Wednesday, August 18, 2021

Trouble-shooting the early vintage Hondas…

I am on several forums dedicated to vintage Honda motorcycles and see a lot of similar questions and complaints about various aspects of owning and reviving these 50-60-year-old machines. I thought I would go over some of the basics once again for newbies that are just getting into the hobby. So, let’s start at the beginning… You bought it and now… See * at the bottom of the story first.


It won’t start… Does it turn over and feel like there is some compression being built up in the cylinder (s)? Test compression first, even if you don’t have a gauge. Put your fingertip in the spark plug hole and turn the engine over, however you can at the same time. Holding the throttle wide open gives the best results. If you do have a gauge, it should be reading anything from 125 to 180psi. If you are looking at 75-90psi, it won’t start and run at all. Try adjusting the valves first to see if they are leaking compression past the valve heads/seats. If that doesn’t help then it is time to pull the head/engine and find out if the valves are tight/burned or the piston is scored/broken from seizures in the past.


It turns over AND has compression… Does it have spark at the spark plug? Determine if the bike requires a battery or if it has a magneto (mostly 50-80cc singles). A bike with a magneto ignition system doesn’t need a battery to make the coil spark, but if you start up a bike that has a battery in it for the lights and horn and the battery is dead, any light bulb that is normally lit will be blown out from the uncontrolled charging system output.


For bikes with battery-powered ignition systems, you MUST put in a fully-charged battery to make the engine run. If you jump-start a dead/dying battery enough to get the engine to run, the light bulbs are also at risk. Most Hondas of that era do not have voltage regulators, preferring to use the fully-charged battery as a buffer to absorb the charging system output and prevent over-voltage situations.


So… now you have compression and spark, perhaps, but does the spark come at the right time? The ignition points are basically just variable electrical switches. Setting the point gap at the highest spot on the point cam is the starting point of the ignition timing exercise. Also, the point’s contact faces must be clean and shiny to maximize the spark energy in the coil. So, set the gap at around .012-.016” and then ensure that the points are closing together and making clean contact with each other. If the point faces show a > > instead of | | contact pattern then the condensers should be replaced. Bad condensers also cause a lot of visible arcing across the point gap when the engine is running. BEWARE of any aftermarket copies of the original Nippon Denso, Hitachi, Kokusan, TEK and other OEM point sets. Many copies have incorrect dimensions for the point rubbing blocks and setting the ignition timing is nearly impossible. Daiichi, SEV, Century and other brands are not recommended. OEM points will have ND or a different symbol for Hitachi and Kokusan stamped on the point bases.


Ignition timing is set by the points just opening at the F (firing) mark. Moving the backing plate back and forth will help you to dial in the correct ignition timing. On some twins, you have to set the left side points to the normal gap, move the backing plate to where they open on the LF mark, then you will have to change the right side point gap open/close until those points open at the F mark.


Once you are sure that the ignition system is setup properly to start the engine, then you have to determine if the mechanical spark advancer is doing its advance/retard function properly. Point cams can get hung up on rust or old grease on the cam base plate shaft and that will alter how the spark timing occurs. If the point cam is slow to return to full retard at idle, then remove, mark and clean the point base shaft and the point cam, so that they both move easily back and forth. Use special point cam grease on the point cam to reduce excess rubbing and friction during operation.


So, if your compression and spark timing are all correct, then it is onto the fuel system for cleaning and testing for full function. For carburetors which have been left standing with old fuels in the float bowl for months/years, the chances of the bike starting normally are very slim. For the engine to start up the idle jet must be clear and the adjacent air bleed ports in the carburetor throat must be open.

The carburetor float chamber must have the correct level of clean, fresh fuel in order to feed the idle and power/main jet circuits. Each carburetor has a specific float level adjustment which must be adhered to for proper operation. Also, there are air bleed ports in the carburetor inlet that must be clean for proper fuel mixing. Also, be sure that the bowl vent passages in the roof of the carb body are open.

If the carburetor needs to be cleaned, use an ultrasound machine to clean out the small passages of the carburetor body and any removed OEM parts. In many cases, the aftermarket carb kits are not accurately made, so try to use as many of the original parts as possible during reassembly.

