Topic: cb750 K4 Idles High After Opening Throttle

[scatman200]

My CB750 K4 idles fine when I start it up, and when cold, seems to react fine to revs.  However, after a long idling session, or while riding, the rev don't want to drop back down to normal (1200 RPM), instead stay pinned around 2500, sometimes climbing.  I have what seems to be an OEM 4-1 exhaust and pod filters. Things I've done so far to trouble shoot:

-Float height set to 26mm
-Cleaned all jets
-Fresh spark plugs
-Throttles cables are brand new and smoothly functioning
-Sprayed WD-40 around the carb boots; nothing changed

What to do next?

[m in sc]

fuel circuit is lean.

[NewOldSchool]

These engines are extremely cold blooded and if set to idle correctly when fully warm they wont idle when cold. You have the opposite problem. You have the little knob that adjusts idle set so that your motor will idle when cold. When it warms up (riding for about 10 to 15 miles) or so it will idle way too high because the adjuster is set too high for normal running temps. Its a trade off. Idle while cold and deal with the high revs while riding, or sit on the bike and keep the throttle cracked a bit until it warms up and enjoy a low/regular idle once out riding. I chose the latter.

[KingCustomCycles.com]

A properly tuned Honda does not involve the compromises described above.  A properly tuned Honda is a joy and a blessing to behold.  It is superior mechanically to anything ever built,  Perfect balance, perfect engineering,,,,,,,,,,,,,,well you get the idea, keep tuning and it will improve. 

[Duke McDukiedook]

Set your idle to running temperature, you'll have to goose the throttle upon start to warm up.

[scatman200]

The problem is that the Revs eventually drop (after sometimes 20 seconds or so), thus, setting the idle lower kills the engine.  I tried ruling out the possibility of the bike running too lean by closing the choke a little, but this seems to be very binary; either no result, or cuts the engine out all together.

[KingCustomCycles.com]

Check "hanging idle" in the search box.  Lots of threads threre.

[m in sc]

hm. if not lean then make sure the timing advancer is not hanging advanced. that will do it also.

[scatman200]

Thanks!  I'm gonna check/set timing and sync carbs this evening, I shall update upon result

[NewOldSchool]

Check the operation of your advance unit, over time the return springs can get "soft" and wont pull back in to idle position as quickly as they should. There are many things which can affect a slow return to idle however so listen to what others had to say and search the forum as well.

[alfego]

I'm having the exact same issue with my 74 750.  I get it good and warmed up and set the idle at 1200 but it still wants to hunt for idle and stays around 2500 and sometimes it comes down, sometimes it won't.   Cleaned the carbs, sealed the tank (rusted out on left side bottom), throttle cables adjusted correctly & look new, new battery, still sorting out various other issues.  Mufflers on 2 & 3 are rusted out with various holes along the bottoms.  Don't have airbox back on yet, as still tuning on carbs a bit.  Gas flowing properly, floats at 26mm, maybe it just needs to be ridden more, being gentle with it for now.  Can you lube the advance mechanism without removing the points backing plate and having to reset the timing and all?  But I'm pretty sure it's working as it should.  Just seems like it's a bit lean, have to feather the throttle to get the rpm up where it belongs to speed up and pull the hills.  Not trying to steal thread here, but Scatman200 & I appear to be experiencing the same symptoms.   
alfego

[scatman200]

Well, I adjust my points system (per the service manual), THEN adjust timing per this guide:

http://www.salocal.com/sohc/tech/timing2.htm

and Ta-da!  The bike is running like a top!

alfego:  In hindsight, I guess my tell-tale indicator that my timing was off was when I tried pulling up my VERY steep driveway after my first shakedown run, the bike barely climbed up the hill(sounds similar to what your experiencing).  Reminded me of when the timing was off on one of my cars, years back.  I'd really try checking the timing before pulling any more hair out, as it's a very clean and easy task once you have access to a timing light.

[alfego]

That crossed my mind today.  Will check it out and report back tomorrow.  Thanks.
alfego

[Duke McDukiedook]

Carb synch AFTER A FULL TUNE UP helps.

There, I unleashed the fury of caps lock to help emphasize the order of things.
 

[alfego]

OK, checked timing this afternoon, spot on, took for a ride around the 'hood', same thing.  Hanging idle, no acceleration.  Called my bud who cut his teeth on these bikes for the local Honda dealer back in the day.  Explained things, he asked if the airbox was on or off, it was off, he said 'put it on and your troubles will be over'.  I'll be, he was right, runs like a top.  Super smooth. 
alfego

[wrenchmuch]

Ahhhh . Air box . 

[Gordon]

OK, checked timing this afternoon, spot on, took for a ride around the 'hood', same thing.  Hanging idle, no acceleration.  Called my bud who cut his teeth on these bikes for the local Honda dealer back in the day.  Explained things, he asked if the airbox was on or off, it was off, he said 'put it on and your troubles will be over'.  I'll be, he was right, runs like a top.  Super smooth. 
alfego

And you'll be amazed at how much better your bike corners once you put some tires on those rims!

[alfego]

Gordon,
I deserved that one!! HAHA!!  I spewed my drink outta my mouth with that one!!  In my defense, it's my first time with a 'Honda', never had one 'til now.  Guess I'm not a virgin anymore, oh oh, mama's gonna be mmaaaaddd!!    alfego;D

[Gordon]

We've all made noob mistakes like that.

[lucky]

OK, checked timing this afternoon, spot on, took for a ride around the 'hood', same thing.  Hanging idle, no acceleration.  Called my bud who cut his teeth on these bikes for the local Honda dealer back in the day.  Explained things, he asked if the airbox was on or off, it was off, he said 'put it on and your troubles will be over'.  I'll be, he was right, runs like a top.  Super smooth. 
alfego

That is because without the stock airbox it was running  lean.

[Retro Rocket]

OK, checked timing this afternoon, spot on, took for a ride around the 'hood', same thing.  Hanging idle, no acceleration.  Called my bud who cut his teeth on these bikes for the local Honda dealer back in the day.  Explained things, he asked if the airbox was on or off, it was off, he said 'put it on and your troubles will be over'.  I'll be, he was right, runs like a top.  Super smooth. 
alfego

That is because without the stock airbox it was running  lean.

And also almost all pods are crap, they have no velocity stacks and cause problems with airflow into the carbs, hanging idle is a common problem with pods among other things.....

[Duke McDukiedook]

OK, checked timing this afternoon, spot on, took for a ride around the 'hood', same thing.  Hanging idle, no acceleration.  Called my bud who cut his teeth on these bikes for the local Honda dealer back in the day.  Explained things, he asked if the airbox was on or off, it was off, he said 'put it on and your troubles will be over'.  I'll be, he was right, runs like a top.  Super smooth. 
alfego

Duuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuude, you totally like had podzzzz on brah, that's so rad, your bike should be able to like drive itself. Podz are like, the best brah, don't you like know?