Topic: brake and carb problems, I'm starting to regret it...

[rbmgf7]

Ok, so while trying to restore my 750F, I went about replacing the entire front brake system: the dual disc MC from Parts N More, Hel S/S lines, Speed bleeders, and rebuilt calipers with phenolic pistons. With that much $ invested, I'd hope that everything would work out fine. Well, nearing day 3, I still can't get any tension in the brake lever, It's all limp and barely makes contact with the pads (this is my first CB so I don't know how it's suppose to feel but compared to all my other brake levers on my other bike and atv, the CB is no where close). I can depress the lever all the way to the handle. If I take the MC off, I can go even further before I begin to get pressure. I've been patiently pumping the system and it's to the point I haven't seen an air bubble for a while. I've followed the FAQ instructions but no dice. I've searched the threads and came upon posts that were reasonably close to my situation but the other's were still using stock components. I'm just wondering if this after market stuff may have something to do with it. This restore is already driving me up a wall and I just need to resolve this so I can make another post about another problem 

Well heck, this is the other problem I'm having. It's my carbs. They're leaking from every hole possible. I've searched this one too but after going through each problem, they're still leaking.

The carbs are off of a 75K. I cleaned them twice and rebuilt them with new parts. I set the petcock on ON instead of reserve to eliminate crud entering the carbs. I've adjusted the floats numerous times. Here's one thing that may be the problem but I don't understand why it wouldn't work. The rebuilt kit came with the metal float needles. I installed those originally. They leaked. Had a spare 78F carb with rubber needles. Swapped them out (caught myself the first time. Had to remove the clips on the needles thinking this would fix it all). Nope, still leaks. Made sure the fuel lines ran above the carbs and didn't drop below. Nope. I'm just completely stumped. The absolute only way, but doesn't remedy the problem, is to just slightly open the petcock so the amount of fuel consumed is about what's being delivered.

~~~\o/~~~    ...help 

Thanks guys

[mkramer1121]

Did you "bleed" the master upon install?  Disconnect the line, place your finger over the hole (cap off of the M/C), and pump, release, watch the air bubbles come out the whole in the reservoir, repeat until you spit liquid out where its supposed to come from.

Also, do the carbs leak when running or just when off and sitting?

[rbmgf7]

Did you "bleed" the master upon install?  Disconnect the line, place your finger over the hole (cap off of the M/C), and pump, release, watch the air bubbles come out the whole in the reservoir, repeat until you spit liquid out where its supposed to come from.

hmm, didn't know to do that. i'll give it a shot tomorrow

Also, do the carbs leak when running or just when off and sitting?

both. I'll turn the gas on before starting the bike and before i can crank it, they're leaking. Whenever I ran the bike with the gas on, they've leaked. i can only counter the problem by giving the bike throttle and backing off the petcock. i've also tapped the carbs with a mallet to try and get the floats moving but doesn't work. i did inspect the brass overflow tubes in the bowls for cracks, all were good.

thing is, i have two 78F carbs ready to go but i can't fit them onto the 75 rubber ducts. that's another $80. jeez, it's getting to the point: my bike or tuition for another semester, lol.

[mkramer1121]

Yup, the bleed the M/C thing got me too the first time.  That and installing one of the seals in the wrong spot during the M/C rebuild.

Make sure your floats aren't hanging up on the of the bowls.  Slightly bend them inwards to make sure they clear.  Also, make sure your floats aren't cracked. 

If you can, drop the bowl on one of the carbs while on the bike (gas off).  Turn the gas on, and raise the float until fuel stops flowing.  Then, if it does stop, adjust your float height.

On my 550, even though the float valves are new, and the floats are set at the correct height, they will still leak if the gas is left on overnight. 

[Patrick]

Gearbot, when you attached the new brake lines, did you make sure the holes in all the banjo bolts lined up with the brake lines?

Patrick

[droopy]

Yup, the bleed the M/C thing got me too the first time.  That and installing one of the seals in the wrong spot during the M/C rebuild.

