Need tuning help before I throw in the towel

[Ksgfx]

I’m hoping this group can help as I’m just about at wits end with my 72 cb450. I’ve done a complete restoration, including a full engine tear down. My problem is while it will run it does not run well. I think I’ve narrowed it down to a carb problem, but I’ve been wrong before.

Here is the issue:
- Starts good and will idle but then starts to accelerate on its own once warm, stops around 4000 rpm and holds
- Also hangs when revved
- Have determine the right cylinder by itself will hang but must be revved. Will then hold at 3000rpm and will not fall
- Left cylinder will also hang on rev but will slowly drop on its own.

Here is what I tried:
- Carbs vapor blasted and cleaned in an ultrasonic multiple times. Also cleaned each passageway with small brushes and drills.
- Cleaned and swapped advancer twice, no change
- Checked and rechecked static timing, all seems good
- Checked and tightened valves to OEM spec, one was a little loose, no change
- Swapped to a spare right carb I had but kept the same jets, no change.
- Removed airboxes, no change
- Set fuel screw at every half turn from .5 to 3 turns out. Issue improves slightly
- Sprayed carb clean all around the intake boots and carb, no change at idle

General Specs:
- main jets:140, pilot:38, both Klein originals
- Fuel screw: 1.5 turns out with new OEM oring
- Using OEM airboxes with old filters. They are a little yellow but still seem to pass air just fine.

Let me know if you need any further information and thanks in advance for your help.

Kevin

[LongDistanceRider]

I haven't worked with any of the DOHC 450's so I'm going to let those who know more chime in. But with the restoration I'm assuming the frame as painted or powder coated. Did you sand off the finish to bare metal where the engine mounting contacts the frame so you have a good ground?
90% of carb problems are electrical.

[ancientdad]

Did you set the timing with a strobe? If the carbs are right then this sounds a lot like advancer. You mention replacing the advancer unit with another but did you check the spring tension at full retard to be sure there's no looseness at all?

[stl360+450]

Does the choke make any difference once the high idle occurs? (If the air boxes are off, would covering the air intake with your hand produce any change in the engine speed?)

Do the slides move freely and return to idle position quickly after the throttle plate closes? (Might check with the air boxes off.)

Symptoms sound like an air leak. Maybe check with starting fluid rather than carb cleaner. Could also check the throttle shafts on each carb.

Are the float bowls refilling quick enough to keep up with demand? Low fuel level in the bowls could produce a lean condition.

[Ksgfx]

Thanks for the quick responses.

- Frame was painted, but ground is good on both engine and chassis
- I have not checked timing with a strobe yet because was hoping I could get it running better first. Advancer springs seem to snap back ok but am curious if there is an actual test I could do.
- Choke does help when it’s at high revs which also leads me to think it’s a lean condition. If I cover the air intake it will come down, but doesn’t idle well.
- Slides do move freely, but will check tomorrow with the airboxes off to watch it closer.
- Will check with starter fluid tomorrow too. Good thought on the throttle shafts
- I set the float bowls at spec, but will check again. Will also try clear tube method to make sure I’m at a good height.

[WintrSol]

Using a strobe light to check the spark timing and advance will reveal any issues with the advancer.

[MrPepper]

Any chance you can pull good carbs off another bike to see if the issue is the carbs or something else?

[WintrSol]

Check for cracks in the intake boots, and make sure the carbs lock into them. Sometimes they feel OK, but the molded ring in the boots doesn't drop all the way into the groove in the carb outlet.

[mikeranger]

Is the throttle cable routed correctly and are the throttle plates closing fully if the adjusting screws are backed right off . Check balance . If the throttle plates (Butterfly) are returning to the idle position with the 38 pilot it should idle and not increase in RPM if it persists I would suspect an air leak on the mounting rubbers or joint with the cylinder.

