Author Topic: carb help on 77 cb750f  (Read 4635 times)

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Offline Bennie1977

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carb help on 77 cb750f
« on: December 21, 2017, 01:11:18 PM »
I bought a 77 cb750f over the summer. I rode it for a bit and now i have to retune the carbs. I bought it with cheap pods on it and im going back to the air box. The carbs need retuned. It gets poor gas mileage, pops and thuds through the pods, and now hardly wants to run. My problem is, i have done a lot of googling on the cb750f and cannot find the carbs that are on my bike. I did some research on the cb550s and, bam, theres my carbs. I need someone to tell me of i have 550 carbs, and then, if possible, tell me where the adjustment screw is on each carb to tune them.
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Offline MikeSimon

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Re: carb help on 77 cb750f
« Reply #1 on: December 21, 2017, 01:18:47 PM »
Most all Honda carbs have a carb-id on the side, usually the No1 carb on the left side, clearly visible. It starts with VB...
This will identify the carbs and tell you what model(s) they were used on.
1973 CB350F -sold
1974 CB350F -218 orig miles, sold
1976 CB750K - in restoration

Other Hondas:
3 x CBX
CB1100R
GB500
Plus Kawasakis, BMws & Ducatis

Offline Bennie1977

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Re: carb help on 77 cb750f
« Reply #2 on: December 21, 2017, 02:34:01 PM »
The only numbers on my carbs are number 4 on the right hand side. I googled the number and nothing came up. Its bugging me at this point. No other markings other than 1-4 looks like 41BAPD then a jappenese sign
« Last Edit: December 21, 2017, 02:39:58 PM by Bennie1977 »
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Offline FuZZie

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Re: carb help on 77 cb750f
« Reply #3 on: December 21, 2017, 02:39:34 PM »
think it would be a pd 41?

Offline Bennie1977

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Re: carb help on 77 cb750f
« Reply #4 on: December 21, 2017, 02:42:33 PM »
I dont know anything about these carbs, so i didnt even know what to look for. Extreme noob here.
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Offline FuZZie

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Re: carb help on 77 cb750f
« Reply #5 on: December 21, 2017, 03:54:38 PM »
PD 41A, seems correct for a CB750F 1977.

Offline TwoTired

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Re: carb help on 77 cb750f
« Reply #6 on: December 21, 2017, 04:03:54 PM »
PD 41 B is correct for a 77 CB750F

http://www.motorcycleproject.com/text/specs.html

If they put on pods, they may have mucked with the internal jets and settings.

The popping and thuds could be dirty spark plugs (soot), especially if it is getting poor gas mileage, it's likely set too rich.  Before you invest time in carb work, do all the routine 3000 mile tune up check list.

Many think bigger jets make more power.  It's just ignorance, imo.

Cheers,
Lloyd... (SOHC4 #11 Original Mail List)
72 500, 74 550, 75 550K, 75 550F, 76 550F, 77 550F X2, 78 550K, 77 750F X2, 78 750F, 79CX500, 85 700SC, GL1100

Those that learn from history are doomed to repeat it by those that don't learn from history.

Offline Bennie1977

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Re: carb help on 77 cb750f
« Reply #7 on: December 21, 2017, 06:17:06 PM »
I have done everything else but messed with the carbs. The guy i bought the bike from made it sound like a nightmare to touch the carbs. My next question that hasnt been answered. Where is the screw to tune the carborators? I looked the over, and theyre not where everything ive googled said they should be. Im so lost. Sounds like he swapped carborators with another bike?
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Offline madmtnmotors

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Re: carb help on 77 cb750f
« Reply #8 on: December 21, 2017, 06:33:25 PM »
Sounds like he swapped carborators with another bike?

The carbs you have are correct for the bike. Carbs are very sensitive to ham fisted and/or random approaches to tuning/adjustment. There is no "tuning screw" but there is an idle mixture screw on each carb as well as synchronization adjustment screws on three of them (#2 carb is the "baseline" that the other three are synced to). I recommend a total rebuild by resident expert harisuluv for carbs of questionable condition. These multi-carb setups aren't the best to learn on.
TAMTF...


