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 Steady cruise AFR problem and pressure at WOT 
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Joined: Mon Nov 30, 2015 3:55 pm
Posts: 47
Location: Jupiter, FL
Other than the weather, nothing changed but...

The carb:
Used to be a PS, was "touched" by Mark. It was set up as follows:
IAB: .067
MAB: .028
IFR: .036
TSR: .081
PVCR: .055
Main Jet: 84
Secondary Jet: 92

The fuel system:
Aeromotive 11203, Aeromotive regulator, fuel cell with -08 to the filter, pump, filter, then to regulator. -06 to the bowls. Deadheaded system. -08 vent from the cell.

11:1 408W. Ported single plane intake, 1" spacer, hydraulic roller 250ish, .640".

At the track, it was losing its ass after the eighth. It went from 104mph (660') to 117mph (1320') on one pass, and from 105mph to only 109mph on another. I was able to notice on the wideband that the AFR was in the high 13's at the end of the track. It didn't noticeably fall on its face, but the timeslip shows it.

I changed from the regulator-mounted pressure gauge to one on the cowl (not liquid filled, 2 5/8" Autometer mechanical) and found my pressure to be off. Driving around, I found that pressure was fluctuating a little bit while putting around, but it dropped instantly from 7 to 6 when I got on it. Then, after cruising around a little, pressure dropped quickly to 3/3.5, bounced to 5, down to 4, up to 6, then would stay there a little before randomly dropping back to 4, etc.

I replaced the pump with the same exact model. It did it again...

I rebuilt the regulator and so far, it seems ok. Need to cruise again, but it was immediately a pound higher after replacing the spring and diaphragm.

Now, my AFR at idle and while cruising in the mains is not an issue, and I'll have to see what wide open is doing if the regulator solved the FP issue. At heavy throttle (not WOT at all), AFR is in the low 12's, but it was getting lean when going full boogie.

Problem I'm having lately is the AFR gauge is going to 16-17+ while cruising around. Tip into the throttle and it fattens right up without an issue.

The idle mixture screws were only 3/4 of a turn out. I just bumped the idle air bleeds from .067 back to .073 (where it was some time ago) and got the mixture screws out to 1.5 screws with a decent AFR and vacuum.

Am I on the right track?

Wed Apr 26, 2017 7:18 pm
Site Admin

Joined: Fri Feb 08, 2013 2:37 pm
Posts: 1744
Is the fuel line from the tank to the engine close to the exhaust at any location? The WOT exhaust heat could be boiling it into vapor in the line ... ? This time of year the pump gas is more likely to cause vapor problems.

Thu Apr 27, 2017 12:50 pm

Joined: Mon Nov 30, 2015 3:55 pm
Posts: 47
Location: Jupiter, FL
Not at all. The line runs outside the frame rail, into the fender outside the engine compartment, and then to the regulator. The bowl lines don't pass over the exhaust, either.

Thu Apr 27, 2017 1:28 pm
Site Admin

Joined: Fri Feb 08, 2013 2:37 pm
Posts: 1744
Vapor is a common problem in the spring and fall, when the pump gas vapor pressure is "adjusted" for the seasonal weather.

If the old regulator diaphragm was even a little bit stiff that may have been the problem.

Does the carb get hot enough it is uncomfortable to hold your hand on it? Have you an infra-red temp gun?

Thu Apr 27, 2017 2:17 pm

Joined: Mon Nov 30, 2015 3:55 pm
Posts: 47
Location: Jupiter, FL
Thank you for the reply and info...Yes. Last night I leaned on it about a half hour after shutting down and it was still really hot. I have a phenolic spacer I can swap in place of the aluminum one on there now.

The pressure fluctuations "appear" to be independent of the lean issue. I mean, without load at cruise, pressure was steady but AFR jumps way up. Drops right back down when I tap the pedal.

With some heat in it, it has "gargled" a couple times. I let off and tap the pedal and it clears out. Again, that after cruising for a bit.

Thu Apr 27, 2017 2:29 pm
Site Admin
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Joined: Fri Feb 08, 2013 5:50 pm
Posts: 2363
This is a 1000 HP? Try going up to 85 in the front, 91 in back.

Fri Apr 28, 2017 9:17 am

Joined: Mon Nov 30, 2015 3:55 pm
Posts: 47
Location: Jupiter, FL
It's a mutt. The main body was replaced, but it's in the same co figuration as when you had it. I dont know if you have any I do on it but I can look at the part numbers when I get home.

I will drive it tonight with the rebuilt regulator and go from there with a new baseline.

