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Update on 1982 CB750c Carb Rebuild - New Carbs?

deeveeance

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To recap: got an '82 CB750C project to work on, traded a laptop I didn't use, and ~$30 for a uHaul trailer. Sweet deal in my books.

First off- cleaned, took things apart, secured everything else, and attempted to start 'er up: engine would only run on Choke, and spewed gas from carb 2/3 area.

Took carbs out to see if things could be cleaned, or start rebuild. The state of the parts was, pretty bad, there was gunk everywhere, and most of the rubber parts were on their way out or torn. During the disassembly, I noticed that two of the Pilot screw tips were broken for carbs 2 and 3, and firmly stuck on the hole inside. Got my thinking cap on and checked idle circuit as well, the nozzle was nasty. Tried my best to nudge the broken pieces out using a 90-degree pick, but no dice.

LINK: Referenced this post regarding broken Pilot Screw tips.

20210912_151850.jpg


Looks like I might be in for a bit of a pickle, but since this project is already underway, I was looking up replacement carbs or alternatives.

LINK: I ended up reading about Murray's Carbs being sold as a plug-and-play kit. I know there is some pushback against pod-filters on these Keihin carbs, but is this a viable alternative route I can go towards and get the project going, essentially doing with different carbs altogether?

Planning on doing stripping, sanding and restoring the paint of the frame/tank after deciding what to do regarding carbs.

Thanks in advance for any and all info!
 

dirtdigger

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If you are going to do aftermarket carbs then you need to get Mikuni rs or keihin cr carbs....4 carbs not the two carb setups. You will lose power if you go with the 2 carbs. They run best with 4,
 

deeveeance

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If you are going to do aftermarket carbs then you need to get Mikuni rs or keihin cr carbs....4 carbs not the two carb setups. You will lose power if you go with the 2 carbs. They run best with 4,

Got it, I did see a few posts regarding the dual Mikuni carbs setup getting around 10hp loss. Not a ton, but still loss.

Got around to a compression check, oil was fine, and everything else is ready to roll, just need to carb this baby up.

Any recommendations for 4-stack carb replacements? Did a bit of Googlin' and found Dynoman's site where they offer #CR29-3026, CR carbs @ 29mm. Is there a difference to going to CR @ 32mm? Any other sites/builders? Sorry, little new to carb rebuilds and Hondas.
 

BB74

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Used stock carbs are cheap keep an eye open for a set locally. You can always use stock ones to get it running and then upgrade to aftermarket ones later.
 

BB74

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Try soaking the carb passages with marvel mystery oil or deep creep for a day or two before disassembling them to loosen up the frozen and gummed up parts.
 

deeveeance

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Used stock carbs are cheap keep an eye open for a set locally. You can always use stock ones to get it running and then upgrade to aftermarket ones later.
I'm not seeing many in Portland, or at least not what my untrained eyes are finding.

Should I stick with the same model or can I get away with anything else?
 

BB74

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1979 to 1983 will work. Only difference will be the floats on the 79 and later ones have a added on (separate part) vacuum port for the fuel petcock.
 

BB74

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I would try soaking the broken tips and then hitting the fuel circuit with compressed air to see if they dislodge.
 

deeveeance

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1979 to 1983 will work. Only difference will be the floats on the 79 and later ones have a added on (separate part) vacuum port for the fuel petcock.

Pretty much all I could find on FB Marketplace (using girlfriend's account), and Craigslist.

1980 Carbs FS.jpg




Did see these as well, but for an '81 CB650

1982 CB650 Carbs FS.jpg


Have compression tester coming in tomorrow, to double check. Bike runs, just spewed gas, so ultimately not well. Might just skip stock and grab the CR's.
 

BB74

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The 650 carbs will be smaller. Keep looking for a local 750 set, used carbs that need rebuilding shouldn't cost more then $100.
 

dirtdigger

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I wouldnt do anything but CR's or Mikuni RS carbs, stock CV carbs are junk. Vastly better adjustability and driveability compaired to stock carbs.
 

jnelson1954

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If you are going to do aftermarket carbs then you need to get Mikuni rs or keihin cr carbs....4 carbs not the two carb setups. You will lose power if you go with the 2 carbs. They run best with 4,
For Whats its worth... I pulled the trigger on the Murray carbs. Man... it woke that bike up. So much so I now had to do some extensive brake work to stop this thing. It was plug and play and the support that murray gives is very good. So from my perspective... a new anything is better than a 42 year old carb setup that is so temperamental and so many moving parts.
 

BB74

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cb900 had 32mm Keihin carbs. cb750 has 30mm Keihin carbs. I'd stick with the 30mm cb750 ones or buy a new aftermarket set.

Any used carbs will need rebuilding so don't spend too much on them, I wouldn't pay more then $150.00 on a set because a good rebuild kit is going to be at least $80. After that your better off buying new.

Did you try some local shops in your area that may be able to fix your carbs? That may be an option.
 

deeveeance

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cb900 had 32mm Keihin carbs. cb750 has 30mm Keihin carbs. I'd stick with the 30mm cb750 ones or buy a new aftermarket set.

Any used carbs will need rebuilding so don't spend too much on them, I wouldn't pay more then $150.00 on a set because a good rebuild kit is going to be at least $80. After that your better off buying new.

Did you try some local shops in your area that may be able to fix your carbs? That may be an option.

Found this:


The floats on here look different than mine, but I think the "missing parts" on it are able to be cannibalized from my current non-working set.

Do you know what rebuild kit would fit the bill on these?

I'll spring for these, get the bike running, and see what else I need to do before dropping any cash for an upgrade set of carbs.
 

BB74

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Those are the 1979 style floats that use adjustable float needle, you can use yours.

Check the 4into1.com website they have carb kits. This one has both types of float needles.


Vintagecb750.com has kits as well. I've ordered from vintage and been happy with the product and shipping.

Amazon and ebay have cheap kits, not sure how good they are.
 

BB74

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Use lots of brake cleaner to clean out the passages and then hit them with compressed air to push all the dirt out of them.
 

deeveeance

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Use lots of brake cleaner to clean out the passages and then hit them with compressed air to push all the dirt out of them.
Received carbs, placed in new parts, grabbed missing ones from the old carbs... Everything went by swimmingly.

Now, after touching the throttle, it hangs at 4k RPM. Cables are good (nothing stuck, or too tight), did the carb cleaner/boot test and rpm was unaffected, Pilot Sync Screws are all at 3 turns out, tried 2.5 turns, no change.

Might just cut my losses and go for the new new carbs to get the project/restore going.
 

BB74

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Sounds like a vacuum leak. Check the carb boots by spraying carb cleaner on the joints while the bike is idling. If it revs up then you have air leaking into the engine at that spot. I had the same problem with mine and needed new rubber carb boots as mine were dried out and didn't seal anymore.
 

deeveeance

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Sounds like a vacuum leak. Check the carb boots by spraying carb cleaner on the joints while the bike is idling. If it revs up then you have air leaking into the engine at that spot. I had the same problem with mine and needed new rubber carb boots as mine were dried out and didn't seal anymore.
The end connected to the engine, or Airbox for replacements.

I did that already and no change, might need to do it more extensively.
 
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