Author Topic: 1977 CB750 Jetting carb after modifications  (Read 15443 times)

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Offline lucky

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Re: 1977 CB750 Jetting carb after modifications
« Reply #75 on: July 15, 2012, 01:51:57 PM »
I think he is talking about using clear tubing on the overflow nubs to do a reality check on the fuel levels in the carbs.  I could be wrong but . . . .

But the float level MUST BE SET FIRST!!!!

Offline lucky

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Re: 1977 CB750 Jetting carb after modifications
« Reply #76 on: July 15, 2012, 01:53:22 PM »
are you running a stock airbox?  I'm not sure dropping your slows is going to help since you're showing extremely lean in low RPM.  What are your fuel screws set to?  I was also having starting problems on my K7 but after resetting my valve clearances and adjusting the choke cable, starts right up now, at least it's a place to start.

Dropping the slows?  Stock is #35 which is what he ordered.

for some reason, i was thinking they were 40s

#40 mm idle jets will not work. Use #42mm's

Offline lucky

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Re: 1977 CB750 Jetting carb after modifications
« Reply #77 on: July 15, 2012, 02:07:22 PM »
This is a video and shows 1978 carbs with:

4 into 2 into 1 exhaust with metal baffle.
#45 idle jets. (try with #42's)
#120 main jets.
Velocity stacks. (same as pods)
Two shims on each needle .022 thousands sitting on top of the pocket in the bottom of the slide making it raise the needles about .054 thousandths.

Just a little to much.
ONE .022 shim on top of the pocket would be better.

In this video you will hear there is no hesitation at all and no popping on decelleration.

1978 CB 750
« Last Edit: July 15, 2012, 02:09:43 PM by lucky »

Offline xnoahx

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Re: 1977 CB750 Jetting carb after modifications
« Reply #78 on: July 15, 2012, 07:02:12 PM »
Thats a smooth runner in that vid

Offline lucky

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Re: 1977 CB750 Jetting carb after modifications
« Reply #79 on: July 15, 2012, 07:13:04 PM »
Thanks xnoahx.
 That was 10 separate R&R's (remove and replace carbs) to get to that point.
One step at a time.

Offline MechSJ

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Re: 1977 CB750 Jetting carb after modifications
« Reply #80 on: August 19, 2012, 10:06:15 PM »
First off, my 77 cb750 came with pods on it already and 120 mains... previous owner couldn't get it to run right, i picked it up for a song. I found a float installed upside down, fixed that still wouldn't run right, so I cleaned the accelerator pump circuit and the primary jets and got curious about the needle position with the 120 mains, needle was on the leanest setting, well that explained the lean misfire at 1/8-1/4 throttle, clip to middle of five position choices...

I now have crisp clean throttle response! and EXCELLENT! low end and mid range. still missing a little something on the plus side of 6500rpm's (probably the stock "restrictive" cylinder head) but I really don't need to go +110mph on my 35 year old bike now do I.

My set up:
1977 cb750K
main=120
primary=35 (stock)
1/2-3/4 out on idle screw (total form seated)
crappy cheep emgo pods (not by choice, air box was lost to fire by previous owner)
4into2 exhaust (mac?) kinda loud'ish free(er) flowing
and running on E10 pump gas (premium!)
My set up:
1977 CB750K
main=120
clip in middle position of five on slide needle
primary=35 (stock)
1/2-3/4 out on idle jet (not air bleed) screw (total form seated)
crappy cheep emgo pods (not by choice, air box     was lost to fire by previous owner)
4into2 exhaust (mac?) kinda loud'ish free(er) flowing
and running on E10 pump gas (premium!)

Offline lucky

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Re: 1977 CB750 Jetting carb after modifications
« Reply #81 on: August 19, 2012, 11:15:15 PM »
First off, my 77 cb750 came with pods on it already and 120 mains... previous owner couldn't get it to run right, i picked it up for a song. I found a float installed upside down, fixed that still wouldn't run right, so I cleaned the accelerator pump circuit and the primary jets and got curious about the needle position with the 120 mains, needle was on the leanest setting, well that explained the lean misfire at 1/8-1/4 throttle, clip to middle of five position choices...

