Author Topic: Increasing idle speed as bike warms up. I've tried everything I can think of!  (Read 3632 times)

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Offline calj737

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"Of all tyrannies, a tyranny sincerely exercised for the good of it's victim may be the most oppressive. It may be better to live under robber barons than under omnipotent moral busybodies. The robber baron's cruelty may sometimes sleep, his cupidity may at some point be satiated, but those who torment us for our own good will torment us without end for they do so with the approval of their own conscience." - C.S. Lewis

Offline Acoustoman

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Will do. Thanks
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Offline flybox1

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Float level is a starting point.  the fuel level inside the bowl will change if aftermarket /new float valve parts are added.
as a best practice, fuel level should be verified with the clear tube test.
Stationary, we shoot for a fuel level of 3mm+/-  below the carb bowl gasket seam.

not our carbs, but a useful illustration.
'78 750K (F3 engine) PD42b's, Modified airbox w/K&N  filter, 40/110 jets, 1 needle shim, IMS@ 1 turn out. Kerker + Cone 18" QuietCore

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Offline Acoustoman

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that's perfect - just what I needed. Thanks. How common is it for the needle spring/plunger to weaken and need to be replaced? The needle tips look smooth, but I'm wondering if I'm having trouble with the fuel levels for another reason. It may just be that I'm not seeing things clearly enough to know exactly when the float tab touches the needle plunger (at 22mm). Hope that makes sense. ???
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Offline flybox1

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The needle and seat are wear items.  Imagine how many times they open and close during a days ride.
springs weaken over time.  aftermarket remakes introduce slight variables in spring strength and post length.
22mm is the starting point.  As long as you understand fuel level is the goal, clear tube test your work, and make adjustments as necessary.

This all can be done off the bike with something to hold the carbs level.
'78 750K (F3 engine) PD42b's, Modified airbox w/K&N  filter, 40/110 jets, 1 needle shim, IMS@ 1 turn out. Kerker + Cone 18" QuietCore

Past Bikes
1974 550K0 (stock), 1973 CB350F (stock), 1983 Yamaha XS400K (POS)
77/78 cool 2 member #3
"Knowledge without mileage equals bullsh!t" - Henry Rollins

"This is my CB. There are many like it, but this one is mine…"

Offline calj737

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Caveat: if you do replace your needles, be dead certain to buy stock or Keihin brand ONLY brass. Aftermarket needles do differ in their taper/profile and will throw off your settings.
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"Of all tyrannies, a tyranny sincerely exercised for the good of it's victim may be the most oppressive. It may be better to live under robber barons than under omnipotent moral busybodies. The robber baron's cruelty may sometimes sleep, his cupidity may at some point be satiated, but those who torment us for our own good will torment us without end for they do so with the approval of their own conscience." - C.S. Lewis

Offline Acoustoman

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Got busy with end of the school-year stuff and am just now getting back to things. I finally got the fuel levels evened out (much easier with carbs off the bike). I had to take some of the bowls off 3-4 times to get things right, but all are 3mm below gasket level. (thanks flybox1 for the diagram). I did another bench sync after finding that slides were not all the way down with the idle screw turned all the way out. Got the carbs installed and bike started within 45 minutes (personal record!). Did a vacuum sync and fine-tuned a couple. Bike is certainly running better, but idle still wants to creep up. Occasionally I have to let out the clutch a bit to force the idle back down. It stays down until I throttle up the next time. O-rings under intake manifolds are nice and supple, and only a couple months old, so I don't suspect any leaks there.
Anyway, not sure what else to do, but I'm happy for now.  :D I appreciate everyone's input.
1973 CB750K (My rider)
1976 CB550K (Next project)
1973 CB500K (Sold)
1981 CB650C (Sold)

Offline HondaMan

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I understand the float level is to be set at 22mm, which is what I adjusted to, but fuel levels in the bowls are not the same. I'm wondering if some needle valves not sealing properly, or float buoyancy is different or something. All floats swivel freely. . . . I thought my 22mm measurements were spot on, but maybe I need to re-visit that.

This can be an issue, but first maybe check: the float angs where the tiny valve tip sits get little dimples in them because the brass is so soft. This makes that valve "stick" a little bit, until the bowl fills far enough to force the tip of the valve to roll and snap shut. This results in a too-deep bowl.

The fix for these is: using an equally tiny file, smooth off the tang until the dimple is gone, then polish the tang shiny again with a bit of 400 or 600 grit emery paper (O'Reilly's Auto). Then reset the float height, as this work tends to bend the tang away from you while you work on it!
;)

On the 750K0-K6and F0/1 with non-PD carbs, this is chronic, making dimples much deeper than on the smaller Fours. On these bikes it can cause bowl overflow, often does. The smaller Fours have much more active bowls because the smaller bikes jounce on road bumps more (lighter weight) and help close the bowls valves.
See SOHC4shop@gmail.com for info about the gadgets I make for these bikes.

