Carbureted 302, regulated elec pump, heat soak causing lean AFR change

[corerftech]

Hey all.
Thought I had my 302 with edelbrock 600 all lined out but I have seen an interesting deviation.
When tuning I nailed what I liked for AFR across the board. The engine bay was plenty hot for 100 miles of test drive and jet work.
A couple days later I hop in on a cool night and AFR is 1.5 richer. Stays that way for very short drives. It soaked briefly and AFR went UP leaner about 0.5. I initially thought air temps as test drive was evening and tune was a warmish/h day.

Today I drove it around town getting parts and many stops. First start was beautiful but AFR was fat compared to my original targets. After first stop, it leaned out a half point. Second stop a bit longer drive, it went 2 full points leaner. Too lean. All in the primaries.

My FPR is a QFT dead head and set for 5.75 lbs. My fuel pressure gauge doesn’t have enough resolution to see small changes effectively.

So my concern: the more heat soak the carb gets, the leaner it gets, to an end point.
The colder it is the fatter it gets, always during an overnight or long break of 12 hours, etc.

Now I jetted it in its last iteration, at 5.75 lbs, relatively cool. I am question if I need to lower fuel pressure maybe one full lb, then Jet to that pressure. Are my boosters leaking just a tick, that I can’t see…. When cold/cool and fuel pressure is at max regulated, then when heat comes on the small leak ends due to a drop below the leak threshold pressure and it then acts like it is jetted?

This is definitely a heat soak condition. All I will need to do to jet change is drop a lighter piston spring in and it will move 1.5 AFR points properly. But….. that’s won’t address the heat soak. I’ll then reverse the condition and have rich for 20 minutes. And then desirable afr.

Any thoughts? Edelbrock says not to let pressure drop below 4 psi but I’m taking about adjusting to 5 even, or a pinch below.
Low performance stock engine. It’s 200 hp, not 400, and that is being generous.

Appreciate the thoughts. Love to hear some wisdom. In any case I’ll drop the pressure and change springs and give it a try. Will report back.

Thanks for your time!

 

[Uncle Gump]

I think if the needle and seat are holding... dialing fuel pressure down a pound isn't gonna fix a thing.

While looking at your AF meter changing... how does it run? How are you driving when you see these changes. In my experience... long steady pulls at mid range or wide open and then just straight idle is the only time I would even look at that meter.

 

[corerftech]

When it goes lean, it runs like it, poorly.
when it’s fatter on the meter, it runs accordingly.
A jump from 12.5 to 15.5 AFR at low speed in all conditions is significant when the shift occurs.

 

[corerftech]

Figured I’d complete this thread with a resolution.

So after some ignition woes were uncovered which affected AFR, and conversion of my AFR meter to lambda- I have determined that there is a temporary shift in AFR caused by heat soak on the edelbrock. It is small and temporary.

It is about .05 lambda and until sufficient fuel and air have passed through the carb it stays that way. The cooling affect around town due to low volume and velocity is not enough to snap the carb cold (150 degrees). Take on a freeway for a couple of minutes and it’s back to a set point. I chased the AFR at idle around the volleyball pole and making matters worse I had some small defects in ignition making it seem larger. Once ignition was ironed out the AFR is repeatable like the sun rising with the exception of a parking lot heatsoak and a temporary shift. If in town grocery store running, it takes a good 3-4 minutes to slide back to normal. I live in the city so 25 mph stop and go.
Anyway- that’s a wrap.
BTW- after all tuning and bug finding is complete I am getting 20 mpg on highway at 72 mph. AFR is a solid 1.05 lambda or 5% lean (call it about 15.0 for gasoline) and the past EFI never thought about 20 mpg. I am quite pleased with the economy. Low speed AFR is in the 1.07-1.12 range and just a tick on the lean end. 1.1 would be the upper limit this engine is absolutely happy with as test drives have shown. I really don’t have any other iterations to throw at the low speed circuit except a metering rod of 2 numbers smaller which will likely alter the whole spectrum. It’s spring and jetted and metering Rodded to very near perfection and I will have to compromise somewhere. Likely the change from the 73 rod to 71.

