1st Gen, Fuel starvation?

[ScottNutter]

I was @ Mid-Ohio this weekend, and other than the severe understeer that I was fighting ( I do believe the extra 2.5psi I put in the tires helped this ), the car seemed to stumble/sputter on both hard rights and lefts. I had read before that the Stock Nikki carb has a problem with hard lefts? But this was both. As far as suppension I believe I have Eibach Pro Sport in front and Racing Beat in the rear, I had both sets and picked the stiffest front/rear pair. And Tokiko Illuma shocks setting 4 in the front, 5 in the rear. My crew says that I lean alot more tham my competitors, so my question is two fold. Has anyone else had this problem and could the location of the fuel pickup in the stock gas tank be related?

The reason for the last part is that it seemed to get worse thru the weekend, as my gas gauge dropped, but then again so did my times. (ie. faster thru the corners )

Thanks

[lateapex911]

I used to get a stumble in the left handers that got very bad as the tank got below 1/4.

When I was on a team that ran the 24hrs of Nelson Ledges in the 80s in stock RX-7s, we had to short pit as the pickup was worthless past 1/4 tank.

Now, after careful futzing, and a different (Carter) fuel pump, this problem has faded. But it still comes up at low tank levels. Many RX-7 guys run thier tanks half full all the time. They say it helps with weight balance.

I didn't see a mention of a sway bar in your post...most feal soem bar combination is the way to go.....

JMHO

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Jake Gulick
ITA 57 RX-7
New England Region
[email protected]

[ScottNutter]

I am running the Racing Beat up front, and currently nothing in the rear. I did have an adjustable in the rear. I took it off to see how the car handled. It was different, but better, although I am toying with the idea of putting it back on. At this stage in the game (first year racing) I am not sure how I want the car to handle. Originaly the oversteer was really scary Now with under steer, it was still scarry, but somehow straight ahead sliding is not as nerve racking as "I'm getting dizzy sliding", I have gotten used to a little slide in every corner. Now I would like to kick the back around in some corners.

Have you looked at the stock pickup? Are we allowed to modify it? Uh Oh better get my GCR out

Scott Nutter

[lateapex911]

Oh, I've LOOKED at it, and I've cleaned it, but no, in what amounts to another strange rules line up, we are not allowed to touch it, as far as I know. BUT.....you CAN add a fuel cell, and put it anywhere you want within 12" of the original location, and add a surge tank and any kind of pickup that you want.

A cheaper way is to call Paul Yaw....they say his carbs don't stumble. And keep an eye on your float level. And make sure your pick up screen is CLEAN, as are your two fuel filters.

This is one of those "If it doesn't say you can, you can't" rules. I'm conjecturing here that the rulemakers were aware of the apparent illogic but felt that they didn't want backyard mechanics modifying and welding the stock tanks. So they gave us the fuel cell option. We can even use the rotary molded cells!

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Jake Gulick
ITA 57 RX-7
New England Region
[email protected]

[Allen Brown]

Just a question....Did the fuel tank change in the later years? I have a 85, but it's a GSL-SE (1st yr of fuel injection). I can run my fuel so low the little yellow idiot light has been on for two or three laps. I race at a track 2.1 miles long that has 6 lefts and 3 right hand corners.

I have seen other small changes in the later years of the 1st gen., so I thought I would ask the question.

[rlekun]

Scott, I struggled with the exact same fuel symptoms you described. What you describe as fuel starvation may actually be too much fuel getting into the bowls caused by too high fuel pressure setting! Even the stock pump and most aftermarket pumps can overwhelm the stock carb. Are you using a fuel pressure regulator? If so, if its set any higher than 2.5, that may be the problem. When I changed my setting a couple of years ago, all my sputtering went away.

Try this before you do anything else! Down the road when your racing budget allows, a fuel cell with a sump tank in it will eliminate any low fuel level & pick-up concerns (although in almost 10 years of racing I've never had a fuel pick-up issue).

I was at M-O this week-end as well...the black #87 RX-7 that was eliminating the West Nile threat by smoke treating every corner

Larry N

[This message has been edited by rlekun (edited September 10, 2002).]

[Skid]

Stumbling in the corners and starvation on the straights are two separate problems - first is carb related and the second I'm still trying to figure out. At anything under 1/3 a tank I get starvation above 6000 in 4th gear halfway down the long straight. The car stops accelerating and if I feather the throttle it will pick-up again after a few seconds -- its like the bowls are empty and need a second or two to refill. I can only surmise its fuel pick-up related. My pump tests out on-spec in the volume test and filter is new (haven't checked other screens).

RE a post above, I can't see that its a problem with too much pressure, then the problem should be greatest with a full tank and be mitigated as the tank level goes down.

Here's an interesting calculation: the hydrostatic head in a full tank might add .25psi to the 2.5psi that the pump generates. So is a 10% reduction in fuel pressure from full tank to empty tank enough to cause such a performance difference?
Or is this a pick-up location/design problem that is only going to be solved with a proper fuel cell?

my yaw carb has baffles in it, has no stumbling problem

[rx7chris]

My mazsport carb has baffles in it. It helped a little, The big improvement in getting rid of the stumble was dropping the fuel pressure down to 2lbs. As for specifics, I'm running a 16gal. cell, Carter low pressure pump, stock line to engine compartment, holley regulator, in line pressure gauge, Mazsport carb. Problem, for the most part is gone. I noticed a little 2 weeks ago at turn 17 at Sebring. I can live with one corner, it used to be almost every corner.


