracked in half and I discovered that the main bearing locating pin had been pushed through the engine case, so that needed repairing. While it was apart, the rest of the top end was taken apart so that an endless camchain could be installed and an oversized piston pin was put in the left side rod. After the engine was reinstalled and run, the clutch pushrod seal dislodged and blew out all of the engine oil during a 15-minute test run. A new seal was installed with Lock-Tite.
3. Engine out again to replace the “repaired” CA77 Dream camsprocket, which was suffering from too much spark advance, too early, causing engine idling issues. When engine was reassembled, the wrong set of timing marks were used as reference points for cam timing, resulting in ZERO compression, discovered when the bike starting attempts failed. 
4. Instead of pulling the entire engine out of the chassis, the bottom rear bolts were left loose and the engine tipped downwards towards the front fender. This left sufficient clearance to remove the cams and rotate the camsprocket 180 degrees for proper cam timing. Compression readings returned to 180 psi on both sides. Bike started up okay, but ignition timing continues to advance out somewhat when engine is operated and returns to idle.

Initial start up was rocky as it wasn’t running on the left side, at idle. After cleaning the idle jet, the start- up improved, but part throttle was dodgy until about half throttle. Closer inspection showed that the carb slides had gotten out of synch and then the ends of the needle tips were not hanging out of the bottom of the carburetors equally. Slides were removed and the right side needle had no numbers on it; apparently an early Keyster kit needle. Replacement of the needle with an OEM version and careful carb slide synch made things much better, but the erratic hot idle conditions remain.

After careful reshaping, the point cam timing ramps are about exactly the same, so that earlier erratic timing problem has been solved. The problem with the whole system is that the return springs for the weights don’t fully pull the weights back into the resting position. There is ALWAYS some slack/slop in the mechanism. The springs are not terrifically strong, so it is easy to twist the point cam and feel the looseness in the parts. Worn point cam shaft shoulders, which ride inside the right side camshaft, can induce timing errors as the point cam wobbles around during each revolution. This is especially noticeable with normal Type 1 engines where one point cam lobe opens two different sets of points that are located across from each other.

Additionally, what I have noticed is that the point cam seal lip grip, enhanced by the coil spring on the back side, grabs the point cam with enough tension to prevent it from returning to resting position at idle. The seal friction, plus the inherent play in the camsprocket components, prevents accurate ignition timing at all engine speeds. This CB77 Type 2 engine can be statically timed at the F mark on both cylinder rotations, but when the engine is started up, the spark timing jumps out five to ten degrees immediately and quickly ramps up to the full 45 degrees, well before the stated 3300 rpms, in most cases.

Controlling the idle speed/quality requires the cooperation of a stable idle spark timing setting, balanced carburetor slide openings, idle mixture screws that are functioning properly, the correct float level setting, idle jets that are right for the application, even compression readings, no air leaks at the carburetor flanges and insulator o-rings and minimized oil consumption. If any of these functions are compromised, the entire engine function suffers, especially at idle. Hampering effective combustion is the fact that the intake ports are not directly mirror images of each other. One intake charge is aimed directly at the spark plug for one cylinder, but the opposite side fuel vapor flow points away from the spark plug tip.

The only way to totally control the spark advance on these engines is either the crank-mounted system from Germany ($500) or an electronic spark advance system using the point cam as an initial trigger. The problem there is that you need to lock up the spark advance system permanently, so the electronics can do their thing correctly. Dealing with a nearly 60 year-old design presents more and more problems as the parts age and wear over time.

1 comment:

  1. Thanks Bill, your analysis provided insights and possible solutions to some problems I've been experiencing. As much as I enjoy mechanical archaeology working on my 305, it can get frustrating.