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Author Topic: 78 CB 750F Carb #2 No fuel??  (Read 4276 times)

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Offline JJmoto

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78 CB 750F Carb #2 No fuel??
« on: May 03, 2015, 05:28:22 PM »
So I have a 78' 750 F that I've been working on for the past 4 years now, recently got it running...it was a seized engine and who knows how long its been since it last ran....I have rebuilt the motor, re-wired the bike, replaced old components, so after fighting with the electrical demons and fixing that mess, the carbs cleaned, on and off a few times to re-check or fix a leak....there is still a slight problem that appears to be traced back to carb #2 not getting fuel? so it appears....Compression is equal all the way across so I don't think its a valve issue.... spark circuit all new and functioning correctly when tested. It will start and run but #2 header runs a lot colder then the rest, using a temp. gun to monitor seems to be through the entire rpm range....I've correctly cleaned the carbs and have read up on them in this forum. The carbs have NOT been separated though. Maybe I need to replace the needle and seats and the pumper diaphram. I did not install any new components in the carbs although some used parts were used where really necessary..
Any input on this would be great, an area of focus maybe??  I was thinking to order a carb kit and replace it all....I would like to take it out for a slap!!!  :D
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Offline flybox1

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Re: 78 CB 750F Carb #2 No fuel??
« Reply #1 on: May 03, 2015, 07:33:28 PM »
Please perform the clear tube test on carb number two.  Take a picture of the results.
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'78 750K (F3 engine) PD42b's, Modified airbox w/K&N  filter, 40/110 jets, 1 needle shim, IMS@ 1 turn out. Kerker + Cone 18" QuietCore

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1974 550K0 (stock), 1973 CB350F (stock), 1983 Yamaha XS400K (POS)
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"Knowledge without mileage equals bullsh!t" - Henry Rollins

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Offline JJmoto

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Re: 78 CB 750F Carb #2 No fuel??
« Reply #2 on: May 04, 2015, 06:50:02 AM »
Okay good call, I will see if I can get on this today, its so nice out I want to ride!!!
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Offline JJmoto

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Re: 78 CB 750F Carb #2 No fuel??
« Reply #3 on: May 04, 2015, 10:32:08 AM »
Okay so here is the result of that clear tube test.
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Offline flybox1

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Re: 78 CB 750F Carb #2 No fuel??
« Reply #4 on: May 04, 2015, 10:36:25 AM »
Yep...your fuel is WAY low.  low enough your jets are probably sucking in air.
Verify your float height is correct.
Fuel level should be 3mm+/-  below the carb bowl gasket seam.

while your carbs are off, set them up so you can do the clear tube test on each one. 
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'78 750K (F3 engine) PD42b's, Modified airbox w/K&N  filter, 40/110 jets, 1 needle shim, IMS@ 1 turn out. Kerker + Cone 18" QuietCore

Past Bikes
1974 550K0 (stock), 1973 CB350F (stock), 1983 Yamaha XS400K (POS)
77/78 cool 2 member #3
"Knowledge without mileage equals bullsh!t" - Henry Rollins

"This is my CB. There are many like it, but this one is mine…"

Offline JJmoto

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Re: 78 CB 750F Carb #2 No fuel??
« Reply #5 on: May 04, 2015, 12:25:52 PM »
really eh.....okay I will pull the carbs and adjust float height to spec this afternoon. I will reply with results
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Offline flybox1

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Re: 78 CB 750F Carb #2 No fuel??
« Reply #6 on: May 04, 2015, 12:34:47 PM »
Quote from: JJmoto on May 04, 2015, 12:25:52 PM
really eh.....okay I will pull the carbs and adjust float height to spec this afternoon. I will reply with results
spec float height is only a starting point, especially if you have aftermarket float needles.
set it, and verify its correct with the clear tube method....adjust it from spec to attain proper fuel level if needed.

not our carbs, but you get the idea.  3mm+/- below the bowl gasket seam.

« Last Edit: May 04, 2015, 12:43:27 PM by flybox1 »
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'78 750K (F3 engine) PD42b's, Modified airbox w/K&N  filter, 40/110 jets, 1 needle shim, IMS@ 1 turn out. Kerker + Cone 18" QuietCore

Past Bikes
1974 550K0 (stock), 1973 CB350F (stock), 1983 Yamaha XS400K (POS)
77/78 cool 2 member #3
"Knowledge without mileage equals bullsh!t" - Henry Rollins

"This is my CB. There are many like it, but this one is mine…"

Offline JJmoto

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Re: 78 CB 750F Carb #2 No fuel??
« Reply #7 on: May 04, 2015, 06:51:15 PM »
Thats a fantastic diagram thanks! So I pulled the carbs and didn't really find much wrong, I re-did all of the float heights to exactly 14.5 mm. There were some that were little bit out and they all took some adjustments to hit the spec. I also did another sync on the slides to get them a little closer.