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Chris
IT7 #88
PowerTrip Racing
http://www.geocities.com/ptripracing/

[Hotshoe]

The faster you go the more you will need to upgrade different systems. You have reached the point that a stock fuel tank will not deliver a CLEAN fuel supply. Key word is CLEAN. No air. That is your biggest problem. You expose the pickup during cornering then after you go about 100 yards the carb gets a supply of air instead of fuel.
A fuel cell with a surge tank is the best cure. I run two fuel pumps, one pumps fuel into the surge tank from the fuel cell, and the other one pumps from the surge tank to the carb. I positioned my surge tank in the fuel cell in the left rear corner. That keeps it in the coolest location. The pickup for the pump that pumps into the surge tank is located in the right rear corner. The only little thing that helps is if you buy a surge tank with the trap doors in it. That allows fuel in but not out.
After I did this to my car I was surprised at how much more power the engine had because the carburator was getting CLEAN not aerated fuel.

#99 IT7 Rick Thompson

[ScottNutter]

Thanks for all the replies.

I have an 83, the tanks did change in 84-85 because we tried to put an 83 tank in an 84 and it didn't quite fit. I have a Carter 7 PSI pump, pretty much in the stock location, stock lines to the front and a Holley 1-4psi regulator set at originally 2.5 and now 2 psi using a Summit Racing 1-15psi gauge (accuracy?).

How close is the pickup to the bottom of the tank? Would a hose clamped on extending it help?

rlekun: Thanks, I will attest to a mosquito free weekend while on the track. I was thinking about installing one of those smoke screen buttons on my car too

[Skid]

What Hotshoe says seems logical when I think of the corners prior to the onset of my straightaway starvation problem.

Heh heh... It also tells me why I sometimes get the low-tank starvation in a 7200 rpm 3rd gear sweeper which comes after a flatout sequence of other broad turns. I really love that feeling of the back end getting light at the apex just when I really need it be planted under throttle.

[rlekun]

Scott, I defer to the other better minds that posted above. From your original post, you said the sputtering was in the turns, not down the straights. Do you get the sputtering at M-O immediately upon exiting the keyhole when you're trying to get on the gas? Same with the carousel and maybe even turn 1?

Baffles in the carb? Where's my GCR....

[ScottNutter]

I would sputter when getting on the gas exiting the keyhole(turn 5), turn 8, turn 9, sometimes turn 11, and the carrousel. I realize that as I get faster, systems will have to be upgraded. However I really wanted to switch to racing springs and a threaded perch so I can get more negative camber and less body roll this winter. Maybe I'll try the 3/4 tank of gas trick at my next tow events BeaveRun 9/28 and Nelson 10/20. If anyone is going to be there look me up.

Scott
#85 Blue/Orange

[bcarter]

Theoretically, Grose jets will ease the starvation problem in the corners.....that's theory. I used them last year, and had fewer problems than most. This year, I had a carb built by Yaw, but just a stock rebuild, nothing beyond stock, no jets, nada. I had trouble getting the float levels right with the Grose jets, so I switched back to the stock needles. ( Paul doesn't like Grose jets anyway) I have had some major starvation in our left hand turning carosel this year, and have to keep pumping the pedal all through the 180 degree turn, particularly in really hot weather, to keep going.

No stumbles down the straight, I think that is a different problem

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G. Brooke Carter
# 10 Challenge Car
Calgary, Alberta

[lateapex911]

I have Grose jets in my carb, but they sure haven't been trouble free.....

One time, at Band Camp, Ok, sorry, at Tech inspection, I left the master switch on, and the fuel pump was running...and as I stood back while they poked and prodded (and came up with objections over things that last years tech guy said was "really cool"...), I noticed a growing puddle under the car.....I tried to casually walk up and turn the swich off, but somebody spotted it and I was told to get the H outa there and don't come back no more! Ugly! lately they've been fine, but the problem I mentioned is a repeat performance.

As for baffles in the carb....I looked but came up empty, and if doesnt say you can do something, well ,then you can't! I've heard mention of those before, and I've always wondered about the legality.

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Jake Gulick
ITA 57 RX-7
New England Region
[email protected]

[ITSRX7]

Why not just corner weight your car with a full tank? Fuel loads tend to be low and if you are around the min weight then it shouldn't matter.

If the weight isn't a factor from a positioning and minimum standpoiint, why not run a full tank and eliminate these issues?

We run a full tank here in NER on a 2nd gen car with the weights all secure.

AB

[bcarter]

Funny thing about running a full tank, after a while in a two hour endurance, the darn thing gets empty. I figure I'm good for about 1 hour and 20 minutes before it's time to stop, and I only want to stop once.

[ITSRX7]

Endurance racing is a whole seperate issue!

AB

[bcarter]

Endurance racing may be a different issue in terms of the style of racing, but a race is a race, and I would sure like to find a way to ease the left turn problem that occurs with most of us. We also run a lot of 15 - 20 minute races.

I have the problem full tank or not, so it really needs some sort of technical solution.

I agree that the full tank lessens the problem initially, but high ambient temperature seems to make it worse. If anybody has a solution that stays within our stock carb rules, I would really like to hear about it.

We aren't allowed to use baffles, but I would be interested in how they work, where they go, and cost, as the drivers in our series can often get minor, inexpensive rule changes through our sanctioning body.

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G. Brooke Carter
# 10 Challenge Car
Calgary, Alberta