New 750f2 owner, member intro & few ?'s.... Carb issues for now

[Matt Story]

Hi, my name is Matt.  I'm a brand new member here.  I'm in Flint Michigan.  Great site.  Already found some useful tips before joining up.  I just picked up a 77 CB750F with about 8000 miles on it.  $700 was the sale price.  Apparently it was stored indoors.  Its pretty clean with little rust, and couple tank dings.  Side covers are wasted.  The PO started to rebuilt the master cylinders and stopped half way.  I have owned a 350four and 550four, and know these are great bikes once sorted.  Was looking for a while waiting for a good or fair deal on a CB750 to come along when I found this one on CL.  It won't currently run well, but I'm making progress.  Carb problems are the primary issue.  The PO also claimed to have had the carbs rebuilt, but 'the shop' didn't do a sync job like they were supposed to.

On a side note, I am also currently neck deep in restoring a 75 Moto-Guzzi 850T.  I picked this bike up partly to have something to ride while I finish the Guzzi, knowing that it should be simple and quick to get the CB road ready, and not have to spend much money on it.

First things first.  I set the point gaps and did a static timing.  Getting spark on all four.  I will look into the valves once I make some headway with the carbs.

As received, the carbs were leaking out the overflow outlets and one thru the intake opening.  My brother and I took the carbs off and gave them a cleaning.  They were fairly clean, but not spotless.  We found that the #3 float was installed upside down.  The Accelerator pump diaphram is wasted and a new one will be ordered.  Thanks to Re-run, we learned that the apparently non-removable slow speed jets are removable.  We took them out and found 1 & 4 were partially blocked, with 2 & 3 fully blocked.  We got those cleaned up nicely with copper wire and cleaner.  We checked that the floats were equally buoyant, and their heights are in spec..

After all that we put the carbs back on, and now it will run with the choke off (would barely run with full choke before) but won't idle.  All four pipes are getting hot now.  Now that the slow speed jets are clean, three if not four of the carbs are overflowing thru the overflow tubes and/or the air intakes.

My first battle is to stop the overflow.  My needles have very slight detents in the rubber where they meet the seats.  They are quite pliable.  Since the seats aren't removable, (or can they be removed like the slow speed jets?) I didn't clean them as well as they should be.  My next task is to clean the needle seats and/or polish them.  I see some other sites recommend using a hardwood dowel to polish the sealing edge.  What does the membership here recommend for cleaning/polishing the seats.

Once I get the needle valves closing up, how high should the gas level be when checked with a clear hose?

I will be placing an order for needles, accel pump kit, if not rebuild kits.  With everything cleaned up, I think the kits are probably overkill.  What brands or distributors (carb parts) would be recommended or which ones should be avoided?  I ask because I saw somewhere there is at least one brand that should be avoided.  The web stores I have found so far don't specify the brand they are selling.

I am going to also buy rebuild kits for the MC's, new bake line sets, the factory tires need to be replaced.  Going to go down the periodic maintenance schedule and hit the whole list.  I'd like to order as many parts as possible at once to save n shipping.

Any comments or suggestions are greatly appreciated.

Thanks,

Matt

[TOOLS1]

Welcome. I usually just polish the needle seats with a Q-tip, and chrome polish. As, for setting the float height, I just set them level with my eye. When, you do your accelerator pump, don't forget to make sure the two check valves are working. As, for kits, I hardly ever use a whole kit. I just use new float bowel gaskets (I recommend the ones Z1 Enterprises sells, as they are molded to fit the bowel.)   I also replace the O-rings on the fuel tubes. They are 10mm X 2mm and I get them at a locale auto parts store.
TOOLS
P.S. I just put a set of F model side covers on ebay. I will take $75.00 for the set shipped.

[pat english]

welcome to the site..lets see some pics of the bikes...i didnt know you could put a float upside down..tools said it all..if youre cleaning all the jets,you just need a bowl gasket and the diafram delio..ive herd you can use rubber o rings as a bowl gasket..im going to the hardware store this weekend..im guna see if i can find some that work..vintage cb750 is a pretty good spot..i think ive only orderd a wire harness from them,but shinys orderd a bunch of stuff and usually reckermends  them..

[Matt Story]

Thanks Tools. I looked for your side covers,but I'm not sure which ones.  Can you send me a link?

[Re-run]

You can drop Tools an email and he can send you some pics if you want. Just click his name and you go to his profile and you can email from there.

