« previous next »
Pages: [1]   Go Down

Author Topic: Idle for 1978 CB750K  (Read 4749 times)

0 Members and 1 Guest are viewing this topic.

bclayton

  • Guest
Idle for 1978 CB750K
« on: March 05, 2009, 02:28:29 PM »
I have a 1978 CB750k, rebuilt the carbs, the airscrews are turned out 1.5 turns, the plugs are gappedat 0.28, number 4 carb backfires periodically, the floats are set at 15mm. It will not idle without benefit of the choke being pulled at least a little bit. I suspect that the airscrews need an adjustment, posibbly trned out some more. Opinions are welcomed.
Logged

Offline HavocTurbo

  • Angry little bastard of an
  • Old Timer
  • ******
  • Posts: 3,738
  • Can you tell?
Re: Idle for 1978 CB750K
« Reply #1 on: March 05, 2009, 02:36:31 PM »
Might be a needle vavle is leaking. Mine did that.
Logged
'48 HD Panhead - Exxon Valdez
'78 CB550K - Fokker CB.3
'78 Honda CB750K - Mavrik
'80 Yamaha XS850G - Kanibalistik
09 XL883L - No Name

Offline eurban

  • Master
  • *****
  • Posts: 1,625
Re: Idle for 1978 CB750K
« Reply #2 on: March 05, 2009, 02:39:03 PM »
Your "airscrews" actually control fuel and would technically be called idle mixture screws.  Clockwise (out) will richen the mix and counter clockwise will lean them.  Your type of issue / question comes up quite frequently on the 77/78 type of carbs.  Often times, even though people think that they have cleaned / rebuilt their carbs, they have not paid enough attention to cleaning the idle circuits.  The pressed in idle jets must be pulled, soaked and cleared with a single strand of copper wire.  All the interconnecting passages for the idle circuits must be confirmed clear with by spaying carb cleaner through them.  Use the search function and you should find a number of threads discussing your type of issue.  Good luck. . . .  


Correction: Clockwise is in not out (obvoiusly) and thus leans the mixture while counter clockwise will richen it. What was I thinking????
« Last Edit: March 05, 2009, 06:59:05 PM by eurban »
Logged

Offline Bido

  • Enthusiast
  • **
  • Posts: 162
Re: Idle for 1978 CB750K
« Reply #3 on: March 05, 2009, 03:08:37 PM »
I agree with the idle jets and be sure their are no leaks around the rubber mounting boots. You need the vacuum to suck up that gas.

Good Luck

Bido
Logged
1978 CB750 Supersport F3
1981 CB750 K

Offline kine8282

  • Enthusiast
  • **
  • Posts: 117
  • "Ceteris paribus, my modis operandi is carpe diem"
Re: Idle for 1978 CB750K
« Reply #4 on: March 05, 2009, 06:02:18 PM »
Had the same problem. I would also check the accel pump and squirters to make sure their clean
Logged
1975 CB550K aka "Nefertiti"
1978 CB750K aka "Kate"

Offline TheHun

  • Enthusiast
  • **
  • Posts: 192
Re: Idle for 1978 CB750K
« Reply #5 on: March 05, 2009, 06:21:25 PM »
Same problem...slow jets....i used a paper clip made it straight as a slow jet plunger :) it worked..carbs are good.

So one the 77/78 carbs... Clockwise is rich and counter clockwise is lean?
Logged
1978 Honda CB750-K

CBR F3 Coils
Kerker 4.1 Header w/ turn down
Maxi-Dwell Ignition
Foam Air Filter Pods
Superbike bars
Starter Delete

CARB CLEANING/REBUILD: http://www.wix.com/TheHun/CarbCleaning-Site

Not Running to Running, doing the work yourself= Priceless

Offline eurban

  • Master
  • *****
  • Posts: 1,625
Re: Idle for 1978 CB750K
« Reply #6 on: March 05, 2009, 06:56:05 PM »
Whoa did I really say clockwise was richer??? This is wrong.  Clockwise tightens the screw down reducing the flow of fuel making it leaner.  Counter clockwise loosens the screw making it richer. . . The accelerator pump has no affect on idle so while it is certainly an important feature of these carbs performance, I don't think that it is the cause of his idle issues.  I also would not recommend cleaning the tiny orifice inside the pilot jet with a paper clip as it can easily damage the jet.  Use a single strand of copper wire.  . .
Logged

Offline TheHun

  • Enthusiast
  • **
  • Posts: 192
Re: Idle for 1978 CB750K
« Reply #7 on: March 05, 2009, 07:51:23 PM »
Well..you just have to be really careful...it didn't do any damage to mine :)
Logged
1978 Honda CB750-K

CBR F3 Coils
Kerker 4.1 Header w/ turn down
Maxi-Dwell Ignition
Foam Air Filter Pods
Superbike bars
Starter Delete

CARB CLEANING/REBUILD: http://www.wix.com/TheHun/CarbCleaning-Site

Not Running to Running, doing the work yourself= Priceless

Offline HavocTurbo

  • Angry little bastard of an
  • Old Timer
  • ******
  • Posts: 3,738
  • Can you tell?
Re: Idle for 1978 CB750K
« Reply #8 on: March 05, 2009, 08:27:36 PM »
E-string works pretty good also. The small one.  ;)
Logged
'48 HD Panhead - Exxon Valdez
'78 CB550K - Fokker CB.3
'78 Honda CB750K - Mavrik
'80 Yamaha XS850G - Kanibalistik
09 XL883L - No Name

Offline PJ

  • Enthusiast
  • **
  • Posts: 191
Re: Idle for 1978 CB750K
« Reply #9 on: March 05, 2009, 08:32:25 PM »
It's the slow jets, they are pressed in but can be removed with care. I used a piece of .023 mig wire and it was still too large. Not sure what size they are suposed to be but mine are a tad larger ;)
Logged

Offline eurban

  • Master
  • *****
  • Posts: 1,625
Re: Idle for 1978 CB750K
« Reply #10 on: March 06, 2009, 03:57:23 AM »
Quote from: TheHun on March 05, 2009, 07:51:23 PM
Well..you just have to be really careful...it didn't do any damage to mine :)

Seriously, I have never seen a paper clip with a wire gauge even close to the miniscule size of the metering orifice in the #35 pilots on these bike.  The hole at the end is not the metering point.  The #35 opening is down inside the jet.  Don't use a paper clip; its way too big!
Logged
Pages: [1]   Go Up
« previous next »