I did get them back on the bike so I was going to just hook up the cables and throw on the tank to give it a try. Maybe I should verify the clear tube method within the +- 3mm , that would be 6mm total. Just shy of 1/4" (6.4mm)

One thing that seems slightly different was the spring inside the seat of #2 carb had a little more spring back on it then the rest but I didn't think it would effect a float height spec at all so I let it go. Also I double checked the circuit with compressed air...all clear. Pilots were pulled already and re-done perfectly. Fuel tank has been re-done and is brand new, its receiving filtered fuel...visually the needle conical rubbers are in decent condition.

Would it be possible that another carb was letting fuel pass through and draining out #2 ??? Bike was up and level when I took the photo of the clear tube.
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Offline JJmoto

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Re: 78 CB 750F Carb #2 No fuel??
« Reply #8 on: May 04, 2015, 06:58:45 PM »
Oh just read that again....it is saying within 3 mm from the split line....also the needles I believe are OEM from 1978, the bike has 17,000 original miles on it must have been stored inside for many years because of how well the carbs cleaned up. I am thinking I would like to buy a carb kit for it to replace the pumper diaphragm and the o ring seals and gaskets. I just wanted to get the bike going first off with what I have on hand right now. The last bike I did was a 78 KZ 650 and the carbs on that were in much worse condition, but once I cleaned them up, they functioned flawlessly. These CB carbs seem to be very finicky and must live in a perfect state which they are pretty much there now...
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Offline flybox1

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Re: 78 CB 750F Carb #2 No fuel??
« Reply #9 on: May 05, 2015, 08:28:34 AM »
No, not WITHIN 3mm of the bowl gasket seam...you want the level AT 3mm from the bowl gasket seam.
(if its much less than that, you'll now be running rich, fyi)
Youre probably not going to find anything wrong inside.  You need to make adjustments in your float height to get the fuel level right.   adjust, test, adjust, test...until its correct.
Did you test all fuel levels to be correct in the float bowls BEFORE your carbs went back on the bike?
You must like taking them off and on  ;D
 
I have no idea where you got 6mm from  :o

TY to goldarrow for the picture
« Last Edit: May 05, 2015, 10:00:31 AM by flybox1 »
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'78 750K (F3 engine) PD42b's, Modified airbox w/K&N  filter, 40/110 jets, 1 needle shim, IMS@ 1 turn out. Kerker + Cone 18" QuietCore

Past Bikes
1974 550K0 (stock), 1973 CB350F (stock), 1983 Yamaha XS400K (POS)
77/78 cool 2 member #3
"Knowledge without mileage equals bullsh!t" - Henry Rollins

"This is my CB. There are many like it, but this one is mine…"

Offline JJmoto

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Re: 78 CB 750F Carb #2 No fuel??
« Reply #10 on: May 05, 2015, 12:00:11 PM »
Yeah its so much fun to do the same repetitive thing!  :o
Anyways, I pulled the carbs again, checked bowl level on each carb, all were just below the split line on the gasket seam.
So I did not make any more adjustments. Installed the carbs, add a little more fuel to tank and started right up.
#2 cylinder has issues still....manifold cold to touch while the other 3 can't touch them....I did notice the motor had slight improvements but it needs #2 cylinder to run....I swapped the spark plug from #1 also...no difference...coils are brand new but the cheap ones...when I tested for spark it sparks large....wtf....maybe I need to pull the pilot out again for #2...I have done this twice already and found no problem....

Your clear tube test seems to be accurate and should have fixed the issue....must be something else?
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Offline flybox1

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Re: 78 CB 750F Carb #2 No fuel??
« Reply #11 on: May 05, 2015, 12:26:16 PM »
What is your jetting and mixture screw settings?
Timing, advance, valve lash done?
Did you twist out the main jet AND emulsion tube to inspect and clean?
did you pull out the press-in pilot jet to inspect and clean.
in both of these, the tiny holes need to be perfectly clean.

when you swapped your #1/2 plugs, what did your #2 plug look like? white, black, wet black?  did it smell like fuel?
It would be helpful to see the insulator tips of #1&2, all would be best.
« Last Edit: May 05, 2015, 12:33:25 PM by flybox1 »
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'78 750K (F3 engine) PD42b's, Modified airbox w/K&N  filter, 40/110 jets, 1 needle shim, IMS@ 1 turn out. Kerker + Cone 18" QuietCore

Past Bikes
1974 550K0 (stock), 1973 CB350F (stock), 1983 Yamaha XS400K (POS)
77/78 cool 2 member #3
"Knowledge without mileage equals bullsh!t" - Henry Rollins

"This is my CB. There are many like it, but this one is mine…"

Offline JJmoto

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Re: 78 CB 750F Carb #2 No fuel??
« Reply #12 on: May 05, 2015, 12:51:13 PM »
jetting, mains 120  pilot 42 mix 2 turns

Main jet and emulsion tubes were serviced, press in pilot jet was serviced, all cross holes serviced.

static timing is set within spec, advance is not set yet. Valve lash completed then check after running.

