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Discussion Starter · #1 ·
I have just completed a top end rebuild on a 1970 CL 175. Last run in 1988. The problem I have is the motor is breaking down under high revs. It starts easily and runs fine up to 50kph, any faster and revs over 4K it stutters badly and wont rev out, like it is running lean but the plugs show ok, maybe a little rich. I have lifted the main jet needles one notch which seemed to improve a bit. Installed a new carb kit and have tested for manifold leaks. I am at a dead end with this now. Anyone had this issue?
 

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Advance mechanism working OK ?
 

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I have just completed a top end rebuild on a 1970 CL 175. Last run in 1988. The problem I have is the motor is breaking down under high revs. It starts easily and runs fine up to 50kph, any faster and revs over 4K it stutters badly and wont rev out, like it is running lean but the plugs show ok, maybe a little rich. I have lifted the main jet needles one notch which seemed to improve a bit. Installed a new carb kit and have tested for manifold leaks. I am at a dead end with this now. Anyone had this issue?
Is it running out of fuel? Float height?
 

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Use the choke while riding when the problem occurs, that’ll tell you if you are rich or lean. If you are idling a lot, the plugs can show rich. You would need to do plug chops at specific throttle positions to trust plug readings %100.

if you used the new brass from the carb kit, that can make things worse as those items are rarely sized correctly even though the numbers are the same.
 

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Discussion Starter · #6 ·
Use the choke while riding when the problem occurs, that’ll tell you if you are rich or lean. If you are idling a lot, the plugs can show rich. You would need to do plug chops at specific throttle positions to trust plug readings %100.

if you used the new brass from the carb kit, that can make things worse as those items are rarely sized correctly even though the numbers are the same.
Yeh tried choking it and it got worse. The brass jets and spray nozzle do concern me. The main sray tube has less holes than original. The air holes are graduated and a couple seem bigger diameter. It would be handy to have a selection to test with.
 

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Clean and replace original jetting......IF problem persists, try with air cleaners removed......Post results
 

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Check out your electrics, and make sure that the coil and condenser are functioning ok, and that the battery is supplying enough voltage at higher revs.

My '72 Triumph Trident did the same thing when the battery got tired and the voltage dropped off. It ran perfectly at low revs but not properly when revved harder. I later discovered one cell of the battery had gone AWOL; a reading of 12.5V after charging fooled me into thinking it was ok, but had I waited for 1/2 hour or so, I would have seen that the voltage had dropped to 10.5V. It was enough to start and run the bike at low speed, and then leave me stranded a couple of miles from home!
 

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I have just completed a top end rebuild on a 1970 CL 175. Last run in 1988. The problem I have is the motor is breaking down under high revs. It starts easily and runs fine up to 50kph, any faster and revs over 4K it stutters badly and wont rev out, like it is running lean but the plugs show ok, maybe a little rich. I have lifted the main jet needles one notch which seemed to improve a bit. Installed a new carb kit and have tested for manifold leaks. I am at a dead end with this now. Anyone had this issue?
Have you checked the tank petcock? could be a bit clogged up and restricting the flow of fuel to the carb'.
 

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Discussion Starter · #10 ·
Ok reporting back. Changed the main jets back to original #90 instead of #98. Replaced original needles, the new kit needles looked fatter. Tore off the old stained original air filter "paper" elements, only guaze wire remaining. New coils and condenser. Big improvement, now the bike is rideable. Still starts and idles easy. Runs up to 50mph. However any quick acceleration in any gear it wants to bog down and there is no response over 50 mph with some twist still left. Gentle acceleration no problems. Plug check shows better than before which to my eye looks about spot on. Where to next? I am running original exhaust, which is near impossible to clean the double internal muffler. Thanks for help so far.
 

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Check your ignition timing - it sounds like its running too far retarded. The engine will indeed start and run very nicely at low speed, which is exactly as it should in the retarded condition. Higher speed needs the spark to advance, by 12 degrees in these engines if memory serves me correctly.
If the timing is spot on (and I can't stress enough that it needs to be), check that the advance mechanism bob-weights are moving freely. If they're stuck in the closed (low speed) position, you'll be forever stuck in 'retard' and never get beyond 50mph.
 

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Discussion Starter · #13 ·
Thanks Jools. Yeh that is something I have looked at before installation but will now check again. I am still amazed that the new Keystar carb kits have not worked properly on the CL 175, which are basically two CT110 carbs and I have used the kits with no problems.
 

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If ignition looks good, it could be lean. you can use the choke when it’s starts bogging to see if it gets better. If so, try larger jet, or raise needle ( lower the clip).
 

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You mentioned that you've lifted the main needle to lean the mix. You should be dropping the needle to lean the mix. If you thought it was rich before, it's now bought a superyaught and a Lamborghini.
 
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