Topic: Idle goes higher as engine gets warmer - 72 CB750

[jerimiah797]

Hi guys, this is my first post dealing with engine performance / tuning issues.  I've got a problem that seems a little similar to this thread:

http://forums.sohc4.net/index.php?topic=37911.0

Instead of the throttle flat spot being the main issue I'm concerned about, it's the rising idle RPM that has me stumped.

The bike runs brilliantly for the first 10 or 15 minutes. Throttle response is great, I can quickly rev the throttle in neutral and the engine dives right back down to 1000 - 1100 RPM, which is where I've set my idle at. So far so good.

Then I ride the bike for another 5 - 10 minutes. Each couple of minutes, when I stop at a light or stop sign, I notice the idle has gotten a little higher. First 1500 RPM. Then 1800 RPM. Then 2000 RPM, then 2300 RPM. The engine is getting hotter and hotter, and idling faster and faster. If I blip the throttle while in neutral, the RPMs will dive to 1200 or so for maybe 1 second, then creep back up to over 2000 in the next second or two. It becomes a chain reaction that I can't stop. I have to turn the bike off and let it cool down for a few hours. Of course as the bike gets hotter, it starts to run like crap, and the throttle response becomes very unpredictable. Like even though the engine is idling at 2500 RPM, there is no power behind it. It's not like I can just slowly let out the clutch from a full stop and the bike will start moving. I have to get it up to 4K, give it a little clutch, and then give it even more throttle to get the bike moving. And there much less power when the bike is hot.

So here's my setup:
'75 carbs, freshly rebuilt.
105 main jet (down from 110)
needle clip in middle position (although still running a bit rich in the midrange)
non-k&N pods
Kerker 4-1 exhaust
Dyns-S ignition and coils
New intake boots, freshly tightened.
Valves done less than 100 miles ago, engine rebuild done less than 1000 miles ago.
Exhaust is black and carbony in the midrange.
Plugs are black and sooty when I pull them, but clean right up.

My hunch is some kind of air leak in the carbs that is triggered by the heat. But a really strange behavior is that if I turn the handlebars all the way to the left when I'm stopped and the engine is racing at idle, the RPMs will actually slow down. Why, I have no idea...

What do you think?

-Jerimiah

See my introduction post for pics!

[mlinder]

Rich idle circuit.

your 5-10 minutes of 1k idle is because it's still cold. Once it gets to operating temp, THEN you should set your idle.
The fact that when you blip the throttle and it will die down to 1200 or so is usually a rich condition.
Lean out your idle circuit half a turn or so on the idel screws, let the bike warm up, and after that, THEN set your idle.
As far as idle dropping when you turn the handlebars all the way, check for throttle or throttle return cable pinching or binding.

[martino1972]

Rich idle circuit.

your 5-10 minutes of 1k idle is because it's still cold. Once it gets to operating temp, THEN you should set your idle.
The fact that when you blip the throttle and it will die down to 1200 or so is usually a rich condition.
Lean out your idle circuit half a turn or so on the idel screws, let the bike warm up, and after that, THEN set your idle.
As far as idle dropping when you turn the handlebars all the way, check for throttle or throttle return cable pinching or binding.

+1 all the way...

[jerimiah797]

Great, thanks for the suggestions, guys. I'll try that right away.

I'm assuming that if the idle is set so low, then until it's warmed up I'm gonna have to stay on the throttle so the bike doesn't die?  I was enjoying the part where I could walk up to my cold bike, start it, and then let it run unattended for a few minutes while I geared up... :-)

-Jerimiah

[mlinder]

You are correct... but you DO have a choke, right?

I start with choke all the way closed, spend 2 minutes slowly opening the choke and blipping the throttle until I get a nice 1500rpm idle, take off, and as it warms, the idle will rise. Open choke a tiny bit more as I ride. Eventually I turn it all the off. Yeah, I may need to pay attention to the throttle for a few miles, but that's the way these work.

[tonycb650]

thats what the choke is for. Use the choke for cold starts, then as the bike warms up, open the choke a little at a time.

[jerimiah797]

Okay, I'll try that. I've never had to use the choke when starting or warming up the bike, probably due to both the high idle setting and the rich idle air screw adjustments. Heheh.

Thanks again. I'm gonna go for a quick spin and see how that works out.

-Jerimiah

[jerimiah797]

Okay, that seemed to help a lot. I backed off all four idle air screws 1/2 a turn. I was still able to start the bike with no choke, although it wasn't totally cool from my previous ride. It idled as normal. I went for a hard ride and 15 minutes later pulled over to find the idle around 2200. Backed off the idle adjuster till I was at about 1200 - 1400. If I went lower than that, the engine would very slowly (about 20 seconds after the last throttle blip ) slow down until it died.

Noticed a little spitting out of the carbs on the ride back when throttling from a longish idle, but not much - as in maybe 2 or 3 times in about four miles with occasional stoplights. Also a little exhaust burp-burp-gurgle-burp at idle when the engine was hot.

Anything sound out of place?

I think I'm still running rich in the middle throttle position, my exhaust is still pretty sooty up there and stinks of gas. I keep suggesting to my mechanic friend that we should lower the needle 1 clip position, but for some reason he thinks that's a bad idea. I can't imagine why since it's obviously running rich in the midrange and that is the fix I see written about over and over again... I don't mind working on carbs at all - maybe I should just do it myself and see what happens.

It's easier to ask forgiveness than permission, right? :-)


-Jerimiah

[martino1972]

yeah,if your meachanic friend thinks that,its time to fire em and do it yourself..
lower the needle one clip(means the clip has to go one up)and give it a try...
note what groove clip was in,and ride it..if much better then do a plug chop and tell us the results..

[mlinder]

How many turns are your air screws out right now?

[jerimiah797]

They are 1.5 turns out now. Looks like my guy set them up at 1 turn out before I started messing with them.  Think they need to go farther out, since the engine slowly dies when the idle is set below 1200?

-Jerimiah

[mlinder]

Possibly a bit further out, yes.
It's trial and error, really.

[jerimiah797]

Okay, cool. I'll play around with it.

Thanks again for your help everyone.  It's so great to get more than one opinion about a problem.

-Jerimiah

[strangedaze39]

Hey Jerimiah,

My bikes idle goes up quite a bit as well once it gets warmer, I either start it give it a very slight amount of throttle and go on a low idle (almost about to die) but then by the second stop sign on my street it's idling great. OR you can turn the idle screw up one full or half turn...whatever is necessary, and then when it warms up at a stop sign or stop light turn it back down a full or half turn.

If I'm trying to just start up and go I normally do the first method, but if I feel like starting the bike and running to go grab my helmet and double check my gear I do the second.

Have fun, and welcome to the forum