Fri Apr 28, 2017 10:51 am

Joined: Mon Oct 21, 2013 12:15 am
Posts: 226
Location: San Diego, CA
If I can make a couple of recommendations:

1. Your supply line to the pump is very very important for maintaining volume and steady pressure. A good rule of thumb with an electric fuel pump is to maintain 3.5psi or higher at sustained WOT. There are two ways to accomplish this. First, you can utilize both outlets on your fuel cell, connect them to a Y-Block with (2) #8 fittings and (1) #10. Run #10 from the Y-Block to the pump. I also prefer not to have any 90-degree fittings from tank to pump. Straight fittings only. Let the fuel line have a gentle bend. Your fuel pressure has been fluctuating all of the time, and dipping low, you just realized it by moving the gauge and watching it. The second way to accomplish this is to simply install a #10 fitting in the tank, and run a #10 to the pump.

2. Use the least restrictive fuel filters as possible. I personally prefer to use System 1 filters, and I know they are expensive, so let your wallet dictate. However, if you can only spring for one, the filter in front of the pump is more important for flow.

3. I would also recommend a return-style regulator. I have heard opinions on both sides of the fence, but I like the return-style for consistent fuel pressure without the spikes that are common with dead-head regulators.

4. Another thing to consider is this: Your fuel pump is pumping to the regulator at 14 psi. When you launch your vehicle, the fuel pump is fighting the fuel in the line that is trying to return to the tank. When you reach the horsepower levels that you are at right now, it becomes imperative to go overkill on the pump, to overcome that. Right now, you are seeing the effects of that. Whatever you can do to combat the fuel trying to run to the tank is in your favor. If you take that into consideration, your fuel pump is effectively providing 3-4 psi to the carburetor during the launch. If you can get into the 18+ psi range with better volume, you will see less pressure drop.

I understand that your tuning issue is most likely attributed to the winter blend fuel that is still lingering. Based on your description of how your fuel system is configured; if you can improve it, you will see way more consistency with your tuneup.

Fri Apr 28, 2017 11:00 am

Joined: Mon Nov 30, 2015 3:55 pm
Posts: 47
Location: Jupiter, FL
Thank you, Revved Up. A return system has been on my mind for a little while, but a very sterile and clean engine bay requires careful planning of where I'm going to route my lines, and that's been my hold back.

I'm in South Florida...Palm Beach specifically. The first time I realized I was leaning out at the top end of the track was November, 2016. I only made one return trip since then and that was February of this year. I don't know if the fuel I recently used is the same VP as the fuel I ran in November, but I don't recall having the very lean cruise issue it has been having lately.

I'll drive it tonight with the rebuilt regulator post back my findings.

Thanks for all the input, guys.

Fri Apr 28, 2017 1:12 pm

Joined: Mon Nov 30, 2015 3:55 pm
Posts: 47
Location: Jupiter, FL
First of all, it's hot and humid today in South Florida.

Now, this is what I've done thus far:
Put the air bleeds and mixture screws right back to where they were to more accurately gauge progress.
Rebuilt the regulator.
Replaced the fuel pump.
Wrapped the lines from the regulator to the carb.
Wrapped the main feed line where it's within a foot of the exhaust (about a three foot run).

Got the idle back to where it was with an AFR in the 13's.

On extremely mild acceleration (just resting on the pedal), the AFR was going lean, into the 15's. Any little bit heavier and it straightened right out. A heavier foot made it go into the high 11's and low 12's.

Cruising in the 2100-2600 range (slow speed, steady pedal), AFR went into the high 15's and fluctuated into the 16's.

Cruising temp never went over 182, by the way.

Turned onto a different road and got on it, but no where near WOT, it accelerated ok, AFR into the 12's, then 13's, then in high gear as I was still accelerating, it went into the mid 14's.

After that, the lightest bit of acceleration made it feel like it was loading up, but the AFR gauge would go into the 16's and 17's. A heavy blip of the throttle made it go right back to the rich side of the gauge and cleared it up.

As I got closer to home (and as the "loading up" feeling and corresponding lean reading continued), pressure started dancing again. Steady at 7, dropped to 5, go up to 6, drop to 3.5, etc.

The only thing that changed from when the car drove ok was the fuel. It runs on 93 and I get it from the same Chevron station all the time. In November, when I ran like crap at the track and I noticed the issue, it was on fuel from the same station.

Does anyone think it's worth draining the cell and starting with fuel from a different station? It's so frustrating. Keep in mind, the tune was pretty close on this thing and no parts (jets, bleeds, etc.) were changed.

Sat Apr 29, 2017 1:24 pm
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