Topic: new bike fuel starved? SOLVED!

[78whiteorbs]

Picked up this gem on trade : 73' CB750

 
After the usual new bike refresh ( change oil , new plugs, plug wires, regap points, ultra sonic carbs, clean petcock, check tank for rust/garbage ) I started her up and she ran great on all fours nice and even and loud with those trumpet muffs. Anyhow my problem is this when taking her for a ride after the gas gets burned oughta the carb bowl she dies. I can drop the bowls and let them fill back up and she fires right back up no problem. First I was thinking there was a kink in the line but brand new lines and no kink- Then I thought I put the floats back in upside down but I do believe I have them in correctly.   The longer part of the float is facing the carb body . Can someone confirm?
no issues with the petcock flowing whatsoever.  It's aggrivating as the bike runs so well til it sucks up all the gas-

[KingCustomCycles.com]

or the tank vent doesn't allow air to replenish and it vacuum locks.  Another possibilty is a plugged screen in that petcock bowl.  I assume the picture was taken without the carbs and they are now installed?  If not, THAT is your problem.

[BeAtheistItsFun]

no, he has a spray bottle filled with gas that he shoots in there as he rides, it even fits in the cup holder!!!


also i have no idea btw sorry

[78whiteorbs]

that's so funny dude^

actually that pic was took while I was cleaning the carb- but thanks for the laugh

I checked the cap and popped it open while I was riding around the neighborhood- no joy- same thing

there is a small amount of fuel in the bowls so I was thinking doh , I put the floats in upside down but from what I have researched they are right.

I can lower the bowls and let them fill with fuel  (not sure why they arent "filling"seems like the problem ,when the bowls are popped on they arent refilling) maybe the float pins are sticking and not allowing the floats to drop but the action seems pretty loose..... It's gotta be stupid simple the bike runs great then no gas, fill em up  runs great and then no gas- it's a viscious cycle

[ekpent]

Are you running inline fuel filters by chance and do you have the lines running through the rack and in the proper orientation.Other than the floats being in wrong or really out of adjustment,sticking float needles,doubtful, or carb bodies not venting properly the only other thing may be not enouph gas in the tank.Try Kings suggestion and run it with the tank cap cracked open to see if its a vent situation.

[78whiteorbs]

tried running with tank open to no avail...first thing I thought of actually . I am running the reserve tank though I might trying gassing here up a bit more and running the open setting . The tank has a failing kreem coating but it isnt super bad failing , I washed it out good and I am running fuel filters until I can MEK out that crap. Shame is the PO just repainted it with that bad liner so I am gonna have to be uber careful dissolving that liner not to ruin the new paint.

I thought about that  and tried rerouting fuel lines through the holes in the carb rack but they seem fine - if I pull the line sure enough spews gas just like it should - and when I drop the bowls and the floats drop it fills back up like it should . This is somethign easy I can feel it..

[ekpent]

Peeling Kreme could be a bad thing. Maybe check to see if the two rubber vent tubes at the top middle part of carbs racked are clear. Bowls also have to 'breathe' through the tiny brass overflow tubes inside the bowl and out the bottom.Other than the inline filters being to restrictive or the gas being to low in the tank to not make a good 'push' or line pressure I may be running out of ammo over here.The large wide part of the float is at the bottom correct? Good luck and let us know the outcome.

[DedHed]

This hasn't been mentioned .... Probably for good reason.... Could it be the two "t's" going to each pair of Carbs? I really don't see how but that is one of the only things that hasn't been mentioned... Of course that would mean pulling the carbs again...
Let us know how it turns out...
Good luck,
Phil

[Spanner 1]

Gummed-up petcock passages ......

[Grabcon]

I just went through something similar. the petcock needed cleaning and the fiber gaskets on the screws that hold the petcock to the tank needed replacing.

I had gas out of one side regardless if I was on open or reserve setting. Pull the petcock and give it a good cleaning.

[wanackg]

Not sure if the older bikes had em but my 81 650 has a vacuum shutoff on top the number 4 carb.  Could be that its malfunctioning.

[78whiteorbs]

Ekpent:  large part of floats are facing up not down. I flipped one of the floats on the end over and it filled with fuel much faster. This may be the problem but I have seen pictures that I went by illustrating that the floats are facing upward (which logically makes no since to me )
http://www.salocal.com/sohc/tech/carb/asmpg_mgs/flthgt.htm

I feel pretty strongly my problem is with the float adjustment somehow ,  as when I drop the bowls the float drops like it should and fuel flows....somehow the floats arent dropping for whatever reason..


may have set float height with the pin compressed ,if that is it than these are some finicky carbs

I already ultrasoniced the petcock and with the fuel lines off tested it and it flows how it should when I change the valve settings.