[Ksgfx]

Got a chance to work on her a bit tonight and unfortunately didn’t come up with a solution. When cold it seems fine but once it warms up it will idle for a second and then race right up to 3000 rpm. It also still hangs after revving. I tried the following:

-added 150 main jets, a little better but still persists
- timed it with a strobe light to dead on perfect, sounds better, but no change
- sprayed starter fluid everywhere at idle. No change
- throttle does close completely when screw is backed off, cables are correct.
- float hieghts checked and set correctly
- switched to a third advancer I had, no change
- if I put the choke half on it will slowly return to idle, so it has to be a lean condition right?

I also ran each cylinder independently and they ran fine. No racing no hanging. Does this mean something’s wrong with the timing?

Anyone got anything else to try?

Thanks.

[stl360+450]

Where do you have the mixture screws? Do they have any effect on the idle speed? (If you gently seat/close them, the motor should stall, unless there is an air leak or the throttle plates are not completely closed, in which case a higher idle would be expected.)

You could test everything with the throttle cables removed to eliminate the possibility of improper cable adjustment. Back the idle stop screws out so that the screws don't touch the stop plate and see where your idle is. It should be running on the idle circuit only at that point.

[WintrSol]

When you checked timing with the light, did you raise the rpm to 3000 to verify the advance was full and stable? Then, if the rpm came back down, did the timing advance return to baseline? If the springs are loose or weak, the timing will bounce around and may not return to the idle position.

[ancientdad]

When you checked timing with the light, did you raise the rpm to 3000 to verify the advance was full and stable? Then, if the rpm came back down, did the timing advance return to baseline? If the springs are loose or weak, the timing will bounce around and may not return to the idle position.

Yeah, I failed to mention the real intent behind using the strobe.

[crazypj]

Do the slides move freely and return to idle position quickly after the throttle plate closes? (Might check with the air boxes off.)

Symptoms sound like an air leak. Maybe check with starting fluid rather than carb cleaner. Could also check the throttle shafts on each carb.

Are the float bowls refilling quick enough to keep up with demand? Low fuel level in the bowls could produce a lean condition.

The slides have zero effect when butterflies are closed
. You can remove carb top at idle and remove the slide completely without the engine 'revving up'
Throttle shaft seals are a good idea, ultrasonic cleaning isn't the 'be all and end all' people seem to think, I've seen it cause more problems than it solves and very rarely use mine when doing 350/360 carbs
To clean jets I use 'old school' soak in vinegar for 15-20 min then wash off and prod with a strand of copper wire to check there isn't oxidation inside the jet restricting flow.
I used to use Yamaha carb cleaner as it was the best stuff available anywhere but since the formula changed to be more 'green' it doesn't work quite as well (but is still good and cheaper in real terms than it was 35~40 years ago)
As for advance problems, the easiest thing to do is cut end off ONE spring then bend up the next coil to fit onto spring post.
It was a very common mod in the 70's to change advance curve but I do it pretty regular 'these days' to compensate for weak springs and make sure advance is going 'full range'
New springs have never been available from Honda without buying a new advance unit ($110.00 ~ $160 last time I checked about 20 years ago)
Advance springs used to be available for Suzuki's but are a slightly different spring rate and probably discontinued as majority of Suzuki bikes stopped using points ignition in early 80's

[Ksgfx]

Big Update. I'm happy to report she is running correctly!
https://photos.app.goo.gl/e8wo338iaCFqh4bg8

What did I do you ask. Well in a last ditch effort I had another 450 engine that came with my parts bike. I decided to take both the advancer and the points plate out of that engine and put it in mine to see if it would make a difference and it worked. Now I know I shouldn't have changed more than one thing at a time, but I didn't want to set the timing twice so I just went for it. Now I'm sure most will be thinking it was the advancer, and it might very well be, but I did try 3 other advancers that I had in my parts pile. It's possible those were all bad, but not likely. My thought is that it was either the plate or the points. The old points were OEM but worn, these are older Daiichi points but they are in almost new shape.

