Flat top carbs. Myth, or.........

[Dobber]

http://www.jetlink.net/~okayfine/su/tunacan.html

http://www.zparts.com/zptech/tech_tips/izccposts/su_differences.htm

http://datsunzgarage.com/engine/index.htm

[SteveJ]

Thanks for the links. I still have the flat top carbs on my 260Z. They perform just fine for me. I haven't found a reason to spend the time and money to swap out.

[Bruce Palmer]

If it isn't broken....... Take the cash and run like heck!!

[Dobber]

I just think that it is interesting that for everyone that says they are ok., there are ten more that say that, they are boat anchors. I guess they are fine as is but are a pain to rebuild and modify for the same performance, out of the box, as the other Hitachi carbs. Some of the rebuild guys will not even take them as cores.

[sblake01]

Some, probably most, of the rebuild guys will not even rebuild them.........

[motorman7]

Having tuned and running flat tops on my 73, and round tops on both my '70 and '71, I have several observations.

When the engine is cold, the round tops run better and are more predictable. The flat tops are a bit more finicky than the round tops and like to let out a backfire or two before they are happy. Once the engine is warm, my flat tops are on par with the round tops.

The round tops are much easier to work on and remove. The flat tops are a chore to remove due to the extra waterline that runs through them. In addition, the 4 nuts that secure them to the manifold are a bit harder to access due to the larger size of the flat top carb.

The flat tops are really a cross between a standard carburetor (the float bowl section, 'Power valve' section) and the side draft carbs (Needle with vacuum chamber). To work on the float bowl, the flat top must be removed from the engine. Although, the nice thing about the flat tops is that there is a sight window on the side that allows you to check your float bowl level without removing tubes and taking measurements. Use a small mirror and you can check the levels very easily. Not so easy on the round tops.

I read all three of the listed references and felt that the zparts article was the most fair. The author made one key point that is often overlooked and is critical; that being the Power Valve. It is the equivalent of the accelerator pump in a standard carb. If your accelerator pump in your standard carb is bad or disconnected, car runs lame. Same with the flat tops. As the zparts article said, this is often the trouble area with flat tops. The 'pump' instead of being a metal piston, like a normal carb is a thin membrane. Membrane tears, then carbs run bad. The fix is pretty easy, put in a new membrane. This can be done without removing the carb from the manifold -but may not be recommended. Also, the holes that the gas pumps through are small and can clog. Make sure these are clean.

Anyway, since I have worked on both types of carbs and am running both types of carbs I felt that I could offer an unbiased opinion based on real life experience.

Peace out

Edited February 4, 2011 by motorman7

[Dobber]

Great input. Thanks motorman7