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Fuel Starvation on left turns

Old 07-11-05, 03:59 PM
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Fuel Starvation on left turns

Hi,
We run a class here in Calgary with gen 1 RX7s that requires stock carbs. I get major fuel starvation on hard left turns which I think cratered my apex seals. Can anybody tell me if there is a setup issue on the carb or thats how it is?

Mike
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Old 07-11-05, 04:29 PM
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Originally Posted by mike horner
Hi,
We run a class here in Calgary with gen 1 RX7s that requires stock carbs. I get major fuel starvation on hard left turns which I think cratered my apex seals. Can anybody tell me if there is a setup issue on the carb or thats how it is?

Mike
I don't think it has much at all to do with your carbs. Are you using the stock fuel tank? During hard turns you're pickup is sucking air and starving the motor which could result in blown motor. Make sure your tank is FULL at the start. Sounds like a Pro7 series you're running in. You should consider a fuelcell.
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Old 07-11-05, 05:57 PM
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A fuel cell in and of itself won't likely solve it. You probably need a surge tank. Some people put one in the cell (I suppose you could put one in your tank), and others plumb one mounted externally from the tank.
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Old 07-11-05, 11:26 PM
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Darn of it is that it does it no matter how full, and only on left turns.
We call it Challenge Car, allowed street ports, adjustable suspension, Illuminas, header and Proxies R1.
Alot of fun, they sure have lively rear ends.
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Old 07-11-05, 11:49 PM
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Not Fuel Starvation -

Trust me, it took a long time to figure this out and then only with the help of a guy who A)races Rx-7's (quite succesfully I might add!), and B)builds carbuerators that don't cause the engine to shut off in long lefthanders. There is some internal baffling that needs to be done, and the short chimney on the stock float bowl vent actually allows raw fuel to come spewing out and dump down the throats causing a nasty super-over-rich condition to put out the fire. He built me a carb, and helped me set up my fuel system properly and "abra-cadabra" problem gone - just like that. I can put you in touch with him if you would like.

(And I used to trade e-mails with Mr Carter fairly frequently, and he was always friendly with sharing his experience. - Does he still run with you guys up there?)

Last edited by Boswoj; 07-11-05 at 11:52 PM.
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Old 07-12-05, 10:31 AM
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Originally Posted by Boswoj
Trust me, it took a long time to figure this out and then only with the help of a guy who A)races Rx-7's (quite succesfully I might add!), and B)builds carbuerators that don't cause the engine to shut off in long lefthanders. There is some internal baffling that needs to be done, and the short chimney on the stock float bowl vent actually allows raw fuel to come spewing out and dump down the throats causing a nasty super-over-rich condition to put out the fire. He built me a carb, and helped me set up my fuel system properly and "abra-cadabra" problem gone - just like that. I can put you in touch with him if you would like.

(And I used to trade e-mails with Mr Carter fairly frequently, and he was always friendly with sharing his experience. - Does he still run with you guys up there?)
The things you learn
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Old 07-12-05, 07:53 PM
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You need stock 12A carb for racing? You need Paul Yaw.

www.yawpower.com

I wonder if he's done any carbs for your Challenge Car series yet.
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Old 07-14-05, 08:44 PM
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Proper fuel level in the carb is critical. What fuel delivery are you using? Pump, regulator, fuel pressure? Return or deadhead?

I personally disagree with the baffles, and they would be illegal for him anyway.
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Old 07-15-05, 12:56 AM
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Float Levels! Float Levels! Float Levels! Check your float levels!

Every single RX7 I've ever driven has had this problem. Adjusted float levels (depends on the car as to the proper level) and bada-bing! No more Mazdafart. Takes a little twiddling, but you have no idea how much it hurts your laptimes until you fix it.
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Old 07-23-07, 02:22 PM
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Hmm, I was just considering posting on this issue when I found a thread which referred to this one. Had some issues with this while racing this past weekend with my Sterling carb.

