Zephyr 750 carb issues

I have a 92 Zephyr 750 Calif version. I took the carbs off to replace the needle jets because the carbs were overflowing. It was running fine prior to this and would rev up freely. Now that I have replaced the needle jets and set the float levels the bike will not rev up past 3-4k rpm. It will idle fine all day long but as soon as I grab a big handful all I get is blah blahh blahhh, as if its not getting enough fuel. Let off and it all goes to normal idle and little revs ok. I have had the carbs off multiple times now checking and double checking, cleaning, blowing air ect. Everything seems ok. The pilots are 38's. Mains 1 and 4 are 92's and 2 and 3 are 95's. So that should be ok. 
When I rev it I notice that the vacuum slides dont really slide up when gassing hard, they just sort of flutter. I would think that they would jerk up. 
I dont have the airbox connected up yet as Im trying to see whats going on. But it will go back on. 
Any Ideas? Im at a loss. 
 

Had the same carb issue on a friends Euro Zephyr 750 after a carb and fuel tank (rust inside) rebuild. Those carbs are running very lean with modern E5/10 fuel. We had to setup the main jets size of#1-4 to 108 and #2-3 to 110 to solve the full throttle problem with stock air box.

 

I assume what you changed is the bowl NEEDLE VALVES,..... NOT the NEEDLE JETS, which are where the main needles sit.   I also assume you did not disturb the emulsion tubes, nor the REAL needle jets on top of them..

Did you do the wet bowl fuel level check, with a clear u-tube???

Are you sure any of the diaphragms is not pierced or torn????

Are you sure the spring/needle head plastic cages are not blocking the vacuum hole in any slider, and the sliders are in the proper orientation?

When you say, won't go beyond 4K rpms, .........that is with the air box installed, right????
 

wrote:

I dont have the airbox connected up yet...

 

This could explain the issue right here.

Agree with Ed, without the air box it will be pulling too much air.  Hook it back up and see how it runs.

wrote:

I assume what you changed is the bowl NEEDLE VALVES,..... NOT the NEEDLE JETS, which are where the main needles sit.   I also assume you did not disturb the emulsion tubes, nor the REAL needle jets on top of them..
Yes, I only replaced the float valves. Everything else is stock and not touched other than to clean. 

Did you do the wet bowl fuel level check, with a clear u-tube???
Yes and the float heights are good. 

Are you sure any of the diaphragms is not pierced or torn????
None are torn or pierced. 

Are you sure the spring/needle head plastic cages are not blocking the vacuum hole in any slider, and the sliders are in the proper orientation?
Yes, everything is setup right. 

When you say, won't go beyond 4K rpms, .........that is with the air box installed, right????
Airbox is in place but I dont have the boots hooked up. I wanted to see how everything is working before I put them back on. 


 

wrote:

Agree with Ed, without the air box it will be pulling too much air.  Hook it back up and see how it runs.
The airbox is in place but I have not hooked up the boots. I wanted to see how everything is working first. 
Do you really think not hooking the boots up could be the problem? They are a pain to put back on but Im working on it for another test. 
 

Yes I do think it makes a difference.  I understand the issue with installing the boots, but the bike was set up (carb jetting) for the stock air box. and when you remove it you should not expect it to operate like it does with it.  I might suggest using tape over half of each carb opening and see it it works better, if so then I would suggest reinstall the air box boots and filter

Would you put everything back and bottomed up the way is supposed to?    Also, for fun,  you may do a synchronizing session with auxiliary tank, and then see how it revs.   Don't try peering into the sliders running.  They will flutter, ....and engine would be way too lean anyway.

wrote:

Yes I do think it makes a difference.  I understand the issue with installing the boots, but the bike was set up (carb jetting) for the stock air box. and when you remove it you should not expect it to operate like it does with it.  I might suggest using tape over half of each carb opening and see it it works better, if so then I would suggest reinstall the air box boots and filter

 Exactly, you can't expect anything to work properly, or the same,  if you don't have it assembled properly, the airbox is a large part of the equation and completely changes the air/fuel ratio.
Fit the boots to the airbox first, all 4, then fit the carb side, use a little bit of lube {i use Lanox, not sure if you can get that in the US}, to help them slide on.I also use a modified small , thin screwdriver,  to help put the rubbers on but make sure its not sharp, I have a small screwdriver with the end cut off, and a 90 degree bend at the end that I've smoothed off anything sharp,   it works well..  I hope I explained that well enough...

Ok, for a test I covered up about half of the vacuum port on the carbs, pic attached. I was able to get it rev out as it should. 
I could not go back to back handfulls of throttle as it would bog. But if I waited 5 sec or so I could get it to rev. 
Just so everyone knows I have always intended to keep the stock airbox. I just left the boots off so I could test 
everything. Thanks gd4now for the suggestion. It appears that once I get all the boots on everything should work
fine...I hope    Its a pain getting those boots on. Once all together I will repost. And if there are still any ideas 
out there Im willing to listen. 

Its been awhile as I have been super busy with work and other bikes, but I got it running and correctly LOL! I jetted the carbs, 1/4 #92 and 2/3 #95. #38 pilots on all. 2 turns out on the air screws. I also installed a K&N reusable air filter. I ended up replacing all the air boots as I found a couple of them were ultra hard and cracked. Left the stock air box in place. 
It now runs very good and has a nice "Hit" in the upper rpm range as it comes on the cam. Im now considering slip on exhaust tips, this bike is very quiet. 
Thanks to everyone for all the help!