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Discussion Starter · #1 ·
Hi all,

Finally got my 68 CB450 running. It starts easily and now idles nicely at about 11-1200.

However as soon as I touch the throttle, the revs rise to about 3-4000 and stick there. The cable is not sticking; I barely have to twist it to get the response.

Both idle screws are sitting on the stop - they snap back nicely - and when I apply a bit of pressure to them, the engine returns to idle and stays there.

I am guessing that something is sticking, but what?

Carbs were out, cleaned and all jets replaced with standard sizes. Air filter is an oiled UniFilter.

All suggestions welcomed.

Sean
 

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Discussion Starter · #3 ·
Thanks Bill

I bench sync'd them as per the common Motor video, and both throttles move at the same time - that's balanced, right?

Sean
 

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no, you need a manometer or I use a digisync. bench syncing just gets them close the same as initial mixture screw setting, then fine tuned when running. Balancing is equalizing the airflow in both carbs.
 

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I recommend a homemade manometer for syncing twin carbs, you can build one from thin clear tubing and use pretty much any oil.
Have a look at YouTube for videos showing how it works, each end of the hose connects to a vacuum port on each carb, when carbs are in sync the oil level is equal, out of sync and the levels go to different levels.
Its an odd process initially but it makes sense after a few minutes. Adjust sync setting, then adjust air screws... repeat.
Keep a fan blowing on your engine to keep heat down too, and use the kill switch if things go awry.

You can do it, and it'll make an unbelievable difference.
 

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Of course, to use a manometer or vacuum gauges, you must first have vacuum ports. My K3 doesn't have them, so I added them to a spare set of intake boots. I make sure the throttle cables are locked down, before removing the modified boots and installing the unmodified ones. Others here have drilled into the intake ports to add permanent vacuum ports, which then must remain plugged when not in use.
 

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And if (after the carbs are right) it still does it, check the points gap/timing and spark advance. If the timing is a little retarded or the springs are weak on the advance unit, it might want to stay revved up to a degree. Or, of course, maybe a vacuum leak
 

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Discussion Starter · #8 ·
So just a quick update.

Redid the balancing etc as per Bill Lane's post on Honda CB450dohc Forum - Forum Index. It has also been posted here previously, but there it is a neat pdf. ("Without vacuum ports, it's the only way!") Thanks Bill. Things got a bit worse, so I gve up for the day.

Next day I took off the bowl, noticing a bit of debris on one side. I cleaned it up, then removed slides, cleaned and polished them. Also removed and cleaned main jet, pilot jet cap, and mixture screw, with parts cleaner and pressurised air. Put it all back together, redid the setup and tried again. It started.

Control was a bit iffy. At first wouldn't idle - it died; then would race; then settled down. As I was tinkering, it died. On kicking, it backfired once at left carb and then wouldn't start again. Left it for a few minutes and it fired up again. Finally got it going reasonably.

Now I have it running, ticking over at +/- 1100-1200, responding to the throttle, but sometimes sticking a bit at 3000 rpm before it settles down again.

So I count that as success because: I'm still using the old - at least 15 years old - points. Setting the gap was a challenge, but I managed thought the gaps are not withing spec.New ones are on the way from DSS, once my lad sorts out his room and finds them :roll: (It's a long story). I'm also using old plugs - they look in good condition, but I'm sure a new set would do no harm - they're also on route.

Thanks for all the suggestions. This is Africa - low tech and patience is the way to go! :)

Sean
 
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Now I have it running, ticking over at +/- 1100-1200, responding to the throttle, but sometimes sticking a bit at 3000 rpm before it settles down again.

Sean
So, now you should hook up a timing light with a clamp-on pickup, and check to see if it smoothly returns to the idle position, or hangs up a bit. If the timing hangs up a bit, then the advancer springs are likely the problem, as Tom suggested.
 

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Discussion Starter · #10 ·
Hi Rick,

I'll do that. Is that the case, even if the points are not the best? Would it be better to leave that until I have new points and plugs?

I still have a few things to do before it's road ready.

Sean
 

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They maybe leaking air around the throttle shafts. Mine did this. I replace the felts, and the idle would still hang. then I lubed the felts with Liquid Wrench White Lithium aerosol and it cured it. To grease them, remove the carbs, tape the head end to a shop vac, turn the vac on and put your hand over the open end and spay the lube into the shaft area. You should see the grease being drawn into the venturi. It worked great the carb action is very smooth.
 

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After the advancer springs, that would have been my next suggestion, Jim.
@Sean, you should be able to test the advancer with the points you have. If it is flaky, you'll have to take the points plate off, anyway.
 
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