I came across a set of twin Weber carbs with a lake city mani and was wondering if tuning these carbs would be different on the 6 port than on a earlier 4 port engine?
My main concern is the difference in size between the main and secondary ports. Since two barrels of the carbs will be running one rotor with two different port configurations will there be need for different venturies, jets, and e-tubes or am i over thinking things?

 

I just test ran a regular 12A Nikki on a 6 port 13B with a frankensteined manifold. It idles kinda high at 1200, probably because it's got a vacuum leak where it's cracked and RTVd back together. Was kind of a poor weld job (I didn't do it). Also the engine is a fresh rebuild, but it feels like it's got some good low end which kinda surprised me considering the aux ports are 100% open without sleeves and they close so darn late. Gotta be a vacuum leak; you rev it and it returns to its high idle kinda slowly just like a vacuum leak is present.

So yes you are over-thinking it. Throw the carb on and see how it runs first.

 

I'd like to hear more about this. I have been thinking of trying this on a GSLSE engine to go in my GSL. Are you planning to start a thread? How did you do the manifold? Pictures?

 

I didn't do the manifold. They started with a 6 port 12A manifold like this.


And turned it into this. Then JB welded it when it cracked, which didn't work too well. I used RTV as a temp fix. No recent pics, sorry, but it looks cleaner at least. lol

 

By the way I have another of these modded manifolds. This time they took an SA manifold and welded on an FC flange cut from an S4 LIM. They didn't hog or smooth anything so the port mismatch at the aux ports is really way off. In contrast, this sick looking thing above flows way more partially because they started with a 6 port 12A manifold so the underlying casting was already set up for the aux ports. Only the runners are smaller, but I think only the primaries are the smaller runners.

Another interesting bit of info for you. When you free rev the 13B, you can get the secondaries to open! They're stock vacuum secondaries with no mods to them. So I know they're not normally supposed to open while free reving due to a lack of load and so forth. Since these can open it shows there's something different about the way this modded manifold pulls on the Nikki. Perhaps it's grossly undersized for the application? I'd say that's a real possibility, all that potential flow just dying to occour but getting blocked by the small venturis, so vacuum builds even at WOT under no load.

It's a fresh rebuilt engine so I can't really get on it yet, but I've so far had it up to around 4k trying to open the secondaries under a load while driving. I've never felt them open. The exhaust is not a restriction as it's a copy of the RB full "streetport" dual long primary with thier presilencers and powerpulse muffler under my REPU. I'm guessing the carb is simply too small.

You know, RB sells a Holley 600 and a 6 port manifold for this engine. Not the 465. Kinda tells me something right there.

 

Hmm although you have two barrels of the carb feeding each rotor their vacuum signal is mixed between the primary and secondary ports at the top of the intake mani unlike the individual throttle bodies while running twin dcoe's. I know that the velocity of the incoming air will be equal between the ports since each carb is feeding one vacuum source with twin dcoe carbs but since the ports are different size I'm worried about different masses of air flowing through them requiring different jetting on each side of the carbs. Has anyone actually tried using twin dcoe carbs on a 6 port engine? I searched and found others asking the same question but no real explanation of whether its actually been done. The best i could find was using twin dcd's which have two different size venturies.

 

I haven't bought this setup yet so unfortunately I can't throw it on and try it. Plus it will require a custom manifold since the lake city is for a 4 port 13b and mines a 6 port. I'm currently running a single 45 dcoe and love it i would just like the better low and high end of twin webers plus they look bad *** I'm still a little apprehensive about using them since I cant find anything giving a good explanation of how it would work having itb's on different sized ports. Whether the carb setup would be uniform across all the carbs or if the two center throttle bodies would be different?

 

I see what you're saying. Having had some degree of success with a fresh rebuilt 6 port and the nasty manifold above, it just might work for you. It won't be ideal, but in my opinion, no 6 port engine is ideal unless you have the stock intake manifolding on it, and we know even those aren't really ideal for max power. That's why lots of people switch to 4 ports in whatever generation they've got, whether 85 and older or 86 and later, which would be the turbo side plates.

But then there are some who've gotten really excellent results on their 6 ports like hyper4mance2k but you gotta realize his intermediate plate is actually a Y from a 4 port so it flows much better than any 6 port int plate. His end plates are ported GSL-SE I think. This is probably the most ideal 6 port setup you could have for max power. He's got a 48 IDA.

Sorry I can't advise you about weber stuff.