Cl100 tuning problems?

benroloff

Member
So I’m new here and there seems to be a lot of vintage Honda 100cc fans out there with a lot of knowledge so I was hoping you all could help. To make a long story short, yesterday I bought a cl100 for very cheap from a friends who didn’t know how to work on it. It had no spark so I just cleaned the points and the coil grounds, and I got the spark back. The tank is pretty nasty so I just hooked up a bottle with some gas and got it fired up. The bike seems to have pretty good compression and ran well at first. It was a little smokey but that was to be expected as I had put some oil in the cylinder and I don’t think it’s ram in at least ten years. It idled really well and seemed to rev out fine. Later I came back and started it again and now it wouldn’t rev out very well. Then next time it wouldn’t idle. Now it’s difficult to even get to run off of choke and it won’t idle at all. I pulled the carb apart (again) and cleaned the jets and all the passages and that didn’t seem to help at all. I’m totally stumped here. I’ve had some experience with almost identical carbs to this one before (1984 TRX200) and never had problems like this. I know the valve lash needs to be adjusted a little but it’s nothing too out of spec. There is a hole in the exhaust pipe and I know these engines run like crap with out a proper muffler. But I covered that up temporarily and anyways the first run, and best run with it was with the hole open. The floats sound like they have something in them and I know that fluid in the floats would make it run poorly, but it’s always had that in them so I don’t think it’s that. I’ve never heard of something like this happening. Maybe a carb rebuild? Just new floats? Whole new carb? Not even a carb issue? Any suggestions are appreciated.
 

irk miller

You've been mostly-dead all day.
DTT BOTM WINNER
Assuming all checks have been made and you've narrowed it down to carbs: Are your plugs wet? Are your float bowls filling up. The float needles can get stuck on old, dirty carbs and not allow the bowls to fill. If the bowls had any white powder (aluminum oxide) there's a good chance this is what is going on. Polish the float hole with a q-tip and clean the float jet. If it's rubber tipped, make sure the rubber is still good and soft. If it's metal tipped, make sure it's nice and polished and not corroded.
 

teazer

Well-Known Member
DTT BOTM WINNER
To build on what Irk just said, it sounds like it is way too rich. First start was cold and it ran. Each subsequent start, it was warmer and finally was so rich it wouldn't run.

Pull the plug and dry it off and then get a new float if that one has fuel in it it will not shut off properly and the motor will flood and die.
 

benroloff

Member
Ok leaning towards a non fuel related issue. I realized last night that it was possible that ignition timing moved somehow, so I went to check this morning and there was continuity across the points no matter if they were open or closed. This obviously suggested a short somewhere else. The problem goes away when I unplug the ignition coil so does this suggest a bad coil? Or am I going to need to chase a short somewhere else in the harness?
 

benroloff

Member
Ok update on the update. I checked coil resistance, but not 100% sure if I did it right. Between the two leads on the coil (black and blue) was about 2 ohms. Between the spark plug wire and the blue wire, as well as spark plug wire and black wire, I got about 3.5 mega ohms, which unless high school physics is letting me down, is equivalent to about 350,000 ohms? That’s like way off right? Sounds like a bad coil to me. Btw spark was like a purple color, compared to white or blue when I did the first start up.
 

irk miller

You've been mostly-dead all day.
DTT BOTM WINNER
Most motorcycle coils are 4 ohms/14k ohms. 2.5ohm coils are not uncommon for electronic ignition bikes, iirc. But 35k ohms seems like a lot.
 

benroloff

Member
Most motorcycle coils are 4 ohms/14k ohms. 2.5ohm coils are not uncommon for electronic ignition bikes, iirc. But 35k ohms seems like a lot.
350k ohms not 35k. I think that has to be my issue. 35k would be within reason 350k is way too much I would think
 

benroloff

Member
Ok so I ordered a new coil, because even if that doesn’t fix the problem, it can’t hurt to replace something that old. So my next question is in regards to the coil mounts. At first it seemed only logical to me that the coil must be electrically grounded to the frame through the “core” of the coil, which on these coils is also where the mounting holes are. But I have read some things online and have seen that not all coils need to be grounded? There is no evidence in my workshop manual that it needs to be grounded? It seems that it should be grounded but maybe the standoffs between the mounting holes and the frame need to act as an insulator?
 

irk miller

You've been mostly-dead all day.
DTT BOTM WINNER
When they need to be grounded, it is often through the mount. The wiring diagram in your FSM will show a ground to coil in the drawing. If it doesn't have one, then you don't need one. That being said, running a ring connector from a coil mounting bolt to the ground circuit in your harness is a great way to add a grounded connection for the rest of your grounding needs.
 

benroloff

Member
When they need to be grounded, it is often through the mount. The wiring diagram in your FSM will show a ground to coil in the drawing. If it doesn't have one, then you don't need one. That being said, running a ring connector from a coil mounting bolt to the ground circuit in your harness is a great way to add a grounded connection for the rest of your grounding needs.
Yea it would seem to me that it must be grounded through the mount, but I thought I ought to check before I went any further as I’d rather not wreck anything and I know that the wrong things being grounded can result in premature failure. I also bought new floats. I was able to patch the old ones but for $13 is it really worth the possible headache? Thanks for all the help!
 
I think the FSM wiring diagrams are a bit confusing in this regard, and the coils are indeed grounded through their mounts. Check out the "Honda Motorcycle Electrical Systems" manual (a fun read and probably available online free with a little searching). It clearly shows the coil for battery ignition bikes as grounded.
 

whynot

~~~If it ain't raining, I'm riding~~{iii}?~~prost~
my little cb100 was the same way, won't run worth a darn.

bought one of these and she fires first kick even after sitting for a week or two.
 

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benroloff

Member
Ok so I got the new coil and the bike does run exponentially better than before, however it has a huge bog when you get on the throttle. If you roll into it if seems to rev out fine, but there is still no power. I first diagnosed this as a fueling issue as it ran better richer, so I set the clip on the needle one set higher, and that seemed to help general performance, but the bog is still there. I have new floats on the way to replace the slightly leaky old ones, but should I also be looking at a whole carb rebuild?
 

teazer

Well-Known Member
DTT BOTM WINNER
Ok so I got the new coil and the bike does run exponentially better than before, however it has a huge bog when you get on the throttle. If you roll into it if seems to rev out fine, but there is still no power. I first diagnosed this as a fueling issue as it ran better richer, so I set the clip on the needle one set higher, and that seemed to help general performance, but the bog is still there. I have new floats on the way to replace the slightly leaky old ones, but should I also be looking at a whole carb rebuild?
Absolutely.

It won't take long and it is a great first step.
 

benroloff

Member
Absolutely.

It won't take long and it is a great first step.
I think your right. I wired up a modern rectifier, just a simple $3 bridge rectifier off Amazon, and that seemed to help idle a little and maybe even power, and with a little more carb tuning I got it to a rideable state, but it still has the bog so I think I’ll do the full rebuild cause what can it hurt for $13 right?
 

teazer

Well-Known Member
DTT BOTM WINNER
Get the carb right and eliminate it as a possible problem. If not you will never get it to run right. Our old bikes are really simple but attention to details makes a big difference to how well they run.
 

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