i am thinking about running a barry grant road demon jr. 525cfm on my 85 rx with a 12a. what jets should i be running? is there a better carb to run? is tis too big? i don't want to run holley because i don't like them.

 

i don't know the ins and outs but this a lot more to just jetting the carb that needs to be done for it to run properly, like the air bleeds must be enlarged considerably.

 

i havent heard that. so far iwas told to plug the power valve and re-jet.

 

Never plug the power valve on any carb.

That said, never run a Barry Grant carb. I like to think that BG's success is a function of good advertisement. I have never had ANY luck with a BG carb, they are complete garbage for metering and atomization. I have had them run horribly lean while at the same time blackening the plugs. Further, nobody I have ever talked to has had any good experiences with BG carbs.

In short, the only people who actually want them or advocate them are people who've never had to deal with one!

All that said... there is a lot more to making a Holley style carb work on a rotary than "rejetting". You have to alter a lot of things that are not adjustable. The easiest and cheapest way to go is to just buy the RB setup. It's sorted and works extremely well.

 

what all has to be done to the carb to make it work right. i dont really want to spend $550 on a carb i can get for $280. i've been trying to find out what mods are needed and so far i was told plug the power valve off and jet to 68/69 primary and 70/71 secondary. if i knew what mods all have to be done, then i would know if i can do it or if i gotta pay rb for theirs.

 

Never plug the power valve on any Holley type carb. That's a redneck thing and all it does is waste fuel.

I haven't dug into an RB carb but you need to modify the air bleeds, the many orifices in the idle circuits, the PCVR's...

Buying a BG carb is just wasting money, they are GARBAGE.

 

Buy a 465 CFM Sterling. www.sterlingmetalworks.com. For starters the 12A will never under any N/A circumstances need more than about 365CFM, so putting a monster carb on isn't going to do any better than a properly sized carb. In fact it will probably run worse and be much more difficult to dial in. Secondly the Sterling is just a stock Nikki heavily modified to outperform most of the other aftermarket options out there. So out of the box you have a carb that was designed to run specifically with the 12A, only significantly improved for performance. Click on my cardomain link in my sig and go to the last page to check out mine. The pictures alone will show you the attention to detail that Holley sure doesn't bother with. Good luck man.

 

I hope the one on Tranquil's car was an anomaly because it was a Sterling and it was one of the worst POS's ever. We fought it and fought it and finally I put a stocker on it and it ran 100x better.

Whoever said the 365cfm thing is full of it. There are a lot of factors in play. One thing you have to figure is that the CFM rating is not only at a constant flow but at a certain pressure drop. The engine may "need" only X CFM on a long scale but it may need more than that on an *instantaneous* level, especially with an independent style manifold like the RB unit or the stock Mazda unit. And then, you have to figure that if you don't want a vacuum at max flow, you need a larger carburetor still. That's why you see the "stockport" RB carb is a 465 and even that is kinda small!

 

Anymore info on the bad Sterling? Is he willing to sell it or swap to a different engine? More info please.

Also what type of exhaust was on the car?

 

Forget the exhaust, probably 100% stock.

As I recall the carb just ran horribly rich (but not dumping from the boosters like the floats were set too high or pressure was too high) and at the same time it wanted more fuel. Like the metering and atomization were all FUBAR.

Plus there was no choke, which is kinda mandatory.

 

Ah, I see. Thanks.

 

Choke actually is not mandatory at all. I do not have one, and although on start up the idle is somewhat unsteady as soon as it warms up she idles beautifully. Saves me a few cents per start up on gas since its not dumping fuel in, and still starts every time. I have the fast idle to help when it's really cold but I haven't used it in months because the cable is twisted and keeps holding the linkage up. If the carb was running horribly rich then I assume you rejetted it. He purposely sends them out rich so people can't blame him for popping an engine saying it ran lean on them and it's his fault. A good tuning kit and some spare time is all you should need to get it running nicely. What fuel pressure where you running? It should have been 2 PSI if it was stockport and 2.5 PSI for streetport. Not that I don't believe you but this is the first I've heard of a bad Sterling, so I naturally want to know why it wasn't working properly.

Here's the mathematical way to tell you what size carb your engine will actually need.

12a rotary max RPM = 9000
12a rotary cid = 70
That's 70 cubic inches each rotation, times 9000 RPM, = 630,000 cubic inches per minute.
12 inches x 12 inches x 12 inches = 1728 cubic inches per cubic foot.
630,000 cubic inches divide by 1728 cubic inches per cubic foot = 364.58 cubic feet per minute.
That means that theoretically, a 364.58 cfm carb would be maxed out on a 12a rotary running at 100% Volumetric Efficiency @ 9000 RPMs
Now I say "theoretical" because the carburetor would have to be absolutely perfect - a "theoretical" perfect, at that. And no carb is perfect

Now I don't take my engine much higher than about 7500RPM, maybe close to 8 if I'm not really paying attention. So with an 8000RPM limit, my VE would actually be closer to 325 CFM. This is the max my engine will flow given perfect conditions. At 8000RPM it is pumping more air than it ever will at any lower RPM, regardless of any other factors (aside from forced induction) it will not flow more. This is hard proof that you do not need a big carb to feed the engine. In fact it would do well enough with a carb that could only flow 400CFM, as long as it was designed properly.

 

Try starting when it's near freezing with no choke. (I dunno about Ontario, but in Ohio, even in the driving season it can get damned cold overnight)

At least, try it if you have your idle adjusted for best economy (pull fuel away until it lean-stumbles just a little - really helps low load fuel economy). If you have it adjusted super rich so that it swills fuel and kills parakeets from 50 feet from the exhaust fumes then you can probably deal with not having a choke.

 

Well that's why I kept the fast idle. I drive the car up until the first snowfall. And yes in the middle of summer there are some snappy cold nights. Not many this year though. Awfully lovely weather we've been getting.

I generally keep my idle mixture a turn or so from stumbling. As soon as the idle smoothes out I turn it back and forth to find the best compromise. But then again, I didn't buy the carb for fuel economy, I bought it to put smiles on my face, and she does. Well that is when the oil cooler isn't leaking. Damn aluminum. Although it still gets about 400KM/tank in the city, and close to 600KM/tank on the highway, and I beat the living hell out of her almost all the time. I personally cannot find any faults with the Sterling. It's well designed from the factory (if not alittle undersized) and well modified after the fact. But to each his own.

 

just to quantify the idle mixture/mpg thing. my last car was a stock 79, even rebuilt the points dizzy.

anyways, with a fresh tune up, and lean idle mix, i got a best of 25mpg. we averaged 23mpg on the 1000mile sevenstock trek last year.

if you have worn point, it wants more fuel, and mpg will drop as low as 16...

its a suprisingly large difference.

an electronic ignition car wouldn't get as bad, cause the ignition is much much much much much much better