Topic: first time carb cleaning issues. CB750 73

[Jinsonphoto]

The carbs & engine are from a 73 CB750K.  The bike hasnt been sitting for years or anything, but I have had issues with getting her started since the weather has gotten cold. So I pulled carbs today for cleaning. This is the first time I have ever done this so I already have some questions & have even ran into trouble right out of the gate. I have the Honda manual which Im following along. My first question is since these carbs havent been sitting for years or anything do I really need to separate each carb if Im not going to dip them? I was hoping that i could just clean the jets & make the adjustments with the carbs together. Now the manual says to separate them, but I have heard others say keep them together.

Anyways on to my first real issue. Once i got the carbs off the first thing the manual says to do is disconnect the individual choke rod & unscrew the 6mm screws. At first I wasnt going to do this since i wanted to keep the carbs together so i went to the next step which says to remove the needle set plate to remove the needle jet from the throttle valve. Now in my manual it doesn't really explain how to do this. Do I go in through the top? Because I dont see this in the bowls? Since i couldn't find the needle set plate I went back to trying to Remove the carb from the stay plate so i could get in through the top. However when I tried to disconnect to choke i got the choke slider stuck. I don't know if Im going about it the right way to remove the choke rod. The manual is vary vague. Or Im just an idiot!

I have uploaded a picture to explain what I have done.

So looking at the picture Im guessing I should have tried unscrewing the actual choke rod itself? I hope I didn't already cause my self a lot of trouble.

Any thoughts or help would be greatly appreciated!!

Thank you in advance for your time.
Best~
Tony

[736cc]

Just replace the pilot jets w/ new ones (#40's are $10 a set), set the air screws to 1 1/4 turn out from bottoming, clean the inside of the bowls and you'll be most likely be ok.

[ekpent]

Don't mess with those screws you have highlighted.If you want to split and remove the carbs from the rack removed the 8 large screws from the back of the rack.Slide the carb bank out and then seperate the choke linkage arms by removing the cotter pins at the two locations and seperate .Unscrewing the large ring at the top of carb will allow you to pull the slide and needle out. Don't do more than you have to and chances are if your carbs are already pretty clean a total disassemble may not be warrented.All in all though it really is pretty simple operation either way you go.

[RSchaefer]

Removing the carbs from the rack w/large 8 Phillips screws and the choke linkage will not effect the sync.  Pulling them apart is the best way to work on them.  I took a scribe and marked each one with its # as you want to put them back in the same position.  You said something about the Needle Set Plate, not sure what that is.  You just unscrew the top collar and the slider w/needle comes out the top.  The choke plate for #1 is just stuck probably due to some twisting in the linkage.  Take apart the linkage from all four carbs and you should be able to move that stuck choke plate back.

[andy750]

"but I have had issues with getting her started since the weather has gotten cold."

Could it be the spark plugs are the issues and not the carbs? Why do you think its the carbs? Did the bike run well in summer? You could also try running the bike minus the external fuel filter.

[thedrewski86]

Everyone else here is right, you really don't have to break the bank apart to do what you need to do. You can without affecting the sync but it's not necessary. Soaking in carb cleaner is great but it may not be necessary. If the o-rings don't leak you really don't need to replace them (unless you have spares hanging around). Take some stranded copper wire to your jets if you don't replace them, that's a real easy way to clean them out.

Good luck, if you really want to take them apart it's a great way to learn! Carbs are really simple.

[lucky]

Just replace the pilot jets w/ new ones (#40's are $10 a set), set the air screws to 1 1/4 turn out from bottoming, clean the inside of the bowls and you'll be most likely be ok.

Don't you mean new float needles?
There would be no reason to replace a pilot jet which has no moving parts.

[736cc]

  If that #4 choke butterfly is stuck in closed position , it slipped off the arm inside the carb body and is a pita to re-install. You have to gently sneak it onto the retaining pin attached to the choke linkage. Carb top needs to come off unless you can poke in there w/ a skinny tool. Been there, done that on the earlier 4 cable carbs which have different carb tops and mechanisms. I am not exactly sure how your later carbs and the choke differ. Anybody?
  Pilots ALWAYS get clogged from dried-up fuel varnish if sitting improperly stored, or if never been replaced since 1973 or so. My experiance w/ these little buggers is if you go to the trouble of removing them, simply replace them, NOT clean them. You can soak them and poke holes w/ wires in the orifices and get them what you think is cleaner (but not really) and at $10 a set for new they'll be guaranteed 100% ready to go.
  New plugs ALWAYS make starts alot easier, too. Add some Startron in a fresh tank of gas, that stuff WORKS!
  Ahhh, new jets and spark plugs in an old CB750, such a wonderful thang.

