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Discussion Starter · #1 ·
Hi guys,

I'm a novice mechanic here so go easy on me...

A year ago, my 1972 CB450 had some idling issues, the notorious 2nd gear pop-out issue, and low compression (140 psi on both cylinders). With those issues present, I decided to go ahead with a full rebuild and basically a frame-up restoration too. The bike was bored out to 0.25 over-sized and dealt with the transmission issues while at it.

Three days ago, I finished putting it all back together and fired it up for the first time. Everything is great, except that same idle problem is back.

The bike revs up to about 3000 RPMs and just stays there. If I reduce the idle, it dies -- and this only happens after it's warmed up. When it isn't hovering at a steady 3000, it's fluctuating up and down on its own. It also has this weird issue where if I lean the bike over to the left, like I'm going to lean it on the kickstand, the idle drops.

I took the carbs back off of the bike and cleaned them out again as best as I could, blowing cleaner through all of the holes in the carb bodies. I put it all back together and it seemed good at first, but then started doing it again. The carb boots are not leaking -- they're new, soft, and flexible. The air filters are originals -- not great, but I wanted everything to be stock so I could eliminate guess work. I'll play with filters later. The jetting in the carbs is all stock (145 main jets for 1972). The carbs are the later style (723A). The carbs were rebuilt using the Common Motor Collective's kits and all of the parts matched my originals in size and shape. Needles were slightly different, but functionally the same as I recall (I did this last year).

Valves were just adjusted during the rebuild. Points were set statically.

Plugs are brown, but kind of charred with soot otherwise -- not sure what that means off hand, but I figured someone might ask. As far as I understand, brown is good. Plugs were new as of 3 days ago. Bike has been ridden 55 miles since. A picture of the plugs is attached (assuming I did it right).

Bart_sparkplugs1.jpg

I also made a video showing the issues. The bike was behaving a bit better than normal while I recorded the video (it figures), so it wasn't doing the 3000 RPM thing, just wavering a bit and doing the side stand idle drop.

Here's the video:

Any help would be appreciated. I've looked all through these forums and found lots of suggestions, but a lot of the threads just end without posts on the final resolutions.

-Mike
 

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If I were you I would reuse all the old jets and needles ect. Those honda parts are light years better than what comes in those kits IMO. A brass jet doesn't wear out.
 

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Discussion Starter · #4 ·
Did you change the throttle shaft felt seals? CoulD be a air leak there.
No, I did not. When I first got the bike (which might as well be last month, even though it's been 2 years since I've only ridden it 355 miles total), I tore the carbs completely apart and pulled those felt seals out. They weren't disintegrated or anything, but I can't say for sure they were still sealing anything either.

I've seen those felt seals on eBay though and watched the seller's videos in the past. I think I'll place an order today. Even if it doesn't fix the issue, it's probably worth the $24 to just do and be done with. I'd still like to hear some additional thoughts or things to try from people though in the meantime.

Thanks!
 

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Discussion Starter · #5 ·
I forgot to mention in my initial post something else I tried with the mixture screws.

After the rebuild, I was trying to get this idle issue worked out and I was trying to adjust the carbs individually.

What I did was pull the wire off of the left spark plug, then start the bike and try to just run it on the right cylinder and adjust it. The bike wouldn't stay running unless I screwed in the throttle stop screw (the idle screw, whatever) a full turn and a half. Then, once it was there and running, the mixture screw had no impact on anything. I could turn it in all the way, or back it out. Again, just running on the one cylinder. I got all excited thinking "YES, it's just a clog somewhere in the idle circuitry of the carb!".

Then I did the same process, only with the left carb. That one's mixture screw had no effect either.

That's when I took the carbs off the bike and blew the cleaner all through them. I did not fully dunk the carb bodies because I didn't want to mess up the felt seals (old ones). I just blew some GUNK brand cleaner through them and compressed air. I did the straw test and that was fine.

What I'm not clear on, is if it's okay to run the bike on separate cylinders like I did or not or if there's a better way. I've read so many posts on the subject of tuning and it's not always clear if both cylinders should be running when you are trying to listen for changes with the mixture screws, or if it's supposed to just be one at a time. And feeling the pressure out of the exhaust pipes? Talk about a vague measurement. I have no confidence I'd be accurate on that.

