HELP PLEASE, Why does my kz400 die when I pull the throttle fast

I have a 1977 kz400A deluxe, I've been working on for 2 years. Lots of custom work done, full rebuild, speedmoto twin Mikuni vm30's with V-stacks, new everything. Anyway, my problem is I can slowly pull the throttle an it will go from idle to WOT no problems, perfect even. But if I pull the throttle quick it instantly dies. Both carbs are sinked and set. Float hight in spec. Valve clearances are in spec. Timing at idle is on point and at full advance. I've tried re-jeting, restrickting air flow, and rechecking and checking everything. I can NOT figure it out. This isn't the first, second, or even tenth bike that I've rebuilt. The only thing I can think of is maybe my advancer CAINT keep up. But there's something I'm missing...... Any help would greatly appreciated. Thank u.

Welcome to KZRider 
Jetting  with the velocity stacks is my first thought 
 Need for accelerator pump  is next along with carb sizing   ( What comes stock on your motor )



OH   And NICE TRUCK

I also thought maybe accelerator pump, I checked the stock carbs an didn't see one on there so I dismissed it. And I took the V-stacks off an put the uni oiled pods on that came with the carbs from speed moto and still the same issue. I've moved needle clip position, up an down. I've tried main jets from 100-200. The crabs were supposed to come pre jetted to my specs. Intake and exhaust and altitude all that. But went threw them anyway trying to figure this out. 

32mm keihins come stock, I think.

Yup...been there.   Basically, at low rpm and small throttle opening your air velocity is high enough to create the Venturi effect and "suck" up fuel from the float bowl.   Opening slowly allows the air velocity catch up.  When you go WFO real fast, your air colum inside the carb has no velocity thus no Venturi to "suck" the fuel up....one possibility.  The other  possibility is your mains are way to lean to handle WFO.   

I would try going stupid fat on the mains as a test and adjust around them on the low side and see if that works.  Without pumpers, that's about your only options....  Maybe go smaller on the carbs might help...or throat restricter's.  

577nitro

32mm stock carb bungs. I put vm30's on it. What would u call "stupid fat" main jets. Stock is 125, I've gone up to 200 and same issue, I've even tried uni oiled pods filters instead of V-stacks an same thing.

Still trying to figure out how to use this forum. Sorry guys. Ok, stock carb size is 33mm I just check. I have 30mm on it now. What do you call stupid fat main jets, stock is 125 and I've gone up to 200 and still same issue 

Not sure why none of my replies are posting.

That short exhaust  isnt helping 
Are their baffles inside those pipes ???
Im sure you are aware that  tuning is a  dark art sometimes  due to radical  changes from stock.
 Im not suggesting you return all to stock  but  differences in air flow from a stock airbox to stacks , to  pods
and then differnt restrictions of pipes in backpressure  can make a spot on tune hard to get right 
Good Luck
Dave

PS your wondering why your posts after the first one didnt appear
it was because non of us Moderators had a chance to  get back here online to approve it before you raised the question .
New members do not have unrestricted posting privliges until their 2nd post is approved

wrote:

32mm stock carb bungs. I put vm30's on it. What would u call "stupid fat" main jets. Stock is 125, I've gone up to 200 and same issue, I've even tried uni oiled pods filters instead of V-stacks an same thing.

your replies are working…no worries.   What kind of carbs were on it before?   CV’s work different differently than round slides..if thats what they were.   Did the stock carbs do the same?   Straight pipes don’t provide any back pressure which can cause all sorts of weird tuning challenges.  
Maybe try richening the idle bleed air and the pilot jet also.   When I’m fighting a new setup that’s not performing and none of the standard methods have helped, I just go to the extreme of the settings to try and get any change.  It sounds like it’s starved of fuel, some how, the fuel circuit in the carbs aren’t allowing the fuel to flow up the main jet.   Check all the parts of that, bowl level, clogged vents, pilot jet circuit, bleed air needle, fuel petcock.  

Good luck..

577Nitro


 

I agree with going extreme to get things figured out.
Clip needles to raise them to max setting.
Adjust floats to get fuel level as high as posible without over flowing.
Adjust idle mixture/bleed screws as rich as possible.
Try leaving the choke 1/2 to 3/4 on.

First off, thank you very much everyone for all the suggestions. I have been at a complete loss and unable to move forward completely out of ideas and now I have some things to try out. I did not think about my exhaust, I've straight piped a couple bikes before but never this short so I will keep back pressure in mind maybe come up with some kind of a baffle. 
stock keihins we're vacuumed carbs, with the billows ontop of slides. 
I guess I'll get another set of main jets 200-300 and work through those too. Also going to mess around with making some throat restrictors. 
Also now that I think of it, it starts first kick with out choke. If I choke it it dies or won't start.