1982 cb750 cv carb swap to pd style carb

[spjr2222]

hello gentlemen, i am excited to ask you guys some question for i have been doing alot of reading and came to the conclusion that you guys, TOOLS especially, are geniuses when it comes to these bikes.  So here we go.

i have a 1982 cb750.  i have had carb issues since day one, 2009.  i have no mods other then pods. Now i know they are no good for cv style carbs but my airbox was cracked when i first got the bike so i not knowing otherwise put pods on it. i jetted it 75 and 110 as required.  i have rebuilt and bench synced the carbs(which is possible). i put new intake boots, plugs and plug wires.  the bike is in good shape.  the bike would race at idle and then stall, which tells me it has and intake leak.  i narrowed it down to the throttle rack.  inside is felt washers which are worthless.  while taking the carbs apart i dropped one carb and busted it and then had problems getting the intake side butterflies to seat properly.  i am at my wits end and want to get different carbs.  i dont want to spend alot either.  i can purchase a decent set of 1978 and prior for around $100 on ebay.  

i want it to be a cruiser with good fuel mileage, no speed demon. My father had a 1978 cb750k and he said his fuel mileage was in the 50's. I've done alot of research and saw cr carbs, gsxr, bst and vms with no interest.  The route i want to go is the old cb carbs, preferably pd's(pd42b 31mm).  Now that I've given you a little history on my bike, heres my questions:

What carbs do you recommend?  1978 pd42b key style with acc pump or 1976 and prior with round top. Next question is will my intake boots mate with the carbs or do i have to purchase 1978 intake boots?  I believe prior to 1977 has bolt on style intake boots. i believe the cv vb42a are 30mm intake so will that conflict with 78 carbs that are 31mm?  Will this conversion compromise my performance for the good or the worse?  Thank you for your time and hope to hear from you soon.

 

[seestheday]

Here is thread over on the cb1100f.net site where a guy details getting the PD41a's to work.

http://www.cb1100f.net/modules.php?name=Forums&file=viewtopic&t=69124&postdays=0&postorder=asc&highlight=sohc+carbs&start=0

Login is probably required to view the thread

Long story short, they will not directly mount and you will have to play significantly with the jetting, but this should give you a template to work from.

[TOOLS1]

What you are proposing is an awful lot of work!!!!! Sudco.com has several different replacement carb sets, for your bike. However they run around $800.00. You might want to give cycle-X a call, and ask if they have a duel carb kit for the DOHC bikes now.
TOOLS  

[Johnnylevi]

you might be better off just grabbing a set of decent CV carbs from ebay & an airbox. preferably a 79 set. They have adjustable floats and the accelerator pump shoots a larger stream of gas than the later years. probably cost around 150-250. Just my two cents.

[seestheday]

Ya, that is the cheapest way to a great running bike.

Did the 79's also have removable idle jets? If so then they were definitely the best year.

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From: Johnnylevi [via Honda CB750'S]

Sent: Thursday, July 4, 2013 3:38 AM

To: seestheday

Subject: Re: 1982 cb750 cv carb swap to pd style carb

you might be better off just grabbing a set of decent CV carbs from ebay & an airbox. preferably a 79 set. They have adjustable floats and the accelerator pump shoots a larger stream of gas than the later years. probably cost around 150-250. Just my two cents.

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[spjr2222]

thank you for your input.  it was very informative.  i am gona go through with it now knowing what i am up against.  my other question would be which carb?  77/78 with key tops and acc pump or 76 and prior with round tops and no acc pump. will i gain fuel mileage but loose power drastically with no acc pump or not much difference.  i dont care about a rocket but would like it to get out of its own way.  also with the roundtops i would be able to jet it easier do to the bowl dropping while carbs are still on.  willl it run properly with roundtops beings i believe they are a little bit smaller.  i heard they are harder to start when its cold out because they dont have the extra spray from acc but i dont ride under 70 degree weather anyway.  any input would be appreciated.  thank you

[Lucky 1]

The engine does not care which carbs are on it.


I would use the stock carbs and get someone to rebuild them that knows what they are doing.
Harrislove comes to mind. E bay carb rebuilding.
This carb changing project is a little over your head at your level.

[spjr2222]

that doesnt answer my question but thanks for your input

[seestheday]

An accelerator pump only takes effect when you hammer on the throttle. Often people flick the throttle to squirting gas when cold starting.

Other than that you're going to be on your own. Honestly, given your questions I'd recommend buying a set that is designed to work. You will have to know a lot about carb tuning ti get those to work right.

