CT70 runs, won't idle?

6

69ST

Well-Known Member
This might come across as being harsh, it isn't meant to be, that's just my process. Okay, onto biz...

If the same carb works properly one one engine, but not another, then it's not the problem...ostensibly. According to what you've posted, so far, neither the pilot airbleed (idle mixture) or idle (throttle stop) have any effect..."holy red flag, Batman!!!" You should be able to see the throttle opening, which corresponds to idle, increase as you rotate the idle screw; if not, something's wrong. The pilot airbleed screw controls the amount of air entering the pilot circuit. Turning it clockwise decreases the amount of air...to a point. At roughly 1/4 turn out from seated, the pilot circuit is effectively shut off; fully seated, the circuit is inoperative. There needs to be a certain amount of airflow to activate it, i.e. create vacuum to draw fuel.

I've no idea what you're talking about with spacing the plug "1/8 inch away from the engine" to get spark. That's .125"!!! If you're getting spark across a gap that big, then your ignition system is damned healthy, right up to the plug. Electrical energy takes the path of least resistance, and the larger the spark gap, the higher the voltage required to arc across it. Based on the (so far) confusing info provided, I'd be looking to throw a plug, points & condenser at this motor...along with verifying compression & valve lash. How's the intake port look? A leaky valve could kill idle quality (power too) and the most common situation is a "tuliped" (worn) intake valve. If the port is black, you're getting reversion and it's been going on for some time.

As for the primary coil impedance value, it's not really that important to verify it. Only rarely will you have something other than a no-spark situation. It does happen but the number of examples I've run across can be counted on one hand, with fingers leftover. I've been working with small engines since the late 1960s...fwiw.
 
kirrbby

kirrbby

Well-Known Member
I'm still stuck on the earlier questions...does the idle screw raise the slide?

Does your idle screw look like this?
(The one on the left)
IMG_20180328_175545147.jpg

When you screw it all the way in does the slide rise up a bit like this?
IMG_20180328_175700115.jpg

Or is it still bottomed out like this?
IMG_20180328_175913038.jpg

Got any pictures??
 
M

mk2davis

Member
I'll try and get pics for everybody.

Yes, the same carb that wont idle in bike A idles in bike B just fine. So I don't think it's the carb either. That's why I've been looking at electrical. "holy red flag, Batman" is right.

When I say the plug is "1/8 inch away from the engine", I am not talking about the spark plug gap. When I check for spark, I hold the electrode against steel on the motor like a screw or bolt, and if things are good then I can see spark across electrodes. In this case, I do not see the spark, but I can hear it. However, if I hold the electrode 1/16 to 1/8 of an inch away from the steel on the motor, then I do see spark, both electrode to electrode and electrode to motor.

I replaced the condenser and lightly sanded the points, with no change. I put in a new plug, and things did change. It sparked as expected (with the plug grounded out on the engine) and started the bike, but died before I could get heat into the motor. Then this plug started behaving just like the other one did.

So it appears that something or something I am doing is killing plugs. The first plug was brand new (bought earlier that day), and the second plug didn't last 7 seconds of running in the bike.

valve lash is .002-.003" for both intake and exhaust. The jug and head is brand new
 
M

mk2davis

Member
20180328_160250[1].jpg This is with the idle stop all the way in.

20180328_160203[1].jpg This is with the idle stop most the way out.

Neither of these positions changes how the bike idles.

Almost everything on this bike is stock. The one thing that isn't is the rectifier. It is an aftermarket modern type. Could this be killing spark plugs/inhibiting idle? This seems unlikely, but I'm open to it all.
 
hrc200x

hrc200x

Active Member
Sorry if you've already answered this, but when you swap carbs between the two bikes are you swapping the carb tops, slides, and needles or leaving those attached to the throttle cables? When the new spark plug quits working or fouls does it look black? Have seen main jets drilled out before, so instead of being a size 58-65 which is what most ct70's run they might be in the 80s or 90s. From troubleshooting in the past, people also get confused as to which way the choke lever goes, should be back or down for normal operating, just trying to throw as many ideas out there as possible.
 
A

allenp42

Well-Known Member
No on the rectifier or even the battery for that matter. Justy to make sure I understand, you get a nice spark with the plug when the plug is close (1/8") to the engine, but no spark when it wrench side touches the head? That is crazy and not sure what to make of it.

