Topic: Warm up and idle speed what's normal?

[flybackwards]

I have two CB750 - K1 and a K7.  I notice on both of them that after cold start using the choke that as things get warmed up I have a fairly drastic increase in idle speed from a kind of lumpy 1100 - 1200 rpm rising to 2000 after a couple of minutes so that I have to go reset the idle stop screw. What's normal on these bikes? or am I just spoiled by the very fine electronic idle control on my new bikes? Both bikes are in good mechanical condition and timing etc set to spec. What think the gurus?

[HondaMan]

I have two CB750 - K1 and a K7.  I notice on both of them that after cold start using the choke that as things get warmed up I have a fairly drastic increase in idle speed from a kind of lumpy 1100 - 1200 rpm rising to 2000 after a couple of minutes so that I have to go reset the idle stop screw. What's normal on these bikes? or am I just spoiled by the very fine electronic idle control on my new bikes?

In one word: yes.
If they are running the idle up that high when warmed up, set the idle when warm and you'll have to help it along until they warm up.

But...
Almost 70% of the time (i.e., somewhat more than 1/2 the time) this happens, it is tied to a bent spark advancer's mounting shaft. These get bent by turning the engine using that big, handy nut by the points. This often happens when someone tries to set the points by connecting a small light bulb across the points with the power on and then repeatedly turns the engine back-and-forth, trying to set the points to open at the "F" mark. Just one time will bend the shaft, particularly turned backward, then forward. The runout should not be more than 0.006" and when it reaches about 0.015" or so, the bikes idle just as you describe (with fresh gas aboard). Turn the engine with the starter/kickstarter or (if available) the alternator's nut while straightening.

If the gas is old, that will also cause these symptoms: setting the idle when hot and then trying it again with fresh gas will help sort out which issue is which, here.

Note: edited the above sentences a little, left something out...

[newday777]

What's normal? The K0-K6 while cold will most times need to be feathered by your right hand until warmed up. Best to be gear on ready to ride when starting, start with choke for a few seconds then off the choke and feather the throttle a minute or 2 at most, then slowly take off riding slow for a couple more minutes to warm the motor before hitting it too hard.
The K7, if it has the PD carbs still should have a fast idle cam to hold the rpms up a bit while the choke is on, again not too long on the choke before taking off, warming up the motor as you ride slowly.
Yes the newer motors with ECM controlling the mixture is much easier but isn't old school cool and makes us appreciate the greatness of the older bikes.

[flybackwards]

Hmm guilty as charged m'lords - spoiled by the new bikes (I'm now remembering starting my 1966 Austin Mini 850 on a cold morning) and also of turning the motor with that handy nut..I must tomorrow check for bent shaft and get fresh gas.

[deeveebee]

I have two CB750 - K1 and a K7.  I notice on both of them that after cold start using the choke that as things get warmed up I have a fairly drastic increase in idle speed from a kind of lumpy 1100 - 1200 rpm rising to 2000 after a couple of minutes so that I have to go reset the idle stop screw. What's normal on these bikes? or am I just spoiled by the very fine electronic idle control on my new bikes?

In one word: yes.
If they are running the idle up that high when warmed up, set the idle when warm and you'll have to help it along until they warm up.

But...
Almost 70% of the time (i.e., somewhat more than 1/2 the time) this happens, it is tied to a bent spark advancer's mounting shaft. These get bent by turning the engine using that big, handy nut by the points, in a clockwise (normal run) direction. This often happens when someone tries to set the points by connecting a small light bulb across the points with the power on and then repeatedly turns the engine back-and-forth, trying to set the points to open at the "F" mark. Just one time will bend the shaft. The runout should not be more than 0.006" and when it reaches about 0.015" or so, the bikes idle just as you describe (with fresh gas aboard).

If the gas is old, that will also cause these symptoms: setting the idle when hot and then trying it again with fresh gas will help sort out which issue is which, here.

Is there an easy way to tell if it's bent?  Other than the suspicion based on the idling symptoms, I mean.   If it is bent, is it . . . unbendable?

[rotortiller]

I set my K7s idle with a fully warmed up engine at about 1400 rpm. I leave it after that.  When I start it cold on full choke it revs to 3000 or so and I then I  back off the choke to under 2000. As the K7 bike warms the engine idle speed naturally climbs like  all normal K7s do so I adjust the choke to bring it down. The K7 has a mechanism in the carb choke linkage that raises the throttle so you don't have to hold any throttle in during warm up. It's just a different beast from the older setups. With the push/pull cables one can force a low idle in the off throttle springy area (normal control bounce over travel). That is my take on how it should work and how to manage it.

[RAFster122s]

I have two CB750 - K1 and a K7.  I notice on both of them that after cold start using the choke that as things get warmed up I have a fairly drastic increase in idle speed from a kind of lumpy 1100 - 1200 rpm rising to 2000 after a couple of minutes so that I have to go reset the idle stop screw. What's normal on these bikes? or am I just spoiled by the very fine electronic idle control on my new bikes?

In one word: yes.
If they are running the idle up that high when warmed up, set the idle when warm and you'll have to help it along until they warm up.

But...
Almost 70% of the time (i.e., somewhat more than 1/2 the time) this happens, it is tied to a bent spark advancer's mounting shaft. These get bent by turning the engine using that big, handy nut by the points, in a clockwise (normal run) direction. This often happens when someone tries to set the points by connecting a small light bulb across the points with the power on and then repeatedly turns the engine back-and-forth, trying to set the points to open at the "F" mark. Just one time will bend the shaft. The runout should not be more than 0.006" and when it reaches about 0.015" or so, the bikes idle just as you describe (with fresh gas aboard).

If the gas is old, that will also cause these symptoms: setting the idle when hot and then trying it again with fresh gas will help sort out which issue is which, here.

Is there an easy way to tell if it's bent?  Other than the suspicion based on the idling symptoms, I mean.   If it is bent, is it . . . unbendable?

Setup a dial indicator to measure the runout.
After it is known mark high spot where you need to apply a few gentle taps with the ball peen hammer. Remeasure.
Repeat as needed to get it to tight tolerances you desire. If you are handy you can get the runout to very tight tolerance numbers.

[HondaMan]

I have two CB750 - K1 and a K7.  I notice on both of them that after cold start using the choke that as things get warmed up I have a fairly drastic increase in idle speed from a kind of lumpy 1100 - 1200 rpm rising to 2000 after a couple of minutes so that I have to go reset the idle stop screw. What's normal on these bikes? or am I just spoiled by the very fine electronic idle control on my new bikes?

In one word: yes.
If they are running the idle up that high when warmed up, set the idle when warm and you'll have to help it along until they warm up.

But...
Almost 70% of the time (i.e., somewhat more than 1/2 the time) this happens, it is tied to a bent spark advancer's mounting shaft. These get bent by turning the engine using that big, handy nut by the points. This often happens when someone tries to set the points by connecting a small light bulb across the points with the power on and then repeatedly turns the engine back-and-forth, trying to set the points to open at the "F" mark. Just one time will bend the shaft, particularly turned backward, then forward. The runout should not be more than 0.006" and when it reaches about 0.015" or so, the bikes idle just as you describe (with fresh gas aboard). Turn the engine with the starter/kickstarter or (if available) the alternator's nut while straightening.

If the gas is old, that will also cause these symptoms: setting the idle when hot and then trying it again with fresh gas will help sort out which issue is which, here.

Is there an easy way to tell if it's bent?  Other than the suspicion based on the idling symptoms, I mean.   If it is bent, is it . . . unbendable?

The easiest way to "see" a bent shaft is with a timing strobe light: the timing marks will 'jitter' back-and-forth under the strobe light quite a bit when the shaft is bent.

[deeveebee]

I have two CB750 - K1 and a K7.  I notice on both of them that after cold start using the choke that as things get warmed up I have a fairly drastic increase in idle speed from a kind of lumpy 1100 - 1200 rpm rising to 2000 after a couple of minutes so that I have to go reset the idle stop screw. What's normal on these bikes? or am I just spoiled by the very fine electronic idle control on my new bikes?

In one word: yes.
If they are running the idle up that high when warmed up, set the idle when warm and you'll have to help it along until they warm up.

But...
Almost 70% of the time (i.e., somewhat more than 1/2 the time) this happens, it is tied to a bent spark advancer's mounting shaft. These get bent by turning the engine using that big, handy nut by the points, in a clockwise (normal run) direction. This often happens when someone tries to set the points by connecting a small light bulb across the points with the power on and then repeatedly turns the engine back-and-forth, trying to set the points to open at the "F" mark. Just one time will bend the shaft. The runout should not be more than 0.006" and when it reaches about 0.015" or so, the bikes idle just as you describe (with fresh gas aboard).

If the gas is old, that will also cause these symptoms: setting the idle when hot and then trying it again with fresh gas will help sort out which issue is which, here.

Is there an easy way to tell if it's bent?  Other than the suspicion based on the idling symptoms, I mean.   If it is bent, is it . . . unbendable?

The easiest way to "see" a bent shaft is with a timing strobe light: the timing marks will 'jitter' back-and-forth under the strobe light quite a bit when the shaft is bent.

Great tip, thanks!  I didn't notice this last time I used a strobe, and my bike's idle behaves nicely anyway, but I definitely HAVE done the turn-the-engine-via-the-big-bolt-with-a-wrench thing, so you got me worried.  Thanks again.

[jwurbel]

So should you set the points by applying force to the kickstarter to move the shaft?

[BenelliSEI]

So should you set the points by applying force to the kickstarter to move the shaft?

Take the spark plugs out and it turns freely…….

[flybackwards]

Just as a follow-up - CMS had some of 30231-300-010 (3) on their web pages - when I tried to buy them they turned out to be phantoms..however Old School parts now has a good looking reproduction available - don't buy them all before I order mine! My timing light does show a fair amount of jitter in the advance setting.

[newday777]

Just as a follow-up - CMS had some of 30231-300-010 (3) on their web pages - when I tried to buy them they turned out to be phantoms..however Old School parts now has a good looking reproduction available - don't buy them all before I order mine! My timing light does show a fair amount of jitter in the advance setting.

You can straighten your advancer shaft using a dial indicator set on the end of the smooth shaft and a ballpeen hammer (light taps on the nut). You will need to remove the spark plugs(so no compression in motor(whenever you turn the motor with the 23mm nut so you don't bend the shaft!), points plate and advancer(or the electronic ignition plate) to have full access to the shaft. It needs to be within .006 thousands according to the service manual. I was able to dial mine to .000
I bought a HF vise clamp dial indicator ($30) because I work on multiple 750s, I have 6 and on friend's bikes, it worked fine for doing this.

OSVINTAGEPARTS is @ 30 miles from me, in the same building as Rick's Electrics(reg/rectifier) but I haven't tried the repop shaft. Good peeps. Rick's does a vintage ride in show in the spring. Lots of great bikes show up.

[pjandrew99]

Just as a follow-up - CMS had some of 30231-300-010 (3) on their web pages - when I tried to buy them they turned out to be phantoms..however Old School parts now has a good looking reproduction available - don't buy them all before I order mine! My timing light does show a fair amount of jitter in the advance setting.

You can straighten your advancer shaft using a dial indicator set on the end of the smooth shaft and a ballpeen hammer (light taps on the nut). You will need to remove the spark plugs(so no compression in motor(whenever you turn the motor with the 23mm nut so you don't bend the shaft!), points plate and advancer(or the electronic ignition plate) to have full access to the shaft. It needs to be within .006 thousands according to the service manual. I was able to dial mine to .000
I bought a HF vise clamp dial indicator ($30) because I work on multiple 750s, I have 6 and on friend's bikes, it worked fine for doing this.

OSVINTAGEPARTS is @ 30 miles from me, in the same building as Rick's Electrics(reg/rectifier) but I haven't tried the repop shaft. Good peeps. Rick's does a vintage ride in show in the spring. Lots of great bikes show up.

Great info. My guess is most cb750's have "developed" this issue over the years. I need to do this. Would make a great Youtube video if one of the experienced members wanted to put one together. Thanks for the direction in dealing with this very annoying issue.

[newday777]

Just as a follow-up - CMS had some of 30231-300-010 (3) on their web pages - when I tried to buy them they turned out to be phantoms..however Old School parts now has a good looking reproduction available - don't buy them all before I order mine! My timing light does show a fair amount of jitter in the advance setting.

You can straighten your advancer shaft using a dial indicator set on the end of the smooth shaft and a ballpeen hammer (light taps on the nut). You will need to remove the spark plugs(so no compression in motor(whenever you turn the motor with the 23mm nut so you don't bend the shaft!), points plate and advancer(or the electronic ignition plate) to have full access to the shaft. It needs to be within .006 thousands according to the service manual. I was able to dial mine to .000
I bought a HF vise clamp dial indicator ($30) because I work on multiple 750s, I have 6 and on friend's bikes, it worked fine for doing this.

OSVINTAGEPARTS is @ 30 miles from me, in the same building as Rick's Electrics(reg/rectifier) but I haven't tried the repop shaft. Good peeps. Rick's does a vintage ride in show in the spring. Lots of great bikes show up.

Great info. My guess is most cb750's have "developed" this issue over the years. I need to do this. Would make a great Youtube video if one of the experienced members wanted to put one together. Thanks for the direction in dealing with this very annoying issue.

It happens when someone doesn't remove the spark plugs before turning over with a wrench on the 23mm nut on the shaft.

[Kaze]

It happens when someone doesn't remove the spark plugs before turning over with a wrench on the 23mm nut on the shaft.

Specifically instances where things like this matter, I wish that manuals were written a bit more clearly for noobs. Written in Dad English...

"Now don't #$%* around and forget to take out the plugs or you'll bend this sonofa#$%* and then you'll be in a world of #$%*."

[newday777]

You can straighten your advancer shaft using a dial indicator set on the end of the smooth shaft and a ballpeen hammer (light taps on the nut). You will need to remove the spark plugs(so no compression in motor(whenever you turn the motor with the 23mm nut so you don't bend the shaft!), points plate and advancer(or the electronic ignition plate) to have full access to the shaft. It needs to be within .006 thousands according to the service manual. I was able to dial mine to .000
I bought a HF vise clamp dial indicator ($30) because I work on multiple 750s, I have 6 and on friend's bikes, it worked fine for doing this.

OSVINTAGEPARTS is @ 30 miles from me, in the same building as Rick's Electrics(reg/rectifier) but I haven't tried the repop shaft. Good peeps. Rick's does a vintage ride in show in the spring. Lots of great bikes show up.

Great info. My guess is most cb750's have "developed" this issue over the years. I need to do this. Would make a great Youtube video if one of the experienced members wanted to put one together. Thanks for the direction in dealing with this very annoying issue.

[/quote]

Yes it would make a good instructional video. Maybe I'll see if I can find time to try. I've never made a video before... so don't hold yer breath....

[drodg33]

My CB500 K0 is the same way.  I have to nurse it for about the first mile with the choke on and then eventually off.  It will idle until fully hot at about 1100 rpm but as it gets hot it will want to run up closer to 2000 rpm.   

[MauiK3]

My K3 has the choke lever down at the carbs. Must have been a mid year change.
My shaft was out as well, I used a small dead blow mallet, works great, small taps.
To anyone reading here, if you don't have Honduran's book, get it, great tool for working on the 750, best tool in the tool box!

https://www.amazon.com/CB750-book-Hardcover-Mark-Paris/dp/B06Y4LP2WW/ref=sr_1_1?crid=1O4FNWL2DZUMC&keywords=Mark+Paris+CB750&qid=1697035895&sprefix=mark+paris+cb750%2Caps%2C304&sr=8-1

[Deltarider]

My CB500 K0 is the same way.  I have to nurse it for about the first mile with the choke on and then eventually off.  It will idle until fully hot at about 1100 rpm but as it gets hot it will want to run up closer to 2000 rpm.   

Except for maybe the K3 model, which had the choke knob at the handlebars, chokeflaps should be fully reopened prior to riding. In the Owner's Manual CB500 Four, Honda advised to have a somewhat increased idle (ca. 2000 rpm) by keeping the throttle slightly twisted inward, untill engine has warmed up enough.

[MauiK3]

Yes, my idle is lumpy until warm up, I usually don't have it just idle when cold, I get ready, get on and gently motor off.