Wilbur



Projects:
"Evolution": http://forums.sohc4.net/index.php?topic=100352.0
"P.O. Debacle": http://forums.sohc4.net/index.php/topic,126692.msg1441661.html#msg1441661
F2/F3 O-rings: http://forums.sohc4.net/index.php?topic=113672.msg1300721#msg1300721
Cam Tower Studs: https://www.mcmaster.com/#93210a017/=t19sgp
Clean up that nasty harness: http://forums.sohc4.net/index.php?topic=137351.msg1549191#msg1549191
http://forums.sohc4.net/index.php/topic,148188.msg1688494.html#msg1688494
http://forums.sohc4.net/index.php/topic,139544.msg1579364.html#msg1579364
                                          
Charging system diagnosis: http://forums.sohc4.net/index.php?topic=1012.msg8345#msg8345
Get the manuals: http://manuals.sohc4.net/cb750k/
The Dragon: http://forums.sohc4.net/index.php?topic=45183.msg1571675#msg1571675
Headlight Switch: http://forums.sohc4.net/index.php?topic=113986.msg1283236#msg1283236
Branden's leak free top end thread: http://forums.sohc4.net/index.php?topic=107040.0
Engine Lifting Made Easy: http://forums.sohc4.net/index.php/topic,58210.msg1684742.html#msg1684742
                                      http://forums.sohc4.net/index.php/topic,100352.msg1675840.html#msg1675840
Static and Dynamic Timing: http://www.hondachopper.com/garage/carb_info/timing/timing1.html
Airbox Gasket Replacement: http://forums.sohc4.net/index.php/topic,114485.msg1290000.html#msg1290000
"Café" : http://forums.sohc4.net/index.php/topic,84697.msg953814.html#msg953814
PD Carb Choke Linkage: http://forums.sohc4.net/index.php/topic,100352.msg1669248.html#msg1669248
                                    http://forums.sohc4.net/index.php/topic,110931.msg1248354.html#msg1248354
                                    http://forums.sohc4.net/index.php/topic,48858.msg515204.html#msg515204
Follow up on your damn posts: http://forums.sohc4.net/index.php/topic,144305.msg1791605.html#msg1791605
Taiwanese Cam Chain Tensioners:  http://forums.sohc4.net/index.php/topic,155043.msg1774841.html#msg1774841
Gumtwo Seat Cover: http://forums.sohc4.net/index.php/topic,164440.msg1897366.html#msg1897366
Primary Drive: http://forums.sohc4.net/index.php/topic,166063.msg1919278.html#msg1919278
Tank Latch: http://forums.sohc4.net/index.php/topic,165975.msg1919495.html#msg1919495
Shorten your forks: http://vintage-and-classic-honda-s.456789.n3.nabble.com/How-to-shorten-forks-td4042465.html DO NOT CUT THE SPRINGS!
Clutch How To: http://vintage-and-classic-honda-s.456789.n3.nabble.com/How-to-change-and-adjust-a-clutch-SOHC-td4040391.html
Late model K7/K8/F2/F3 front sprocket cover removal: http://forums.sohc4.net/index.php/topic,178428.msg2072279.html#msg2072279
630 to 530 conversion: http://forums.sohc4.net/index.php/topic,180710.msg2094423.html#msg2094423

Sent from my Tandy TRS-80!

Offline madmtnmotors

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Re: carb help on 77 cb750f
« Reply #9 on: December 21, 2017, 06:43:32 PM »
Here is an idea of what you would be in for should you attempt to rebuild them yourself:

http://forums.sohc4.net/index.php?topic=146893.0

And here is a post (with picts) of how to correctly assemble just the maddening choke spring/linkage assembly:

http://forums.sohc4.net/index.php/topic,100352.msg1669248.html#msg1669248

Then once the carbs are properly refurbished/cleaned/adjusted then you need to synchronize them once the bike is running to get the best performance from them.
TAMTF...


