20 Most Recent 1970 Honda CB 100 Questions & Answers

Does it still have the original sparkplug cap? If so, from it's age, the resistancein the cap has increased past it's optimum performance. Replace the cap or (if possible) pull the resistor out and run it direct from the coil only if it's apoints style ignition not a CDI.The resistor cap was put on all engines back in those days so it wouldn't interferwith radio and television transmission signals.Now days with CDI (capassitor discharge ignition) they need resistance on the wire or cap or both, to clean up the high voltage going to the sparkplug so it don't hurt the CDI. Running a old engine with points with no resistance wont hurt a thing.

Sounds like a bad coil. Did you first put new plugs in.? If you did then it is time to get coil tested. If not coil could be voltage regulator

Congratulations on owning a fine classic motorcycle. Best to start about 1 1/2 turns out. Maybe you can get some better advice from the guy or gal at the parts counter at your Honda Shop.

It is possible for cells to go bad in a battery. Anything that remotely resembles mold in a motorcycle battery is not mold, but it is bad news. It should be a 12 volt system.- 6 fill caps on conventional batteries.

I am assoming that you are talking about the marks on the points plate.
Before you put the points and point plate on, check the spark advancer for proper function. Twist the point cam and it should snap back to the correct original location.
If not MARK the relationship between the point cam and the advancer base... HINT.. you can put it together backwards and the bike will NEVER run!

The points should mount up on the plate with no problems, other than check carefully where the point wire goes onto the insulated bolt and don't let the terminal end get grounded out on the point base/plate. Point plate and points should install so the points are at about 10-11 o'clock location when the plate is secured to the cylinder head. The point wire enters the point base plate at the far right and runs underneath the point cam to connect with the contact breaker on the left side.

When installing new points, be sure to check and clean the point faces of any grease or oil that is a preservative. Put a drop of oil on the felt and a light coat of point grease (if you can find it anymore at auto parts stores) on the point cam. Because the point rubbing block is new, the little high spots will wear down initially in the first couple of hours of operation, so you will probably have to come back and recheck the gap and timing after some run-in time. The gap needs to be about .014-.016" checked at the highest point of the point cam. Turn the engine over with a wrench and watch the points open and close, then observe where they are open the widest and check the gap with a flat feeler gauge. If you set them about .016", then they will close down to around .014" or so after they have seated in. This is how it is done in a perfect world of new, unworn parts.

The inherent difficulty here lies within Honda's choice to cutaway the cam bearings to facilitate camshaft installation. This area tends to wear rapidly, causing the camshaft (and point cam) to wobble around during rotation, even when the camchain tensioner is fairly snug. If you grab the end of the point cam or camshaft with your fingers, try to move it from side to side/up and down. ANY movement will translate to a change in the point gap, which makes a change in the ignition timing. If it is really loose, the timing will be difficult to set and the running timing will be erratic. The only way to fix it is to put on a new head and cam and/or POWROLL may still be able to convert the head/cam to a needle bearing design... all a lot of work and expense for a 30 year old tiddler bike.

The timing, once the gap is set properly, is adjusted by moving the point plate back and forth until the points JUST BARELY open when the F flywheel mark is aligned with the pointer mark on the case. You can check this with a 6v test light or a ohmmeter or audible test unit. Recheck the point gap when you move the plate, as it tends to change the gap. Go back and forth until you have both the proper gap and the correct point opening moment set.
This bike MUST have a fresh 6-volt battery installed to run. It is NOT a magneto ignition.
Hope this helps out. Let us know how it goes.

The hose on the bottom of the carb is just a drain hose. It connects to the bottom of the carb and nowhere else.

float adjustment honda 100 1978

You don't have to split the case to replace kick starter shaft,and you don't have to remove motor,drain oil,remove kicker,then side cover.really not that hard,go for it,you will learn alot.

You may want to get a service manual to work on this one. The expanded parts diagrams that are available for ordering parts online will show you all the parts. If you can not find some picture of the 100cc models the 1974 CB125 is basically the same. The design of your motor dictates that you will have to take the engine out of the frame, pull the top end off, and split the cases to get in to the area that houses the mechanism you want to replace. You may be able to pull good used parts out of a salvage motor locally or even purchase a used motor or donor bike if the price is right. You will need a complete gasket set for the motor. As for the change in the kick shaft size, it may be that the shaft was replaced before with a shaft from the CB125. You may have to measure your shaft and then measure the shaft on the motor you are taking the replacement part out of. Chances are you need to replace several parts to repair the kick mechanism. You will have to disassemble the motor to find out everything you need to properly fix your classic.

Mating surface at the bottom of the carb to the bottom of the float.

You need to check for 12 volts on the coil with the switch on. If the points are open the coil should have 12 volts on both sides, if the points are closed you should only have voltage on the coil positive side. recheck all wiring from the coil to the points/condenser plate, check the wire at the points connection should be hot with the switch on and the points open. The point plate should be grounded. Check for any oil or grease in between the points contacts, the contacts have to be dry. Last check for the rotor shaft to be turning, as you turn the engine the rotor shaft should turn with the engine to open and close the points. recheck your gap and be sure the plate does not move up/down.

Either dirt in the tank is getting into the carb or the fuel line is deteriorating and pieces of the inside of the line are getting in the bowl. Disassemble the petcock and replace o-ring, screen, and four hole washer. Put on a new fuel line and consider adding a small inline filter typically used on a lawn mower. This should even take care of rust that may have formed in the tank over the years. If you have bad rust in your fuel tank a chemical cleaning kit can be purchased to soak the inside of the tank and clean out the problem.

JP Cycles, but not sure of the size of Carb. but they will know.

The float in the float bowl is stuck open. It could be a bit of trash from the gas tank stopping the float needle from sealing shut. Remove the float bowl and gently lift the float up and down while spraying carb cleaner at the float needle. Install an in-line fuel filter to stop the problem in the future.

Remove the points cover. Make a scratch mark with a pin or nail across join of points backing plate and the outer housing and loosen two screws holding the backing plate for the point breaker just enough to rotate the plate 1mm anti-clockwise. Tighten retaining screws. Try starting now. If no joy, repeat procedure moving clockwise 1mm forward of the mark. This can be achieved with the slot end of a screwdriver between the two lugs on plate. Cheers.

make sure kill switch is not killing spark even in the run position. Chech all fuses (test them) make sure power to and from coil is there. the CDI is not repairable but could need replacing.(not sure if the cb 100 has this with points) is the condenser good? if it is it can be charged and discharged with a mulitmeter.(use ohms resistance to charge it and volts dc to read as it discharges CAREFUL it can BITE)

Are you saying it pulls strong when in 1st gear but doggish on 2nd and 3rd?
If yes, check your carb jets.

That a loaded question,how can I tell you what to replace without seeing the bike .The normal thing if a bikes been standing for a long time is tyres and oil /petrol .brake lines any rusty parts I/E bolts and nuts .Brake pads or shoes .And a carb stripp down .and petrol tank flush out

Hi and welcome to FixYa,

For the wiring diagram of a Honda CB100, please click here.

Good luck and thank you for asking FixYa.

when you turn the drain screw you will see fuel run out this drain line, its meant to keep fuel from just wetting everything when carbs are purged.