Like Tools, I use a qtip as well.I'll usually wet the seat down with carb cleaner and then stick the qtip and and twist back and forth to get crap off. A wooden dowel would work I suppose but the issue would be getting the correct point on it to be sure you are getting the whole surface.

You might look into replacing the needles too. Get good rubber tipped ones again. I got some cheaper ones and then are OK but never fully seal. I can get a few drops of gas on the ground after a few hours. I never had that with the old honda needles.

With the accel pump, make sure that diaphragm has both ears opened up. Should be a hole in each one for proper function. Some only come with 1 ear open.

Do NOT get carb kits with round 0-rings for the float bowl. Those SUCK! I got those once, ONCE and never again. You have to glue down the o-ring to keep it in place. Screw that, get the formed ones and save yourself a headache any time you remove the bowls.

And finally, how far have you turned in the idle screw? OH OH OH!!! did you set the Fast Idle Cam?
When the choke is pulled, it opens the throttle a bit to increase rpm while warming up. If this is not adjusted, you need to keep a hand on the throttle till the engine warms up enough to stay running on its own.

And, float height is measured right from the side, at the middle of the float and should be 14-15mm. if I remember correctly, the clear tube test should put the level just below the top of the float bowl.

[TOOLS1]

I would be willing to sell them direct to you, but here are the auction links.
http://www.ebay.com/itm/300758202944?ssPageName=STRK:MESELX:IT&_trksid=p3984.m1555.l2649
http://www.ebay.com/itm/300758204999?ssPageName=STRK:MESELX:IT&_trksid=p3984.m1555.l2649
TOOLS

[Matt Story]

I have been pretty busy lately, but have had some time to mess with the bike.  I have been doing some inspections and of the Master cylinders that a PO tool off the bike.  I'm mostly set on the carb parts, except for the acellerator pump which seems to be a bit harder to get.  At least the online discount sites (vintageCB750, CB750supply, PartsNGo) are out of stock  They all seam to list somewhere around $18, but none in stock.  Is there a similar assembly with a longer rod that could be cut?  I see you can cut the stock part for my bike to make one for some other bikes.

Heres a pic of the bike.



Matt

[Piute]

          Beautiful bike almost as nice as mine
Welcome Story ,Those diaphragms are from 35.00 to 65.bucks so try to be patient and get the less priced one,cb750 (think was)sent me the wrong one at 1st but did make it right by sending correct 1 right away before I returned wrong item ,So there my go to guys most the time.
   Theres enough carb post on here to get lost for to long but all have great info,I learned to check Carb outside the bike,rather then putting in N out ,,I'd say about 5-6 times ,pump that accl. outside bike also should shoot across the room.That 1 "o" ring was my biggest prob.BE patient may not work the 1st couple time ,had to mod mine but been working great,no leaks,.
   I got tired of trying to set them dang floats so there eye-balled at just below specs I'd say.
I used a tech-tip from TOOLs (use a drill bit to start . set / sliders) I actually used an 1/8" Allen wrench,Bought the sync gauge set and haven't even used,
                               

TO BUSY RIDING TO STOP NOW

[Matt Story]

I haven't ordered any carb parts because I need some other stuff and I want to minimize the number of orders.  vintagecb750 offers free shipping for orders over $150.  I'm thinking I'll probably have to do that twice.

Other than carb work, the brakes need to be gone thru.  A PO had taken the master cylinders off.  I managed to get both front and rear fully apart.  I was able to press the front assy apart thru the hose connection.  It didn't come aprt easy, but after apart and cleaned up, things look really good except for the wasted boot.  The clymer pics show more parts than what I came out of mine.    It looks like I might be missing a piece at the bottom of the bore that Clymer calls a check valve.  A separate rebuild kit is available for 77 and 78 750f bikes, and they are a bit different.  The 78 version  includes an extra piece that may be a check valve.  The 77 kit does not.  Can anyone offer comment? is the check valve version better?  Is is compatible?

The rear was not so easy.  The piston was stuck in crud.  I tried to use air pressure, but it refused.  It would move in, not out.  I let it rest for a while to think it over.  I got an idea to hydraulically push it out.  I didn't have any handy way to connect it to another M/C.  What I came up with was to fill it with grease, plug the extra hole and push a 'piston' into the the hose connection.  I didn't get that far.  I very loosely cross threaded my grease gun fitting into the hose port.  The hose popped out once, and I snugged it a tiny bit more.  I pumped the cavity full.  Before I knew it the piston popped out onto the floor.  Once apart and clean, everything looks pretty good except that clevis link is fairly corroded.  Below is a pic of it disassembled.  I have a couple points to consider looking at the parts.



