Air Fuel Ratio

[80Merccapri]

Okay motor is in and running smoothly, carb is still going lean.The air fuel ratio is at 11.5-12.0 under WOT in 1st thru 3rd but as soon as your in 4th gear it goes lean to about 16 plus. Now the fuel pressure is at a constant 11-12 psi. Thru a 105 gph fuel pump. If I am just cruising in 4th gear and go to WOT it will pull for a second and go lean, only in 4th gear though. 1-3 are fine. Any advice? More jet, or more fuel pressure? At idle is at about 12.5 already so its a little rich and light cruise is at 11 to 12

[shadowracing]

What carb do u have on it if it's a holly carb u need to open up the high speed air bleeds

[80Merccapri]

It's a Holley 5200. Won't even hit 3 lbs without going lean in 4th and 5th but can hit 12 pounds in 1st thru third with a constant 11.5 air fuel.

[80Merccapri]

Alright did some jetting and some more testing and this is what I came up with, Holley 5200 primary jet is a 150 with a 185 air corrector secondary is a 235 main and a 160 air corrector. Car is to rich in 1st-3rd now so I am going back to a 210 secondary main jet. Car did not richen up in 4th or 5th gear at all with the 235 jet. It goes slowly lean and will not even build 4 pounds of boost at wot in 4th gear. Now if I only go half throttle to 3/4 throttle in 4th gear the car will pull and build 10 pounds before I let off prob would build 12 pounds at redline. So for some reason it goes lean under WOT only in 4th gear. In assuming this is because the engine is under a lot of load, but why pull at part throttle? Going up a hill in 4th and it goes lean instantly when I put it to the floor. On a flat road t will take a few seconds to go lean. I'm sure this is a carb problem, and I will most likely be switching to a bigger carb down the road but I want to get this running decent before spending a bunch more money on a carb. Motor runs great other than that and pulls down low like it should just weird that 4th gear only pulls part throttle. Wideband goes to 15-16 than jumps to 19 at WOT under part throttle stays right at 10.8-11.6. I'm stumped on this one so any advice would help me at this point. Thanks everyone

[Ethyl Cat]

Fuel volume. Do you have a pressure gauge on the car?

I figure you need a pump that can supply 130lph at operating pressure to keep up.

[80Merccapri]

Fuel pressure is set at 11.5 psi and I have a 105gph electric pump and a regulator on it. Thought maybe the bowl was running dry but even if I'm cruising at 2000 rpm and floor it the car goes lean and you would think the bowls would be full than.

[Ethyl Cat]

Is the fuel bowl boost referenced?

[Ethyl Cat]

Is the fuel gauge stable during a hard pull?

[Ethyl Cat]

105 gph at 11.5 psi or free flow?

[80Merccapri]

I'd have too look but I believe it's 105 at free flow and I cannot see the gauge under a pull but with it feeding the carb enough to make a WOT pull in 1st thru 3rd i thought it should be keeping up. Carb is not boost referenced either.

[80Merccapri]

Yes it's 105 gph free flow, and the carb is a stock 5200 that came off my n/a motor. Same one I used with my nitrous system but I jetted it much higher for the boost obviously. Car will pull from any rpm in 1st thru 3rd gear to 6000 rpm than as soon as I grab 4 th you can watch the air fuel just drop into lean territory and the car just cuts out all together. I tried increasing the fuel pressure that is why I am at 11.5 now when I used to be at 9. I jet higher and it makes the car really rich than still goes lean in 4 th gear. Gauge goes to 19. I can try more fuel pressure maybe. Power valve is stock. Car doesn't like WOT in high gears but like I said part throttle is fine and will take me to red line no problem. thanks again stumped on this

[Ethyl Cat]

You need to verify the pump is keeping up somehow. What pump is it?

Highest load demands the highest fuel flow. In the lower gears you have gear reduction which decreases the load in the engine. In fourth the reduction goes away and load is increased.

Another thought what is the plug gap? Wonder if the spark is being blown out by the cylinder pressure. Try gaping them down .005-.008" or so and make a pull. See what happens. Easy to do and might give info we can use for diagnosis.

[Ethyl Cat]

I am pretty sure that the bowls need to be boost referenced to keep the balance correct for fuel flow. The power valve needs boost referencing as well.

[Ethyl Cat]

I am assuming this is blow thru or are you running it as a draw thru set up?

