Topic: Idle issue

[Magilla]

Today was my first ride of any length on my bike. It starts on the first kick and rode down the highway excellently. After the bike is well warmed up and you come to a stop the idle drops really low, almost to a stall.  Then it kicks back up and seems to run ok.  When I take off there is a slight hesitation and it picks up and seems to run right.  Each time I stop and restart it repeats this pattern.

Any thoughts on what might be the cause of this?


Bike= 76 CB750k. 15k miles on it.

[nancy]

Well I'd start with the carbs. Actually - make sure you have clean fuel 1st...no crud in the tank.
Then - check for air leaks in the carb rubbers. Then...
Remove, clean out the carb float bowls, remove the float valve/filter that you see in there - clean that. Remove and clean main and idle jets. Check float levels are at 26mm. I can do all this with the carb rack still on the bike - with a bit of a neck stretch and knuckle scrape. To do float levels the correct way - rack needs to come off - but u can do it in situ by using a tube as a level from the drain on the bowl once refitted.
Mark
PS: Then I'd look at plugs, coils, points.....beer...

[lucky]

Today was my first ride of any length on my bike. It starts on the first kick and rode down the highway excellently. After the bike is well warmed up and you come to a stop the idle drops really low, almost to a stall.  Then it kicks back up and seems to run ok.  When I take off there is a slight hesitation and it picks up and seems to run right.  Each time I stop and restart it repeats this pattern.

Any thoughts on what might be the cause of this?


Bike= 76 CB750k. 15k miles on it.

No diagnosis is possible unless we have more facts.

type of intake?
Type of exhaust system?
Idle jets?
Main jets?
Slide needle position?
Mixture screw setting?


But the carbs is the last thing to work on after all other parts of the motorcycle are adjusted.
If the carbs are clean, and the floats and float needles are correct then you can sync the carbs.

[Magilla]

Today was my first ride of any length on my bike. It starts on the first kick and rode down the highway excellently. After the bike is well warmed up and you come to a stop the idle drops really low, almost to a stall.  Then it kicks back up and seems to run ok.  When I take off there is a slight hesitation and it picks up and seems to run right.  Each time I stop and restart it repeats this pattern.

Any thoughts on what might be the cause of this?


Bike= 76 CB750k. 15k miles on it.

No diagnosis is possible unless we have more facts.

type of intake?
Type of exhaust system?
Idle jets?
Main jets?
Slide needle position?
Mixture screw setting?


But the carbs is the last thing to work on after all other parts of the motorcycle are adjusted.
If the carbs are clean, and the floats and float needles are correct then you can sync the carbs.

To the best of my knowledge everything is factory stock.  Stock manifold.  Stock pipes. 

[ADW]

Today was my first ride of any length on my bike. It starts on the first kick and rode down the highway excellently. After the bike is well warmed up and you come to a stop the idle drops really low, almost to a stall.  Then it kicks back up and seems to run ok.  When I take off there is a slight hesitation and it picks up and seems to run right.  Each time I stop and restart it repeats this pattern.

Any thoughts on what might be the cause of this?


Bike= 76 CB750k. 15k miles on it.

Sounds like the idle mixture might be too lean. Easy to change, give it a try. Check your manual for what the setting is supposed to be.

[iron_worker]

My thinking is that the idle mix is too rich and it's "loading up" at idle.

My reasoning is that a cold bike needs more enrichment to run properly ... as it warms up it needs less fuel since the fuel atomizes better due to the heat of the engine. If your mixture is rich then it would run well cold and get worse as it comes up to temp (at least at idle).

Does your bike smell rich? Unburnt fuel smell? Black smoke? Does it require very little/no choke to start when cold?

IW

[sir funk]

Assuming everything else is OK (clean fuel, carb idle circuit clear and adjusted properly, timing set properly, proper float bowl level, carbs synced etc), it could be related to a mechanical problem.  Your symptoms don't really sound like this is it, but if its dropping at each stop or intermittently, this may help.   

Check that the carb throttle linkage is against the stop  when you are not applying throttle.  If you turn the throttle the opposite direction (from where you are not applying any throttle) and revs drop,  you should give one or both the throttle cables a tiny bit more slack.  Once the throttle cables are adjusted properly, Wait till the motor is warmed up and then adjust the idle screw. 

[Magilla]

Bike requires little to no choke to start and run.

Today I pulled to fuel bowls and cleaned them, the floats and the carbs.  I found very little if any sediment in The bowls.

I pulled the plugs and they seem a bit black to me.  I cleaned them and reinstalled them.  I also checked for air leaks before I took anything apart.  No leaks.  I put everything back together and took it for a ride.  Problem still exists.  Runs bad if not stalls at low rpm.  Runs well at 2000 or above.

Don't know what jets are in it.  I believe everything is 100% stock.

[spanner boy]

Does the stall happen if the bike is just rev'd in neutral ? If not are you draining the carb bowls under load? The other thought I had is are the bob weights on the advance and retard sticking advanced?

[Magilla]

It does not stall when in neutral.   It happens after its warm and when I come to a stop.

[Magilla]

Let me start by saying I do have a manual.  I have a Clymer manual.

I need to identify what a screw on the carb does.  There is a crew on the left side of each carb, slightly above the float bowl.  3 out of 4 of these screws have a plastic stop on them that only allows them to be turned one full turn.  The fourth does not have the stop and was backed out 4 full turns.  What is this screw?  I don't see it in my manual.

Also in my manual it does not tell me how to lean my idle.  I am sure I am running rich so I need to lean it out.

[nancy]

Magilla... I assume you are looking at the idle air control screws - very small. If so - standard setting to start from is 1 full turn out from fully closed...ie turn them clockwise to close..and then turn them anti-clockwise to open by 1 turn....and u are not supposed to go beyond 1 & 3/4 turns.
Mark

[Magilla]

So how do I lean out the idle on this bike?

[ADW]

So how do I lean out the idle on this bike?

Nancy just told you how. Set the screws as described by Nancy; turn them in all the way clockwise (don't screw them in HARD like a nut/bolt, just until they bottom), then turn them all out 1 turn. See if the bike is better. If it is, ride it, then turn all the screws out 1/4 turn more, ride it, is it even better? If not, go back in 1/4 to where you where, then turn them in 1/4 turn further. Ride it. Better? Keep doing that in smaller increments each time (i.e. once you find the spot where it's pretty good, keep doing the "more out or more in" with the screws, but 1/8 turn at a time).

As Nancy said, don't go further out than your manual specifies for the idle mixture screw setting (that's what we're talking about here, idle mixture screws...those are the ones on each carb just above the float as you said).

[Magilla]

Which way is leaner, in or out?

[TwoTired]

Your carbs have screws that control air flow from the air jet.  Outward passes more air, inward passes less.