All the pieces look good except the spherical plunger that connects to the pedal.  The ballstud is pretty corroded, and I'm not sure about the washer that is captive on the stud.  It is pretty rusted up.  Its hard to tell  from the pic, but it is also dished out.  I can't tell if it is that way by design, or because some ham-fisted mechanic deformed it trying to pull it without removing the snap ring.  Is it supposed to be dished?  The ball stud and washer don't come in the M/C rebuild kit for a 77 750f.  The Honda kit for a 75-76 750f includes the ball stud.  Will  the earlier kit fit a 77 model?  Here is a pic of the early kit:



Strangely, the early kit includes what looks like the check valve in the later ('78) Front M/C rebuild kit.  I am curious why Honda would remove the check valve from the rear assy after '76, and add the check valve to the front assy after '77.  Am I seeing it correctly?

Here is a pic of the kit desiganted for 77 750F



What do you guys think?

I am headed out to the garage to start my Guzzi tank rust removal using electrolysis.  I let you know how it goes.  I expect well.

Sorry for the pic sizes, and thanks for your time.

Matt

[Piute]

  You got the right idea shop around and be sure of the lowest price brake piston are up to 80bucks at times .
   I like to abuse Bike Bandit (I don't buy anything) But have great OEM pic's
http://www.bikebandit.com/houseofmotorcycles/1977-honda-750-super-sport-cb750f/o/m151839#sch405622
  This I always have in favorites to go back in forth  ..
  Wished I could help you out but haven't dug that deep into my brakes ,did change front piston /all seals though,GOOD LUCK you seemed very competent ,

[Matt Story]

Bit of an update.  Over the last couple days I finally got to spend a few hours on the bike.  It now has a really good initial tune on it.  New needles and float adjustment really did the trick.  Unfortunately, the needle seats on this bike aren't removable, and the #2 and #3 seats were corroded pretty badly.  They needed a good lap and polish to seal.  I used small bits of adhesive backed fine grit sand paper stuck to the end of a wood dowel and just twisted and twisted until the shoulder was polished smooth and flat.  Probably only took off a few thou.  Then I took another dowel and ground a point on it and twisted that around to polish the sealing corners clean.

I was worried that I might not get a good seal with the new needles and the reworked seats, so I foolishly set the floats about 2mm low to get a stronger seal.  Fortunately, they did seal up.  Set-up like this was the best it had run for me yet, but still not good.  I rechecked the timing, set the valves, and did a quick carb balance.  That was better but still not it.

I took the carbs back out and carefully reset the floats in the middle of where they're supposed to be.  Put them back in, and re-synced again. WOW!  Now it will idle down to about 850rpm once warm.

I was pleased to see that when I checked my static time job with a timing light, it was really close - at idle and at full advance.  It close enough that I'm not going to bother readjusting with the strobe light.

A couple years ago I bought a hand held electronic tachometer.  I used that to check the accuracy of the bikes tach.  It looks like the bikes gage runs almost 200rpm faster than the pocket gage.  Tomorrow I'm going to use the hand held tach to check and set the idle mixture screws.  Will adjusting the idle mixture screws have any effect on the carb balance??

Above, I asked if the 75-76 rear master cylinder kit would work on a '77.   The earlier kit comes with the spherical ball stud where the later one doesn't.  I kin d of rolled the dice and bought the early kit because my ball stud was rusted up.  Now I have it, and everything looks compatible.

After a little more carb tuning, all I have to do is rebuild the the brakes and the bike will be on the road!  Just a few more hours of work and I'll be riding.

[Hoko]

Re-run wrote

 

And finally, how far have you turned in the idle screw? OH OH OH!!! did you set the Fast Idle Cam?
When the choke is pulled, it opens the throttle a bit to increase rpm while warming up. If this is not adjusted, you need to keep a hand on the throttle till the engine warms up enough to stay running on its own.

And, float height is measured right from the side, at the middle of the float and should be 14-15mm. if I remember correctly, the clear tube test should put the level just below the top of the float bowl.

What is the clear tube test?

[Lucky 1]

Set the float levels as described in the Clymers manual.

Use a ruler or float gauge or calipers.

The "clear tube thing"  is only AFTER you have set the float levels using the right tools.
Page 125 bottom of page Illustration #13  Float level CB750F 1977 14.5mm


READ THAT AGAIN PLEASE!!!

Also change the float needles and seats. They probably have not been replaced for years.
It is one of the MOST important parts of the carb.