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Discussion Starter · #1 ·
I have been trying to get my 1974 Cb360 to idle.

What happens is that when the throttle stop screw is disengaged totally it wont idle at all. Starts then dies with no throttle. Can keep it running with a little throttle. No over rev.

I give the stop screw like half a turn and the bike revs to the moon. Very sensitive. It's like its either on or off. Its not the original set up for the stop screw. I had to replace that with just a regular screw that turns with a screw driver.

When it revs like that it backfires through the carbs and the pipes, smoke comes heavy from the breather tube
and points cover and the carburetors have a little fit and start jerking around.

I have to kill the bike with the choke.

It is like when it gets to a point where it wants to idle is when it will take off like a crazy bike.

Last night I let it rev and tried to adjust it a little but that is when it started smoking like crazy. Got nervous and shut it down.
Could I have burnt the crap out of one of my pistons letting it rev like that?

Anyhow, what could cause this.
 

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Sensei
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27,173 Posts
we'll need pix of your carbs/idle screw modifications, although I suspect other problems....
Runaway revs can be caused by an air leak....hung carb slide....hung cable.....etc....
Let's see what you have at present....
 

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Registered
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175 Posts
Not enough information. Since the motor will idle with a little hand throttle opening, but revs off scale with a turn of the minimum throttle setting screw, you could start by mechanically balancing the carbs. Hand throttle and MTS screw both operate on the same mechanical part of the carbs, the screw only limiting how far the throttles can close. If, however, your carbs are not mechanically balanced, your hand throttle may initially only be opening a throttle on one side, the MTS screw opening the throttle on the other side. This should still not result in extreme revs. Keep in mind that, to rev up, the motor needs air and fuel. You can see (if this is unilateral) which side is the problem by trying this with one plug wire or the other disconnected and grounded. Sort of sounds like a flooding carb and an air leak - you don't need much above the idle throttle plate position to give you a lot of no-load revs. Try re-starting while you're on the bike with first speed engaged and the clutch disengaged. See if you actually have any power or if you're just getting no-load revs.
 
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Discussion Starter · #4 ·
The carbs are not flooded but an air leak is entirely possible. That was my first thought. The carbs are cleaned and assembled correctly. They work together as they should. Float levels are correct. Floats are not stuck. Will try your suggestion of starting in first.
As you requested 66Sprint here are some photos. From bottom to top 1. original mechanism for the stop screw. 2. the screw that I put in its place. 3. the right carburetor.
 

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Discussion Starter · #5 ·
There is definitely power behind the rev.

Keep in mind that I have only had the bike for a few weeks so I am still discovering things about it.

For instance, I have just discovered that under the rubbery goop that is covering the intake boots are numerous
cracks. I can't tell if they go all the way through but they are quite deep. Could that be enough to cause it to rev?

Why does a lean condition cause the engine to rev anyhow?
 

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Premium Member
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1,612 Posts
Imagine a turbo set-up on an engine. The turbo kit has a fan that pushes added air into the mix which in turn creates more power. Kinda like blowing on the coals of a fire, the energy increases. So an intake leak will add more air to the fuel mix and in turn create more power..though..at risk of over-heating, which is why turbo engines have intercoolers.
 
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Discussion Starter · #8 ·
there were a lot of cracks in the intakes so I replaced them. that helped a little bit. it still races but not so high. it was also suggested that I try to see if only one cylinder was racing. sure enough only the right cylinder is racing. I have a good intake boot with good gaskets. so i think the air leak was taken care of. it is also still sensitive to the minimum throttle screw. if the screw is off the throttle plate there is no idle. if the screw is on the throttle plate the least little bit then the engine races like crazy with no hand throttle.
what should be my next step?
why would only one side rev like this?
 

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Registered
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175 Posts
Because there's only something wrong with one side. Have you done the full tuneup drill - point gap, time each cylinder seperately, mechanically balance the throttles, set the float levels... Eliminate all the standard malfunction issues and work from there. Then analyze the problem. For instance, if you have high float level or leaking float valve combined with a significant air leak, nothing happens as long as the throttle is completely closed because no air passes through the venturi to carry fuel into the air stream - the air leak means there's always air going into the cylinder. As soon as you crack the throttle, a small amount of air going through the venturi carries a lot of fuel into the air stream and the motor revs up. As far as burning the piston goes, that won't happen. The motor has to be doing work to generate that much heat. You could eat a valve, break piston rings, other not nice things, though.
 
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Discussion Starter · #10 ·
float levels are right.
point gaps are right.
leaky float valve i am not sure. what would be the signs of that? the carbs are not flooding.

i am going to take the carbs off and recheck that both sides are working together.

i have noticed that the fins on the engine are getting hotter than any of my other bikes get. is that a sign of something?

also, the stay for the throttle cable is a tiny bit loose. i know that it is not supposed to be and that is something I am going to fix but is that something that could have anything to do with what is happening?

if there is an air leak it is coming from somewhere other than the intakes.
what about valve clearance? is that something I should be looking at for this problem?

i should also mention that the petcock on this bike will not close fully. when it is off there is still a stream to the carbs.
my thinking was that the float valves will stop that when the bowls fill. what is your opinion on that?

thanks for the help
i just gotta keep looking
 
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