I have a mostly stock 85 rx7 12a all that is done is holley 650 carb, racing beat intake, pace settee header, and custom air intake. As far as I know there is no return fuel line and no regulator just an autozone fuel pump. So my question is with this set up should I have an fpr and return line? And also the bottom plug on the first rotors keeps fouling any idea what could cause this?

 

get a regulator! you dont really need a return line.

 

Will it actually make a difference?

 

a carb needs to run at a specific fuel pressure. Yes having a regulator will make a difference. Im not sure what the holley needs to run at but say your pump puts out 10PSI and your carb needs 6 to run right. With 4 more psi than you need you are going to have problems. A return isnt needed. Having a regulator with no return is called dead heading, Im not 100% sure why you would want this, but to me (from what ive read) seems like the only thing not having a return does is put more stress on your fuel pump. Im sure someone with holley carb experience will chime in, but they are going to say that you need a regulator lol

 

a. What psi does the carb require? (I believe this is like 6psi)
b. What psi does the pump put out?

If b>a then yes...




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Your problem is the enormous carb on a stock port engine. The engine only flows 313cfm so theres no point in going above 465. A stock replacement pump doesnt need a regulator but is a miracle to run the engine with that carb. The jetting is probably through the roof too. That explains your plugs fouling. Strange that the trailing plugs dont foul.

Nikki and webber carbs need to be set at 4.5psi with a high flow pump. Holleys need 6psi. My sterling nikki technically flows 465cfm with a stock replacement pump and it runs great with no regulator.

 

That's the way I bought it and the guy I bought it from had it running perfect. Then this cold weather hit and its acting funny now and then. But i do think the carb is wayyyy to huge for a stock port but for now I'm broke. And kentetsu I think your right it is 6 il double check when I get home.

 

Take a good look at your ignition too. A lot of times, carb problems turn out to really be ignition problems. The fact that it was running fine before is something to consider before you start making more changes.

 

While I agree that 465 is in the right ballpark for a stock, street-driven 12A, I have to point out that this is a myth.

 

What about all the years of research racing beat and mazdatrix have done?

 

Well, as far as I know, Mazdatrix is just parroting Racing Beat's info. And like I said, I'm not doubting that number; that size is probably about right for a street car.

The myth I'm talking about is that you can calculate an ideal carb size based on the amount of air your engine ingests, or that there's no benefit in going over a certain size. Oh that it were that simple.

 

Well do any of you have a specific carb cfm that work well? I doubt il change the carb anytime soon and i just put accel super stock ignition coils in and it made little improvement and buying plugs once a month is too expensive

 

Unless you are racing, a stock carb is just fine, and matches the performance of the stock engine pretty well too. For racing, I like a modified Nikki.

The problem with running a carb that is larger than you need has to do with the velocity of the air going through the carb. Big carb = low speeds (air flow), which are harder to control with any sort of precision.



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Dont waste money on new plugs. Clean them. Spray contact cleaner in the crevises and use a soft brass wire brush on a drill to remove the buildup on the outside.