Flat Top Mods?

[wantazee]

I'm looking for some guidance to help modify the 76 260z carbs.

I've read some posts by Tony D that give some hints, but I'm looking for specific information. Can anyone post the Nissan suggested modifciations for the carbs?

I think having seperate idel and primary circuits would be advantagous.

Any help would be appreciated.

http://www.zcar.com/forums/read.php?f=1&i=497908&t=497807#reply_497908

"making some of the Nissan Suggested modifications and richening up the idle circuit backfires go away."

"You can slightly richen the idle circuit by making some of hte passages larger allowing more fuel. Once this is done, you can run fine, AND pass emissions with a properly set AFR at idle and transition.

This will allow the "fat needles" to be run for a MUCH richer power delivery curve, WITHOUT the "damn pig rich idle" EVERYBODY with round tops complains about when they have the alternate needles in them!"

"The REAL problem with the carburettor is the LACK OF FUEL RESERVE. The 74 carbs have little to no fuel capacity, so unless you have a damn good fuel delivery system (and almost to the point of using bigger jets to let more fuel in faster) you run the chance of sucking the float bowl dry on a 74 carb when drag racing due to it's small capacity. "

"The lack of reserve was meant to stop percolation after shutdown, and to limit residence time in the bowl where it will pick up heat and be easier to vaoprise/percolate. When you increase the volume by altering the die-cast intrusions into the bowl, you end up getting like the 73 carb that may percolate over-rich after shutoff. But insulation, removal of the EGR manifold, NOT REMOVING THE HEATSHIELDS OR USING HEADERS will keep themsp much lower minimiszing the problem."

[240ZX]

The general concensus of most folks is to circular file the flat tops for a set of round tops.....very simple modification and does not involve dorking around with the internals of an ill designed carb.

[Zedrally]

Take it from one that has flat tops on one of his vehicles, that the ONLY mod that you would want to do is a changeover with round tops.

Perhaps a complete strip down and overhaul will solve your problem?

Believe me, I've thought about how to do it and the only way is to engineer so system that allows the mixture to be varied. ie variable height jet that the needle sits in. All that you would achieve is a complicated round top SU.

All the time and effort that you have expended would have brought you a set of round top SU's.

That the reason they (the flat tops) where ditched in favour of FI. YOur other solution is to alter the float level, but be very careful as it's very time consuming to remove the carbies all the time to make minute adjustments.