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Discussion Starter #1
Hi all,

I'm new to working on bikes, just picked up a '72 CT70HK1 a couple weeks ago. Seems to be in good condition, starts, idles, and runs well up until I'd say 80-90% throttle while riding. Once throttle is opened up past that point, the bike begins to cut out intermittently and I lose power until reducing throttle and letting the bike run happily again.

Things I've noticed/tried:
  • Carb is a new repo unit and looks very clean, not sure where from as there's no branding, but it looks like a clone of stock to an untrained eye.
  • This does not appear to happen in neutral, can rev very high when not under load.
  • Putting the choke on helps while riding near WOT, the bike doesn't seem to cut out as much if at all.
  • Original spark plug was totally sooted.
  • Checked and re-set the carb idle and mixture screws, seemed to be well tuned already.
  • Good fuel flow through the carb drain tube while petcock is on.
  • Adjusted carb float down to 25/32" with goal to increase fuel loading and prevent starving (this seemed to make the bike harder to start, with no other effects).
  • Replaced main jet (originally #60) with #58, bike seemed to lug even earlier with the smaller jet, felt very different.
    • I had a new spark plug in at this point but stupidly did not pull to check if it was running lean or still rich...
  • Replaced main jet (#58) with #62, bike seemed to run better overall but still cuts out near WOT.
  • Checked new spark plug, already starting to get sooty. Spark color is a purple-blue.
At this point I feel like I've reached my extent of what to try from reading the primary threads I've found so far, other than maybe to try cleaning the points and getting a slightly better spark, or replacing the carb entirely. Any insight or suggestions here would be much appreciated!
 

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Have you tried lowering the clip/raising the needle to make it richer?
Pars #3

Your plug could be getting sooty at idle...too big pilot jet, and be lean at WOT...too small main jet.
Are you checking the plug after a hard run, or after a long idle?
Stock 72cc engine? Cylinder stamped 72cc?
Stock airbox?
 

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Discussion Starter #3
Thanks for the reply!

Have you tried lowering the clip/raising the needle to make it richer?
Pars #3

I have not tried this yet, I think I've read the stock position to be #3, but I'm not sure whether to count from the top or bottom slot?​

Your plug could be getting sooty at idle...too big pilot jet, and be lean at WOT...too small main jet.
Are you checking the plug after a hard run, or after a long idle?

Good point, I'll check the plug after a long idle next time and investigate the pilot jet size. Would it be unheard of to require a main jet larger than #62?​

Stock 72cc engine? Cylinder stamped 72cc?
Correct, stock engine, where can I sanity check the cylinder stamp?​

Stock airbox?
Looks OEM to me with a foam filter.​

Not sure what the stock carb settings usually are, but:
Existing idle screw setting is 3.5 turns out.
Existing air screw setting is 0.75 turns out.
 

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Discussion Starter #5
It should have a 72 cc stamped on the jug. Check pic.
Ah got it! Mine shows "72cm3".
I suppose I could also keep sizing up the main jet, but feels odd to me to need higher than a #62. I think DrATV typically recommends a #60 with this carb.
 

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Two things you said make it sound like the main jet is the problem. If it runs well till the top 80-90% of throttle, and runs better with choke closed, your main jet is too small or dirty. The main jet size is the biggest factor on the top 25% of throttle (pilot jet affects first 25%, needle position and profile affect middle 50%.) Before you get a larger main jet, check the emulsion tube small openings (on the side of the tube) for cleanliness.The emulsion tube can be removed by taking out the main jet and pressing the top of the tube with the flat end of a pencil or dowel rod from the carb barrel side. If you are not familiar with the part, it is the brass tube the needle fits in, and has small holes on the side with a larger central bore. The smallest guitar string is good for poking it out. The other things to remember is that after changing the main jet, there may be some adjusting of the air and idle screw as well as needle position to make it pull strong all the way to WOT. The other thing is that if you are doing plug chops, the only way to do it correctly is to turn off the ignition while riding at WOT while pulling in the clutch at the same time and check the electrode for color. You also have to use a clean or fresh plug each time. Lastly, there is an old saying: "95% of carb troubles are electrical." Make sure your timing is on and your points are clean.
 

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Discussion Starter #7
Before you get a larger main jet, check the emulsion tube small openings (on the side of the tube) for cleanliness.The emulsion tube can be removed by taking out the main jet and pressing the top of the tube with the flat end of a pencil or dowel rod from the carb barrel side. If you are not familiar with the part, it is the brass tube the needle fits in, and has small holes on the side with a larger central bore. The smallest guitar string is good for poking it out.
Tons of good info there, thanks. I did a visual check on the emulsion tube earlier on but admittedly did not use a piece of wire to guarantee it's cleanliness. The guitar string is a good tip, I'll see what I have around.

The other things to remember is that after changing the main jet, there may be some adjusting of the air and idle screw as well as needle position to make it pull strong all the way to WOT.
I did a rudimentary re-tuning of the idle and air screws with the up-sized #62 jet, this was all done with the engine idling warm. Should that be sufficient?

The other thing is that if you are doing plug chops, the only way to do it correctly is to turn off the ignition while riding at WOT while pulling in the clutch at the same time and check the electrode for color.
Really interesting about having to turn off ignition to get a good plug read. Is that because the coloring of the electrode will change in the short period of coming back down to idle?

Lastly, there is an old saying: "95% of carb troubles are electrical." Make sure your timing is on and your points are clean.
I've seen this around the forums :) Once I get my flywheel puller I'll be checking timing and points!
 
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