Wilbur



Projects:
"Evolution": http://forums.sohc4.net/index.php?topic=100352.0
"P.O. Debacle": http://forums.sohc4.net/index.php/topic,126692.msg1441661.html#msg1441661
F2/F3 O-rings: http://forums.sohc4.net/index.php?topic=113672.msg1300721#msg1300721
Cam Tower Studs: https://www.mcmaster.com/#93210a017/=t19sgp
Clean up that nasty harness: http://forums.sohc4.net/index.php?topic=137351.msg1549191#msg1549191
http://forums.sohc4.net/index.php/topic,148188.msg1688494.html#msg1688494
http://forums.sohc4.net/index.php/topic,139544.msg1579364.html#msg1579364
                                          
Charging system diagnosis: http://forums.sohc4.net/index.php?topic=1012.msg8345#msg8345
Get the manuals: http://manuals.sohc4.net/cb750k/
The Dragon: http://forums.sohc4.net/index.php?topic=45183.msg1571675#msg1571675
Headlight Switch: http://forums.sohc4.net/index.php?topic=113986.msg1283236#msg1283236
Branden's leak free top end thread: http://forums.sohc4.net/index.php?topic=107040.0
Engine Lifting Made Easy: http://forums.sohc4.net/index.php/topic,58210.msg1684742.html#msg1684742
                                      http://forums.sohc4.net/index.php/topic,100352.msg1675840.html#msg1675840
Static and Dynamic Timing: http://www.hondachopper.com/garage/carb_info/timing/timing1.html
Airbox Gasket Replacement: http://forums.sohc4.net/index.php/topic,114485.msg1290000.html#msg1290000
"Café" : http://forums.sohc4.net/index.php/topic,84697.msg953814.html#msg953814
PD Carb Choke Linkage: http://forums.sohc4.net/index.php/topic,100352.msg1669248.html#msg1669248
                                    http://forums.sohc4.net/index.php/topic,110931.msg1248354.html#msg1248354
                                    http://forums.sohc4.net/index.php/topic,48858.msg515204.html#msg515204
Follow up on your damn posts: http://forums.sohc4.net/index.php/topic,144305.msg1791605.html#msg1791605
Taiwanese Cam Chain Tensioners:  http://forums.sohc4.net/index.php/topic,155043.msg1774841.html#msg1774841
Gumtwo Seat Cover: http://forums.sohc4.net/index.php/topic,164440.msg1897366.html#msg1897366
Primary Drive: http://forums.sohc4.net/index.php/topic,166063.msg1919278.html#msg1919278
Tank Latch: http://forums.sohc4.net/index.php/topic,165975.msg1919495.html#msg1919495
Shorten your forks: http://vintage-and-classic-honda-s.456789.n3.nabble.com/How-to-shorten-forks-td4042465.html DO NOT CUT THE SPRINGS!
Clutch How To: http://vintage-and-classic-honda-s.456789.n3.nabble.com/How-to-change-and-adjust-a-clutch-SOHC-td4040391.html
Late model K7/K8/F2/F3 front sprocket cover removal: http://forums.sohc4.net/index.php/topic,178428.msg2072279.html#msg2072279
630 to 530 conversion: http://forums.sohc4.net/index.php/topic,180710.msg2094423.html#msg2094423

Sent from my Tandy TRS-80!

Offline TwoTired

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Re: carb help on 77 cb750f
« Reply #10 on: December 21, 2017, 08:21:45 PM »
I have done everything else but messed with the carbs.

If you say so.  What check list did you use?

The guy i bought the bike from made it sound like a nightmare to touch the carbs.

If you don't know what you are doing, and what bit does what, any machine is a nightmare.

My next question that hasnt been answered. Where is the screw to tune the carborators?
The is an Idle Mixture Screw just in front of the carb bowls.  It adjusts the mixture at idle RPM.
Don't expect that to be the only adjustment to the carb's mixture.
There is a main jet, and a throttle jet needle valve adjustment, too.  These meter fuel at throttle positions other than idle.

Have you checked the vacuum sync?  Have you looked at the spark plug deposits?

Do check the Honda CB750 Shop manual supplements.  They have added chapters that address the carbs you have, including the location of the IMS and how to tune it.