I did not try the separate cylinder mixture screw thing since I cleaned the carbs again. It just felt like I was hurting it in some way haha.
 

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The felt seals really seemed to improve my bikes running(escpecially when stopping quickly with the throttle closed). Just a warning, be extremely diligent when disassembling. Use a block of backup wood and the correct jis screwdriver. The screws are staked so they may wreck the threads when you take em out unless you relieve em somehow. And they're likely to raise burrs that'll put some nice scores in the shaft or bush. Jamespal has a method where he sucks grease in with a shopvac. Also a strobe timing light may reveal a host of issues.
 

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Discussion Starter · #7 ·
The felt seals really seemed to improve my bikes running(escpecially when stopping quickly with the throttle closed).
I sure hope I have the same experience. I ordered them this morning and hope to have them in a few days. I'll report back if they help once I get them and get them installed.

Just a warning, be extremely diligent when disassembling. Use a block of backup wood and the correct jis screwdriver. The screws are staked so they may wreck the threads when you take em out unless you relieve em somehow.
I've had mine apart before like a year ago. I followed the felt seal guy's video -- I just didn't replace the felt seals. When I reassembled, I used blue loctite on the screws, so hopefully they'll come right out.

Also a strobe timing light may reveal a host of issues.
What's the strobe experience like? I have a timing light, but only used it maybe once years ago on an old car -- long enough ago that I don't recall how I used it.

I don't really have a means to lean the bike to the right while it's running. Am I in for an oil shower if I have it just on the center stand? Using the strobe in this case wouldn't be to set timing so much as analyze the advance, right? Are you (or someone else) able to walk me through what I'd have to do and what I'd look for? Pretend I'm a complete noob with a timing light (because I am hahaha) and that the only timing I've ever done was static with a test light on this motorcycle. One thing I'm definitely not clear on is if I find it's not timed right dynamically, do I move the plate or the point itself? AND, do I do the adjustment while running like on a car (pretty sure on the car years ago the distributor was moved while running... hmm or I remember it wrong lol), or adjust, then fire it up again?

Thanks for all of your help so far, I appreciate it. I'm happy to have stuff to try next that's a little more than just randomly going at it.
 

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Maybe tape a heavy plastic bag with a notch enough to see the index marks? The strobe will tell you if cheap points are wobbling on their shafts causing erratic timing/if it's timed correct/ if it's advancing properly/ if the shim is worn causing erratic timing. I'd study the fsm on procedure but I don't use Allen screws because it's easy to torque em to much and strip the threads. If you snug em enough to pry em s little with a screwdriver while running I find its the easiest way to get em spot on. Try to get the gap as close as you can buy I don't think it's too critical. Setting the gap can also be used to fine tune the timing.
 

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Discussion Starter · #9 ·
Maybe tape a heavy plastic bag with a notch enough to see the index marks? The strobe will tell you if cheap points are wobbling on their shafts causing erratic timing/if it's timed correct/ if it's advancing properly/ if the shim is worn causing erratic timing. I'd study the fsm on procedure but I don't use Allen screws because it's easy to torque em to much and strip the threads. If you snug em enough to pry em s little with a screwdriver while running I find its the easiest way to get em spot on. Try to get the gap as close as you can buy I don't think it's too critical. Setting the gap can also be used to fine tune the timing.
I think I can rule out cheap points -- I have OEM Honda points on it (My local Honda dealer was actually able to get them in for me when I first got the bike).

I think I'm going to try the throttle shaft seals first, and see where that gets me. I'll dunk the carbs too while the seals are out and see if I can't clean them even more. If a deep clean and new seals are no help, I'll try the timing light and see what I find there.

Thanks!
 

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What are the #s on ur carbs? Are they a matched set? Have they been Effed with (swapped parts)... Idle adjustment screw parts..
Get the timing set proper... then deal with the carbs..
 

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Discussion Starter · #11 ·
What are the #s on ur carbs? Are they a matched set? Have they been Effed with (swapped parts)... Idle adjustment screw parts..
Get the timing set proper... then deal with the carbs..
The carbs are a matched set. The newer style, 723A. Parts inside were inside are all original spec. I have the originals, but when this problem first came about, I swapped out with parts from a Common Motor Collective rebuild kit, which were all the same size jets and such. Nothing is screwed up with the parts. Timing is proper to the best of my knowledge, done statically, but spot on. Even the points gap was within spec.

