1973 ct70 struggling at higher rpms

KennyCT

Member
So, I just got my ct70 running after replacing the contact points in the magneto. She starts fine, idles alright, but then when I give it gas it goes to a certain point before you can here the engine bogging down. Is this a sign that it's running to lean? I have no air filter in the airbox, is that a must have in order to have it run alright? I havent checked the valves yet, so I'll have to get on that soon. Any suggestions as to what the issue may be?
 

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69ST

Well-Known Member
Yes, those are classic symptoms of lean mixture. Lack of an airfilter is a bad idea. It can result in leaner A:F ratios but, even worse is all the abrasive crap the motor can inhale...and the damage that will soon follow.

First thing I'd do is pull the carburetor, unscrew the main jet holder/emulsion tube, then run a thin wire through every single one of the wall orifices. Not sure how many there are on a K2 model but, there will be at least 8. Those minuscule orifices are easily obstructed and can, in some cases, be a little difficult to clean properly. Outside of the engine, itself, these are the most high-precision items on the entire bike. They need to be completely clean & unobstructed to a.) flow the correct amount of fuel and b.) atomize is finely & uniformly.

Next things I'd do are adjust the point gap, then order a new airfilter element, sparkplug & points.
 

KennyCT

Member
Would an aftermarket airfilter be fine? Or should I get an oem since the engine is not modified in any way. I'm simply working with a tight budget, so if I can save money, I will.
 

cjpayne

Well-Known Member
Alright, wasnt sure if it made that much of a difference. Thanks.
It really does. Even if the snorkel on the back of the aircleaner, that fits up into the frame, isn't there, it will make a difference.

Btw, if the fuel lines aren't situated properly, they can interfere with that snorkel and close/pinch it off. They can also pinch it off just enough, that it only does it at higher rpm's.
https://lilhonda.com/index.php?threads/gas-line-install-to-clear-air-snorkel.19750/
 

KennyCT

Member
Yes, those are classic symptoms of lean mixture. Lack of an airfilter is a bad idea. It can result in leaner A:F ratios but, even worse is all the abrasive crap the motor can inhale...and the damage that will soon follow.

First thing I'd do is pull the carburetor, unscrew the main jet holder/emulsion tube, then run a thin wire through every single one of the wall orifices. Not sure how many there are on a K2 model but, there will be at least 8. Those minuscule orifices are easily obstructed and can, in some cases, be a little difficult to clean properly. Outside of the engine, itself, these are the most high-precision items on the entire bike. They need to be completely clean & unobstructed to a.) flow the correct amount of fuel and b.) atomize is finely & uniformly.

Next things I'd do are adjust the point gap, then order a new airfilter element, sparkplug & points.
I took the carb off, ive already cleaned it once, what orfices should I try to clean that I may have missed? Also, does anything look abnormal or out of place?

*edit* Just cleaned all of the orfices in the walls of the float bowl, jets, and throttle body. Everything is looking as clean as can be. The float also seems to be in good condition. Gave it a good shake and didnt hear or feel any gasoline in it so thats good. I'll also see if I can come up with some kind of test air filter like a sponge, until I can get a genuine one. I have inline fuel shutoff valves coming tomorrow so I'm going to wait to reinstall the carb then. I will be sure to update y'all when I start it up again.
 

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69ST

Well-Known Member
Se that hex-headed piece into which the main jet is screwed? That's jet holder/emulsion tube. Unscrew it from the carb body. That will expose the metering orifices. Run a thin wire through all of them, followed by carb cleaner & compressed air. There really is no substitute for physically "rodding out" those orifices. Probably 95% of high-speed carb problems are the result of one, or more, of those minuscule orifices being just a little clogged.
 

KennyCT

Member
Se that hex-headed piece into which the main jet is screwed? That's jet holder/emulsion tube. Unscrew it from the carb body. That will expose the metering orifices. Run a thin wire through all of them, followed by carb cleaner & compressed air. There really is no substitute for physically "rodding out" those orifices. Probably 95% of high-speed carb problems are the result of one, or more, of those minuscule orifices being just a little clogged.
Alright, I'll double check it once I get home from work.
 

cjpayne

Well-Known Member
Alright, I'll double check it once I get home from work.
When you get that tube out and cleaned, check it looking at it sideways with a good light behind it. The tiny holes have to be perfectly clear. I just strip a bread tie and use the wire. Thats also good for the main jet itself, but not the separate idle jet(wire is too big for it).
 