Cheers,
 
Lloyd... (SOHC4 #11 Original Mail List)
72 500, 74 550, 75 550K, 75 550F, 76 550F, 77 550F X2, 78 550K, 77 750F X2, 78 750F, 79CX500, 85 700SC, GL1100

Those that learn from history are doomed to repeat it by those that don't learn from history.

Offline Bennie1977

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Re: carb help on 77 cb750f
« Reply #11 on: December 22, 2017, 01:36:13 PM »
I have replaced spark plugs, don't remember what ones i put in it, but i did a few days of research to find the right ones and the correct gap. They werent bad, but i wanted to make sure. I did an oil change, oil filter, and oil lines. I got the carbs off now and the gaskets are pretty shot. Basically what i would exspect for carbs that havent been touched in 40 years. Carb one was the worst. I took the boul off and it was full of rubber gasket material. Also, in carb one, the float pin was toast. Should i go with the same ones, or order the ones feebay lists? Also, id assume these are the screws mentioned to sync the carbs at an idle?
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Offline Bennie1977

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Re: carb help on 77 cb750f
« Reply #12 on: December 22, 2017, 01:43:09 PM »
Sorry. Still learning the website
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Offline evinrude7

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Re: carb help on 77 cb750f
« Reply #13 on: December 22, 2017, 04:08:59 PM »
i'm sure glad i have 657 carbs.  those pds look complicated.  bennie, you'll want to find the manual for that bike and do the 3000 mile maintenance as TT and Calj mentioned.  valve tappet adjustments, cam chain, timing, etc. after that you'll have a better chance at dialing the carburetors in.   

i think this ought to help you.  don't think it's specific to the year and model but seems to contain what you'll need. 

https://moto-odejda.ru/file/CB750SOHCShopManual.pdf
« Last Edit: December 22, 2017, 04:14:59 PM by evinrude7 »
cb750 k6 - ugly

Offline Bennie1977

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Re: carb help on 77 cb750f
« Reply #14 on: December 22, 2017, 07:03:55 PM »
These carbs are fing atrocious. I have taken everything apart, cleaned the insides. Destroyed a f ton of gaskets and o rings. Still have no idea how to tune 1,3,4 because #2 looks the same, but has a small what apears to be a vacuum on the bottom. YouTube is my best friend right now. I dont want to pay someone to do it, but even if i did, i dont think theres to many people around me that know how to work on these bikes. Thats why i like it though. Not common at all. Just stressed about it at this point. You guys are the only ones i can ask that have experience on these bikes. Working on something a little over twice my age is satisfying and stressful. Dont want to sink all this effort into it and it not run. The links aren't opening for me. Ill have to jump on a computer one of these days.
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Offline 750K

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Re: carb help on 77 cb750f
« Reply #15 on: December 22, 2017, 07:20:36 PM »
Pd's are really simple carbs to set up, clean them replace the gaskets and o rings. Bench sync, make sure the accelerator pump works, adjust the fast idle, vacuumed sync and plug chop and tune.
77 Cb750, 78 Kz1000

Offline Bennie1977

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Re: carb help on 77 cb750f
« Reply #16 on: December 22, 2017, 07:37:14 PM »
At this point i dont know what adjusts the butterflies to bench sync it. Ive played with the smaller jet that shows on the outside of the carbs. Took them out and cleaned them. Then put them back as close as i could to the way they were before. I see the idle screw obviously raises all 4. I just want to raise one at a time. Then ill worry about the air/fuel mixture once i get it running. Not that i can tell how to tune it. Ive looked up downloadable manuals, but they dont tell me what i need to known
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Offline evinrude7

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Re: carb help on 77 cb750f
« Reply #17 on: December 22, 2017, 08:20:52 PM »
does this help at all?  http://forums.sohc4.net/index.php?topic=113931.0  sometimes you have to read thru it and use what works for you. 
cb750 k6 - ugly

Offline dave500

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Re: carb help on 77 cb750f
« Reply #18 on: December 22, 2017, 08:31:10 PM »
the one and first thing you must do before trying to tune carbs is to be sure the ignition settings are spot on,make sure the advance mechanism is free to move,correct point gaps with good points and perhaps fresh condensors,check the plug caps resistance,youll just be fighting #$%* if the igniton isn't squared away first and the poor old carbs will get the blame,good choice returning the stock airbox its the best way for a good streetable bike,once tuned etc any old bike goes years trouble free.

