Topic: Uneven Idle

[jwurbel]

Rebuilt carbs.  All jets cleaned. New needle valves and seats.  New boots.   Jetting 110 pilot and 40 main.  Bench synced and floats set to 26 mm. Mixture screws at 1 turn. New plugs and caps.  New air cleaner . No leaking from carbs   Idle unstable.  Throttle blipping during Carb Tune sync and almost or will die.  Checked for vac leaks with carb cleaner and propane.  Nothing shows.  Running out of ideas and looking for any advice.

[Scootch]

Cable free play at the top of the carbs set to spec? All slides open and close the same amount and together? When closing throttle do all slides hit idle screws simultaneously and then open together? Gaskets at the top of the carb housings? Make sure you don't have vacuum leak at carb sync ports.

[jwurbel]

Found a loose plug, #3.  Tightened and was able to sync.  Was able to tighten a smidge the carb tops but there wasn’t much there.  Carbs seem to rise and fall in unison.  Idle issue is still there.

Would floats be an issue?  Although they measure at 26 mm bike seems to act like it’s fuel starved and idle goes up and down on it’s own.

Will probably pull everything off and go through again for the umteenth time

[Rosinante]

Does everybody know what kind of bike this is? 

I have been dealing with similar issues with my K8 CB750.  I found loose set screws attaching carb linkages to the main throttle shaft, but idle has still been a problem.  What I am finding is that the factory 'starting' point for the idle screw (1.5 turns) seems to be not enough.  At 2.0 turns, the bike idles better.  When I am finished fiddling with it, I will report back in the nearby thread I started regarding idle screw settings.

[jwurbel]

Bike is 72 CB 750 K 2

[HondaMan]

Can you post a picture of your fuel hose routing to the carbs? It almost sounds like an air bubble in one of the fuel lines. This will intermittently fill/not-quite-fill the carbs. I have seen this happen in clear fuel lines before, although the trouble in that situation was that the hoses were too big (6mm) so they were also letting the downstream fuel lose its hydraulic suction on the tank (they were not sealed well at the ends).

Also: did you keep the Keihin jets when rebuilding? If you used the jets in the [whatever brand] 'carb kits' they will NOT be the correct size, despite the numbers on the jets. This is a big issue nowadays.

[jwurbel]

Only used Honda parts.

Photo attached as using auxiliary fuel tank.

[stocky]

Found a loose plug, #3.  Tightened and was able to sync.  Was able to tighten a smidge the carb tops but there wasn’t much there.  Carbs seem to rise and fall in unison.  Idle issue is still there.

Would floats be an issue?  Although they measure at 26 mm bike seems to act like it’s fuel starved and idle goes up and down on it’s own.

Will probably pull everything off and go through again for the umteenth time

Try the clear tube method if you suspect float levels are off before pulling it off again just to confirm levels are either good or off. A good, quick and simple test to do after you rebuild carbs and especially anytime you mess with the floats.

[smee]

My fuel lines between the carbs are routed under the airbox (versus above in yours). This may affect the equalization of fuel between each carb.

Airbox sealed tightly? Sometimes the bottom piece is warped and doesn't sit well against the rubber seal.

All carb passages squeaky clean?

[HondaMan]

What is your idle speed? On the K2 pre-3/72 builds it should be about 950 RPM, or 1050 RPM if made 3/72 and later.

[dave500]

you stated new plugs and caps,hows the rest of the ignition?points correctly set and timing strobed?square away all the ignition first before you tackle carbs!

[jwurbel]

Idle speed 1200 rpm’s

Valves, points are to spec.

Today couldn’t bring idle down from 1800 using idle screw.  Sometimes I can.  When in first gear and accelerating with the tach reading 4500rpms, back firing on deceleration.  Doesn’t happen in any other gear.  Have checked for vac leaks and none. 2 & 3 plugs usually foul. 1 looks ok and 4 tends to look rich but not fouled.  Coils within specs.

Air mixture at 1 turn to start with. Have tried 1/8 out, leaner, and 1/8th in, richer.  Still get the hesitation on acceleration.

Frustration meter starting to get pegged!

[dave500]

ok,you say points to spec?have you timed using a strobe?

[dave500]

also check mechanical advance isnt sticking,it must be free.