I now have crisp clean throttle response! and EXCELLENT! low end and mid range. still missing a little something on the plus side of 6500rpm's (probably the stock "restrictive" cylinder head) but I really don't need to go +110mph on my 35 year old bike now do I.

My set up:
1977 cb750K
main=120
primary=35 (stock)
1/2-3/4 out on idle screw (total form seated)
crappy cheep emgo pods (not by choice, air box was lost to fire by previous owner)
4into2 exhaust (mac?) kinda loud'ish free(er) flowing
and running on E10 pump gas (premium!)


What position are the slide needles??

Offline 736cc

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Re: 1977 CB750 Jetting carb after modifications
« Reply #82 on: August 20, 2012, 09:34:49 AM »
   I've owned this 78F since June and been dialing it in ever since: fresh Bridgestones, new 630 chain, Norman Hyde handlebars, lost the black rear fender, detailed its cosmetics w/ S-100 Cyclewash and rejuvinated the black motor w/ S-100 Shiny Engine Bright, removed the ugly useless plastic clips on the Comstars, removed seat strap, threw on an old Action 4's header w/ a new Supertrapp muffler, plus a few more details. But its always had a carb hesitation/stumble and its super cold-blooded, so I've been reading this post looking for clues. I also noticed signs of the engine being removed from the frame in its past, so today I started checking engine mounts- the were all loose! Soooo, now I'm checking every nut, bolt and screw, when EUREKA! ALL 8 SCREWS ON ALL 4 INTAKE BOOTS WERE LOOSE! Torque them down and PRESTO! No more hesitation or stumble, and the idle is higher!
   I like fixes like this. ;D
   So don't forget to "check the intake boot screws for looseness" to the list of remedies for hesitation and lean running before we yank the carbs. ::)



« Last Edit: August 20, 2012, 09:37:00 AM by 736cc »
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Offline Tugboat

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Re: 1977 CB750 Jetting carb after modifications
« Reply #83 on: August 20, 2012, 01:50:29 PM »
^Those headers are badass!
If it's worth doing, it's worth doing twice.

Offline 736cc

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Re: 1977 CB750 Jetting carb after modifications
« Reply #84 on: August 20, 2012, 06:00:08 PM »


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Offline MechSJ

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Re: 1977 CB750 Jetting carb after modifications
« Reply #85 on: August 21, 2012, 06:52:06 PM »
First off, my 77 cb750 came with pods on it already and 120 mains... previous owner couldn't get it to run right, i picked it up for a song. I found a float installed upside down, fixed that still wouldn't run right, so I cleaned the accelerator pump circuit and the primary jets and got curious about the needle position with the 120 mains, needle was on the leanest setting, well that explained the lean misfire at 1/8-1/4 throttle, clip to middle of five position choices...

I now have crisp clean throttle response! and EXCELLENT! low end and mid range. still missing a little something on the plus side of 6500rpm's (probably the stock "restrictive" cylinder head) but I really don't need to go +110mph on my 35 year old bike now do I.

My set up:
1977 cb750K
main=120
primary=35 (stock)
1/2-3/4 out on idle screw (total form seated)
crappy cheep emgo pods (not by choice, air box was lost to fire by previous owner)
4into2 exhaust (mac?) kinda loud'ish free(er) flowing
and running on E10 pump gas (premium!)


What position are the slide needles??

Clip is in the middle position of five. When I found it, it was on the top (leanest) position and I had a lean misfire from 1/8-1/4 throttle and a crapy idle
My set up:
1977 CB750K
main=120
clip in middle position of five on slide needle
primary=35 (stock)
1/2-3/4 out on idle jet (not air bleed) screw (total form seated)
crappy cheep emgo pods (not by choice, air box     was lost to fire by previous owner)
4into2 exhaust (mac?) kinda loud'ish free(er) flowing
and running on E10 pump gas (premium!)

Offline lucky

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Re: 1977 CB750 Jetting carb after modifications
« Reply #86 on: August 22, 2012, 11:33:17 AM »
First off, my 77 cb750 came with pods on it already and 120 mains... previous owner couldn't get it to run right, i picked it up for a song. I found a float installed upside down, fixed that still wouldn't run right, so I cleaned the accelerator pump circuit and the primary jets and got curious about the needle position with the 120 mains, needle was on the leanest setting, well that explained the lean misfire at 1/8-1/4 throttle, clip to middle of five position choices...