Yesterday’s carburetors are deeply affected by today’s fuels, which are generally loaded up with 10% alcohol to reduce emissions and stretch out the fuel supplies. Because the fuel is somewhat diluted, there is less energy released when the fuel vapors burn. This situation creates a lean ratio mixture condition that generally needs to be corrected with larger-sized jets, at least for the main jet circuits.

You must check all the carburetor components carefully as they can be damaged by the old fuel vapors and solids that remain inside the float bowl after months or years of neglect Most older carburetors used small brass floats which can be compromised by the acids in old fuels which etch into the metal and create pinholes. These tiny holes will allow gasoline to enter the float lobes causing them to lose buoyancy and start to sink into the float bowl. When that occurs the float cannot control the fuel level in the bowl, which creates an overflow condition.

When fuel flows out of the overflow tubes at the bottom of the bowl, you have problems with either the float valve not shutting off or the float itself. Figure out which is at fault and repair it before moving forward with the startup cycle. If you are sure that the float level is set correctly and the float valve is doing its job, then look carefully at the overflow tube in the float bowl. They have a tendency to split along the lengths of them, causing persistent fuel leaks at the tube exit.


When setting up the carburetor body, install the idle mixture screw and back it out about 1-1/2 turns to begin with. Turn the idle speed screw in until it contacts the bottom of the slide and just starts to raise it upwards. Be aware that idle screws come in two functions: Air screws and fuel screws. When you back out an air screw, more air is added to the idle mixture. When you back out the needle-like fuel screws, more gasoline is added to the mixture. For carburetors that bolt onto the back of the cylinder head, look for warped flanges and flattened out o-rings that seal the carb to the insulator, which seals to the cylinder head. Insulators will either be sealed with an o-ring or a flat gasket.

Obviously, if you are working on a twin or four-cylinder model, your work will be multiplied and you will have to make sure that the carburetors are synchronized properly on all cylinders. Twin-cylinder bikes can be synchronized by watching the slides lift off of the idle speed screws. To begin, turn the speed screws in until you see the slides just begin to lift. Then, adjust the cables at the top of the carburetor using the screw adjusters. For 350-450 CV carbs, you will have to watch both of the carburetor cable arms to ensure that they both move at the same time. REMEMBER: For twins with standard slide-type carburetors the slides are side-specific. You must see the slide bottom cutaways in the throat of the carburetor. They must always face the air filters. If you reverse the slides, you will get fouled spark plugs and a very high idle when you start the engine. Yes, they will install backward!

Fuel: When the 1960-70s bikes were built, fuel quality was much higher in octane ratings. Regular fuel was 90 something octane and premium fuel was 100-105 octane and all of it contained lead to protect the valve seats. Unless you are buying your gas at a boat marina, airport or from a race gas station, you will have to deal with the usual 10% alcohol infusion with octane ratings of 87/89/92. Gasoline chemistry has changed radically in the last century and the octane ratings used in the last century are not necessarily a match for today’s fuel ratings. My rule of thumb is to use premium fuel for vintage Hondas, either with or without alcohol. What I have discovered is that with alcohol-infused fuels, the engines run leaner and often need a 5-10% increase in the main jet sizes to compensate for the alcohol additives in today’s fuels. My recent experience is that most CB77 Super Hawks run best on a #140 main jet vs. the stock #135 main jets specified from the factory. If your machine is tuned to full normal specs and is still giving performance problems, try stepping the main jet up a size or two.

*Of course, always start the process by checking to see if the engine has enough oil to meet the marks on the dipstick. Low oil levels can often lead to seized and damaged engine parts, so making sure that the engine has sufficient oil in the engine. You might want to drain and flush the engine oil FIRST before you go forward with the above steps. If chunks of metal or a lot of aluminum specks come out of the oil, there may be expensive issues going on inside the engine that won’t be helped with the above troubleshooting and tune-up steps.

If you have followed all the above steps, you should have a running motorcycle once again.

Bill Silver 8/2021

1 comment: