Topic: Inconsistent Idle - Over a year later - ALMOST THERE!!! 9/24/11

[KB02]

It's a carb problem, I'm sure of it... or maybe ignition...  I don't know...

THE BIKE:
1978 CB750 bored out to 850.
Stock carbs rejetted to 130 main, stock slow speed jet, idle mixture screw 1 1/2 turns out (I think)
Cycle X Optical ignition
Pod filters.
Anti-Pod Filter box
4 into 4 aftermarket exhaust.

THE PROBLEM:
I have a very inconsistent idle .
I just finished the rebuild on this bike (Check out my build thread below) and am still getting some of the quirks worked out. Right now, the biggest thing that is bugging me (other than a small, pesky oil leak) is the idle. It is either idling so low that it stalls out, or racing up to 3-4000 rpm. When coming down from cruising speed, the bike will stay at that engine speed and not come down. If I load up the clutch a little bit, the idle will drop right down and it will stall out. If I adjust the idle to stay up in the slower state, I can never get the engine to come down again once I have sped it up.

The carbs are clean (I know the process - Clean them once, clean them twice, clean them a third time, and then clean them once again before you're done with the first round...). I just did a carb sync yesterday as well. The bike runs very smooth, I just can't seem to get it to that happy idle place.

It's the hanging idle that bugs me. I can get a good idle speed, but once I go away from that speed, I can never get it to come back to it. 

Any thoughts?

[Nikkisixx]

Things that can cause your symptoms include (but are not limited to)
-throttle cable pinched or hanging
-vacuum leaks
-timing components wandering (plate lose, advance springs worn out, advance not retuning at low rpm, etc)  <- i'm not familiar with your ignition system

[yozzie]

My experience with idle problems (if you have a good synch) has been either improper idle mixture or leaking intake gaskets/o-rings.   

[BobbyR]

After all of the excellent advice above. There is one other consideration. When do you adjust your idle? The 78K had a fast idle cam attached to the choke lever and it is adjustable. Check for adjustment and readjust if needed.

The idle speed should be set using the large knob when the bike is fully warm and preferably run at cruising speed for some distance.

Cold idle should be set at the fast idle cam on the choke lever.

I know this because I had it happen to me. If you use the knob when the motor is cold you will be chasing the idle all over the place.

[KB02]

Sorry, I've been away dealing with my new baby. 

I have been checking it with the engine at full temp. I didn't realize there was an adjustment o the fast idle cam; I will check that when I can. I'll also check for vacuum leaks.

Thanks

[Spanner 1]

Congrats, new Dad , ..... bike might fade into the background now for a bit... good luck with getting any sleep for the next 6 months!!

[Nikkisixx]

Congrats, new Dad , ..... bike might fade into the background now for a bit... good luck with getting any sleep for the next 6 months!!

Dammit spanner, I wish people would quit saying stuff like that!  My wife and I are expecting in 3 weeks 

Congrats KB02! 

[KB02]

Dammit spanner, I wish people would quit saying stuff like that!  My wife and I are expecting in 3 weeks 

Congrats KB02! 

Advice from one new Dad to another: The salmon colored pee is normal. No one told me that. I was freaking out during a diaper change and had to call for a nurse.
Congrats, Man. It's been a hell of a ride so far (and it's only been 4 days so far).

[BobbyR]

Congrats on the the new Baby. Every day is a new adventure.

[KB02]

SO I finally got back out to work on the bike. I pulled the carbs, made sure they were fully seated and sealed and the problem still exists. I'm riding down the road with the engine happily singing at 3-4000 RPM, I pull in the clutch, release the throttle and the engine speed stays right where it is. IF I ease out the throttle the RPM will drop slightly to match the engine speed to road speed, and stay there. but as soon as I power back up again, the idle hangs.

The only thing I can think of left to try is the throttle return spring. I eased it down to reduce the hand fatigue of the heavy pull. I can reset it to see if that helps, but I doubt it will have any effect as trying to slam the throttle closed with my wrist has no effect what so ever.

Any other suggestions?

[Nikkisixx]

Well, you are up at 6 AM and it has been a month since you worked on your bike.  Sounds like life around here  

It kinda seems like a cable hanging up.  This can be tricky to identify because it may only hang up under tension.  When the cable is disconnected (like when you are checking it out) it can slide nicely, but a slight kink can give you grief when in use.

edit: just looked to see how much cables are - $16 for each.  I've used motion pro and had good results -
http://www.bikebandit.com/1978-honda-cb750k-motion-pro-cables/c/a658106a623931?m=9458&t=1

[KB02]

Cables are brand new.  Besides, if it was the cable, why would letting out the clutch while the bike is slowing or stopped have any effect?

