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High to return to Idle? (Complex ??)

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Guest

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Well my bike is acting up a little...

I recently got the carbs synched at a shop and watched them. They did a great job, bike starts and runs/idles like a champ. (they did not fiddle with the idle mixture screws)

however it still is slow to return to idle. When i am riding or pulling up to a light, the rpm's will stick at 3500-4k for a few seconds. There are no intake leaks, and the orings behind the boots are new. I am running the stock airbox with a k&n drop in, with no lid. Once it drops to idle, it holds it nicely.

I have read not to adjust these idle mixture screws, should i? I am also still running very rich at idle, due to trying to keep the A/F somewhat rich and keep the heat down until i can get it on a dyno.

Any help would be appreciated.
 
No lid on the airbox and you are running rich? :shock:

Two things that just don't go together, there, unless you have re-jetted the carbs. The 'hang' in the throttle return is a classic lean condition. I would start with putting the lid on the airbox and tweaking the mixture screws for highest rpm. Most likely have to turn them out (counter-clockwise) about 1/2 turn or so. Before you turn them out, turn them in slowly, counting the turns to where they seat lightly. Now go back to where they started and tune for highest rpm. When you have the highest rpm for each carb, go back and count how many turns out you are. Record those numbers and file them for future reference.


.
 
Are you sure your throttle linkage isn't hanging up on something or the throttle cable?
 
No binds in the cable, or linkage. And yes, i'm running rich. I need no choke to start the bike and it'll idle at 3-4k until it gets warm.

The bike is not at it's stock displacement, so i have been playing with the jetting for a few weeks now...

I will try playing with the air mixture screws i guess
 
brveagle said:
Well my bike is acting up a little...

I recently got the carbs synched at a shop and watched them. They did a great job, bike starts and runs/idles like a champ. (they did not fiddle with the idle mixture screws)

however it still is slow to return to idle. When i am riding or pulling up to a light, the rpm's will stick at 3500-4k for a few seconds. There are no intake leaks, and the orings behind the boots are new. I am running the stock airbox with a k&n drop in, with no lid. Once it drops to idle, it holds it nicely.

I have read not to adjust these idle mixture screws, should i? I am also still running very rich at idle, due to trying to keep the A/F somewhat rich and keep the heat down until i can get it on a dyno.

Any help would be appreciated.
Click to expand...



It is that rich mixture at idle (which adds to the overall fuel charge at higher rpm) that is causing your high-hang before curb idle occurs. This is a classic carbureted engine problem when misadjusted by whatever means. A carb has to offer the engine a correct fuel mixture when cold (richer), idle (low air flow), transition to high airflow and at high airflow.....the last 2 under varying load conditions. This happens somewhat in-addition with many carbs and thus altering the mixture rich, especially in the idle circuit, affects the rest somewhat.
 
Your CV carbs commonly don't respond well to filter housing changes.
Removing the lid can effect how vacuum enters the diaphragm chamber that raises the piston that allows the throttle plates to operate smoothly. Many times the vacuum port to the chambers must be enlarged. That is also a requirement in jet kit info.
If not vacuum related, a hanging idle is caused by a lean condition. A lean mixture causes uneven combustion because the mixture often fires too soon. Uneven combustion equals uneven engine speeds and the bike can't return to idle as it should. You may be rich at other various throttle positions but your idle sypmtom is a lean condition.
A rich condition is the opposite. Once warmed up, if you rev the bike and the idle momentarily drops BELOW the factory recommended idle, that's a rich condition. The bike can't immediately burn off the richer mixture with the plates returned to nearly closed and so there's that momentary drop in rpm.
What exact mods do you have, what are your stock jets and what have you got in there now?? You'll have to re-jet all 3 jetting circuits for a removed lid. Mains, jet needle position and probably pilot jets, though you may be able to to richen the mixture screws per the highest rpm method and avoid changing pilot jets. The jet needle and mains will have to be changed for sure.
 
And just to suggest, I think the power increase is so minimal by removing the air box lid and running stock exhaust, that you should just return the stock jetting and replace the lid. Intake changes as this will not be worth it on a 550. You may even lose some lower rpm torque that will far outweigh any very small top end gain. Just being honest with you.
 
KEITH KRAUSE said:
And just to suggest, I think the power increase is so minimal by removing the air box lid and running stock exhaust, that you should just return the stock jetting and replace the lid. Intake changes as this will not be worth it on a 550. You may even lose some lower rpm torque that will far outweigh any very small top end gain. Just being honest with you.
Click to expand...

It's a 740... It needs all the air it can get.. Sorry, should have mentioned it earlier.. .:)

Also i'm running 122.5 mains and i'll have to check when i get home what the other jets are...
 
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KEITH KRAUSE said:
And just to suggest, I think the power increase is so minimal by removing the air box lid and running stock exhaust, that you should just return the stock jetting and replace the lid. Intake changes as this will not be worth it on a 550. You may even lose some lower rpm torque that will far outweigh any very small top end gain. Just being honest with you.
Click to expand...

I couldn't agree more. I see lots of posts talking about rejetting, changing to individual air filters and exhaust changes (especially on smaller bikes) and know these fine people, many without a mechanical background or the ability to measure or test their results, are chasing a dream and flushing their wallets.

I remember putting a Kerker 4-1 header on my first bike....77 CB550 and being surprised first at how it narrowed the power band.....even if it peaked a little more at 6000 or so, where I seldom drove it.
 
I have dyno time set up with the local shop, so i will be making sure everythign is running right, just want to get it close before throwing down some cash on dyno time.
 
So what other mods do you have? Usually, if someone takes the time to increase a 550 to a 740, to take advantage of the extra displacement it's typical to install pods and a quality pipe. That way your mods compliment each other. Taking the lid off isn't taking advantage of the bigger pistons.
 