[80Merccapri]

It's a blow thru setup and it's a mr gasket 105 gph. I can move the gauge with longer fuel lines to check it and the gap is set at .036 I will try closing it and increasing the fuel pressure a little. What's the easiest way to boost reference that carb?

[embler]

Where do you have the pump mounted?

[80Merccapri]

It's about level with the gas tank under the car

[Ethyl Cat]

It's a blow thru setup and it's a mr gasket 105 gph. I can move the gauge with longer fuel lines to check it and the gap is set at .036 I will try closing it and increasing the fuel pressure a little. What's the easiest way to boost reference that carb?

I was thinking of a roots blower for some reason when I mentioned the boost reference.

Ok, I do not mess with that stuff very much but here is what you should have:

A return style fuel system

A boost referenced pressure regulator. Your 105p came with a regulator that is not boost referenced.

Drilled out PVRC in the metering block. These must be tuned to size for added fuel under boost. Otherwise jetting it up will ruin the low end and midrange afr.

Standard fuel pressure of about 7psi.

No brass floats that will not collapse

I would add adjustable air bleeds for tuneability

A fuel pump good for about 400 hp at 22psi (125lph) Your 105p is rated to 14 psi max. It has a great free flow rating but pressure kills volume when it comes to fuel pumps. You need plenty of both.

Your fuel system should behave like a N/A engine until boost comes in. That means normal settings. Once boost starts coming on the pump pressure needs to rise 1:1 with boost to maintain fuel flow into the carb because the bowl is being pressurized to whatever boost is and you need to have a higher pressure to keep filling the bowl.

IE You have 11.5 psi set at the regulator and 12 psi pressure in the bowl. How is fuel going to get into the bowl when bowl pressure is higher? It cannot!!!!!

Can you tell me what the symptom would be from a situation like this? I bet you have seen it before.

[80Merccapri]

The carb has adjustable air bleeds on it, and has a plastic float. I understand what your saying about 11.5 psi of fuel pressure and 12 pounds of boost but why am I not having problems before 4th gear with the bowl pressure? So basically I need 7 psi of fuel pressure than an extra psi for each pound of boost I run so I would need 19 pounds of fuel pressure to feed the 7 to my carb and another 12 for the 12 pounds of boost? My regulator is a return style system, I'm looking at regulators for boost now though.

[Ethyl Cat]

You need to get a pump as well.

[80Merccapri]

I think I am getting the walbro 255 lph that is rated at 60 psi next week and a new regulator as well. For now I am going to lower the boost down to 7 pounds and crank the fuel pressure up to 14 psi to see if it will be enough to pull in 4th gear, so I can see if the fuel pump will fix this problem at all. I am switching to 1/2 lines from 3/8 also i believe. Now for the pvcr you said drill it out which I agree with and have heard from many people so that will get done as well.Thanks for the info so far, if you think I will have problems with the 255lph fuel pump what one would you suggest? I wanna get one and be done and not have to worry, I plan on seeing 18 pounds of boost in the future and the car still has a 75 hp wet shot on it. So with a boost ref regulator if I have it set to 7 psi for the carb and the car hits 18 psi of boost it will dump 18 more psi of fuel into it for the boost, and this is all from a vac line that read how much fuel to put into the car id assume the same way my boost gauge reads off the intake?

[80Merccapri]

I was told to drill out my power valve restrictions and wanted to make sure I was on track before drilling anything here. What I always thought was the power valve spring is pulled up with vac and when you start accelerating hard it pushes the spring down in return pushing the pin down releasing gas from the bowl of the carb past the pin and into the smaller holes. Giving you more fuel into the mix. So the second smaller holes would be my power valve restrictions correct? Fuel always has access to the larger holes on top because it is in the bottom of the bowl, but only when the pin is pushed down does it allow fuel thru the smaller holes and into the motor? Correct me if I am wrong just wanna double check before drilling anything

[Ethyl Cat]

I applaud your tenacity on getting this thing back running and all, but I think you should get your fuel system sorted out and gather data before you start drilling

Your assessment on that component seems correct. I have no experience with that carb though.

[80Merccapri]

Got a boost referenced regulator and backed the boost off to 4 psi still goes lean in 4th gear fuel pump can most definitely handle the 11 psi so drilling the pv is the next option. fuel pump is on the way should be here next week. Runs like crazy until 4th gear than falls on its face.

[Ethyl Cat]

11psi at what volume? You have to have both pressure and volume.

So the gauge stayed at 11 psi in fourth gear?