[XLerate]

I've done an awful lot of carb cleaning on small engines and bikes, many hundreds and probably well over a thousand. I'd very strongly suggest that you don't use copper wire to clean orifices. It fatiques and breaks very easily. It can break off flush in a hole, then no way to grab it, plus the part where wire broke is all gnarled up, fatter, bent etc. Then you're stuck with trying to drill it out without doing major damage on your carb.

Over the years I've collected a set of various size hard wires I use for this or you can order a set of jet cleaning wires online, pretty cheap. Just be gentle when using them and don't ream out a jet or passageway, the aluminum is quite hard and won't easily get damaged. If you make up your own hard wires run the tips across some fine emery to smooth and slightly round them in case there's a jagged edge that could gouge the aluminum. If you do that it's pretty difficult to do any damage with them.

[Jinsonphoto]

Wow thanks for all of the info! I wish I would have just gotten on here first when I couldn't figure out how to undo the choke linkage. Since I unscrewes the wrong screw & caused the #4 choke butterfly to slip off the linkage I guess I have no choice now but to pull the whole thing apart. - I wish the manual would have just stated to pull the cotter pin off the choke linkage arm to remove it! I hope I will be able to get the choke butterfly back on the arm!

[Jinsonphoto]

"but I have had issues with getting her started since the weather has gotten cold."

Could it be the spark plugs are the issues and not the carbs? Why do you think its the carbs? Did the bike run well in summer? You could also try running the bike minus the external fuel filter.

Yes i tried changing the spark plugs. The reason i decided to pull & clean the carbs was because the petcock was leaking & when I put my finger under it I got this amber colored gas on my finger. So it seamed there was maybe rusty gas in the bottom of my tank.

Although I didnt try running the bike without the external fuel filter. Pardon my ignorance, but what wold this have shown me?

[Jinsonphoto]

Removing the carbs from the rack w/large 8 Phillips screws and the choke linkage will not effect the sync.  Pulling them apart is the best way to work on them.  I took a scribe and marked each one with its # as you want to put them back in the same position.  You said something about the Needle Set Plate, not sure what that is.  You just unscrew the top collar and the slider w/needle comes out the top.  The choke plate for #1 is just stuck probably due to some twisting in the linkage.  Take apart the linkage from all four carbs and you should be able to move that stuck choke plate back.

Wouldnt I have to remove the carb from the set plate to take off the top to get to the needle?

[Jinsonphoto]

One more question. Instead of buying new jets or cleaning them with wire. Couldn't I just soak the jets alone in carb cleaner & then blow them out with a compressor?

[XLerate]

One more question. Instead of buying new jets or cleaning them with wire. Couldn't I just soak the jets alone in carb cleaner & then blow them out with a compressor?

I never had any luck at all with soaking and compressed air. The problem over time is a shellac or similar buildup that needs mechanical manipulation to move it. Only thing that's ever worked for me is jet cleaning wires, used sensibly with plenty of carb cleaner spray like Berryman's etc. Pretty much the same as washing your hands after working on the bike. You don't set a bar of soap in the palm of your hand, run some water on your hands and call it good: friction/manipulation is needed.

[Jinsonphoto]

One more question. Instead of buying new jets or cleaning them with wire. Couldn't I just soak the jets alone in carb cleaner & then blow them out with a compressor?

I never had any luck at all with soaking and compressed air. The problem over time is a shellac or similar buildup that needs mechanical manipulation to move it. Only thing that's ever worked for me is jet cleaning wires, used sensibly with plenty of carb cleaner spray like Berryman's etc. Pretty much the same as washing your hands after working on the bike. You don't set a bar of soap in the palm of your hand, run some water on your hands and call it good: friction/manipulation is needed.

Do most part stores sell carb cleaning wires? Or are they a special order item. Is there anything else that works good to get down in the jets?

[andy750]

Although I didnt try running the bike without the external fuel filter. Pardon my ignorance, but what wold this have shown me?