Offline Bennie1977

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Re: carb help on 77 cb750f
« Reply #19 on: December 22, 2017, 09:09:29 PM »
All spark plugs have been changed, all spark plugs appear to be getting good spark. They put garbage pods onto the carbs. By the looks of them, the last person to touch them was the guy in japan who put them together. Im starting to understand how to bench sync. The video below has helped me understand how to troubleshoot my problems. Honestly, as much as research as ive done, which isnt a lot compaired to which is only hours bot years, every issue im having is due to the pods and the tune. Everything is stock inside the carbs, the floats worry me a little, because they dont open or close a whole lot. Not much play as im used to on other engines. So id assume a stock air box should help a lot, and then tuning. Ive been looking at my carbs, and the slides look like they're close to where they should be. Time to order new gaskets, o rings, and stock main jets, just because im not 100% sure the carbs havent been touched, and theres no numbers on the main jets. I dont k ow how much farther I want to dig past the carbs before i see how it runs after reinstalling them. Ive had some really bad luck trying to fix a 4 stroke enduros timing. Never got it right and it never ran again. Sold it dirt cheap. This is one big experiment to me.
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Offline Bennie1977

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Re: carb help on 77 cb750f
« Reply #20 on: December 22, 2017, 09:24:35 PM »
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Offline kerryb

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Re: carb help on 77 cb750f
« Reply #21 on: December 22, 2017, 09:25:20 PM »
Bennie, the first carbs I ever worked on were pd 42's, very similar to yours.  I never worked on a carb before.  First you need a manual that describes your carbs parts and procedures.  Then you need to read all the threads on here that deal with '77-'78 carbs cause each one will teach you a little.  My carbs took a few weeks to clean and set up.  A few orders to sirius and oem honda, a bunch of removing and reinstalling, a lot of pondering and swearing.  Overall a very gratifying experience cause they work well now.  I like syncronizing carbs now, its fun!  So go for it, but in the right order. 
Get the book, study the parts, maybe even buy a junk carb on ebay to practice on.  Don't buy any carb kits, just the o-rings and seals.  If you need brass parts ( you might not ) make sure they are genuine keihin brass.  Oh...and don't use carb cleaner on rubber parts, it ruins them.  That's how I learned how much an accelerator pump diaphram costs!
p.s. a clymer manual is a good start, cause you can bookmark the pages and write notes in the margins.  And I'm still usiing my original gallon of carb dip cleaner many carbs later.

Have fun...its all part of the adventure.
intrigued by the wail...seduced by the scream.

Offline dave500

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Re: carb help on 77 cb750f
« Reply #22 on: December 23, 2017, 01:19:57 AM »
if theres no numbers on the main jets how the #$%* do you know what size they are without a jet drill set to guage them?even #$%* ones have a number on them look again,the factory have the size and the kiehin logo,keyster and other brands will have the size on them but be wary of their precision!the size is in millimetres,a 100 jet is 1mm,a 98 jet is point .98 of a mm and so on,just out of interest some jets in other makes are marked in area of the hole,the factory jets if not drilled out or damaged are always the very best to use full stop!

Offline Bennie1977

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Re: carb help on 77 cb750f
« Reply #23 on: December 25, 2017, 12:11:55 PM »
Just nervous tinkering with things i dont know about. I already have to much money invested into the bike, i dont wanna f something up and be out a bunch of cash. Good news though. I fn found what ive been looking for. Currently bench syncing it and i have my parts on the way. Parts are coming together and this isnt as crazy as i thought. Nervous noob here.
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Offline evinrude7

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Re: carb help on 77 cb750f
« Reply #24 on: December 25, 2017, 12:16:42 PM »
sometimes you can learn by fuking up.  it's served me well. 
cb750 k6 - ugly

Offline Bennie1977

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Re: carb help on 77 cb750f
« Reply #25 on: December 26, 2017, 07:46:45 AM »
Lol. That is life.
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