It seems that I can reduce it by lowering the fuel pressure a bit (float levels look good). However, I think I have an issue with either my regulator or my gauge. Every day I have a different fuel pressure reading. And I had the problem fixed during test and tune on Saturday, but then had it again Sunday during the race (same course). Further reduction of the fuel pressure helped some more, but now I'm only reading like 1.1 psi

The regulator is a holley, and the gauge is the one from Respeed. Not sure where to start with this issue, so I'm open to suggestions... Thanks.
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Old 07-25-07, 01:58 PM
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I'll be at Gingerman this weekend with an IT car turned budget EP car, and I could probably walk you through it. There is a whole variety of things going on, but is is usually fuel slosh and not starvation.

Where exactly is your regulator located, and is it the low pressure Holley spring or the regular one?
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Old 07-26-07, 02:16 PM
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I'll be in Kalamazoo this Saturday for an autocross at Wings Stadium, but thanks for the offer (I wish I could take you up on it).

My regulator is located about 8 - 12 inches upstream from the carb. It is the Holley 1-4 psi (or something like that). Carb was modified by Sterling.

I understand that it is fuel slosh, you can see the black smoke when it happens. I think I have a problem with my regulator, since the pressure keeps changing on my gauge. I'll try a new one and see if I can get it to read consistently. Thanks for the input and the offer!
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Old 07-26-07, 04:04 PM
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Get the regulator directly behind the carb, or as close as possible. 12" to the side of the carb will make your fuel pressure go up or down 1/2 psi depending on which way you turn. (at 1.4 G. Don't know what you have for tires.....)

The Holley rebuild kit for the regulator is cheap, comes with a diaphram and both the low and high pressure springs. Can't remember which one is which right now, but the instructions say which color is which. Summit racing used to have the kits. Make sure its the low pressure.

The varying pressure suggests you are operating below the bottom range of the spring.

Verify that the return line is functional. the check valve can stop functioning, it is more than 20 years old....

If you have a big pump and a small engine, you can open the metering orifice up a bit in the carb banjo thingy....

Should be shooting for around 1.5 psi.

Lastly, evaluate the driving style. The longer you are off throttle turning left at max G, the worse it will get. In most cases, you really shouldn't be off throttle for that long. Because no matter how nicely you get it to work, there will always be a hint of it left, and you can use it to remind you that you probably just overcooked the entrance a bit if the problem is getting worse.

get the float levels down below the stock setting. Edge of the meniscus lined up with the stock line is a good starting point.

Make sure you haven't capped the fuel tank vent line that used to go to the charcoal cannister.
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Old 07-26-07, 05:05 PM
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I have this same problem with left sweepers. I thought it might have been fuel slosh the first autox so the next one I went in with a full tank and still had the problem. Guess its time to check the floats.
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Old 07-27-07, 01:16 AM
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Thanks for the advice Dyno, I'll keep working on it...
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Old 08-01-07, 11:11 AM
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Does anyone know of a better fuel pressure regulator than holley? One that can consistantly handle low fuel pressure like 2.25psi or lower?

And what would capping the fuel tank vent line do? What should we hook it up to when we remove the charcol cannister?
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Old 08-01-07, 07:44 PM
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The Holley works fine with the low pressure spring. I've been running 1.5 psi for years, no problems. The build quality can be poor, so its a good idea to pull the diaphragm out and clean it up or de-burr as necessary whenever you get a new one.

Capping the vent line will pressurize the tank. Run the vent line to some type of filter, away from anything hot, preferably back near the tank. A sintered bronze exhaust filter works well. It can cause some fuel spillage out the vent during sustained lateral loading.

The same problem exists if you leave the bowl vent valve in place but pull the power lead. The float bowls become pressurized and vent out the emulsion tubes.
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Old 08-18-07, 10:11 PM
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is there a way to adjust the floats without removing the carb from the motor or pulling it apart? I can see the sights but couldnt see a way to adjust them and the Haynes didnt mention a way other than bending the tabs when assembling the carb
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Old 08-19-07, 01:12 PM
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Hey guys, trust what the Dyno says. Not only dose the man know, he is connected.

Anyone like to buy a Yaw carb ? See "Parts for Sale" forum for a bunch of 1st gen OEM & race care stuff including a ZERO rust 1982 RX-7 GSL from Oregon or pm me.
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Old 08-19-07, 01:18 PM
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I sent you a PM yesterday about the yaw carb, let me know!
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