Maybe I will pull the carbs again because it takes me 5 minutes and take apart #2 completely again for the 3rd time and see if something has blocked off the passages for some reason...

1&2 plug swap....well it appeared that #1 was black from running, #2 had a small amount of black on it and you could tell it had not been burning anything, it did not appear to be wet and flooded.

I'm going to check into this further...
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Offline flybox1

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Re: 78 CB 750F Carb #2 No fuel??
« Reply #13 on: May 05, 2015, 01:18:28 PM »
what intake and exhaust are you running?
42's + 2turns out is really rich...do you have pods and an open exhaust?
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'78 750K (F3 engine) PD42b's, Modified airbox w/K&N  filter, 40/110 jets, 1 needle shim, IMS@ 1 turn out. Kerker + Cone 18" QuietCore

Past Bikes
1974 550K0 (stock), 1973 CB350F (stock), 1983 Yamaha XS400K (POS)
77/78 cool 2 member #3
"Knowledge without mileage equals bullsh!t" - Henry Rollins

"This is my CB. There are many like it, but this one is mine…"

Offline BobbyR

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Re: 78 CB 750F Carb #2 No fuel??
« Reply #14 on: May 05, 2015, 02:12:42 PM »
I have a 78k so I think the carbs are the same. Before you pull them again try a simple experiment. There is a threaded screw used for attaching the vacuum gauges for synching the carbs. Take the one out from carb in question. Spray some carb cleaner or starting ether into that hole. If the cylinder comes to life you know fuel is not getting in the cylinder. If by sticking your finder over the hole you should feel some vacuum when you crack the throttle, if you don;t you have a bigger problem.   
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But we were boys, and boys will be boys, and so they will. To us, everything was dangerous, but what of that? Had we not been made to live forever?

Offline flybox1

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Re: 78 CB 750F Carb #2 No fuel??
« Reply #15 on: May 05, 2015, 02:30:44 PM »
good tip, BobbyR...
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'78 750K (F3 engine) PD42b's, Modified airbox w/K&N  filter, 40/110 jets, 1 needle shim, IMS@ 1 turn out. Kerker + Cone 18" QuietCore

Past Bikes
1974 550K0 (stock), 1973 CB350F (stock), 1983 Yamaha XS400K (POS)
77/78 cool 2 member #3
"Knowledge without mileage equals bullsh!t" - Henry Rollins

"This is my CB. There are many like it, but this one is mine…"

Offline JJmoto

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Re: 78 CB 750F Carb #2 No fuel??
« Reply #16 on: May 05, 2015, 03:37:42 PM »
Hey yeah that is a great tip for sure thanks! Okay...so I GOT HER BEAT!  ;D
Heres the story....pulled carbs again, tore down #2...may have been a tiny burr in the pilot jet, but maybe not, so I cleaned that, pulled everything out of that carb and found no problems, I left all the float levels at the 14.5mm setting as the clear tube test came back with decent results. I then put the carbs back on. I checked for spark. I have spark on all 4 plugs. (brand new coils and points assembly, all new wiring on bike) so then I noticed the spark may possibly be intermittent on #2 so would burn only sometimes. (I had previously by-passed the wiring and went direct to this coil to test if it was a wiring issue and nothing changed) This time I switched terminals (yellow wire and black/white) on the # 2&3 coil and noticed the spark was weaker on one plug then the other. SO with that being found I went to the old parts bin and pulled out the original 1978 coil, installed it...put it back together and fired it up. Well it is running on all 4 cylinders now!! The only thing I changed after is was running was opened up the mixture screws to 2 1/2 turns out and that cleaned it up really well. WTF is the problem these days with the low quality brand new parts eh?? I'm going to have to go back on my receipts and figure out where I bought them from its been months ago...

You have all been of great assistance in helping me out with this issue to resolving it!  ;D Hope this thread can help someone else having similar issues...
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Offline flybox1

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Re: 78 CB 750F Carb #2 No fuel??
« Reply #17 on: May 05, 2015, 04:35:31 PM »
OEM coils rarely go bad.  Be sure your resistance checks ok in your plug caps.  Those do go bad...
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'78 750K (F3 engine) PD42b's, Modified airbox w/K&N  filter, 40/110 jets, 1 needle shim, IMS@ 1 turn out. Kerker + Cone 18" QuietCore

Past Bikes
1974 550K0 (stock), 1973 CB350F (stock), 1983 Yamaha XS400K (POS)
77/78 cool 2 member #3
"Knowledge without mileage equals bullsh!t" - Henry Rollins

"This is my CB. There are many like it, but this one is mine…"

Offline BobbyR

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Re: 78 CB 750F Carb #2 No fuel??
« Reply #18 on: May 05, 2015, 04:37:15 PM »
Congratulations, we love to see a problem solved. Happy riding!
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Dedicated to Sgt. Howard Bruckner 1950 - 1969. KIA LONG KHANH.