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Offline Acoustoman

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I will do this when I pull the carbs off again. I did notice dimpling on the float tangs and will sand it off. I should probably replace the float needles/seats at the same time because the springs may be weakening-I don't know how old they are.
The jet needles were set at the 3rd clip down position and things seemed to be running lean (based on symptoms I'm reading from other posts). I moved clips to 4th position (stock) and I seem to have better pull at more open throttle.  :)
I'm still dealing with some burbling off idle, and a variable idle speed which tends to want to creep up. I need to vacuum sync my carbs one more time, and play with the idle/air screws. I have no evidence of air leaks anywhere.
I still need to find a tool tray to go over the air box. This is probably messing with the air flow a bit, although I've put a rag over top of the filter element to restrict air flow just a bit. I'm using MAC 4-1 exhaust, which I understand is close to stock in terms of back pressure. I don't know how much these 2 things could be affecting performance. Thanks again - I really appreciate the light you guys are shedding on my inexperience ???
1973 CB750K (My rider)
1976 CB550K (Next project)
1973 CB500K (Sold)
1981 CB650C (Sold)

Offline HondaMan

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I will do this when I pull the carbs off again. I did notice dimpling on the float tangs and will sand it off. I should probably replace the float needles/seats at the same time because the springs may be weakening-I don't know how old they are.
The jet needles were set at the 3rd clip down position and things seemed to be running lean (based on symptoms I'm reading from other posts). I moved clips to 4th position (stock) and I seem to have better pull at more open throttle.  :)
I'm still dealing with some burbling off idle, and a variable idle speed which tends to want to creep up. I need to vacuum sync my carbs one more time, and play with the idle/air screws. I have no evidence of air leaks anywhere.
I still need to find a tool tray to go over the air box. This is probably messing with the air flow a bit, although I've put a rag over top of the filter element to restrict air flow just a bit. I'm using MAC 4-1 exhaust, which I understand is close to stock in terms of back pressure. I don't know how much these 2 things could be affecting performance. Thanks again - I really appreciate the light you guys are shedding on my inexperience ???

The genuine Keihin springs ARE weak, and for a purpose: this helps the bowls fill better. Today's Keyster versions are usually way too stiff, and this then requires a deeper bowl setting, lest it run low while running at higher RPM. For example, the 750 carbs with the Keyster float valves need a float setting of +2mm to get the same fuel depth at 4000+ RPM as the stock setting. Since we don't have fuel pumps, and the most PSI we EVER see if from a full fuel tank (less than 0.2 PSI then), it's tricky to get the right float depth if the parts are not the OEM style.
;)
See SOHC4shop@gmail.com for info about the gadgets I make for these bikes.

The demons are repulsed when a man does good. Use that.
Blood is thicker than water, but motor oil is thicker yet...so, don't mess with my SOHC4, or I might have to hurt you.
Hondaman's creed: "Bikers are family. Treat them accordingly."

Link to Hondaman Ignition: http://forums.sohc4.net/index.php?topic=67543.0

Link to My CB750 Book: https://www.lulu.com/search?adult_audience_rating=00&page=1&pageSize=10&q=my+cb750+book

Link to website: www.SOHC4shop.com

Offline Acoustoman

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Well folks, I have a satisfying update. My increasing idle is no mas problema. I did 2 things: I put new float needles and seats in and made sure my float levels were spot on (~3mm below bowl gasket). Also, I bought new air screws, which I think was the real culprit. The old ones were grooved, apparently from being seated too tightly in the past (see pic). I think this was causing inconsistent air flow and an irregular idle speed. I also noticed that one of them seemed to bottom out, when it really wasn't bottomed out. I'm not sure it was this way with the old air screw, but if so, this one cylinder would have been getting too much air via the idle circuit and it could have caused the idle to creep up. Not sure, but turned past this to gently seat, then turned all screws out 1.25 turns. Simple fix - hope things stay this way. For now, I'm mighty happy with the way she's running. :D
« Last Edit: November 25, 2015, 07:23:58 PM by Acoustoman »
1973 CB750K (My rider)
1976 CB550K (Next project)
1973 CB500K (Sold)
1981 CB650C (Sold)

Offline HondaMan

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Good find! I have seen once in a while where a spring on those air screws has been lost and replaced with [something else], resulting in a screw that tightened on the SPRING, but not the SEAT. This will really confuse the rider!
See SOHC4shop@gmail.com for info about the gadgets I make for these bikes.

The demons are repulsed when a man does good. Use that.
Blood is thicker than water, but motor oil is thicker yet...so, don't mess with my SOHC4, or I might have to hurt you.
Hondaman's creed: "Bikers are family. Treat them accordingly."

Link to Hondaman Ignition: http://forums.sohc4.net/index.php?topic=67543.0

Link to My CB750 Book: https://www.lulu.com/search?adult_audience_rating=00&page=1&pageSize=10&q=my+cb750+book

Link to website: www.SOHC4shop.com

Offline Acoustoman

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Thanks HondaMan. Do you think the idle problems I described earlier (idle creeping up, or being generally inconsistent) could be caused by the grooving on the screws? Thanks for the expertise! Bill
1973 CB750K (My rider)
1976 CB550K (Next project)
1973 CB500K (Sold)
1981 CB650C (Sold)