It would have shown you if the fuel filter was blocked and causing a shortage of gas to the carbs. However this may have been expected to occur in all temperature ranges. But it could have been a progressive thing - your filter in the photo looks quite orange so maybe a cleaning of the fuel tank is also in order. At the very least change the fuel filter or clean it.

Btw what were your symptoms of not running correctly?

Good luck!

[Jinsonphoto]

Thanks!

& yeah when it was running it wasn't running good. If i pulled back the throttle quickly it would skip. But mostly It just got harder & harder to start until I couldn't get here to start. And like you said I started to see the amber color in the in-line fuel filter. Im putting a new tank on her anyways. Although I have to get the rust of out of the new tank first!

Im going to try the electrolysis method.

[RSchaefer]

OK looking through the micro fiche again I see that Honda calls, what I have been calling the rack, the set plate.  Basically the two large countersunk Phillips screws holding each carb to the set plate.  Yes you would have to removed the carbs from the set plate too unscrew the top collar and remove the slide and needle.

For cleaning the jets Hondaman recommended a pin vise which I picked up at the local arts & crafts store for $9.  The pin vise looks like say an ink pen and has an assortment of various size mini drills under its end cap.  You unscrew the tip of the pin vise, like a Jacobs chuck, and tighten in your desired drill.  I started with a small drill and worked my way up until I felt a little resistance as I worked it back and forth.  You can increase the size of these holes if desired.  Hondaman had suggested increasing the diameter of the 8 holes at the top of the emulsifier tub to around .037".  The openings at the top of the emulsifier were larger on the early Honda's and got smaller over time due to EPA restrictions.  You really only need the carb wire for the one air jet in the main throat that runs back to the needle valve seat.

[736cc]

S'more cold-start gremlins:
-Pertcocks passages get clogged, sometimes only 2 carbs get fuel; dissassemble and poke around and blow in it (tastes awful)
-Sticky float; hit/tap carb bowl w/ a hammer (it worked for me more than once)
-Bad coil; start bike in darkened garage and watch for fireworks arcing from petrified cracked plug wires/boots; (I've seen it)
-Clogged pilot jets (replace, don't clean, spend $9.95 + postage)
-Remove mouse nests and piles of acorns from airbox (maybe it was a squierrel?)
-Remove mouse nests from open HM300/341 pipes (preferably by suprise when it starts-up)
-Crummy stale fuel mixed w/ water/rust/varnish (keep a clean, spare tank around)
-Points not grounded (wiggle/tighten every possible connectio)

[Jinsonphoto]

OK looking through the micro fiche again I see that Honda calls, what I have been calling the rack, the set plate.  Basically the two large countersunk Phillips screws holding each carb to the set plate.  Yes you would have to removed the carbs from the set plate too unscrew the top collar and remove the slide and needle.

For cleaning the jets Hondaman recommended a pin vise which I picked up at the local arts & crafts store for $9.  The pin vise looks like say an ink pen and has an assortment of various size mini drills under its end cap.  You unscrew the tip of the pin vise, like a Jacobs chuck, and tighten in your desired drill.  I started with a small drill and worked my way up until I felt a little resistance as I worked it back and forth.  You can increase the size of these holes if desired.  Hondaman had suggested increasing the diameter of the 8 holes at the top of the emulsifier tub to around .037".  The openings at the top of the emulsifier were larger on the early Honda's and got smaller over time due to EPA restrictions.  You really only need the carb wire for the one air jet in the main throat that runs back to the needle valve seat.

Wow thanks for the great info! A question tho.. What exactly is the Hondaman you are referring to? I have the CB750 manual I downloaded here, but Im guessing thats not what your referring to?

[Jinsonphoto]

S'more cold-start gremlins:
-Pertcocks passages get clogged, sometimes only 2 carbs get fuel; dissassemble and poke around and blow in it (tastes awful)
-Sticky float; hit/tap carb bowl w/ a hammer (it worked for me more than once)
-Bad coil; start bike in darkened garage and watch for fireworks arcing from petrified cracked plug wires/boots; (I've seen it)
-Clogged pilot jets (replace, don't clean, spend $9.95 + postage)
-Remove mouse nests and piles of acorns from airbox (maybe it was a squierrel?)
-Remove mouse nests from open HM300/341 pipes (preferably by suprise when it starts-up)
-Crummy stale fuel mixed w/ water/rust/varnish (keep a clean, spare tank around)
-Points not grounded (wiggle/tighten every possible connectio)

736cc Thanks for taking the time to share this with me! This will great to refer back to. In fact i already know now that i have a float sticking & my petcock has been leaking so maybe its clogging in passages as well.