Phil , thanks I was suspecious of the t's as well- gonna pull the rack and blow them out with compressed air again and try and reset the float height.  But just for the record, so the long end of the float goes down? Not like the photo?

[Gordon]

may have set float height with the pin compressed ,if that is it than these are some finicky carbs

If that's how you set them, then you just discovered the problem.  Float height is set with the tang just touching the needle, not pushing it up.  If you set the floats with the needle pushed up then your fuel level is way too low. 

And yes, these carbs have to be finicky in order to do all the things that are required of them. 

[ekpent]

The picture shows the float orientated correctly.Set them as Gordan said with the tang just touching the pin at 26mm.Also double check that the pin and spring are still working,they can get gummed up and loose the spring effect. Think you are on the road to Victory.Let us know--- Eric

[Spanner 1]

Gummed-up petcock and 'prolly wrong float height IF that's how you set your floats ( Pic. ).....can't measure float height with the floats weighing down on the valve like in your pic..... carbs MUST be held sideways with the float pin  very close to vertical and the float tab BARELY touching the valve ( no weight on it AT ALL ).... Do it right once and you will never have to do it again for the 'life' of your bike !!

P.S., sorry for capitols for emphasis... but it's important to set them correctly first time....

[78whiteorbs]

Yes I set them with the carbs upside down so the weight of the tang and gravity. Would be compressing the pin
Oother carbs I have cleaned have a retaining hook like piece to keep the float needle in the valve when the carb is rightside up. These do not so how am I to set the float height  without the carbs being upside down compressing the needle pin.   There isn't anything holding the needle in the valve but the ta there is no little wire to keep it from falling out.

[78whiteorbs]

The petcock is crystal clean- gonna pull the carbs this evening and reset floats with the cabs on the side. Will post results and thanks fellas for the input!

[ekpent]

Like Spanner mentioned if you balance the rack on its side you can hold the gauge,carbs and manipulate the float to see if its just touching the tab on pin.They,the needles usually will not flop around.Someone to hold the carbs may help also. You can put the carbs upside down in a soft chuck vise and do it also but you have to hold the float up while measuring to determine the sweet spot. Hope you don't wear bifocals like I do though as I am usually walking away cross eyed after checking a set,hard to focus and see     #$%* gettin' old---------

[WarwickE36]

I made that mistake setting my floats in my 550 the first time.  No gas to 2 bowls...  Reset them doing it on the side, keeping a watchful eye on the tangs and pins.  Works like a charm.

[78whiteorbs]

In theory wouldn't having set the floats at 26mm. With the pins compressed make your floats to hi not to low resulting in to much gas and overflow puking ,not no fuel/low fuel level like what I have going on .

[TrueSpin]

In theory wouldn't having set the floats at 26mm. With the pins compressed make your floats to hi not to low resulting in to much gas and overflow puking ,not no fuel/low fuel level like what I have going on .

The opposite, actually.

Consider the distance for the float line to be a constant.
You're only moving the tab that contacts the float needle. If the needle tab is too far up when you set the float line, you will get early shut off. If it's too low, you'll get late shut off (or no shut off).

[Magpie]

Have you routed the overflow tubes down the back of the motor and between the motor and the swingarm? They can get pinched causing a fuel problem - happened to me years ago. Idles fine but wouldn't go when I rode it. Cliff.

[78whiteorbs]

Could have just learned something I havent read anywhere else before(clearly at least) Thinking back a few carbs I have cleaned when someone else looked at them the said that the floats were off a little . I am a master printer at a sign and screen printing shop so I have plenty of practice with measurement and those comments stuck in my head as funny when I heard them but it was from people who "knew there stuff" . I am pretty dang sure this is the reason why! It is misleading in all the illustrations of people measuring there carb floats with the carbs upside down like that picture I posted above and I havent read mention of this in any manual I have . Well this could be a revelation! I bet all my bikes in the future run much nicer    

[my78k]

put on some clear line to the bottom of the over flows bend (slightly) so that the top os above the float bowl, open the drain screw and get a visual check of what the height in the bowl is...

[78whiteorbs]

Magpie, yes ! I made sure the overflow tubes were not pinched

True Spin: thanks, that's what I wanted to hear! Somethings are hard to visualize for me sometimes and it is a reoccuring theme for me that things would be right  but they are  backwards. i guess I have some mild mechanical dyslexia I am trying to work out      but in  infamous words , "I get by with a little help from my friends!"

again thanks for all the thoughts guys , it's very encouraging !  It's gonna be a race to pull the carbs off and reset that after work just to see if I can cruise today.  It's the little things you know that can make your day .