Make sure your floats aren't hanging up on the of the bowls.  Slightly bend them inwards to make sure they clear.  Also, make sure your floats aren't cracked. 

If you can, drop the bowl on one of the carbs while on the bike (gas off).  Turn the gas on, and raise the float until fuel stops flowing.  Then, if it does stop, adjust your float height.

On my 550, even though the float valves are new, and the floats are set at the correct height, they will still leak if the gas is left on overnight. 

I'll second the float height sounds like the float frame is bent
had same problem had to set all four with drop the bowl turn gas on set float by eye. problem fixed  d@mn P.O.'s

[Jay B]

Gearbot, when you attached the new brake lines, did you make sure the holes in all the banjo bolts lined up with the brake lines?

Patrick

That shouldn't make a difference, the banjo is hollow all the way around the bolt to allow fluid to access the line no matter what position.

[UnCrash]

Another bleeding trick is to turn the handlebars so that the MC is at its highest point. 

Then clamp the brake ON.  I did this with a bungee.  Leave it overnight.

This trick got the last of the air out of my lines and the brakes felt great after.

Don't give up!!!

[bender01]

can you tell if its just one carb or all leaking?. I had a stickicng float no matter how clean so i sanded all the float pins so theyll never stick. That solved that problem. Of course they never stick when theyre off the bike.Hope that helps.
 Ben

[hymodyne]

I discovered that a float bowl gasket had been crimped off outside its channel on my carbs, causing a small leak down one side of the offending float bowl. Where is the gas leaking from exactly? from the air intakes? the feed lines, overflow lines? I don't know the 750 carbs, but I'd check passageways in the emulsion tubes and other super small orifices for blockages.
I used a fine stiff wire from a new wire brush to probe and clean out those types of openings on my carbs when I have had leakage issues.

hym

[doobiebro]

Gearbot, when you attached the new brake lines, did you make sure the holes in all the banjo bolts lined up with the brake lines?

Patrick

That shouldn't make a difference, the banjo is hollow all the way around the bolt to allow fluid to access the line no matter what position.

I think what Patrick was eluding to is that some banjo bolts are made to work on specific brake lines due to the thickness of the brake line banjo.  For instance, either Galfer or Speigler lines are of a certain thickness that only the same manufacturer banjo bolts will work.  As an example, if the brake line banjo hollow groove is perhaps 2mm from the edge, but the bolt hole is 4mm from the head of the bolt, the two may not line up for the fluid to pass sufficiently.

[scondon]

 I've bled dry front brakes a few times on my '78's. The first time I had much the same experience where I could not get pressure at the lever even after flushing 1/2 bottle of brake fluid through. I've since adopted the method of bleeding each line from the top down starting with the master cylinder as others have suggested.

     At the "T" splitter I put a solid bolt in one side to block it and attach one of the longer lines leading down towards a caliper and I pre-fill each caliper through the banjo bolt opening.I then attach the short line to the MC and squeeze the free end between fingertips and bleed the line. Once bled, quickly attach it to the "T" splitter and continue bleeding the lower line in same fashion, then quickly attach line to caliper. Remove plug bolt from "T" and do last line. Bleed system as one normally would, then centerstand the bike and ziptie the brake lever and let sit overnight. Brakes are nice and firm.

   I know my method allows air into the lines but it seems that having a large volume of fluid throughout the system to begin with really helps.


   How fuel enters the carbs is a pretty simple system once you've gone through them a bunch of times. If your carbs are overflowing it's either the float height or the needles/seats. Any tiny bit of gunk on the seats will prevent the needles from seating. The new metal needles in rebuild kits are hit or miss, I usually have an extra set on hand to swap out until I find four that work. Used needles develop wear points, rubber tipped or not, so swapping in used needles from another bank of carbs may not be the answer. What height did you set your floats at? and can you confirm fuel level in each bowl?

[USN20]

Well heck, this is the other problem I'm having. It's my carbs. They're leaking from every hole possible. I've searched this one too but after going through each problem, they're still leaking.