To answer one of previous questions. You'll read ~1.5 to 1.8 ohms even with the condenser in place and points open, & less than 0.5 ohms if the points are closed. I expect you'll see something very close. If you have a new condenser, would assume for now that your condenser is ok. I too agree with racerx - Doubt you've got a problem with the Stator Pri coil.

I've hard carb problems, ignition problems, but nothing as strange as this one. For sure, I would take racerx's advice and do a compressor check.

Honestly, I am struggling with the fact that it's taking extra throttle to maintain an idle -and- you don't have a leak -and- it's not running rich at ~1300 rpm. Maybe I'm overthinking it or in the weeds.
 
M

mk2davis

Member
Allen, I feel your frustration. It is a bit of a head scratcher.

Does anyone know the thread size for the spark plug hole? My compression tester doesn't go that small.
 
OLD CT

OLD CT

Well-Known Member
I would install brand new points, as a bike ''can'' show a visual spark and still run like :poop:SH*T:poop: because the points are fouled. Been there done that.
A compression test is always the first place to start on any newly acquired bike, just to know the bike is healthy. You will have the answer soon.:) New points and set em at .016 gap.
 
M

mk2davis

Member
Compression test shows 80-85 pounds. This is a bit discouraging on a brand new top end. This was done with six kicks, some drops of oil in the piston to help the rings seal, and throttle open. I guess the next step is to do a leak down test to see where I'm losing pressure.

My best guess is that plugs are fouling because of an overly rich condition, and this same fuel ratio is what is taking the bike out of range to find idle. What I don't understand is how low compression would lead to a rich condition.

I'm confident that the carb itself is set up correctly, as the carb works well in another bike. I COULD adjust the carb to compensate, but that would only mask the true issue.
 
6

69ST

Well-Known Member
mk2davis said:
Compression test shows 80-85 pounds. This is a bit discouraging on a brand new top end. This was done with six kicks, some drops of oil in the piston to help the rings seal, and throttle open. I guess the next step is to do a leak down test to see where I'm losing pressure.
Click to expand...

That's weak, as in beyond anemic. I'm surprised that it even runs. 110-120 usually means soggy/weak running, 90-100 is the ragged edge, below which they usually won't even fire.

Retry the compression test, with the carb off the engine. A closed throttle can result in low cranking compression. If you suspect that the pressure gauge is off, do the shadetree compression test...rotate the kickstarter, slowly, by hand. If you feel sharp resistance on the compression stroke, the top end if probably healthy. If you have a tough time figuring out when it's reached the compression stroke, you've got a massive leak. Best guess would be the valves.
 
A

allenp42

Well-Known Member
X2 on source of parts? 85 lbs is below the threshold for even being able to run.

Did the kit include a cylinder, piston and head?
 
hrc200x

hrc200x

Active Member
I bought a bike that someone had installed the "china ebay rebuild kit" on that came with piston, cylinder, and head for around $130, it runs, no smoke, but compression is around 80 or so. I believe the ebay china kits aren't correct for the '69-81 ct70 rod length and are for the newer 70cc rod length, resulting in low compression when installed on a 69-81 ct70.
 
M

mk2davis

Member
It was from ebay, and it was piston, jug, and head. It was missing little things, like the o-rings and stuff, so I had to source them. It did run, and it would do an indicated 30, although it never felt sharp.

Leak down test confirmed blow by on both valves.

I'll try and remember to do a leak down once I do some valve work. I'm curious to know if the pressure difference is due to design or (I'll be generous) loose tolerances.
 
hrc200x

hrc200x

Active Member
Do you have a link to the kit you bought, unless they have changed the kit to be for the 69-81 short rod motors the combustion area is probably too large and won't generate the compression it should. How did you test the valves for leekage? Guess its possible the valve adjustment is set to tight.
 
6

69ST

Well-Known Member
Not much choice, at this point...the head's gotta come off. You might be able to get by with lapping the valves. IMHO, the odds are not favorable. Considering what you've seen, so far, I'd want to verify the valve seats, faces & contact bands. Alternatively, you might consider rebuilding the original head. Whilst the head is on the bench, verify compression height. If the piston sits down in the bore, at TDC, you've got a 12v/"long rod" piston.
 
M

mk2davis

Member
RacerX nailed it! 20180403_120830[1].jpg

A comparison between stock and ebay available... 20180403_124951[1].jpg

The issue initially was that this motor didn't exactly come with a head. Or jug. Or piston.
 
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