Motorcycle Forum banner

1 - 4 of 4 Posts

·
Registered
Joined
·
7 Posts
Discussion Starter · #1 ·
I recently rebuilt the the top end an my newly acquired 1966 C100 due to it being seized when I got it. It's been bored 1st over with new piston and rings and the valves were lapped and seating perfectly. Put it back in the frame, reconnected everything and she fired up on second kick. I was super excited but short lived ever since.

Here's the scenario:

Will start 1st kick hot or cold, choke or no choke.

Get it to idle somewhat good by adjusting the idle screw, rev the throttle and idle hangs again.

Back off the idle screw so it won't idle, rev the throttle and the revs do not hang but it will die if I do not hold the throttle open enough to keep it running.

There is no hesitation in throttle response from low rpm to as high as it can go.

The hanging isn't from a sticky throttle or cable carb slide and the float is working fine also in stopping gas flow

The air adjustment screw does nothing no matter where I set it and adjustments of the idle screw on the slide is super touchy.

Spark plug is black like it's running rich and the bike will start with or without the choke when cold also indicating running rich.

I rode it around for about twenty minutes and it ran fine except for the hanging idle.

What I have done: Took the carb apart and soaked everything in ChemDip for hours, blew everything out with compressed air and reassembled several times now. Tried new jets (sometimes one at a time) and also a total carb rebuild kit and the results are still the same.

New points, condenser, spark plug, battery. Compression 140 lbs, valves adjusted to .002" cold originally, backed off to .003" at present. Timing is dead on.

Also checked for vacuum leaks with carb cleaner and propane with no luck although that would indicate a lean problem.

Where the heck do I go from here? Any help appreciated.

Maybe a new carb from Thailand? Anyone ever use one?

I have restored several '60's Honda twins but this is my first single and I am totally stumped .
 

·
Six-String Jockey
Joined
·
1,860 Posts
Well, I had a very similar problem on a CT110 the other day. Turned out to be the main jet needle. It was set on the improper "Slot" at the top of the needle where it hangs down from the sliding barrel. If your plug is really getting black you need to lean out the mixture . . . we did it on the CT110 by moving the needle to where it hangs down the farthest it can from the barrel. Make sense?
 

·
Registered
Joined
·
7 Posts
Discussion Starter · #3 · (Edited)
Figured it out. Finally! Been at it for several weeks and just about ready to throw the carb out the window.

For those of you familiar with the downdraft carbs on these C100's, where the float is attached to the carb, is where the main jet is located in the carb body. In the same area is another hole that if you look closely, runs up to the pilot jet. In that hole on mine is a brass plug so I assumed that the pilot jet was drawing fuel from somewhere else but no matter how many times I looked at the carb, I couldn't figure out where the pilot jet was getting the fuel from.

I decided to manually drill a very tiny hole through the plug with a pointy thin piece of hardened wire. It took about 30 minutes hand twisting back and forth with easy pressure to get through the plug. Flushed the hole with carb cleaner and the fluid came out from the hole just below the threaded part where the pilot jet screws into the carb just as I expected it should.

Reassembled the carb, and put it back on the bike. Fired on the first kick, a few tweaks and adjustments and she is purring like a kitten. No more idle hanging, the idle adjustment for the slide and needle works flawlessly. the air adjustment screw works great. It's like a whole new bike.

Which brings me to another question...Do all these carbs for the C100's have a brass plug in the hole going to pilot jet next to the main jet?
Are they supposed to have a hole in them so the pilot jet can draw fuel? Seems only logical to me that it has the hole. Did someone that didn't know what they were doing plug that hole?

I knew that pilot jet wasn't getting fuel and it didn't make sense to me that that hole was plugged but I thought it came from the factory like that so I kept ignoring it. Glad I figured it before I tossed the bike in the lake behind my house. It came very close to happening.
 
1 - 4 of 4 Posts
Top