Topic: RPM hanging up

[Scott S]

 '71 CB500

This bike sat for 28 years before I bought it. I'm nearly finished restoring it and heard it run under it's own power yesterday afternoon.
 I've done all the "usual" stuff you'd do to a bike that's been dormant for three decades:
 -oil and filter
 -adjusted valves
 -adjusted cam chain
 -set points
 -static timed (timing light coming today)
 Etc, etc.

 The carbs were in horrible shape with fuel in the float bowls that had turned to molasses. I cleaned them myself, but ended up having them ultrasonically cleaned and four new rebuild kits installed. Bench synched, mixture screws 1.5 turns out.

 The bike will start right up and I can adjust the idle. It will idle all day long at 1000 rpm. BUT, as soon as I give much more than a tickle of throttle, the RPM's climb and hang up around 3-5K.
 I've double checked the (new) throttle cable and it's definitely not the culprit. What I find odd is that if I tap on the carb tops with a wrench, it can actually make the revs go up. Especially the #1 carb, which is telling, because the #1 cylinder is running cooler that the other three by about 100 degrees (using my IR temp gun on the exhaust pipes).

 I'll check the fuel line routing today, but a cursory look doesn't show anything that is interfering. I'll also check for a vacuum leak at the manifold boots.
 It's almost like a slide is sticky or something, but I don't see the linkage being held open and the carbs were ultrasonically cleaned.
 Someone suggested lubing the throttle rod, but where?

 I have some vacuum gauges and have not done an actual synch yet. I wanted to check the timing first. Should I find out what's causing the RPM to hang up before synching or could that be part of the problem?

 Any thoughts?

[ekpent]

Different bike but similar problem.K4 750 throttle sticking. Had cleaned the carbs well,were very gummy.Idle would hang up around 3 grand,go back down with a quick twist or blip of throttle,checked all possible reasons. Tried stronger spring etc. Went back in and thoroughly cleaned the slides and throat again and never a problem since.A very tiny hangup can change the idle drastically as you know with the idle adjustment screw. Good luck on it.

[Scott S]

 I've traced it all to the #1 cylinder. I'm getting compression back through the carb and it's not fully running on that cylinder. I pulled the tappet covers and made sure the valves were moving and not sticking. It's too hot to do an adjustment on them right now.
 Moving the mixture screw, I was able to make the "chuffing" come and go, but at times it has no effect on it.
  Gonna do a compression test today. Still not sure if it's the carb or a valvetrain issue.

 Any ideas are appreciated.

[Scott S]

 OK, the one and only Pamco Pete rode over today to help me sort this thing. We found a couple of issues with the timing. The 1/4 points were way retarted. The 2/3 points were way advanced (I had static timed it, but pretty much put it back the way I found it).
 Also, I found an error in the Clymers manual. It said the points gap was .04mm (.0015 in). That's TINY. We realized it should be .015in.
 Set the points and timing best we could, The mounting post for the 2/3 points is wobbly, but Pete has promised me a Pamco ignition as soon as I get the bike sorted.

 Found the #1 exhaust valve a little tight. Fixed that.

 There's still something up with the #1 carb. I can hear a chuffing noise and the RPM are sometimes reluctant to fall. Sometimes the chuffing goes away and, when it does, the #1 cylinder runs. There's an awful lot of fuel in that carb. I think there's a vacum leak somewhere but CANNOT find it. I suspect its actually around the carb top or the linkage arm that moves the slide.
  The other 3 cylinders were/are pretty damn close as far as synch goes. #1 is close at idle, but it all goes wacky with throttle.

[Scott S]

 Oh yeah, did a compression test on 1 and 4 (can't get the gauge into those deep spark plug holes in the center two).
 150 psi on #4 and 130 on #1. A little off, but not so much that the #1 cylinder shouldn't fire.

[Deltarider]

Scott, you mention:

Bench synched, mixture screws 1.5 turns out.

I would like to see a reliable source (Honda America for instance) for this 1.5 turn out. I've seen it before in schemes that I suspect being corrupt (mixing up 550 and 500 models).
I don't know any better than for the 500 it is: 1 ⁺/_- 1/8 turn.

[Scott S]

 I've seen everything from 1 (+/- 1/8) to 2 (+/- 1/2). That leaves a WIDE range, from 7/8 of a turn to 2.5 turns. I was told on this forum that 1.5 was a good starting point.
 Think 1 would be better?

[Mdub]

Did you download the service manual?
It says 1 turn out +-1/8 turn.

[Jarhead12]

all that is wrong is it runs at that high of rpms to get the engine warm. Then put he choke to the open position and that is where the choke is off and your bike should idle at about 1000rpm to 1500, if it is too high unscrew the idle screw and visa versa. Just make sure your engine is warm when you turn the choke to open or off

[Scott S]

 It wasn't the choke....I was aware of the choke plate position.
 
 Turns out I had two problems....the slow jet had actually come unscrewed and was loose in the float bowl. That was the cause of the flooding.

 The throttle shaft that raises and lowers the slide...the one under the carb top...the set screw was loose. That was the cause of the RPM's not dropping. The cable would raise the slide, but it would slip on that shaft and not lower properly.

 We cleaned, inspected and measured the old jets and re-used them. We also installed some better o-rings on the jets and the float bowls.
 All I need now is some fresh oil and gas and I'm ready to try it out again.

[FunJimmy]

Thanks Scott.
I have a simular issue and will look at these areas too.

FJ

[Don R]

You may have a sticky valve. If so, a little marvel oil might cure it, It worked well in our old flathead fords.
 My 750 idles high after a run on the highway until it cools down a little, there is a burble in the exhaust sometimes too, I suspect a vacuum leak of other carb issue.

[Lars]

If you have cleaned your carbs well, did you use an instrument to set the float height correct? If timing etc. is correct, tyr to synch the carbs, that usually have an amazing effect.
Lars