Topic: slow to return to idle

[huyzer]

Hi Guys,

I have a '78 750K.  Running with:

Pods
rejetted the idle jet to #40, and main to 125. 

tuned the bike, synced, timed and all around running really good.  no revving issues 

but then I started to fiddle (I know I probably shouldn't have...but I am a guy..and this is what I do).  I drilled out the stock (4-4) muffler dampers.  Sounds awesome I might add...but now once the bike warms up...the revs if given even a small blip of the throttle will just hang at 3-4K RPM. 

before she warms up...no problem...it'll rev up and down without hanging.  This happens once the bike is nice and warm.  and didn't have this issue prior to drilling out the dampers. 

Any thoughts as to why this is??? I suspect a lean condition due to the change in the exhaust profile after I drilled the damper out. 

any ideas I can try??? 

Thanks guys!!
 

[mrfish2]

I suspect a lean condition due to the change in the exhaust profile after I drilled the damper out. 

Definitely too lean. First, do some plug chops. I would suspect your idle jets or your needle clips need to be adjusted one size richer.

[flybox1]

^^^ +1
idle Plug chops....we also need to know what your fuel screws are set at.
Throw in a new set of D8EA's once the bike is warm, and let it idle for 4-5 minutes with a fan on the cases, then cut the engine.  No blipping of the throttle.  Just idle and then cut.  Line up your plugs 1-2-3-4, take a similar picture of their tips, and post it here for us to see
Ceramic insulators SHOULD come out looking like this. light brown.


Also, check for vacuum leaks around the carb insulator boots, spray some wd40 on them one at a time, and listen for a change in idle.

[70CB750]

Check your advance springs too.  I cut my advance springs shorter half a turn, that made the return to idle faster.

[huyzer]

What/where are the needle clips? Didn't know there was that adjustment.

[huyzer]

Flybox1. If by fuel screw you mean the idle mix screw, mine are set at 1 turn out i believe.

Also 70cb750: what/where are the advance springs? Not too sure I want to mess with that just yet, rather look at air/fuel ratio first.

Thanks for the suggestions. I'll make some changes and let you guys know.

[Airborne 82nd]

What/where are the needle clips? Didn't know there was that adjustment.

Don't feel bad. If your carbs are stock then you don't have needle clips.

[mrfish2]

What/where are the needle clips? Didn't know there was that adjustment.

Don't feel bad. If your carbs are stock then you don't have needle clips.

Oops, you're right. I'm used to the 550's haha

[70CB750]

Also 70cb750: what/where are the advance springs? Not too sure I want to mess with that just yet, rather look at air/fuel ratio first.

Under the right cover on the end of the crankshaft, it looks like this:



Over the years, those springs - they hold back advance weights - stretch a bit due to heat and age.  Shortening them makes the return to idle snappy.

I learned it from Hondaman's post, I am not that smart

But if you check those springs, you will clearly see what shape they are in.  When the engine is not running the advance weights should be tucked in with reasonable force - mine were actually loose.

Chances are that your problem is idle related, this is just another posibility.

[huyzer]

Also 70cb750: what/where are the advance springs? Not too sure I want to mess with that just yet, rather look at air/fuel ratio first.

Under the right cover on the end of the crankshaft, it looks like this:



Over the years, those springs - they hold back advance weights - stretch a bit due to heat and age.  Shortening them makes the return to idle snappy.

I learned it from Hondaman's post, I am not that smart

But if you check those springs, you will clearly see what shape they are in.  When the engine is not running the advance weights should be tucked in with reasonable force - mine were actually loose.

Chances are that your problem is idle related, this is just another posibility.

Thanks 70CB750...I actually cut mine as well...did this while tuning up the idle a while back.  the springs on mine were not too bad but did not quite have the "springiness" like they probably did when new.

you're right though...there is something idle related.

What I have found is that the Idle Mix screw had wandered on its own to being very open...so very lean condition.  I've seen this before.  Adjusted the screw back to 3/4 turns out, but still experiencing a slightly hanging idle when throttle is blipped.  Not to the extent as it was before.  Next step is to close the screw completely (ie 0 turns) and see what will happen. 

any suggestions on how to keep the screws in place after the adjustment is made?  seems to like to wander on their own...plus when turning the screw out its seems to "easy"...like there should be some resistance so that they stay in place. 

in any case I feel that it a lean condition...may also have to play with the idle jetting too.   

[flybox1]

mixture screws on PD carbs adjust fuel, aka fuel screws
what are the results of your idle plug chop?
clear tube test your bowl fuel levels.  3-4mm below the top of the bowl on all?



do you have the proper stack on the fuel screws? 
the oring/spring will create enough resistance to hold it in place.
screw->spring/oring/washer-> carb body

[huyzer]

mixture screws on PD carbs adjust fuel, aka fuel screws
what are the results of your idle plug chop?
clear tube test your bowl fuel levels.  3-4mm below the top of the bowl on all?



do you have the proper stack on the fuel screws? 
the oring/spring will create enough resistance to hold it in place.
screw->spring/oring/washer-> carb body

Thanks Flybox1.  Haven't had a chance to do the plug chop...and it'll probably be a couple weeks as well.  But I will do this one. 

as for the fuel level.  I will check this too...the floats have been set to be all the same..and that was bench set at 22mm. 

so if on the PD carbs the fuel screw controls fuel...then would turning them in all the way reduce fuel? (lean the mix?) or reduce air? (richen the mix?).  My understanding was that it reduced air and richens the idle mix.  I could be wrong though.

and on the screws I do have the right stack...screw, spring, o ring, washer carb body...but they are a cheap aftermarket...hmmm...maybe time to replace those with the proper OEM part. 

Thanks
Huy

[flybox1]

Thanks Flybox1.  Haven't had a chance to do the plug chop...and it'll probably be a couple weeks as well.  But I will do this one. 

as for the fuel level.  I will check this too...the floats have been set to be all the same..and that was bench set at 22mm.  What model PD carbs do you have?  check the casting outside the #4 carb bowl.  floats should be close to 14.5mm/top of the pilot jet for ours.

so if on the PD carbs the fuel screw controls fuel...then would turning them in all the way reduce fuel? (lean the mix?) or reduce air? (richen the mix?).  My understanding was that it reduced air and richens the idle mix.  I could be wrong though. dont confuse yourself    OUT is richer, IN is leaner.  that's all you need to remember

and on the screws I do have the right stack...screw, spring, o ring, washer carb body...but they are a cheap aftermarket...hmmm...maybe time to replace those with the proper OEM part.   good idea

Thanks
Huy

edit...I am assuming you have the proper PD carbs for your K8, that is why i ask you to check.
If you do, please see this for proper float height checking/setting.
http://forums.sohc4.net/index.php?topic=138727.0

[huyzer]

So just wanted to close the loop in this thread.

So in the end the cause was definitely a lean idle circuit.
The pilot/idle jet was too small for the set up.

Final set up is:
#48 pilot/idle jet
1.2mm spacer on the needle
1 turn out on the idle mix screw.

Plug chops not done but will do soon. I suspect it to be slightly rich in the end but I can live with that.

The bike runs very well through the throttle bands. And revs the way I'd expect it should...basically returning to idle after a nice blip of the throttle.

The above set might not work for every bike but....and most importantly it worked for me.