Allright I've been fighting this since I had the mikuni on and it's getting worse and worse and now its driving me nuts.
Heres the deal once I hit third gear @ WOT anywhere from 4500 rpm to 7000 the motor starts missing like a mother. When this started it was real random sometimes it would happen sometimes it wouldn't, now it's un-advoidable when I want to get on it.
I have enough fuel, I've also tried multiple jetting setups on the delloroto. Swapped the ignitors for good ones. Plugs are good, cap, rotor, wires are practily all new, timing looks good.
One thing I've noticed is the tach seems be getting more and more delayed, my shift light with a 6.8 pill in now doesn't come on until well after 7500 now?!

It runs awesome thru 1st and 2nd... just when third comes on the load just starts to kill it...

thanks,
matt

edit: I have checked for vaccum leaks and fuel filter is brand new

 

Are you using fuel pressure regulator? If so do you see the gauge jumping when you try to set fuel pressure?I've had a very similar problem, even with a fuel pressure regulator, it turned out it was a bad fuel pump.

 

Sounds like fuel starvation to me, too.
The fact that it happens in third gear and not first or second reinforces that notion.

 

Check the output of the fuel pump. Disconnect the fuel line at the carb, put the hose in a graduated container, turn the key on, and measure the amount of fuel it pumps in a minute.
Not sure what the spec is, look in your haynes manual.

 

Check the voltage at the fuel pump also, you might have a short or something.

 

Yeah I have a fuel pressure regulator which I have set at 4.5 right now. I also have a Carter pump. All the hardware is pretty much new.

The Pressure stays steady even when the miss kicks in. I've tired settings from 2.5 to 4.5. (I just installed a new external guage)

Something I haven't looked into is the float setting tho..
I just don't know at what point to messure the float at on the dellorto.

Heres a little bit more info. The miss started to show up in january like I said it was a bit random at times with the mikuni. For weeks I wouldn't have a problem it would come and go

In late march I stopped driving the car to do bodywork, suspension work etc... the dellorto is on there now and it's horrible.

thanks,
matt

 

The pressure may be okay, but the volume may not be up to par.

 

check the shutter valve, It has 1 large and one smaller vaccum . the large hose goes[or used to] to the air breatheron top of the carb. Plug the BIG hose and see what happens. Once upon a time I got a car real cheap because the ''Mazda'' dealer mechanics couldn't even figure that one out. That car was doing exactly the same as yours, shift to 3rd and it died almost completely. I got lucky.

 

O.K., For ***** and giggles here's a long shot-
You say you have a shift light wired in...Could the fuel pump relay be cutting out? Try bypassing it.

 

nipper: I have a dellorto48.... my car is far from stock.

sterling: I wish that could be the problem, the shift light is hardwired. but tonight I noticed something, when the miss starts; when I shift into the third the light doesn't come on, unlike during the 1-2 shift. wierd...

matt

 

You might want to ground or reground your fuel pump.

 

Called up mariah today and almost stumped them hehe.. but they too agreed that is sounds like volume issue too.
But to double check I should be getting the beast up to their dyno.
Soooo.... I'm gonna start checking out the fuel pump wiring, trying another pump, and since I planned to do it anyway upgrade my fuel line 3/8"

Thanks for all you help guys, the input was helpfull now how do I stick 3/8" line into my tank?

matt

 

3/8" line? I think the line coming out of my tank is 1/2"

Then after the fuel pump it is 3/8".