I now have crisp clean throttle response! and EXCELLENT! low end and mid range. still missing a little something on the plus side of 6500rpm's (probably the stock "restrictive" cylinder head) but I really don't need to go +110mph on my 35 year old bike now do I.

My set up:
1977 cb750K
main=120
primary=35 (stock)
1/2-3/4 out on idle screw (total form seated)
crappy cheep emgo pods (not by choice, air box was lost to fire by previous owner)
4into2 exhaust (mac?) kinda loud'ish free(er) flowing
and running on E10 pump gas (premium!)


What position are the slide needles??

Clip is in the middle position of five. When I found it, it was on the top (leanest) position and I had a lean misfire from 1/8-1/4 throttle and a crapy idle


Sorry i missed that the first time.
So the needle is moved upwards two clip positions. That will work well.
Others including myself have had good results with this same combo.

BTW..That top clip position on the needle was the stock position same as the 1978 model but the 1978 model only had one clip groove and was therefore non adjustable. The 1978 needle also had the top of the needle shortened.
A 1976 needle is the same as the 1977.



Now I would just up that idle jet a little.
« Last Edit: August 22, 2012, 11:37:05 AM by lucky »

Offline lucky

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Re: 1977 CB750 Jetting carb after modifications
« Reply #87 on: August 22, 2012, 11:41:17 AM »
   I've owned this 78F since June and been dialing it in ever since: fresh Bridgestones, new 630 chain, Norman Hyde handlebars, lost the black rear fender, detailed its cosmetics w/ S-100 Cyclewash and rejuvinated the black motor w/ S-100 Shiny Engine Bright, removed the ugly useless plastic clips on the Comstars, removed seat strap, threw on an old Action 4's header w/ a new Supertrapp muffler, plus a few more details. But its always had a carb hesitation/stumble and its super cold-blooded, so I've been reading this post looking for clues. I also noticed signs of the engine being removed from the frame in its past, so today I started checking engine mounts- the were all loose! Soooo, now I'm checking every nut, bolt and screw, when EUREKA! ALL 8 SCREWS ON ALL 4 INTAKE BOOTS WERE LOOSE! Torque them down and PRESTO! No more hesitation or stumble, and the idle is higher!
   I like fixes like this. ;D
   So don't forget to "check the intake boot screws for looseness" to the list of remedies for hesitation and lean running before we yank the carbs. ::)


Good point 736, never assume anything.
A good visual inspection can catch many flaws.
Beautiful bike!

Offline Finksies

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Re: 1977 CB750 Jetting carb after modifications
« Reply #88 on: September 03, 2012, 06:00:37 PM »
Got a 1977 CB750F Supersport. Modifications include... K&N Pods & 4-1 exhaust with a modified baffle.

Symptoms.. Poor idle and running at low speeds.

Right now Im running 125 mains, 1.5 turns on the air screw. Pilots were removed and cleaned, accel pump is working as it should. No air leaks.

Today I ordered #42 pilot jets. I also have a set of needles with adjustable slots. Once I get these installed I will post back with results.

**Lucky thanks for all the Posts, hopefully the pilot jet helps!

Offline Finksies

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Re: 1977 CB750 Jetting carb after modifications
« Reply #89 on: September 19, 2012, 02:01:19 AM »
Got a 1977 CB750F Supersport. Modifications include... K&N Pods & 4-1 exhaust with a modified baffle.

Symptoms.. Poor idle and running at low speeds.

Right now Im running 125 mains, 1.5 turns on the air screw. Pilots were removed and cleaned, accel pump is working as it should. No air leaks.

Today I ordered #42 pilot jets. I also have a set of needles with adjustable slots. Once I get these installed I will post back with results.

**Lucky thanks for all the Posts, hopefully the pilot jet helps!


Update 9/19/12

Installed #42 Pilot jets
Air screw 2 turns out
Main Jet #125
Needle one clip down (richer) from middle
Carbs synced

The bike idles like a champ now and am pretty happy with the results. Its not running 100% mint but Im close. Next time I have time, I think im going to drop the clip one more notch on the needle

 

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