Sent from my BlackBerry device

From: spjr2222 [via Honda CB750'S]

Sent: Thursday, July 4, 2013 3:48 PM

To: seestheday

Subject: Re: 1982 cb750 cv carb swap to pd style carb

thank you for your input.  it was very informative.  i am gona go through with it now knowing what i am up against.  my other question would be which carb?  77/78 with key tops and acc pump or 76 and prior with round tops and no acc pump. will i gain fuel mileage but loose power drastically with no acc pump or not much difference.  i dont care about a rocket but would like it to get out of its own way.  also with the roundtops i would be able to jet it easier do to the bowl dropping while carbs are still on.  willl it run properly with roundtops beings i believe they are a little bit smaller.  i heard they are harder to start when its cold out because they dont have the extra spray from acc but i dont ride under 70 degree weather anyway.  any input would be appreciated.  thank you

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[TOOLS1]

Since you are going to do it, I would say use the round tops. Keep us posted on the project. I am very interested on how you do it, and how it works.
TOOLS

[seestheday]

Ditto what tools said. If you're set on doing it I'm very interested in seeing what you do. I would love a pre/post dyno test even more

[spjr2222]

Thanks will do. about a half an hour before you replied to get round tops i had bid on a set of key style with the acc.  i wound up winning the bid for $95.  i read a forum earlier that showed a guy had did the same thing im getting ready to do.  his main jet now is 170.  sounds kinda high.  i have a jet kit that goes up to 125.  im gonna start with 110, thats where i left off with the cv carbs and pods and i was happy with its performance,  other than the intake leaks throughout the carb.. i understand the style of the carb makes a difference but like i said gotta start somewhere, if it winds up being 170 then thats what it is.  i do thanks you guys for your time and ill keep ya's posted.  keep any pointers or tips coming if you have them.  i will check this forum periodically.  thanks

[TOOLS1]

170 sounds way too high! I wonder if it was a typo. I'm more interested in how you are going to mount the carbs, then the jetting.
TOOLS

[Lucky 1]

That does answer your question.


"I would use the stock carbs."

[shinyribs]

TOOLS1 wrote

 I'm more interested in how you are going to mount the carbs, then the jetting.
TOOLS

Radiator hose. Sounds goofy,but it works. And it'll hold up to gas pretty durn good. Of course,I've never done it on a bike,but I have seen it done. Can't remember where. But I have used it to mount a carb on an old riding mower when the factory mount died. You might have to rig up some sort of support to the frame to hold the weight of the carbs if you go this route. Maybe not the best,but it's cheap,easy and fast. Can't go wrong with cheap,easy and fast.

The catch would be the size difference between the carb outlets and the intake spigots on the head.

[Black 750K8]

What set did you end up with 77 or 78 A or B.  77 has adjustable needles as far as cold starts that is where the fast idle cam comes into play. on both sets. To much work for me but good luck.

[Lucky 1]

A #170 main jet would be way too much. DOHC or SOHC.

You cannot make it run right by just changing the main jet.
You must alter the needle clip position on the slide and the idle jet
 if the intake or exhaust has been changed from stock.

[Lucky 1]

The 1978 carbs do not have adjustable slide needles. They have one clip and only one clip groove.
You would have to shim the needles OR get adjustable needles from a 1977 or 1976 model.

[spjr2222]

today was the first day i was able to mess with the carbs.  they came a few days ago.  they are pd41a.  i pulled them apart to clean and put washer/shims on the needle of each slide.  the needles have clips.  it is my understanding that 78 have no adjustable and 76/77 have clips/adjustable.  so either they're 77 carbs or in the past 35 years adjustable needles were put in.  it does have an acc. i dont know if they're any noticeable differences between 77 and 78.  

the clips were all the way up top, in the most lean position.  two shims is equivalent to 1 notch.  so i put the needle back in the middle then dropped it one(to make it richer) because i have pods. i later read that stock for these bikes is one notch from the top.  my experience, dealing with dirtbikes and three wheelers all my life, was the stock position was in the middle. ill try it like it is and go from there.  im going to start with 35 pilot and 110 main. im going to the parts store tomorrow to get radiator hose to use as my intake boots.  i got lucky with the motor and carb being the same size.  i also have to find a throttle cable to match my wrist throttle that is also  1 1/2 inches longer than stock.  im open to any suggestions that anything i just noted.

[spjr2222]

UPDATE:

researched the model number pd41a.  my carbs are off of a 1977 cb750k.  they are a good set of carbs in good physical condition.  i think im going start with 115 main jets after reading a little more.  i also purchased a throttle cable for that year and model of bike off of ebay, hoping that will fix my throttle cable issue.