Hello every one.

[hondacb750f]

 I currently have two bikes my daily rider which is a 86 Yamaha radian and a new to me 1981 cb750f ss
with kei hin round top carbs.

 My plan for the Honda is to bring it back to operation,  it runs ok but I do need to do some carb work. Then I am planning on changing exhausts to  deep and loud, as well as raising the bars.


When I got the bike it had the pods. I took the necessity time and found the original air box. I was told by the man I got it from all he did was take off the float bowls and cleaned them real good. and then replaced them.   Only mechanics experience I have is the synch job I did to the radian.

I hooked up the air gauges to the Honda and found 3 and 4 dead on.  while two was the lowest. I am assuming this means it need more vacuum.  I did rev it several times and 3,4 stayed together whiled 2 went down and one went up.

what inspired me to do this was after running  maybe five mins the 1,2 jugs where so incredibly hot I mean crazy hot. while the other two I could almost still touch.

there was some mild popping on the decal from the 3,4 exhaust and carb bowls 123 were leaking from the top of the bowl.  Now  this maybe my fault. the Honda gas tank has  several pin holes in it so I hooked my radian tank to it. having done this I had to leave the tank on prime because it is a vacuum petcock.  which means the gas was continuing until changed to reserve I did change it but to me I did not see a difference. but by this time it was all wet.

one last issue. the man told me he did change the crush gaskets to the exhaust. a friend suggested I temp them and I did. I noticed a 70 degree difference between them.  is this because they are not tight or could it be linked to the jugs getting so hot so fast?  

avid motorcycle rider for a while now. I just want to start doing some to most of my work so I can become more experienced. I have started learning the concepts of the combustible engine. at this time I am focusing on the carbs,  then I will spend time learning the difference between the two stoke and four.

maybe simple. but I am just now putting forth the effort to learn. I would like to learn quite a bit. from identifying the issues: to timing, to values, to carbs, charging system and whatever I may not have mentioned. I know I can not learn every thing about every thing. but truth be told I do not even know most principles.   Part of the reason for getting the Honda was to have a hands on experience a 400 dollar bike fits my budget better than school does.

long winded I know and apologies for it.  but thanks for all the stuff here I have read and wwill continue to read. and please feel free to suggest stuff, cause I may not know the right questions to ask.  I will post a youtube link after a lil while.  

thankyou

[LukeM]

Hi, and welcome to the forum.

Step 1: feel free to download the Factory Service Manual (FSM) at the home page of our forum. It's an excellent reference for the bike.

Regarding the carbs: They can't be too clean internally.  There are lots of little passages that need to be cleaned, and then cleaned again. Don't forget the jets. There are very small holes that can be cleaned with a .008 or .009 thousands guitar string.

Here's a link to info on CARB SYNCHRONIZATION. Basically, the idea is this:  Carb #2 is the reference.  Balance #3 and #4 (which sounds like they're already balanced).  Then balance #1 against #2.  Then balance the #1/#2 pair against #3/#4. You may have to adjust the throttle down when your carbs get closer to balanced.  It'll smooth out nicely as well.

I hope this helps. Keep us in the loop, and we'll help you if we can.  There are a LOT of guys working on these bikes.

Luke M

[Lucky 1]

Good answer to a complex question LukeM.

[shinyribs]

Welcome to the forum!

If you have a way to post pics of your bike I'd be interested in seeing a set of roundtop carbs mounted to an '81 model bike.

Your carbs may not actually be what is commonly referred to as round tops. Here are the three types of carbs found on these bikes:

Round tops: 69-76 SOHC

Early

Late


Keyhole: 77-78 SOHC


CV carbs (what should be on your bike): 79-83 DOHC

[hondacb750f]

thank you every one for the warm welcome and the reply's.

shiny ribs you are right the cv carbs are what I have. I will get with my girl and have her figure out how to load pics.  as far as computers go. I can find web sites and post but that about it. As far as technology goes I try to stay away from it  I am a bit  of a simpleton and currently still own a phone from 05.

My mom tells me I born 30 years to late. I still believe in hand shakes. looking some one in the eye, and  not living above my means.

but we will get some picks up.


thanks again.

[hondacb750f]

I can only assume after all the reading I have done here that, that hose in the pictures is a vent hose.
well I pulled the carbs today, cleaning the jets. then when I get half way thru. it downs on me that one of these are a bit small/short so I checked the whole. well two my amaze ment there were what appears to be another jet lodge in there. As a guess man I think the small one is the main and the inner one is the secondary?

