Part throttle stumble
 

Oh wise and powerful rx7 gods I humbly come to you with an issue that the search button does not help. I have been having an issue with my car for quite some time that has so far just annoyed me but now I want it to go away. My car starts and runs fine. When i just start to push on the gas (as in just gently taking off) just after the rpms start to rise I am getting a minor stumble at around 2300rpm. Like it is getting too much fuel or just not mixing right. The carb was rebuilt in the spring (had the issue before and after). Everything looks normal operating the carb by hand and looking down the barrels. This does not affect how it runs when already moving. Oh, almost stock nikki with deleted choke and all emissions have been removed btw.
All vacuum lines are newer and in great shape. No leaks. Fuel pump and filter are new. I was thinking accelerator pump but no amount of tweaking changes the stutter. Any ideas guys? possible timing issue? I am lost on this one.

Clogged transition circuit ?

Sounds like a guess as good as any. Now where would I find said circuit?

Download a copy of the carburetor service manual via the link in my signature.

Does the stumble only occur under load? As in, if you sit in neutral and rev it, does it have the same or similar stumble? While I have a Holley 465 on mine, I experienced a similar issue until I upgraded to a fuel pump capable of a higher pressure. WOT was fine from start to redline, but partial throttle always had a crappy and annoying stumble, kind of surging until the vac secondaries kicked in to offer more fuel. It was especially bad while in parking lots. The Holley (red) fuel pump and higher pressure fixed it. Not sure if the Nikki can be affected in the same ways, though, or if it would/could ever need a stronger pump.

Nikkis drown easy; stock fuel pump only throws 3.7-4.7psi.

Now I was under the impression that the nikki needed 2.5lbs of fuel pressure so that is where my regulator is set. The problem is almost completely without a load however it is getting worse. As of yesterday the car doesn't want to take off anywhere under 2500rpm without trying to die. I have also discovered I can get the car to backfire on command by revving the engine over 3500 and when it returns to 2500 or so punching it. Although I do think backfires and the fireball are cool the car did not do this before. Feel free to correct me but I am assuming this is caused by a temporary rich condition. I am going to download the service manual (thank you) and try working on it again this weekend.

crap. my computer wont let me download the manual. Claims that my adobe is running when it is not. This is just not my week for things to work as they were designed to

Sounds to me like a rebuild could do some good. or at least pull it apart and clean out all the jets and e-tubes.

www.sterlingmetalworks.com

has a good website that helps with tuning and taking it apart.

The carb was rebuilt last spring and other than this last few days the car has seen daily driver status. I am honestly fed up with this issue and I think she is going to sit for the winter. Will see if I can get my mikuni in there and if it works than the crap nikki is going to the scrap yard where it belongs.

i would personally pull the carb off and pull the top of the carb off, pull the sight glasses, pull the jets, and the accelerator pump off and clean everything. most commonly you'll find that if the rebuild was not thorough enough, there will be crud in your float bowls and the jet(s) may be partially clogged. clean each bowl and each jet thoroughly.you may want to remove and clean each air bleed, one at a time as well, just for good measure. toss on some new gaskets and a new accelerator pump diaphram, recheck the fuel filter, plugs, wires, cap, and rotor, and if need be, recheck the fuel pressure and keep us posted.

& don't lose the check balls and weights blowing out the bleeds!