You guys tell me, what do you think? Possible it was the points? Maybe the plate itself was warped (I did notice when trying to time the engine it would tend to float a bit back and forth without moving the plate).

Thanks again for everyones help!

Next up is determining why the shifting must be done very deliberately, but that's a story for another post.

[crazypj]

It could be the wire to points was grounding on something or not grounding when it should?
Check how the wires are attached to pints connector bolt, it's VERY common to be assembled wrong at some time in the past.
Points plate 'floating' in housing is very common, the OD of plate and ID of housing are not the best size match (built for ease of assembly in the factory, timing was supposed to be checked and re-set if needed by dealer before sale)
Glad everything worked out but it's still a good idea to find out why the 'original' parts didn't work. (the difference between a parts changer and a mechanic)

[ancientdad]

...but it's still a good idea to find out why the 'original' parts didn't work. (the difference between a parts changer and a mechanic)

Absolutely true, which is one of the many reason we're here.

[crazypj]

Blimey, that looks like it just came from 1972!
Cool.

[Ksgfx]

That is a great point PJ and I plan to figure it out at some point. I just wanted to take the win and ride it around a bit before I took it back apart and determined which part was causing the problem. I did notice when i removed the other plate that I did have the yellow wire around the wrong way and it was a little loose so you may have just figured it out for me.

[ancientdad]

Blimey, that looks like it just came from 1972!
Cool.

Jeez, I didn't even watch the video until now and it does look like we took a time machine. Beautiful bike, exactly like the one my Dad had while I was in high school and had my CL450K4. Very nice!

[ancientdad]

That is a great point PJ and I plan to figure it out at some point. I just wanted to take the win and ride it around a bit before I took it back apart and determined which part was causing the problem. I did notice when i removed the other plate that I did have the yellow wire around the wrong way and it was a little loose so you may have just figured it out for me.

It's the little details that make the difference, which is why we always say to post lots of pictures... sometimes we can see stuff that one might not think to mention or realize might be an issue.

[Forden]

Glad to find this thread as I'm having a similar problem--throttle hanging up around 3K.

As for advance problems, the easiest thing to do is cut end off ONE spring then bend up the next coil to fit onto spring post.
It was a very common mod in the 70's to change advance curve but I do it pretty regular 'these days' to compensate for weak springs and make sure advance is going 'full range'

Reading this thread, I think the advance is the most likely culprit, as I thought the springs seemed a bit weak when I serviced it. After doing a bunch of needed maintenance (carb rebuild, new manifolds, new points/plate, valve clearances, oil filter clean, etc) the problem is still there, I learned yesterday. I'll give modifying the springs a try. As far as I can tell the throttle cable seems fine (will recheck), and the throttle plates themselves didn't seem to be off-center or hanging up when I had the carbs apart, they snapped shut nicely.

It is a bit erratic at idle as well--I suspect throttle shaft seals need replacing, so will test for a leak there before replacing them, which I probably should've just done when I had the carbs off, if I'd known... live and learn.

[ancientdad]

The very reason archived content is so important. If you don't start a thread for it, at least come back here and let us know how things turned out.

[Forden]

Before I modify the springs, wondering if leaking throttle shaft seals could also cause the hanging throttle--I'm replacing the seals anyway in the next day or two, and just want to do things one at a time so I know what's at fault. Thanks
cheers
Mike

[ancientdad]

Any vacuum leak can cause or contribute to a hanging idle, yes, and it is absolutely wise to only change one thing at a time.

[Forden]

New throttle shaft seals fixed the erratic idle, but not the hanging throttle, so I moved on to the advancer and shortening one of the springs did the trick. I still need to check dynamic timing to see exactly where the advance is, but she felt great out on the road.

Just for reference, I found a thread elsewhere indicating that the springs from Mike's XS for an XS650 work also, if tightened a bit. Just in case that helps anyone.

Thanks everyone,
Mike

[ancientdad]

Glad it's sorted now, one of those possibilities is always the cause.