I have not checked the other two at this point because the two I did check have been striped so I can not get them out, but they are clean.  two clarify which ones I am talking about, there are two jets, one sits higher than the other,  I am talking about the one that is lower, or closer to the carb body, the taller one I know has two and they both came out of all of them.

while waiting for my friend to get home so I can use his tool to measure the 15.5 cm for the float I am going to check the other two.

here's the kicker. the two I did notice are carbs one and two. and I remember them being out of synch, I wonder ..... what if I can not sync them. because that is tonight's goal.  

over all there was a small amount of gook in number three, however as a whole, everything was pretty much clean.    on the side of the small/short jets they read 68, and on the top they read 125. I am not sure how to intrepid  this?  
if I can not sync the carbs any idea on how to get them out.? I do have a small die set or will this hurt some thing?

[hondacb750f]

ok just checked three and four. they both came right out and were as clean as the rest of every thing. starting to think the guy that sold it to me, might have known this....hmmm...

[hondacb750f]

ok I got one and two out, I be leave  with no issue's, this is because I put three and four in those spots. so tomorrow I will get/order two new ones.... any body know what they are called?

[shinyribs]

The jets you asked about,the one marked 68 and the other 125, that 68 will be your pilot jet and the 125 will be your main. The number is the size of the jet. Jetting will not affect your synch whatsoever. That is a separate adjustment altogether.

Your #2 carb is non-adjustable as far as synching is concerned. Whatever vacuum reading it produces on your synch gauge just adjust the other 3 carbs to match.

Here are couple links you'll want to check out. Looots of info concerning your carbs in here. There are a lot of things that you can cut corners on,but in these bikes,carbs is not one of those areas. They are extremely important to get right. Clean,clean,clean and synched! But dont let it scare you. It's actually a really easy job once you know what you're working with. And with a good,quality inline fuel filter in place ,once you get them right, they will hold up well and works reliably for years and years.

Links:

http://honda-cb750-s.456789.n3.nabble.com/Cleaning-and-adjusting-a-carberaters-best-videos-for-dummies-This-ought-to-be-pinned-at-the-top-td3905817.html

http://honda-cb750-s.456789.n3.nabble.com/DOHC-carb-cleaning-procedure-td867809.html

[seestheday]

Shiney, you have me second guessing myself, but isn't the stock size for the main jet 105?

Here are mine.  I have a 75 instead of a 68 (I think I'll switch it back to 68 or go to 70 though).

[icerigger]

Stock jets are 68 and 102.

Take as much time cleaning as you need, don't rush it especially om the idle circuit and low speed jets. Use an old guitar string .008/9 to clear the idle jet. If you're more mechanically inclined and have access to a drill press and taps you may want to remove the press in low jets and clean the emulsifier holes as well.

We can't tell you too many times RTFSM. The Factory Service Manual is available for download on the main page here. There's also a really good carb manual here

http://www.cb750c.com/publicdocs/SeanG/Honda_Carb_Manual_revF.pdf

The CV carbs on these things can't be too clean.

[hondacb750f]

I did try to get the service manual from here, it would not download but it did open.   I am waiting on the parts, needle holders to come in.   they are obsolete but I found a junk yard that's going to send them, while I am waiting any good advice on repairing several pin holes in the tank?  

  I do know to get a wire brush and bring it all back to medal. but I don't really know what else?   also has any one heard about the tank treatment called Kreme?


thank you again every one!!!!

[TOOLS1]

I myself do not trust tank liners. I have seen too many fail, and then they case even more problems with the carbs plugging up. If you just have some pin holes fill them with JB Weld. I prefer to use the putty rather then the liquid when doing this.
TOOLS

[hondacb750f]

is jb weld gas restraint?

[TOOLS1]

Yes it is!
TOOLS

[Truck]

I was talking to a mech. over the weekend and the subject of the tank liners came up. Basicly with the higher ethenol that is now in our gas it is eating away at the liners. He recommends useing the etching treatment the keeping a full tank of gas.

[LukeM]

I had the same thing happen to my old gas tank. There was rust under the liner, so it kept flaking off and gumming up the rest of the system. I replaced the tank with a NOS unit, and keep it as full as possible with some SeaFoam (maybe 1-2 ounces per 3 gallons of fuel). So far, no rust or issues.

Luke M

[hondacb750f]

gonna get to sanding it down over the next week or two.  gona jet some jb weld?


and look into etching liner for tanks, thank you every one.

[LukeM]

There are a couple of threads on our forum where a Zinc plated bolt was used to "galvanize" the inside of the tank.  It involved a 1-2amp battery charger, some lye and water, some bolts, and some time.

Luke M

[shinyribs]

If it gives you any peace of mind I have a very poorly executed JB Weld repair on one of my tanks and it's still holding strong 3 years later. I jabbed it in there in a hurry while the gas was actually still seeping out. Right over top of rust and all. Somehow...I forgot all about it and painted over it and all.

I also have one float bowl overflow tube, a float bowl drain screw that was stripped and a carb body jet holder that was cracked...all three got fixed with JB Weld and are still doing fine. (I didn't realize how much I use the stuff!)

If it holds that well when you're not even trying just imagine how good it would hold up if you actually prepped the area!