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Offline 1978CB750SS

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1978 CB750F carb questions
« on: March 08, 2017, 11:14:30 AM »
What is this smaller tube on the carb? I bought the bike incomplete but does have original tank. I don't see anywhere to put the tube. I'm guessing it's a fuel line. The bike does start when I apply at least half throttle but does not idle I have to hold choke tight closed and give it half throttle to run.. The bigger tube I have going to petcock. I took carbs off to clean.
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Offline 1978CB750SS

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Re: 1978 CB750F carb questions
« Reply #1 on: March 08, 2017, 11:17:51 AM »
I checked the manuals site, nothing there. I checked youtube, could not find anything. I will order a Clymers when I can afford one. I hope the info is in there.
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Offline flybox1

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Re: 1978 CB750F carb questions
« Reply #2 on: March 08, 2017, 11:28:56 AM »
Atmospheric vent to help carb bowls maintain fuel level.
It should be unblocked.
Some modify and plumb it into the front side of the airbox (for a light vacuum)
« Last Edit: March 09, 2017, 07:14:26 AM by flybox1 »
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'78 750K (F3 engine) PD42b's, Modified airbox w/K&N  filter, 40/110 jets, 1 needle shim, IMS@ 1 turn out. Kerker + Cone 18" QuietCore

Past Bikes
1974 550K0 (stock), 1973 CB350F (stock), 1983 Yamaha XS400K (POS)
77/78 cool 2 member #3
"Knowledge without mileage equals bullsh!t" - Henry Rollins

"This is my CB. There are many like it, but this one is mine…"

Offline flybox1

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Re: 1978 CB750F carb questions
« Reply #3 on: March 08, 2017, 11:31:35 AM »
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'78 750K (F3 engine) PD42b's, Modified airbox w/K&N  filter, 40/110 jets, 1 needle shim, IMS@ 1 turn out. Kerker + Cone 18" QuietCore

Past Bikes
1974 550K0 (stock), 1973 CB350F (stock), 1983 Yamaha XS400K (POS)
77/78 cool 2 member #3
"Knowledge without mileage equals bullsh!t" - Henry Rollins

"This is my CB. There are many like it, but this one is mine…"

Offline 1978CB750SS

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Re: 1978 CB750F carb questions
« Reply #4 on: March 08, 2017, 12:11:45 PM »
Thank you for answering Flybox! I see that the big hose on mine is hooked to the acc pump. Is that correct? Or are the hoses backward? I have an old cb750 screwtop carb bike out back that has a t bar connecting two hoses.. I understand it's two different designs but it would make sense why the bike only runs when giving it more than half throttle.
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Offline flybox1

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Re: 1978 CB750F carb questions
« Reply #5 on: March 08, 2017, 12:15:46 PM »
There is no external accel pump nipple to connect to.   ???  The accel pump is a self contained system on the bottom of your #2 carb bowl.
The larger of the two hoses pictured above goes to your tank petcock.  The other smaller one can be left where it is.
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'78 750K (F3 engine) PD42b's, Modified airbox w/K&N  filter, 40/110 jets, 1 needle shim, IMS@ 1 turn out. Kerker + Cone 18" QuietCore

Past Bikes
1974 550K0 (stock), 1973 CB350F (stock), 1983 Yamaha XS400K (POS)
77/78 cool 2 member #3
"Knowledge without mileage equals bullsh!t" - Henry Rollins

"This is my CB. There are many like it, but this one is mine…"

Offline 1978CB750SS

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Re: 1978 CB750F carb questions
« Reply #6 on: March 08, 2017, 12:18:13 PM »
It don't let me translate.
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Offline 1978CB750SS

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Re: 1978 CB750F carb questions
« Reply #7 on: March 08, 2017, 12:20:20 PM »
ok Thank you.
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Offline Jnel

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Re: 1978 CB750F carb questions
« Reply #8 on: March 08, 2017, 05:58:55 PM »
1978CB750SS.... I have a 78 F.  I also have the same carbs.  I have a manual too.  If you get in a jam i can photo the pages in the manual and send to you.  Just a thought

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Offline trenchgrinder750

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Re: 1978 CB750F carb questions
« Reply #9 on: March 09, 2017, 05:48:23 AM »
Quote from: Jnel on March 08, 2017, 05:58:55 PM
1978CB750SS.... I have a 78 F.  I also have the same carbs.  I have a manual too.  If you get in a jam i can photo the pages in the manual and send to you.  Just a thought

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which manual do you have.  just wondering if its one that I don't have.   ???