KennyCT

Member
When you get that tube out and cleaned, check it looking at it sideways with a good light behind it. The tiny holes have to be perfectly clear. I just strip a bread tie and use the wire. Thats also good for the main jet itself, but not the separate idle jet(wire is too big for it).
So I cleaned out the carb very carefully, made sure all the jets are completely clean, and reassembled it. Last night when I first started it, it seemed to run beautifully. But now for some reason its back to the same issue I was having where it bogs down when I give it more gas. Am I doing something different I just am not realizing? Why would it run fine last night but not now?
 

69ST

Well-Known Member
Next things I'd want to check are fuel flow to the carb, i.e. fuel level inside the float bowl and valve lash. Fuel flow can be restricted in a number of ways, up to and including a blocked tank vent. Valve lash will tend to decrease as the engine reaches peak operating temps, due to thermal expansion.

See how it runs next time. If if starts out okay and the situation repeats, one of those two issues is probably what you have to sort.
 

KennyCT

Member
Next things I'd want to check are fuel flow to the carb, i.e. fuel level inside the float bowl and valve lash. Fuel flow can be restricted in a number of ways, up to and including a blocked tank vent. Valve lash will tend to decrease as the engine reaches peak operating temps, due to thermal expansion.

See how it runs next time. If if starts out okay and the situation repeats, one of those two issues is probably what you have to sort.
I know that the carb is getting fuel since I currently have clear fuel lines running to it, I'll have to check and adjust the valves later today.
 

69ST

Well-Known Member
I know that the carb is getting fuel since I currently have clear fuel lines running to it, I'll have to check and adjust the valves later today.
That does not guarantee that fuel level inside the float is remaining where it needs to be. IF...and that is the operative word here...it drops too low, the mixture will go lean.
 

69ST

Well-Known Member
That can be difficult, since time is a factor. If the underlying problem is inadequate fuel flow...the float bowl will have time to at least partially refill, once the engine is shut off.

Testing methods:

Switch the petcock to "off", then drain the float bowl via the drain valve, into a small container. There should be a few ounces of fuel; a smaller quantity, like a tablespoon, would indicate a flow problem.

Open the drain valve, observe fuel flow. There should be a steady stream.

Fill the tank, open the drain valve and see how long it takes to completely empty the tank. It should take less than 5 minutes.
 

KennyCT

Member
That can be difficult, since time is a factor. If the underlying problem is inadequate fuel flow...the float bowl will have time to at least partially refill, once the engine is shut off.

Testing methods:

Switch the petcock to "off", then drain the float bowl via the drain valve, into a small container. There should be a few ounces of fuel; a smaller quantity, like a tablespoon, would indicate a flow problem.

Open the drain valve, observe fuel flow. There should be a steady stream.

Fill the tank, open the drain valve and see how long it takes to completely empty the tank. It should take less than 5 minutes.
My fuel drain valve is stuck on really well. I was able to barely loosen it to get fuel to slowly drip out. Any tips on getting it unstuck? I'll keep working at it and try to get it loosened more.

*edit* Well, fml. Was loosening the drain bolt and the head snapped right off. Also seems as if there was not much fuel in the bowl. Pics attached below.
 

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allenp42

Well-Known Member
I had this happen once. Assuming you've had it out while cleaning your carb, I would suggest vice grips and gently apply some persuasion. If you've not had the drain screw out recently, I would remove the carb bowl, spray some carb cleaner in the direction of the drain hole to get rid of the gas, then shoot some PB Blaster or equivalent into the hole, let it soak for a few minutes than try vice grips. Just don't be too aggressive. You may have to rock it back and forth a few times just like you do with any stuck screw.
 

KennyCT

Member
I had this happen once. Assuming you've had it out while cleaning your carb, I would suggest vice grips and gently apply some persuasion. If you've not had the drain screw out recently, I would remove the carb bowl, spray some carb cleaner in the direction of the drain hole to get rid of the gas, then shoot some PB Blaster or equivalent into the hole, let it soak for a few minutes than try vice grips. Just don't be too aggressive. You may have to rock it back and forth a few times just like you do with any stuck screw.
So I got the drain screw out, how do I adjust my float? Is it a matter of bending the little tab on it? I believe not enough fuel is going into the bowl causing the engine to run lean.
 
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