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CB450 high idle, revving engine keeps rpms climbing

dlicata

Member
Joined
Jun 21, 2020
Total Posts
21
Total likes
0
Location
Lee's Summit, MO, USA
My 450 readily starts but has a high idle at around 3000 rpm. Revving it causes the engine to rev way high, and not come back down, hangs around 5000-6000 rpms. I need help figuring this out, I've tried everything and can't get the dang thing to run right.

I've been working on this 71 450 for a few years now and had gotten to a finishing point, but have had carb trouble consistently, and the engine eventually stopped firing on the right side. After my limited diagnostic knowledge, I determined the only thing left for the no spark issue was a bad PAMCO electronic ignition. I replaced that with the common motor collective Shockwave system and the bike readily started with spark on both sides. The engine should be perfectly timed, and the valves should be adjusted correctly at about .003 of clearance.

Things I know:

-Carbs totally disassembled and cleaned with ultrasonic

-Felt gaskets in throttle shafts replaced and upgraded

-Throttle plates set with guitar string method, both the same

-New intake manifolds, rtv'ed to the head rather than paper gaskets

-No air leaks that I can tell

-Properly timed and adjusted engine (starts effortlessly)

-Engine slows down/struggles if choke is on

-Mixture screws set to 1.5 turns out

-Throttle cables slacked, not holding the plates open

-Cable not caught on anything

-Vintage hooker 2-into-1 exhaust

-Pod filters

-Mechanical advancer springs replaced with century spring B3-5, extremely similar to stock, but a tad stiffer making for a slightly later advance

If anyone has ideas, please let me know. I can provide any additional information about the bike that's needed, as well as pictures.
Thanks!
 
What happens when you back off the idle speed screws? Not sure what setting the throttle plate with a guitar string does since each carb may have different idle speed screw adjustments to get a sync of the carbs.
Throttle shaft felts were lubricated with what?
1.5 turns out on the mixture screws gives the highest idle?
 
I used a light oil on the throttle shaft felts, they're also slightly larger so they have a better seal. Turns real smooth.

I'm actually somewhat ignorant on the mixture screws, I've just been told that it should be 1.5 turns out, starting from screwed all the way in. Does fewer turns lower or raise idle? and vise versa?

The carbs have 145 main jets and 38 pilot jets if that means anything.

other than synchronizing as a baseline on the bench, I don't know how synch these either without vacuum ports or two seperate exhaust pipes
 
"New intake manifolds, rtv'ed to the head rather than paper gaskets"
In case you didn't know, RTV degrades in the presence of gasolene, it will eventually be dissolved, causing future issues. You might want to install some proper gaskets when you get a chance.
BTW, is compression fairly even, on both sides?
 
I just had high idle problem on a CL450 I just got running. Found that throttle butterflys on both carbs were off center in bores and leaving air gap between butterfly and bore. Had to loosen two screws on shaft and re center.
 
I just had high idle problem on a CL450 I just got running. Found that throttle butterflys on both carbs were off center in bores and leaving air gap between butterfly and bore. Had to loosen two screws on shaft and re center.

I was about to suggest the same thing. On mine the butterflys were not quite centered and the plate would hang up on the inside of the carb venturi. You have to get them centered and right side up. There is a right way and a wrong way, but I don't recall which is which. There's just a slight bevel in the edge of the butterfly that should go toward the bottom, but I don't remember whether it goes toward the intake or toward the engine.
 
I was about to suggest the same thing. On mine the butterflys were not quite centered and the plate would hang up on the inside of the carb venturi. You have to get them centered and right side up. There is a right way and a wrong way, but I don't recall which is which. There's just a slight bevel in the edge of the butterfly that should go toward the bottom, but I don't remember whether it goes toward the intake or toward the engine.
The bevel faces the direction of travel when closing. Bottom bevel facing the filters, top bevel facing the engine.
 
The bevel faces the direction of travel when closing. Bottom bevel facing the filters, top bevel facing the engine.

I agree with the first sentence, but shouldn't the beveled edge face the filters on the top of the plate and the engine on the bottom?

My understanding is that the fit is so tight (and crucial to function) that the plate couldn't rotate freely without the opposed beveled edges, which is why the plates don't close fully when incorrectly oriented or misaligned on the shaft.
 
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