Either way with all of this great info Im bound to get to the bottom off it! Ill be sure to report back if i get into trouble.

[RSchaefer]

Dude, Hondaman is the Man!!  Hondaman worked in a Honda shop circa late '60's to mid/late '70's and documented every change/issue w/CB750.  He sells a book which IMHO is a mandatory accessory to any CB750 rebuild/restoration.  I think he offers a discount to forum members.  His name is Mark Paris and he lives in the Denver area.  He also has a web site and offers some rebuilds, he did my swingarm, electronic ignitions, updated fuse boxes, etc.  I think if you do a search on Hondaman and his book in particular you will find a thousand threads.

[Jinsonphoto]

Dude, Hondaman is the Man!!  Hondaman worked in a Honda shop circa late '60's to mid/late '70's and documented every change/issue w/CB750.  He sells a book which IMHO is a mandatory accessory to any CB750 rebuild/restoration.  I think he offers a discount to forum members.  His name is Mark Paris and he lives in the Denver area.  He also has a web site and offers some rebuilds, he did my swingarm, electronic ignitions, updated fuse boxes, etc.  I think if you do a search on Hondaman and his book in particular you will find a thousand threads.

Thanks for the info! I guess I better get the book then. Im at the beginning of going over my cb750. And since I have no experience this might be just what I need. As I said earlier, the shop manual is pretty vague at times. It tells you what to do, but not always how!

[Stev-o]

Hondamans website and book.  The book can also be found on lulu.com and frequent discounts are offered.


http://sohc4shop.com/catalog/product_info.php?products_id=30

[Jinsonphoto]

I have one more question this a bit off topic, but still has to do with this subject. I'm going to do electrolysis on my new tank & I'm curious after i get all the rust removed do I need to coat the tank with anything or just get it filled with gas asap?

[Stev-o]

After derusting, you could coat with 2 stoke oil if you do not plan to use.  Or yes, fill with fuel, and keep it topped off as much as possible. 

[736cc]

An ounce or 2 of Startron mixed in a full tank will keep your fuel systems happy all winter. Equal amounts of 2 stroke oil added insurance against rust

[RSchaefer]

Interesting question and many have differing opinions.  I think in general the opinion would be to do a tank liner (coating, i.e Dragons Blood) after electrolysis/etc. as you will have removed the factory protective layer on the steel.  Others would recommend immediately coating the inside of the tank after electrolysis with 2-cycle oil to prevent almost immediate rust.

I'm thinking that if you had rust and had to remove with electrolysis, etc. then you should be considering a tank liner.

[Jinsonphoto]

Okay, I know that this thread is old, but I finally have gotten the carbs cleaned and put back on the bike & new fuel in the cleaned tank, everything is as it should be I think. So I tried to start it up for the first time and I can't get it to start? It seems like fuel is not getting to the back side number four carb? That fuel line coming from 1&2 to 3&4 has no fuel at it and when I tried to drain the the #4 carb there is no fuel in it? Any ideas what the problem could be?

I haven't had a chance to check the spark plugs but I just replaced them before I did this carb cleaning.

Any thoughts or advice would be greatly appreciated. Thank you in advance your time!

[andy750]

Option #1
Assuming you have fuel going to the other carbs then Id say its likely the float valve sticking and not letting the fuel through. Pop off the bowl and press the float up...fuel will hopefully come out - this indicates your float valve is sticking.

If so remove float pin, remove float, catch float valve as it drops...use a Q-tip to clean out the hole and then reinstall in reverse order...should be good to go.

Option 2: no fuel getting into the fuel line suggest you have a blockage coming from the tank - unscrew the fuel petcock bowl and inspect the filter, clean if necessary and reinstall. Check fuel flow with fuel lines off the tank.

Good luck
Andy

[Jinsonphoto]

Andy -
Well I had already tried most of option #1 although I didnt remove the float. No fuel came out when I press the float up. I suspect fuel never made it that far. After doing some research I think that i have found that the lines all have to be going down hill. I also found out that there are holes are two holes in the stay plate that i can run my fuel lines through. So Im going to shorten the lines so they are going downhill & through the stay plates. I will also inspect the petcock.

Hopefully that will get fuel into the carbs so she can start up!

[andy750]

Good luck and let us know how it works out!

Andy