The carbs are off of a 75K. I cleaned them twice and rebuilt them with new parts. I set the petcock on ON instead of reserve to eliminate crud entering the carbs. I've adjusted the floats numerous times. Here's one thing that may be the problem but I don't understand why it wouldn't work. The rebuilt kit came with the metal float needles. I installed those originally. They leaked. Had a spare 78F carb with rubber needles. Swapped them out (caught myself the first time. Had to remove the clips on the needles thinking this would fix it all). Nope, still leaks. Made sure the fuel lines ran above the carbs and didn't drop below. Nope. I'm just completely stumped. The absolute only way, but doesn't remedy the problem, is to just slightly open the petcock so the amount of fuel consumed is about what's being delivered.

This may help as it's often overlooked during carb rebuilds - carb mounting posts. That is the INSIDE area of the carb mounting post - where the float pin & float attach to the carb body. Sometimes they will have wear marks present from the friction of the floats up and down movement. Check the posts for any casting roughness present. Either condition can prevent the float from reaching it's full range of intended travel. My 750K4 had this float sticking problem causing high/low fuel bowl levels.

Take a piece of fine emery cloth and carefully smooth (gently by hand) the INNER post hole pin area. It won't take much to accomplish and check with a magnifying glass. The idea is to dress up the post smooth without removing any more metal than necessary. Do this to all four carbs - a total of eight mounting posts. Inspect the brass metal edges of the float itself where it will contact the freshly polished carb post, if it feels sharp or rough you want to smooth those as well and don't forget to check the pins.   

A tip I learned from Hondaman       

[rbmgf7]

i wanted to check the MC out by connecting it to one caliper. It works. Can only press the lever about halfway to the bars. so at least the MC isn't faulty.

reconncted the entire system back together and followed through with the FAQs guide to bleeding. started bleeding around 10am and worked to 2pm. i'm going to try the "leave the lever pressed overnight" approach and see what happens.

i skipped the carbs today. just want to take a break from the bike. running short on money for the bike. i still need to buy xmas presents so the bike may have to wait even longer (unless i get more money for xmas  )

[fuzzybutt]

make sure the folat pivot pin isnt bent either, that one bit me in the ass last year

[rbmgf7]

hmm, that's crazy. leaving the lever pressed overnight sure has firmed it up. i can still squeez it down to the handlebars but it's a lot better than the day before. would leaving it squeezed longer be better? what about trying to bleed it some more?

[mgmuellner]

Do you understand the concept of bleeding brakes?  It's odd that if you did it would take days to get the air out. 

1) Open resevoir, fill with brake fluid.
2) put pressure on brake lever.
3) Open spigot on caliper, close spigot when hand goes back to bar
4) Do # 2 & 3 until when you do #2 instead of a fart of air you get a steady stream of fluid.

Total time should be <15 min, unless you're high as a giraffe ass.

[scondon]

Do you understand the concept of bleeding brakes?  It's odd that if you did it would take days to get the air out. 

1) Open resevoir, fill with brake fluid.
2) put pressure on brake lever.
3) Open spigot on caliper, close spigot when hand goes back to bar
4) Do # 2 & 3 until when you do #2 instead of a fart of air you get a steady stream of fluid.

Total time should be <15 min, unless you're high as a giraffe ass.

  Never bled a dry system on a '78 Supersport have ya? Easy enough to assume that all cb750's are the same, but some have four pipes comin' out the back, some have only one, and some got metal rear fenders and some got plastic. And some even have the ignition key hidden under the tank, WTF(took me a year to figure how to start the damn thing)

   Mine's black, woohoo!!!!!! Hey, quit hoggin' and pass that thing(insert red eyed,droopy lid icon)

[rbmgf7]

Do you understand the concept of bleeding brakes?  It's odd that if you did it would take days to get the air out. 

1) Open resevoir, fill with brake fluid.
2) put pressure on brake lever.
3) Open spigot on caliper, close spigot when hand goes back to bar
4) Do # 2 & 3 until when you do #2 instead of a fart of air you get a steady stream of fluid.