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78 CB750F

Offline trenchgrinder750

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Re: 1978 CB750F carb questions
« Reply #10 on: March 09, 2017, 05:54:25 AM »
Quote from: 1978CB750SS on March 08, 2017, 11:14:30 AM
What is this smaller tube on the carb? I bought the bike incomplete but does have original tank. I don't see anywhere to put the tube. I'm guessing it's a fuel line. The bike does start when I apply at least half throttle but does not idle I have to hold choke tight closed and give it half throttle to run.. The bigger tube I have going to petcock. I took carbs off to clean.


looks like you are missing half of that hose.  that will help you get to the top of the airbox.  if you need any for your carbs I have parts for the PD42A
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78 CB750F

Offline Jnel

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Re: 1978 CB750F carb questions
« Reply #11 on: March 09, 2017, 07:07:51 AM »


This one.

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Offline flybox1

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Re: 1978 CB750F carb questions
« Reply #12 on: March 09, 2017, 07:08:17 AM »
^^^
That short hose does not go to elbow inlet at the top of the airbox. 
It is the carb atmospheric vent, and it stays right there.
The hose attachment on the top of the airbox is to be attached to the outlet of the vapor separator attached to the bottom of the airbox.
« Last Edit: March 09, 2017, 07:13:30 AM by flybox1 »
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'78 750K (F3 engine) PD42b's, Modified airbox w/K&N  filter, 40/110 jets, 1 needle shim, IMS@ 1 turn out. Kerker + Cone 18" QuietCore

Past Bikes
1974 550K0 (stock), 1973 CB350F (stock), 1983 Yamaha XS400K (POS)
77/78 cool 2 member #3
"Knowledge without mileage equals bullsh!t" - Henry Rollins

"This is my CB. There are many like it, but this one is mine…"

Offline flybox1

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Re: 1978 CB750F carb questions
« Reply #13 on: March 09, 2017, 07:10:25 AM »
Quote from: Jnel on March 09, 2017, 07:07:51 AM


This one.

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Pitch that and get the official HONDA workshop manual  ;)
http://www.honda4fun.com/materiale-documentazione-tecnica/shop-manual
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'78 750K (F3 engine) PD42b's, Modified airbox w/K&N  filter, 40/110 jets, 1 needle shim, IMS@ 1 turn out. Kerker + Cone 18" QuietCore

Past Bikes
1974 550K0 (stock), 1973 CB350F (stock), 1983 Yamaha XS400K (POS)
77/78 cool 2 member #3
"Knowledge without mileage equals bullsh!t" - Henry Rollins

"This is my CB. There are many like it, but this one is mine…"

Offline Ericcb750

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Re: 1978 CB750F carb questions
« Reply #14 on: March 09, 2017, 07:16:06 AM »
Hey Guys, Just wanted to say Flybox is the man when it comes to these PD carbs. I have a 78K (PD42B) and a 78F (PD42A) and he's helped me with getting every aspect (especially the accel pump) of those things cleaned and functioning correctly. The carbs are literally perfect now, not that he's the only one who knows about these, but his advice is solid and worked 110% for me with my PD's..... Once they're setup they are perfect

That hose is a vent like he said to the atmosphere.
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1978 CB750F super sport
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Offline madmtnmotors

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Re: 1978 CB750F carb questions
« Reply #15 on: March 09, 2017, 07:21:50 AM »
Quote from: Ericcb750 on March 09, 2017, 07:16:06 AM
That hose is a vent like he said to the atmosphere.


Correct. Of the three K8's and three F2's that I have each of them have this vent hose. They are each done in two pieces with the little plastic connector you have shown joining the two. The second piece of tubing is only another six inches long (approx) and extends back to a little clip on top of the airbox to hold it into position with the open end facing to the rear.
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Offline flybox1

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Re: 1978 CB750F carb questions
« Reply #16 on: March 09, 2017, 07:45:11 AM »
Quote from: madmtnmotors on March 09, 2017, 07:21:50 AM
Quote from: Ericcb750 on March 09, 2017, 07:16:06 AM
That hose is a vent like he said to the atmosphere.


