Only running with choke fully closed

[mrtnhdtmnn]

Hi and hello this is my first post if I do something wrong I'm sorry. I have this same issue since I'm working on the engine.

My bike: CB750 KZ RC01 from 78
Carburetor: Keihin VB52A with POD filters
4 in 1 exhaust
Bought as ratty project did a full on restauration disassembled every bolt didn't touch the inner engine

Issue: Can't even get it to idle when choke isn't fully closed

My background: I have a little experience with old cars but not carburetors. Changing simple things here a little welding there. Not a mechanic but an electrical engineer and I like learning new stuff.

Spent like 50 something hours trying to get it running now. I'm doing lots of research with AI in combination with the owners workshop manual.

Things I already checked chronologically:
Fresh oil
Engine compression test and valve clearance.
Checked float level.
Checked ignition for spark.
First run went okay but lastet only a minute.
Tested different mixture screw settings and idle screw settings - same issue.
Tested different float levels from very low to very high - same issue.
Tested bigger jets .8 and 1.4 - same issue.
Got back to .4 and 1.2 (current setup)
Carburetor fully disassembled then ultrasonic clean and blowed compressed air through canals. Fresh mixture screws fresh float bowl gasket checked membrane. Everything back together - same issue.
Checked static (ignition) timing of the pick ups.
Replaced rubber intake manifolds - same issue.
Replaced old OKI MPS-200 with Ignitech TCIP4 - same issue (current setup)

Things that I find confusing at the moment are that I bought it with some rubber hoses connecting the drain screw with vacuum nipple. From what I researched this shouldn't be connected. But makes no difference if I close the vacuum port (at the engine intake).
I can blow (low pressure) air through the drain port of each float bowl.
There's a rubber hose coming from the not-fuel-rail of the carburetor bank I haven't connected it to anything since I have no air box but if I blow into it some gas comes out of a hole the carb throats. If I start then the engine revs a little higher and dies after a few seconds.
When I have the choke fully closed the engine runs but I got white grayish smoke (unburnt gas?!) coming out of the exhaust and a misfire every 10 seconds or so. As soon as I open the choke a little bit the engine dies after a few seconds.

My next step is replacing the needle valve and making sure that really no port in the carb is blocked (with compressed air) but I have the feeling that this wouldn't change a thing. Another clue could be off camshaft timing but haven't checked.

If anyone has another clue what I'm doing wrong I would be very helpful for any advice.

Thanks,
Martin

 

[93trident]

Hi and hello this is my first post if I do something wrong I'm sorry. I have this same issue since I'm working on the engine.

My bike: CB750 KZ RC01 from 78
Carburetor: Keihin VB52A with POD filters
4 in 1 exhaust
Bought as ratty project did a full on restauration disassembled every bolt didn't touch the inner engine

Issue: Can't even get it to idle when choke isn't fully closed

My background: I have a little experience with old cars but not carburetors. Changing simple things here a little welding there. Not a mechanic but an electrical engineer and I like learning new stuff.

Spent like 50 something hours trying to get it running now. I'm doing lots of research with AI in combination with the owners workshop manual.

Things I already checked chronologically:
Fresh oil
Engine compression test and valve clearance.
Checked float level.
Checked ignition for spark.
First run went okay but lastet only a minute.
Tested different mixture screw settings and idle screw settings - same issue.
Tested different float levels from very low to very high - same issue.
Tested bigger jets .8 and 1.4 - same issue.
Got back to .4 and 1.2 (current setup)
Carburetor fully disassembled then ultrasonic clean and blowed compressed air through canals. Fresh mixture screws fresh float bowl gasket checked membrane. Everything back together - same issue.
Checked static (ignition) timing of the pick ups.
Replaced rubber intake manifolds - same issue.
Replaced old OKI MPS-200 with Ignitech TCIP4 - same issue (current setup)

Things that I find confusing at the moment are that I bought it with some rubber hoses connecting the drain screw with vacuum nipple. From what I researched this shouldn't be connected. But makes no difference if I close the vacuum port (at the engine intake).
I can blow (low pressure) air through the drain port of each float bowl.
There's a rubber hose coming from the not-fuel-rail of the carburetor bank I haven't connected it to anything since I have no air box but if I blow into it some gas comes out of a hole the carb throats. If I start then the engine revs a little higher and dies after a few seconds.
When I have the choke fully closed the engine runs but I got white grayish smoke (unburnt gas?!) coming out of the exhaust and a misfire every 10 seconds or so. As soon as I open the choke a little bit the engine dies after a few seconds.

My next step is replacing the needle valve and making sure that really no port in the carb is blocked (with compressed air) but I have the feeling that this wouldn't change a thing. Another clue could be off camshaft timing but haven't checked.

If anyone has another clue what I'm doing wrong I would be very helpful for any advice.

Thanks,
Martin

Mate unless you pull those pilots it will run like crap especially with pods added to the equation. My 81 had been standing a few years and when I cracked the bowl I had never seen jets like it. I thought had cleaned them but after x3 goes I was told that the pilots are infamous for blocking if stood. My carbs have push in pilots so I had to use the screw and washer method. But that turns out to be a good thing because after you can remove them easily next time. The holes on the jets are tiny i used a single strand from a wire brush. Check your pod intakes too. If they have a rubber ring/ rim inside them they won't work as they block the cut outs in the carbs( kidney shaped hole and drilling on top intake side) Hope this helps. This is a cracking site full of information and it has helped me many times.

 

[mrtwowheel]

Maybe this will help. The "start up procedure" for one of these bikes when finely tuned is always the same at first start of the day regardless of the temperature. Full choke to start. Full choke for about 3/4 miles. 2/3 choke for about the next 2 miles. 1/3 choke till it slightly bogs down, then open choke.

 

[Dandy Man]

Might just be paranoia becauss i have vacuum issues but have you thoroughly checked for a vacuum leak?
You said you replaced the boots but i did the same and still had issues, to test just get the bike running and start spraying something flammable around the carb (but not the intake/pod filters)
Starting fluid or carb/brake cleaner works well.
Spray that shiz all over the carbs, between them, at the throttle and choke shafts, and the boots.
If the idle picks up and the engine starts to sound happy then you have a vacuum leak wherever it was you had sprayed. Hope this helps, this is something i wish i had done last november when i got my 750 and couldnt figure it out for months lol

 

[MP-x]

https://drive.google.com/drive/folders/1sD40NnB1wWTzDq19P1B85o1qvhA0kBKH?usp=sharing
thanks to danadalis for sharing.
This helped me, when I my bike had similar problems, especially the pilot jet cleaning part.
You probably have pressed in pilots as well, since you have early MY, so check out youtube videos about screw and washer method on removing them.

This is also a very good source for info on VB (and other) carbs:
https://motorcycleproject.com

 

[Motogrady]

The small passageway on each carb, that goes from the air screw, after the jet, and dumps fuel in the throttle body, is very small. The passageways are clogged. No fuel is reaching the air being sucked past the little hole in each carb as it heads to the intake manifold.
No matter how many turns in or out the air screws go.

The fix is easy.

Pull the carbs.
Pop the domes.
Pop the float bowl.
Remove the air screws.
Using guitar string, pick as much of the hardened varnish and crap from the idle passages. Spray carb cleaner into the passages using the tiny red straw that comes with the cleaner.
Blow out passages, both ways, with 100 lbs psi air.
Repeat 4 times.
Install carbs. (A tie down ratchet strap, wrapped around the carbs/cylinders helps wiggle them back in).
Turn air screws seat/back out, start maybe 1.5 turns.

See what happens.

Good luck guy.

 

[Motogrady]

ps. Each carb, 4 times.