Total time should be <15 min, unless you're high as a giraffe ass.

  Never bled a dry system on a '78 Supersport have ya? Easy enough to assume that all cb750's are the same, but some have four pipes comin' out the back, some have only one, and some got metal rear fenders and some got plastic. And some even have the ignition key hidden under the tank, WTF(took me a year to figure how to start the damn thing)

   Mine's black, woohoo!!!!!! Hey, quit hoggin' and pass that thing(insert red eyed,droopy lid icon)

lol, this whole time i was starting to doubt myself. trying to figure out what i was doing wrong. i've bled many systems before and never ran into this much difficulty. it brings me a little satisfaction knowing others have this same problem. next time i should just get a double banjo and just two long hoses straight to the caliper, bypassing that junction. i bet there's all sorts of air gathered in there. i'm sure there's a little air trapped in there whenever i had the helicoil one thread.

i let the lever stay closed another night seeing if it would make anymore difference. i would say yes. it's a little bit more resistive the closer i get to the bars. i don't know the explanation behind this but it's apparently working.

[754]

Gearbot, can you go back to your original float needles, instead of mixing parts.. what was wrong with them?

Cant remember if you said  braided brakelines.. regardless though try this. I always would go along the lines and tap with say a 7/16 wrench.. start at the bottom and work up, repeat a few times.. this gets out bubbles clinging to the side. Also I think the F has a log manifold for a splitter.. can you unbolt this so it is not level while bleeding?? should let any bubbles out easier.

I know its new but your master cylinder may be hooped, if you could hook a blocked line up to it you would know very quickly..

[techy5025]

I have found that it is better to bleed the brakes by forcing the fluid into the bleed valve on the calipers. This way the air bubbles are forced in the direction that they naturally want to go...up. This is the way cars are bled at the factory.....starting at the wheels.

Jim

[754]

its called backbleeding...

[rbmgf7]

reviving this thread from the dead, lol.

i got the brakes working as best as i can. they stop the bike from decceleration and can hold it on an incline. i guess it'll work.

as for the carbs, turns out the petcock filter had a leak and debris was getting through causing the float vavles not to close properly.

now i've been trying to ge the bike running properly. this has been a pain so far.

i finally narrowed down as to which symptom the bike was encountering. at first, the bike was running so lean, the plugs appeared to be rich. black, sooty, some wet, and spitting smoke. so this whole time i was working to fix richness. well, after reading the carbs FAQ, the characteristics while riding fell under leaness. so again i reverted myself and corrected for leaness. wow, the bike rides a little better. the engine still hesitates after 4K (from opening the throttle slowly), the exhaust sputters, the bike accelerates whenever i close the throttle, and whenever i open the throtthle really fast, the bike rockets around 4K and just dies (as if i turned the kill switch off). however, the plugs began turning to a brownish tan color.

right now i have the main jet at 115 (up from 110 which was horrible), needle clip in the middle (haven't changed), and 3/4 turn out on the air screw (was 1 but runs better at 3/4). the temp outside is mid 60's but lately i've test riden the bike when it was in the 40's. i am running pods and a 4-1 exhaust. does this sound about right?

thanks

[Kevin400F]

Regarding the brakes, you DID put the OPEN end of the brake pistons against the brake pads, correct? 

[rbmgf7]

Regarding the brakes, you DID put the OPEN end of the brake pistons against the brake pads, correct? 

yessir

[Blueridgerunner]

High up a What?

[rbmgf7]

ok, i can bring an end to this thread. i drilled out an old set of jets which would be equivelant to 130's (up from the 115's currently installed). took it out for a spin (today just happen to be in the 70's  ) and it flat out hauls-my-balls. it's a little hesitant around 4.5-5k but once it overcomes it, better hold on. i can tell the exhaust sound is throatier with the bigger jets. once i get my new credit card, i'll order some 135's and new slow's.

ok, i'm back in love with my CB. just needs a paint job and i can call it quits.