CT70 runs, won't idle?

[mk2davis]

This one has me stumped. I bought a bike in a box which needed a new top end. Put that on with a stock Keihin carb, and it runs decent, but the sucker will not hold idle. I'm holding the throttle about a quarter turn open. LET ME CLARIFY: The bike itself will idle, as long as I hold the throttle in position. If I let go, the throttle rotates closed about a quarter turn and the bike shuts off.

Carb has been cleaned with float level at .780", pilot jet is new, main jet clean, stock needle at it's middle position. The air/fuel mixture has zero affect (effect?), and the idle set screw is all the way in.

Cam timing is set, valves set at .002-.003". Ignition points break right at the F on the flywheel.

I'm going to swap the current carb with another Keihin because I'm running out of ideas. I don't like not knowing what the issue is, and see this as a valuable learning opportunity.

Because the air fuel mixture and idle set screws have no impact on the bike, I'm assuming that something is drastically wrong. I feel like I'm not even in the ballpark.

After some tinkering, I've "accomplished" the following:

Carb cleaner around the head, manifold, and carb yielded no change.

I have a second bike that I using to troubleshoot
- Bike A with carb A - ran fine, idled while being managed but didn't even think of idling on its own
- Bike B with carb B - ran and idled well - this will be used to swap parts
- I took the carb top off of bike A (A stands for "axcellent) and put it on bike B (B is for "beater"). Idles and runs like a champ. Bike B doesn't have an air filter either.
- Carb B on bike A - bike runs and idles real low for a few seconds, and then dies. Out of adjustment for both idle and mixture screws

Any suggestions would be bonus

 

[69ST]

The pilot jet is probably clogged...which doesn't take much. The fuel metering orifice is minuscule. If you're lucky, you may be able to run a thin wire through it. Try the wire from a twist tie, the kind used for bread loaves. You'd still be well-served by sourcing a minty-new replacement pilot jet a quick, inexpensive and permanent fix.

 

[mk2davis]

Unfortunately, I am not that lucky. I have a new pilot jet in carb a, which runs fine in bike b. And carb b which ran good in bike b, runs like sh!t in bike a. It appears that whatever my issue is, it's not in the top part of the carb.

So it's .... something else?

 

[SteveB]

Are your Cable lengths the same.Short Cable for short Cap Long Cable for tall cap. Next time you take the Carb off the bike look in the intake side to see if the slide is retracting all the way closed. you may find Carb B from on bike A there is a gap.

 

[Slyclone]

Are your Cable lengths the same.Short Cable for short Cap Long Cable for tall cap. Next time you take the Carb off the bike look in the intake side to see if the slide is retracting all the way closed. you may find Carb B from on bike A there is a gap.

I was thinking the same thing. Are the cables good?

One could be cracked and freyed. Or at the least after 40 years streched.

Also the throttle setup, in the bikes handle bar small cable holder wears. I have a handful of them here worn out on bottom where screw inserts. Need a new one ($3) to get the throttle back to where it once was on my z50. I borrowed one from my bike to put on my CT 70 years ago..

 

[mk2davis]

Thanks for the lead! I know my current cables are new, but that doesn't mean they are right. I found lengths on CHP website:
Short cable with short cap: A= 34.4375" (861mm), B= 31.0625" (776.562mm)
Long cable with tall cap: A= 35.0625 (876.56mm), B= 31.562" (789.05mm)

B is sheath length, A is total length of cable. I'll measure it out and let you know what I come up with.

 

[kirrbby]

Sounds to me like you're saying that it's NOT a carb...or cable problem. If the carb runs and idles in another bike... the jet must be clear, and you don't even need a cable to set the idle. You're down to maybe a clogged fuel line, bad gas, could it be electrical?? or maybe low compression?
Intake leak?

 

[hrc200x]

There was different slide lengths too, that probably changed when the cap height changed.

 

[kirrbby]

The cable is not what sets the idle, the idle screw does. When you screw it in, it hits the slide. The slide has a bevel along that edge, the screw hits the bevel, forcing the slide up...the more you turn it in, the higher it raises the slide. When the slide goes up, it allows air to flow under it, the air also is what sucks fuel up thru the pilot jet and sends it into the intake/engine...the higher it goes the more air, and fuel, and the higher the idle.

Soo, is the idle screw Workin???
I assume it's workin if you can set the idle in a different bike/engine.

 

[kirrbby]

Then, when you twist the throttle, the cable pulls the slide up even higher. This allows the engine to suck a LOT of air thru the carb...enough air to be able to suck fuel thru the MAIN jet...more gas...and more air...more revs.

 

[69ST]

Have you verified that there are no vacuum leaks? Doesn't take much of one to kill the idle and they don't always manifest in hard-starting (especially when cold).

The other possibility is more ominous...a blockage in the carb body itself. If the pilot airbleed circuit is obstructed, no fuel can be drawn through the pilot jet.

That said, before tearing the carb apart, visually check the idle stop. Look into the carb throat, then turn the screw clockwise; you should see the gap, between the venturi & slide, grow larger. If not, there's your problem and it's an easy fix.

 

[mk2davis]

Sounds to me like you're saying that it's NOT a carb...or cable problem. If the carb runs and idles in another bike... the jet must be clear, and you don't even need a cable to set the idle. You're down to maybe a clogged fuel line, bad gas, could it be electrical?? or maybe low compression?
Intake leak?