[HondaMan]

The symptom here: 1200 RPM idle that wanders or 'hangs' high.
The things that cause that are:
1. Incorrect ignition timing: specifically, timing is not on the "F" marks at engine speeds below 1200 RPM. The advancer should not be advancing timing at all until 1250-1300 RPM, reaching full advance by 2000-2200 RPM (2500 RPM is better).
2. Widely unbalanced carbs: don't even attempt to vacuum-balance these carbs until the engine will idle at 1000-1100 RPM at the least. If you try to "adjust" the carb vacuum for engine idle when the engine won't idle in the first place, you're chasing your tail instead of tuning the bike. Keep in mind that once the engine leaves the 1200+ RPM range, the vacuum readings mean nothing: they are merely a diagnostic tool above idle speeds, not a tuning device.

Your K2 likely has the spark advancer marked "AD125" from Hitachi (their stamp is a pair of concentric circles with an "H" in the middle). These advancers, by today, ALL need to be tightened up by cutting off at least 1/2 turn of both springs, or by removing one coil of one spring and removing the slack in them both by re-coiling the springs' end loops. The timing should not start advancing until 1500 RPM, but it is likely already advancing several degrees by 1100 RPM now, if this has not been done to the advancer. This spring looseness will usually cause either "hanging high idle" or "won't return to idle", or both.

[tofan]

In addition to what everyone else has suggested, I would check that your coils are good and getting good spark.

Also, I'm not clear if you vacuum sync'd the carbs yet. If not, then I would definitely do that.

[grcamna2]

also check mechanical advance isnt sticking,it must be free.

+1

[jwurbel]

Used a timing light and dwell meter.  However I did not check the idle speed when doing this.
Carbs synced but idle was at 1200 rpm’s.
So regardless of the above, I should address the spark advance spring first and than proceed?

[HondaMan]

Used a timing light and dwell meter.  However I did not check the idle speed when doing this.
Carbs synced but idle was at 1200 rpm’s.
So regardless of the above, I should address the spark advance spring first and than proceed?

On nearly all of these engines I have rebuilt in the last 15 years, the spark advancer springs have been a problem. Part of the reason is our modern gasolines: they burn far slower than the 1970s versions, so even Regular acts like Premium used to, in many areas. When there is an excess of octane it causes an unburned-fuel issue that collects in the combustion chamber(s) and near the exhaust valve, causing 'wet' combustion. Then when the engine warms up, this starts adding some unburned fuel from the intake tract and, on older existing engines (i.e., not those that were just rebuilt with clean pistons, etc.) from the carbon collected on the tops of the pistons. Then when the engine warms up this starts adding a bit of extra fuel to the mix that wasn't there when cold, and the engine acts differently. Most of the time this shows up as a wandering-idle speed issue. While riding the bike for a few hundred miles will often burn away this situation and make it more 'normal' again, for those engines that have been tinkered with a long time in a stationary garage setting tend to start having these sorts of issues.

In other words: if you can, put it all back together and try riding it for 25-50 miles, then go back after the tuning issues. You might be surprised to find that it then behaves better and reacts to your tuning efforts more predictably. Seen this many times...

[rotortiller]

Regardless to what the shop manual states for published a idle speed I have found over the last 50 years that these engines prefer 1200 -1500 rpm idle. Even with a well setup and tight motor it will sound better at the higher idle with less rattle and mechanical noise. You can have a stable idle speed in this range easily. I would never idle one below 1000 rpm as it does the bike a noisy public injustice, I like sewing machine smoothness. Most fuel injected modern inline 4 cylinder bikes idle around 1200-1500 for this same reason, granted emissions will be lower at 900 but it will often sound like a rattle trap. The bike will not overheat or become unstable at the slightly higher idle, nor will it grow two heads . If the idle shifts position 100-150 rpm during the day, not a big deal after all it is a carb fueled engine. The idle speed within reason is the users choice and preference as the manual is but a guide, one likely written by a graphite commando. Idle speed preference within reason for many has to do with enjoying a well setup and maintained bike. Lots of good advice by others on fixing a hanging idle.

[Scootch]

Agree totally with rotortiller. These are carb motors with no compensation for temp, humidity, baro pressure. What seems good one day can change somewhat the next. Higher idle speed helps smooth it out. Trying to force the motor to idle below 1000 just isn't going to work IMHO.

[jwurbel]

Will remove point plate and advancer.  Will inspect springs for obvious lack of tension.  If they “appear” ok, will probably clean, lube, and reinstall.  Hate to start cutting if not necessary.  If no improvement will cut per Mark’s comment. 
Would an aftermarket advancer solve the issue?  4 into 1 has one.
When resetting timing after that, I still use the same marks?
Can’t do anything now as away from bike for next month. However will post results.

[RAFster122s]

Mark has mentioned issues with the springs on the aftermarket advancers that he has dealt with...

If you have a tach and timing light you can see what rpm you are achieving full advance...