KEITH KRAUSE said:
So what other mods do you have? Usually, if someone takes the time to increase a 550 to a 740, to take advantage of the extra displacement it's typical to install pods and a quality pipe. That way your mods compliment each other. Taking the lid off isn't taking advantage of the bigger pistons.
Click to expand...

I have a 4/1 Vance and Hines pipe and a k&n drop in. There really isn't anything much more that i can do to this bike to make it fast, short of a set of custom cams and some work with the timing. I don't run pods due to the overwhelming amount of trouble they tend to give.

I know alot of people think that they can take the airbox lid off and gain horsepower, and "feel" the difference, but that simple isn't me. I took it off due to the fact that the engine was starving for air at the high rpms. It would stutter and fall flat on it's face at 8k. Removed lid and changed to k&n and it pulled like a totally different bike.

This whole project is still in it's sorting out stages, and i'm really happy with the way the 740 pulls, i just am still working out the quirks of jetting for something that hasn't ever really been done. (that i could find on the net atleast)
 
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Well, I undertand if you don't want pods, though that's the mod you should do to make the most of the pistons and pipe. You would see a power increase at higher rpm's with the pods.
Just thinking out loud...it seems that increasing your 550 to a 740 could cause a flow problem. I mean, we're talking approx' 35% bore increase. That's excessive for street use in my opinion. The motor must run quite a bit hotter than stock. The stock head can't possibly flow enough to satisfy your specific mods.
I just sense a poor flow match and that could explain your jetting needs. If the mods don't truly compliment each other, unusual things happen.
 
KEITH KRAUSE said:
Well, I undertand if you don't want pods, though that's the mod you should do to make the most of the pistons and pipe. You would see a power increase at higher rpm's with the pods.
Just thinking out loud...it seems that increasing your 550 to a 740 could cause a flow problem. I mean, we're talking approx' 35% bore increase. That's excessive for street use in my opinion. The motor must run quite a bit hotter than stock. The stock head can't possibly flow enough to satisfy your specific mods.
I just sense a poor flow match and that could explain your jetting needs. If the mods don't truly compliment each other, unusual things happen.
Click to expand...

Yeah, the bore is a little excessive, but that's what makes the GS's fun right?
:) The bike does run hot. I run an oil cooler and refuse to ride the bike anytime there is a remote chance of getting in stuck in traffic. ( i mostly cruise on this bike at night with buddies)

Thanks for your help though. I'm gonna keep plugging away on the jets. I'll get it right eventually and then i'll throw a dyno sheet up for grins...

:)
 
I have the same problem except that my bike sticks at higher idle at the light until I crack the throttle and its goes back to idle but is low. It fluctuate high and low and I cant get a happy medium no matter what I do. I am running 32's on it tho. So I figure that has something to do with it.
 
brveagle said:
Thanks for your help though. I'm gonna keep plugging away on the jets. I'll get it right eventually and then i'll throw a dyno sheet up for grins...

:)
Click to expand...
OK. I re-read this thread from the start and your original complaint was a hanging idle. You seem positive it's not due to mechanical interference(throttle plates or linkage) and you say it's running rich. Because you believe it's rich you wanted advice on adjusting the mixture screws.
You also haven't provided much info about the jetting, except to say you have 122.5 mains. Going on that, I don't see why it would be rich. Your hanging idle, if mixture related, is a classic lean condition of the pilot circuit. Also, since you don't say otherwise, I assume the stock pilot jets are in there which would certainly be lean with pipe and intake mods.
Typically, you would need to run pilot jets at least a step (2.5) larger. I don't think richer mixture screw adjustments alone would work. You do have to get the right pilot jet in there in order to adjust the mixture screws using the highest rpm method. If the pilot jet is correct, the mixture screws WILL respond to the highest rpm method and you can be sure the pilot circuit is good. So that's all I can say about your mixture screw question. Whether it cures the hanging idle, I don't know. If adjustment does nothing...
You can of course try some quick tests to see how mixture effects the hanging idle. If you temporarily cover the top and force it to breath through the smaller factory openings, how does the idle respond?
How does the idle respond if you go further and block half of the factory opening(s)?
Does it do it cold and hot? Be sure the choke is off when testing.
If it is mixture related and not mechanical interference, it seems that the tests above should show some obvious change. If no change, I'd really look at the throttle plates and linkage and throttle cable condition and slack next.
Is there any change if you move the bars at different points? Does it get worse?
As for the jetting, I suggest a stage 3 kit if you can get it and let that bike breath with some pods. If no, you'll have to re-jet all 3 circuits anyway. As I said, I think a step up on the pilots will work. I have no idea if your main jet choice makes sense because I don't know what the stock mains are. Air box mods are more difficult to jet for than pods in my opinion. The CV carbs can show all kinds of mixture/vacuum problems. Just ballpark guessing, I'd allow 1 to 2 full sizes (5-10) for the pipe, maybe 1 step or full size (2.5 to 5) for the pistons and about 3 sizes (15) for the lid removal. So that's 4+ to 6 sizes or 22.5 to 30 larger mains than stock. If I had to guess from that range, I'd go 25 larger.
The jet needles will also have to be raised. That's where you need the jet kit and its superior jet needle.
 
I had the same hanging idle problem on my 550 with 4-1, pods and jet kit. After much tinkering (carb syncs and valve shimming) I finally cured it by adjusting the pilot jets with the highest idle method. Runs like a champ now
 
Thanks for your help Keith. I do appreciate it. I'm at work now so i can't post alot, but The bike's getting a new battery this week and i will start tinkering this weekend.
 
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