It doesn't appear to me that it is the carb. The A carb works well in bike B, and the B carb that worked well in Bike B has the same issue in Bike A. I don't think it's a bad fuel like because the bike runs fine off idle. It maybe electrical, but I'm not sure what the issue would be. I don't think its an intake leak as carb cleaner between the carb/manifold/head doesn't have an effect.

I do have another "long" slide I am playing with now, but the cable lengths between bike a and b are the same. Just for shits, I built a tall cap carb, but that isn't proving useful.

The idle screw is all the way in, and doesn't seem to effect the idle one bit. I know it's supposed to. It is not. And the A carb runs in bike B like a honey.

With the fuel off, I was running the bike to rid the bowl of fuel. As the fuel level in the bowl was dropping, idle was rising as things were leaning out. How stupid is it to run a smaller pilot jet and see what that does?

 

[69ST]

The idle screw is all the way in, and doesn't seem to effect the idle one bit. I know it's supposed to. It is not. And the A carb runs in bike B like a honey.

With the fuel off, I was running the bike to rid the bowl of fuel. As the fuel level in the bowl was dropping, idle was rising as things were leaning out. How stupid is it to run a smaller pilot jet and see what that does?

To which screw are you referring...the idle speed/throttle stop, of the pilot air bleed?

Don't bother even looking for a smaller pilot jet, for a stock carb. Turning the airbleed screw counterclockwise leans-out the idle mixture and there's more than enough range to go terminally lean.

 

[mk2davis]

The idle speed screw is all the way. It started about 2 turns out, but any position has zero effect. The pilot air bleed starts at 1.5 turns out, and that can usually get me in the neighborhood, but again no effect.

I did find one thing that has an effect. Through my troubleshooting, I have no spark. I had spark, but no longer. If I pull the spark plug away from the engine an eighth of an inch while turning over, I get spark. So I tested the secondary side of the coil, 11.5k ohms. Isn't spec (both of these reading with the spark plug boot) supposed to be 19k+/-? It's a new coil, too. Bummer. I'm going to swap in another coil, and see what she do.

If someone can tell me how rpms effect resistance in a coil, I'd love to hear it.

 

[cjpayne]

You switching out the carbs to no effect, made me wonder if you have a weak primary stator coil. I had one, that when the engine warmed up, it would not idle or just fail. If you get desperate for answers, you could always swap out the entire stator plate assembly. Its a pain, but will give you more info. I'm assuming that both bikes are 6volt 3speeds.

 

[allenp42]

If both carbs work fine on the "other" bike, neither carb works on the "problem" bike, and you don't have a vacuum leak, then I tend to agree CJ. The part that makes me wonder, though, is that it seems to idle fine when lean?

So I tested the secondary side of the coil, 11.5k ohms. Isn't spec (both of these reading with the spark plug boot) supposed to be 19k+/-? It's a new coil, too.

I just checked 2 ignition coils and both measure about 10k ohms. However, I'm not using a resistor type of plug cap, which I think adds about 5k ohms. You may also want to check the primary of the coil as well. It should measure ~2.5 ohms. Just from personal experience, coils rarely fail. Key word is rarely.....

Since I didn't read anything about points, condenser, or a new plug, No harm and not a lot of money to rule these out.

 

[mk2davis]

All good thoughts. Thank you guys for a wealth of ideas and suggestions. Both of these are 6v 3 speeds.

I am no electrical engineer, but I took a class, many years ago. With the plug grounded on the motor, no spark. If I give that "circuit" more resistance by an 1/8" of air between the plug and the engine, I get spark. Now I am skeptical, because as CJ noted, coils rarely fail. The fact that I do get spark once the circuit is given appropriate resistance, I am leaning towards the stator coils being okay. I did get shocked when holding the bare plug while kicking it over. I was desperate for an unforgettable confirmation, and I got one!

Allen, I'm leaning towards a bad ignition coil. My readings were with the resistor style plug, so it should be around 15K, and I am 1/3 off of that. Tomorrow I'll pull the tank and swap those out and see what happens.

Points seem to be functioning as intended, 0.4 ohms closed, 1.6 ohms open, which is consistent with what I've seen in the past on other bikes. Plug was brand spanking new. Condenser is not a suspect right now. I haven't seen a reliable way to test that part, if someone does know how that would be appreciated.

 

[allenp42]

Your stator pri coil resistance is spot on to what I normally see. You mention that the coil is new, but I assume aftermarket? What were the symptoms with the original coil?

No good or reliable way to check the condenser. Sometimes you can tell if it's bad but no good way to tell if it's good. Since we're going down a path of weak spark, or at least it smells that way, I would not rule out the condenser. It plays a key role in how hot a spark you get.

Just out of curiosity, you have to give it some throttle to keep it idling? By "idle", are we talking normal idle speed of ~1,300 or something higher? Any black smoke with the extra throttle?

 

[mk2davis]

The coil is aftermarket. The original coil was missing (a bike in a box). By "idle", I mean a normal idle speed, although it does tend to lope just a bit.

Turns out it's not the coil. The swap had no effect. One thing I did notice was that with the plug grounded out, I hear the spark but I sure as heck don't see it. Again, I see the spark when the plug is about 1/8" away from the engine.

 

[mk2davis]

The flywheel is coming off! I'm going to inspect and clean the points, and swap out the condenser. I know the body and the center of the condenser are supposed to have resistance, anyone know how much?

Speaking of resistance, I'd also like to know the resistance of the coils under the flywheel. Might as well check those too.