But we were boys, and boys will be boys, and so they will. To us, everything was dangerous, but what of that? Had we not been made to live forever?

Offline JJmoto

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Re: 78 CB 750F Carb #2 No fuel??
« Reply #19 on: May 05, 2015, 05:23:43 PM »
Just got back from its first ride in probably 20 years...happy about that but there is still tuning to be done as it doesn't like the throttle opened much. I need to do the timing for sure and that should clean that up as it must be the advance that is not right. (i've only done the static set-up) Anyone have a good way to do timing set-ups??

So now the exhaust manifold temps are higher on 2&3 but lower on 1&4 (1&4 has the new coil now). What should I see for normal operating temperatures on motor and exhaust?

Maybe I should put the other old coil back on now....The NGK caps are brand new so were the wires and coils but I would have to fix up the old coils with new wires....

To answer Flybox this bike is running pods and has 4 into 1 exhaust with a modified harley muffler, the muffler has a custom small baffle I put in there so it wouldn't be obnoxious...
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Offline flybox1

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Re: 78 CB 750F Carb #2 No fuel??
« Reply #20 on: May 05, 2015, 08:56:31 PM »
Does the throttle response improve if you turn your mixture screws in to just 1 turn out?
I still think your rich in pilot and mixture screw setting.
Your plug color Will be a good indicator.
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'78 750K (F3 engine) PD42b's, Modified airbox w/K&N  filter, 40/110 jets, 1 needle shim, IMS@ 1 turn out. Kerker + Cone 18" QuietCore

Past Bikes
1974 550K0 (stock), 1973 CB350F (stock), 1983 Yamaha XS400K (POS)
77/78 cool 2 member #3
"Knowledge without mileage equals bullsh!t" - Henry Rollins

"This is my CB. There are many like it, but this one is mine…"

Offline JJmoto

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Re: 78 CB 750F Carb #2 No fuel??
« Reply #21 on: May 06, 2015, 08:17:32 AM »
Hey, yes I will be working on it again today. I ran out of time yesterday...I have to pull the plugs out and take a reading to see where its at....I know all 4 manifolds were hot this time but 1&4 were now colder then 2&3. 1&4 has the brand new coil, I bought the coils from Vintage CB750. I hope they will take a return on them.
I suppose it would be the easiest thing to do simply lean out the mixture screw before digging into other things..

I will keep coming back here to follow up progress good or bad!!
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Offline JJmoto

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Re: 78 CB 750F Carb #2 No fuel??
« Reply #22 on: May 06, 2015, 11:04:52 AM »
Here are the plug colours....2&3 running 37 year old coil with a black to tan colour....1&4 has new coil, #1 almost looks like it is works sometimes (but this plug I swapped out with #2 previously to test) #4 has a grey/white appearance which means its lean I think....

I just took a look at the other old coil and tried to remove the old plug wires and they busted off inside so now I can't use that one....Vintage CB750 won't take a return or exchange on the coils, they are sending me another on of the same type which I almost think is the main problem now?

Any thoughts? What a run around here!
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Offline flybox1

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Re: 78 CB 750F Carb #2 No fuel??
« Reply #23 on: May 06, 2015, 11:23:09 AM »
1/4 look a tad lean, and 2/3 look a little rich.
we cant really start fine tuning mixture until you settle on a set of coils to use, and THEN, choose the appropriate spark plug to match the coils and wires.
Your use of Resistor and Long plugs might be creating more issues for you.  Its not an area i'm qualified to speak about.  Might need to PM Hondaman about plug type once a set of coils has been chosen.

I run D8EA's with stock coils and 5kohm caps.
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'78 750K (F3 engine) PD42b's, Modified airbox w/K&N  filter, 40/110 jets, 1 needle shim, IMS@ 1 turn out. Kerker + Cone 18" QuietCore

Past Bikes
1974 550K0 (stock), 1973 CB350F (stock), 1983 Yamaha XS400K (POS)
77/78 cool 2 member #3
"Knowledge without mileage equals bullsh!t" - Henry Rollins

"This is my CB. There are many like it, but this one is mine…"

Offline JJmoto

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Re: 78 CB 750F Carb #2 No fuel??
« Reply #24 on: May 06, 2015, 11:23:37 AM »
Might I also add the bike was rewired, new everything including switches, ignition, electronic voltage regulator, new battery, grounds etc all redone, new plug caps etc...stator seems to be good, battery charges up fine etc...
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