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cowboykp 05-25-2010 09:41 PM

1970 Honda cb 450 idle problems
I have a 1970 Honda 450. While at idle if I hit the throttle, the engine revs up to about 3000 rpm. and stays there until I turn the Ignition off. Any ideas why? Thanks

Giles 05-25-2010 10:04 PM

Most likely cause would be a kinked or dry throttle cable. Also check the throttle twist grip on the handlebar.

cowboykp 05-25-2010 10:17 PM

I checked that, they are ok and free. Do you think it might be ignition timing?

Giles 05-25-2010 10:32 PM


Originally Posted by cowboykp (Post 447038)
I checked that, they are ok and free. Do you think it might be ignition timing?

:huh:Are you sure carburetor "butterflys" are BOTH returning to adjustment screws. I see no way the engine could turn 3K rpm with both throttles closed.

firehawkmph 05-26-2010 10:40 AM

The old 450's if I remember right, had vacuum diaphram carbs. Also, the valve adjustments were carried out by torsion bars, not screw and locknuts. Everything had to be set right or these bikes would have a hard time idling. Because of that, owners would turn the idle speed up a bit but as soon as you crack the throttle, the diaphrams control the slide lift and would lift the slides and you would be caught in the mid range circuit of the carbs. The other thing that was really touchy on these bikes was the float level setting. We used to get bikes in back then that had extended fork tubes for that chopper look. It was enough to throw the float levels off and cause problems like yours. Lastly, the carbs need to be synced with each other to make sure the butterflies are being pulled by the cable at the same time. We used to hook up vacuum gauges to check this.
Lastly, if your pilot jet(s) are plugged up the bike will have a hard time idling. These jets have a very tiny hole and are easily plugged by leaving gas in the carbs over the winter or an extended period of time. Let us know what you find. Almost forgot, check the rubber intake manifolds that the carbs fit into. If they are cracked from dry rot, you may be sucking air there, causing the bike to run lean. If they are, either replace them, or try smearing a thin layer of silicone rubber over them and let them dry.
Mike Hawkins:)

cowboykp 05-28-2010 12:20 AM

Okay, I replaced the rubber insulators, I cleaned out the jets, replaced the inlet valves, and set the float level, and I disconnected the cables from the carb. The bike will start and idle okay, but as soon as you open either carb, the motor will take off. The only way to slow it down is to back off on either idle screw, but the motor will stall out when it idles down, unless I turn the idle screw back before it stalls. Where did you hook the vacuum hose up to set the idle? Thanks

shumakerscott 05-29-2010 04:53 PM

Your carbs need to be ultrasonic tanked. Your passages inside are clogged. CV carbs with diaphrams are very sensative to small changes. Old school was to change over to Mikuni smooth bores. Much easier to tune. dorf dude...

firehawkmph 05-29-2010 08:37 PM

To add to what Dorf said, it sounds like your pilot jets are still plugged up. You might have missed them. They are ususally tucked up in side the carb. If you took the carbs off, removed the float bowls, look up into the top half of the float bowl and you will see the pilot jets up inside a little tunnel shaped piece. They have a screwdriver slot on them. Remove them and hold it up to the light and see if you can see through it. Very tiny hole. If not, soak them in some gumout for a few minutes and take a single strand of copper wire from something small gauge like speaker wire. Twist and push gently and it should push its way through. Don't force anything bigger through. Once you can see through both of them, put it back together and try it again. If that doesn't do it, then the carbs need to be soaked, either in an ultrasonic tank like dorf mentioned. Or you can buy a good carb cleaner at your local yamaha dealer. It is mixed half and half with hot water. Take the carb apart and remove any plastic, rubber, or nylon parts. Soak the rest for 20-30 minutes. Use rubber gloves, the solution is pretty strong. Rinse everything off and dry and reassemble. Do this outside, not indoors.
Mike Hawkins:)

shumakerscott 05-30-2010 12:22 AM

A vacuum leak at the mounting rubbers would also cause this. Start the bike and spray starter fluid around the flanges. If the bike revs up then you have a leak. On the saftey side have a fire extinguisher on hand which you should have anyway. dorf dude...

firehawkmph 05-30-2010 10:12 AM

Good point Dorf,
only I would use wd 40 instead of the starting fluid. Less likely to need the fire extinguisher. I would hope to think his new rubber manifolds are sealing up all right. We'll see what happens next. Good thread.
Mike Hawkins:)(ret. firefighter)

High Gear 06-19-2010 12:33 PM

Boy some of you guys are really showing your age . :laughing:

That's OK I had a 73 CB450 I bought new.

They had Keihin CV type carbs.

You could try running some Seafoam in your gas to loosen up the carb.

That has saved me a couple of times over the years.

firehawkmph 06-20-2010 04:27 PM

3 Attachment(s)

Originally Posted by High Gear (Post 458557)
Boy some of you guys are really showing your age . :laughing:

That's ok HG,
Know what's in my garage now?
6 hodakas 1974 is the newest.
4 husqvarnas, 1972-1983
2 honda elsinores 1975-6
suzuki trials bike '74
2 honda 70's, 70-71
yamaha wr250f 2002 (only newer bike; I ride this one every so often, but prefer the older bikes)
Mike Hawkins:wink:

shumakerscott 06-20-2010 04:41 PM

Not only do you have great old dirt bikes but your a wood worker too! I could see your equipment in the background. Are we related? The name "Dirt Squirt" keeps popping up for some reason. Dorf dude...

firehawkmph 06-20-2010 05:13 PM


Originally Posted by shumakerscott (Post 459023)
Not only do you have great old dirt bikes but your a wood worker too! I could see your equipment in the background. Are we related? The name "Dirt Squirt" keeps popping up for some reason. Dorf dude...

Hey Dorf,
We could be related.:laughing: Dirt sqirt was one of the hodakas. Had a couple of them a few years ago and sold them when I was thinning out the herd. I do a lot of woodturning, and mix in some cabinets and flatwork type projects. Retired from the fire service five years ago. I do kitchen and bath remodels now but not full time. Gets in the way of the fun stuff.
Mike Hawkins:)

High Gear 06-20-2010 06:02 PM

Very nice those bikes look showroom new.

I had fun with a Yamaha DT250 (1975) back in the day.

Your pic reminded me that I need to get a dust collector , dang garage gets full of dust.

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