Correct. Of the three K8's and three F2's that I have each of them have this vent hose. They are each done in two pieces with the little plastic connector you have shown joining the two. The second piece of tubing is only another six inches long (approx) and extends back to a little clip on top of the airbox to hold it into position with the open end facing to the rear.
Yes.  It shouldnt be confused with ATTACHING it to the the elbow inlet on the top of the airbox.
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'78 750K (F3 engine) PD42b's, Modified airbox w/K&N  filter, 40/110 jets, 1 needle shim, IMS@ 1 turn out. Kerker + Cone 18" QuietCore

Past Bikes
1974 550K0 (stock), 1973 CB350F (stock), 1983 Yamaha XS400K (POS)
77/78 cool 2 member #3
"Knowledge without mileage equals bullsh!t" - Henry Rollins

"This is my CB. There are many like it, but this one is mine…"

Offline trenchgrinder750

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Re: 1978 CB750F carb questions
« Reply #17 on: March 09, 2017, 08:28:25 AM »
Quote from: flybox1 on March 09, 2017, 07:08:17 AM
^^^
That short hose does not go to elbow inlet at the top of the airbox. 
It is the carb atmospheric vent, and it stays right there.
The hose attachment on the top of the airbox is to be attached to the outlet of the vapor separator attached to the bottom of the airbox.

oh no I wasn't talking about the elbow inlet.  http://www.hondapartshouse.com/oemparts/a/hon/506b8d9df870023420a1e9c8/air-filter

part's 44 and 6 is what I was referring too. 
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78 CB750F

Offline Jnel

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Re: 1978 CB750F carb questions
« Reply #18 on: March 09, 2017, 07:34:07 PM »
Can anyone tell me a good starting point for the pilot screw on my 78 F crabs?  I didn't have the foresight to find the original setting before i backed them all the way out for cleaning.

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Offline harisuluv

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Re: 1978 CB750F carb questions
« Reply #19 on: March 09, 2017, 07:36:41 PM »
1.5 turns out
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Offline Jnel

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Re: 1978 CB750F carb questions
« Reply #20 on: March 09, 2017, 08:06:53 PM »
Great!  I appreciate the info.  Thank you.
Jnel
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Offline 1978CB750SS

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Re: 1978 CB750F carb questions
« Reply #21 on: March 10, 2017, 05:16:39 PM »
Thank you man! I'm working so much I don't reply right away. I'll be tinkering with the carbs this weekend.
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Offline 1978CB750SS

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Re: 1978 CB750F carb questions
« Reply #22 on: March 11, 2017, 07:42:01 AM »
Quote from: trenchgrinder750 on March 09, 2017, 05:54:25 AM
Quote from: 1978CB750SS on March 08, 2017, 11:14:30 AM
What is this smaller tube on the carb? I bought the bike incomplete but does have original tank. I don't see anywhere to put the tube. I'm guessing it's a fuel line. The bike does start when I apply at least half throttle but does not idle I have to hold choke tight closed and give it half throttle to run.. The bigger tube I have going to petcock. I took carbs off to clean.


looks like you are missing half of that hose.  that will help you get to the top of the airbox.  if you need any for your carbs I have parts for the PD42A


Thank you.
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Offline 1978CB750SS

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Re: 1978 CB750F carb questions
« Reply #23 on: March 11, 2017, 07:44:02 AM »
Quote from: trenchgrinder750 on March 09, 2017, 05:48:23 AM
Quote from: Jnel on March 08, 2017, 05:58:55 PM
1978CB750SS.... I have a 78 F.  I also have the same carbs.  I have a manual too.  If you get in a jam i can photo the pages in the manual and send to you.  Just a thought
 Thanks Sir. Appreciate that.
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Offline 1978CB750SS

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Re: 1978 CB750F carb questions
« Reply #24 on: March 11, 2017, 07:45:20 AM »
Quote from: flybox1 on March 09, 2017, 07:45:11 AM
Quote from: madmtnmotors on March 09, 2017, 07:21:50 AM
Quote from: Ericcb750 on March 09, 2017, 07:16:06 AM
That hose is a vent like he said to the atmosphere.


Correct. Of the three K8's and three F2's that I have each of them have this vent hose. They are each done in two pieces with the little plastic connector you have shown joining the two. The second piece of tubing is only another six inches long (approx) and extends back to a little clip on top of the airbox to hold it into position with the open end facing to the rear.
  Thank you very much.
Yes.  It shouldnt be confused with ATTACHING it to the the elbow inlet on the top of the airbox.
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