-Mike
 

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Is the pin & piston moving freely up and down on the carbs... my wife's once in a while will "stick" with a high idle.. she has to put it in gear and slowly release the clutch to slow the motor.. it allows the needle to drop..
Idles fine after that... and yes it happens after it gets "warmed up"...
 

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Discussion Starter · #13 ·
Is the pin & piston moving freely up and down on the carbs... my wife's once in a while will "stick" with a high idle.. she has to put it in gear and slowly release the clutch to slow the motor.. it allows the needle to drop..
Idles fine after that... and yes it happens after it gets "warmed up"...
I don't believe those pieces are hanging up. I did the straw test where you hold your finger over one of the ports and blow into the other and watch the slide go up and fall back down. That was good for both carbs. I also, as a precaution on the last cleaning, flipped the top covers around 180 degrees on the carbs just in case maybe there was binding in one position vs. another. I did not, however, try spinning the part inside that goes up and down around to try it in all positions... like different degrees of rotation inside. Maybe I'll test that when I have the carbs out for the throttle shaft seal replacement this weekend (if I get the new ones by then).

Thanks!

-Mike
 

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^^^+1^^^ thought that myself , but figured with the "CMC" kit and the rebuild it would have been done... who knows , sometimes it's the simplest easiest things that get overlooked..
 

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Discussion Starter · #17 ·
I checked the float height. I made a template from an old health insurance card using the specs someone posted in these forums awhile back.

I set the float height last year, and then checked it again after I cleaned out the carbs last week and it seems right.

I'll check the float heights again though just to be 100% sure they're good when I take the carbs off to do the felt seals. I admit, I did a really quick check on them this last time, since I figured they'd still be fine from the time before when I set them.

My new felt seals came in the mail yesterday and I bought a new paint-can of carb cleaner (to fully dunk the carbs since the felts are coming out), but I'm working on getting a 1975 CB125 ready to use as my "pitbike" transportation at the AMA Vintage Motorcycle Days in Ohio next weekend and it might be another week before I get to digging into the CB450 again.

I will definitely post results after the seals and float height re-check.

-Mike
 

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Wow.. hope you "kill that gremlin".. keep us posted... anychance you can "borrow" a set of (good-known) carbs from someone?
 

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You might want to check this out.
I've got the same issue, random high idle, but I push down on the RIGHT set screw and the revs come down (left or right has the same affect). The bike seems to start and idle ok, but only on the right carb. I can open the throttle so the main jet lets fuel through and both sides are firing smooth. So my problem seems to be with the left side slow jet circuit.
I did find the left slow jet clogged, cleaned it, but that did not help. I adjusted the idle screw with no change. The only chance it has is if I turn in the set screw a few full turns to open the main jet.

What kind of tolerance do we have with the float height? If the main jet is submerged and it's not overflowing, is that it?

My next steps are: change the fuel line route (I have them non-standard), disassemble and clean the fuel petcock filter, check the clutch cable routing. I'll also attempt to make a float height template if that doesn't work. My last resort is a full rebuild kit.
 

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I've got the same issue, random high idle, but I push down on the RIGHT set screw and the revs come down (left or right has the same affect). The bike seems to start and idle ok, but only on the right carb. I can open the throttle so the main jet lets fuel through and both sides are firing smooth. So my problem seems to be with the left side slow jet circuit.
I did find the left slow jet clogged, cleaned it, but that did not help. I adjusted the idle screw with no change. The only chance it has is if I turn in the set screw a few full turns to open the main jet.

What kind of tolerance do we have with the float height? If the main jet is submerged and it's not overflowing, is that it?

My next steps are: change the fuel line route (I have them non-standard), disassemble and clean the fuel petcock filter, check the clutch cable routing. I'll also attempt to make a float height template if that doesn't work. My last resort is a full rebuild kit.
Try adjusting with the throttles disconnected. pushing down on the idle screw sounds like something holding it open. The cables can get a curl in them over the years and can hold your throttle open. All idle adjustments can be made with the throttle cables disconnected.
 
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