 

[superj]

why did you go with a 600cfm?

for a stock 302, that is kind of large. i know you adjusted the jets to read properly but maybe a 390 to 450 cfm would help out on the trouble?

maybe you need to get some of those hood stand offs to allow hot air to vent out the back of hte hood?

i saw you said its fixed, just throwing out ideas for people to think about later in case someone else also has this trouble

 

[Brain75]

One thing we figured out with other carb'd systems to combat heat soak is to put a wooden (typically a hardwood) spacer between the carb and intake - literally an insulator between the two pieces of metal. On my 48, the studs are long enough I could get away with a full 1/4" + a little bit. On some of em where the studs aren't long enough folks have said they get away with just 2 extra thick gaskets and even that helps. Also cuts down on the bowl boiling dry after parking for a few minutes / vapor lock when you stop for just a few minutes somewhere and it isn't long enough to completely cool off / evaporate and start easy again..

FWIW, as a point of comparision, I am running a 6# pressure on a Holley 847 (1948) on a stock 226 straight 6... those have a heat shield to keep exhaust heat from over-warming the intake (both intake and exhaust are on the passenger bank, intake over the top of exhaust - duh Ford why??) and are notorious for hot intake air due to the bad design.

 

[19Walt93]

I run a 1 inch, 4 hole plastic/phenolic spacer under my carbs. It improves the signal to the boosters and insulates the carb from manifold heat. Most carbs are happy with 3-5 psi, if it's not flooding your 5.75 is probably OK. A 600 isn't too much for a basically stock 302, I tried a 390 cfm Holley on one I built years ago and it had less power and got less mileage than the 600 did. I also run Holleys becuase the throttle response is much better. I swapped my son's Edelbrock onto my Mustang years ago and it cost me 3 tenths on the drag strip and had soggy response. They work OK but if OK was good enough I would leave it stock.

 

[85_Ranger4x4]

A stock mustang HO 4bbl was smaller than 500.

I have a 500 Edelbrock on my HO and the heavy lifted brick on aggressive AT's flirts with 20mpg running heavy on the way to offroading trips, carb is nowhere near tapped out. If you got yours dialed in that is great, a smaller carb may help throttle response though.

I have a 1/2" phenolic spacer, Edelbrock bowls are mounted low and transfer heat very well. I also blocked off the exhaust crossover under the carb in the intake and noticed no ill effects.

 

[corerftech]

I deployed the composite edelbrock spacer which is maybe 3/8 inch at the start. It was available, cheap and reportedly up to the task. My engine bay has cooled off with all of the tuning and I am running the stock 195 tstat. It is quite cool, but having an “instrument” that catches the eye is a two edged sword. Too much info that is available for interpretation. Granted it helped me sort out the ignition woes that were affecting AFR stability, but now that my attention has been drawn there, I scrutinize quite tightly and now likely too much scrutiny. There isn’t a defect (any more) as the AFR/heat issue also had ignition issues compounding.

I have no operational issues, no defects- I had wanted to post a final follow-up to the thread. Yes there is a slight uptick in AFR with an edelbrock on heat soak that has a fairly specific timing to reverse while driving. Would you feel it- nope. But an electronic device has measured it and it’s reliably consistent and repeatable.

The AFR meter will be removed next month as it will only detract from the joy of driving. Later maybe it will be used for a diagnostic tool of engine wear kicks in. Else, it’s done

The wood spacer is on a bucket list to place into service. Due to other issues needing debugged, it has been low priority. I know it will remedy the minor shift.

As for a 600 cfm, it was a:

Pawn Shop Covid 19 NIB 1905 and Pawn Shop/Covid CHEAP.

It was easily tuned/jetted to perfection. I can pull over and in 3 minutes change rods and springs and in 20, change jets which is only once during tuning.

Did I mention it was cheap? Pawn shop cheap, like $250 bucks off cheap. And new in the box. My wife and I rolling down the interstate at 70 to some weekend destination will never know a throttle response change. It is as gentle as an efi system and when you go WOT the air door does its job as gracefully as an efi fueling increase. It is smooth, linear, not abrupt and hold an AFR of 0.86-0.91 the whole time from part throttle to WOT at 5000 rpm. I need a car/carb/tune my wife can drive and not be afraid of. Economy, not performance is the driving force with no lack of driveability.

I’ll end with this: I am scrutinizing a jet/rod combo to .001 inch. And that is to obtain exactly 1.08 to 1.1 with .95 lambda enrichment at tip in from idle to the point that low speed operation/part throttle ends. That is efi realm behavior and it has required 1 number Jet/rod diameter changes. It’s been awe full easy to calibrate this carb following debugging other influencing factors.