[Nikkisixx]

Well, that is frustrating then.  For me, goofy RPM problems and always turned out to be cable related provided there is no air leak or ignition problem.
As for springs, the '78 has two springs off a bear trap - one on the shaft and one by the cables.  They are darn stiff, but maybe they can go bad.    Now I'm curious to know how it works out.  When you get it fixed, keep us updated!

[KB02]

And Survey Says!~

It was the throttle return spring. 

I have now reinstated the elevator spring used to pull the sliders shut and my issue has appeared to go away.

DAMN.

I was really hoping not to have to put a throttle rocker wing on this thing because I HATE those. But the pull is just soooooooo strong that my hand is cramping up less than a mile from my house.

Anyway, problem is solved.

[vance]

get a vista cruise throttle lock. made it possible for me to ride. i have numbness in my hand from the slightest wrist pressure.

[hapakev]

Damn KB, you feel my pain.  I tried so hard to get the bike running before my daughter was born, in May mind you, and couldn't finish it with all the buying of baby stuff going on.  Its Sept and I just got it running.  Sneaking out for 15-20 minutes at a time evoked attitude from the wife. Nikki, tread lightly my friend.

[Shooter]

On my CB550, I decided to use only one throttle cable.  Dont know if your bike has two or not but it sorted mine out fine and throttle action is light and responsive..

And Survey Says!~

It was the throttle return spring. 

I have now reinstated the elevator spring used to pull the sliders shut and my issue has appeared to go away.

DAMN.

I was really hoping not to have to put a throttle rocker wing on this thing because I HATE those. But the pull is just soooooooo strong that my hand is cramping up less than a mile from my house.

Anyway, problem is solved.

[KB02]

Damn KB, you feel my pain.  I tried so hard to get the bike running before my daughter was born, in May mind you, and couldn't finish it with all the buying of baby stuff going on.  Its Sept and I just got it running.  Sneaking out for 15-20 minutes at a time evoked attitude from the wife. Nikki, tread lightly my friend.

My wife knows that working on my bikes in my Zen time and there are times that she has kicked me out of the house.   

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I think the throttle pull would fine (heavy but fine) is full stock form on the bike, but with the clip ons I am using, it gets a little overwelming for the hands. Just gives me another excuse to consider a set of CR's... 

[MRieck]

Damn KB, you feel my pain.  I tried so hard to get the bike running before my daughter was born, in May mind you, and couldn't finish it with all the buying of baby stuff going on.  Its Sept and I just got it running.  Sneaking out for 15-20 minutes at a time evoked attitude from the wife. Nikki, tread lightly my friend.

My wife knows that working on my bikes in my Zen time and there are times that she has kicked me out of the house.   

----------

I think the throttle pull would fine (heavy but fine) is full stock form on the bike, but with the clip ons I am using, it gets a little overwelming for the hands. Just gives me another excuse to consider a set of CR's... 

You really have to experiment with the cable routing Rob. You shouldn't that much spring to close the carb slides. I'd lube the linkage too.

[KB02]

Okay, so it wasn't the return spring.    Rode the bike to work yesterday and the problem resurfaced about half way there and suck with my the whole way home. I just don't get it. What am I missing?

You really have to experiment with the cable routing Rob. You shouldn't that much spring to close the carb slides. I'd lube the linkage too.

It's not cable routing. When the cables are not attached to the carbs they just as free as if I were holding them in mid air. That's not the issue.

[MoMo]

I have had very similar problems-the culprit was always gummed up slides.  I had to totally disassemble the carbs, remove the lacquer from the slides and throttle bodies and fine sand so that the slides moved freely.  Good luck with the bike and baby,  Larry

[Duke McDukiedook]

I say knackered slides, something is hanging up in the carb mechanism causing a sticking point on your throttle.
Check the slides, needles, you know the routine.

[Nikkisixx]

The pink pee is one thing, but I think the black poop newborns crap would be a good substitute for epoxy.  It is quite possibly the sh!t the black tiles on the space shuttle are made of.  

Anyways, this sounds like a broken record but I've been down the road you are on a couple of times.  Brand new cables, rebuilt carbs, good return springs, new grip and plastic thing under it, grease everywhere and it was like butter when the cables were not hooked up to the carb.  When attached at both ends and under tension the throttle would stick.  I finally loosened the jam nuts that hold the cable to the carb slightly and routed the cables goofy - around the outside of the headlight in as big an arc as I could.  THEN the darn thing worked great.  I'd try that first, it is a good 1 beer project and costs nothing  

If the cable is still routed like this, it will be grumpy:

[BobbyR]

I have had very similar problems-the culprit was always gummed up slides.  I had to totally disassemble the carbs, remove the lacquer from the slides and throttle bodies and fine sand so that the slides moved freely.  Good luck with the bike and baby,  Larry

Clean the slides, don't sand but polish them up. They are chromed.

[Scott S]

 Polish them with what? 0000 steel wool?