 

[85_Ranger4x4]

That stupid thin Edelbrock spacer... did not work at all for me lol.

Good find on your carb, new ones are so stupid expensive they are hard to justify vs EFI.

We have slightly different end uses too, mine being a 4wd low end power is my endgame. Higher air velocity thru a smaller carb at lower RPM helps keep things moving. MPG/drivability is big too, I am like 2-3 states away from anywhere fun to wheel.

 

[corerftech]

Agreed on the edelbrock spacer. I don’t even like the way is is assembled with the steel bushings at corners.

I really think wood is the way to go.

I have a backwards mounted carb due to linkage (front is back/back is front) making idle and mixture adjust cumbersome and the GM TPS (Hughes) system is a bitch to align with my linkage issues. So removing any carb is the biggest PITa and needs lots of finesse to get happy. The wood spacer is on hold until I have patience for that whole process to be repeated.
I would like to make the spacer a bit 1.5 inch as I have almost 8 inches of vertical air cleaner in play. Lots and lots of free airflow. And it clears hood. I would delete a 1.5 round filter from the vertical stack to make room for a 1.5 spacer but that will make the manifold become more single plane than dual- I think?

Yes two diff applications. My zero dollar car is more comfortable to drive in than my Eddie Bauer expedition for my wife’s RA. So the road tripper car is the garbage can. It also gets 5 mpg better than the Exped 5.4. I can only do 16 tops downhill in that pig. And only 150k miles—- modular junk shit. Needs a pushrod to work right.

best

 

[85_Ranger4x4]

Why did you mount your carb backwards?

 

[corerftech]

The 2001 explorer throttle cable is really really long- like 6 feet. And the factory cruise control is equally long. Not having a factory linkage or cable support to fall back on, and not wanting to spend $$$ on aftermarket cables and things, I used a universal throttle cable plate/bracket kit for a carb and installed the carb 180 out to facilitate using the hugely long factory cables. This put the carb linkage in the correct geometry and placement to clear everything. So the carb linkage gets pulled forward not rearward. Mixture screws hidden a bit behind the carb now and idle/linkage on passenger side. Makes access to tps more convenient and its geometry is more difficult to get correct. All trade offs or zero cost alternatives. It also puts the fuel inlet in a better position, the choke cable (manual choke) in best position. It was a hinderance until the whole package started coming together, then it began to flow well around all the idiosyncrasies of the Explorer engine bay.

 

[85_Ranger4x4]

The 2001 explorer throttle cable is really really long- like 6 feet. And the factory cruise control is equally long. Not having a factory linkage or cable support to fall back on, and not wanting to spend $$$ on aftermarket cables and things, I used a universal throttle cable plate/bracket kit for a carb and installed the carb 180 out to facilitate using the hugely long factory cables. This put the carb linkage in the correct geometry and placement to clear everything. So the carb linkage gets pulled forward not rearward. Mixture screws hidden a bit behind the carb now and idle/linkage on passenger side. Makes access to tps more convenient and its geometry is more difficult to get correct. All trade offs or zero cost alternatives. It also puts the fuel inlet in a better position, the choke cable (manual choke) in best position. It was a hinderance until the whole package started coming together, then it began to flow well around all the idiosyncrasies of the Explorer engine bay.

This is in a second gen Explorer?

I figured it was your '87 in your profile.

 

[corerftech]

Got home from work and pin gauged with minus gauges, all the jets I have not installed in a carburetor. They are definitely not pairs that match and they are defintely not to spec. Does it really matter.... no.... but it gives me some latitude to select candidates for reaming to alternate sizes. I need a 102 or a 99, they dont make one.

92 pair is a 91/92
95 pair is 94+
100 is 100+ (101-)
104 is 104 ++
107 is actually 107 within .0002

Not nitpicking and I know the orifice diameter is not all that is entailed in flow, but the 91/92 pair is an issue.

I will likely not use ever the 91/92 so they will be reaming candidates. Curious to know if the jet pairs in the carb are matching.

I did confirmed at least one pair of metering rods is dead nuts within .0002, both steps. So I guess to get the last iota out of the carburetor I will be reaming custom jets. I am guessing all the rods will be very tight tolerance based on how they woudl be manufactured and easily measured, compared to a small hole.

I ran a spread sheet to create relative "numbers" of rod/jet/step parings so that I can visualize jet/rod changes rapidly. If you put an odd nunber jet in with an even step rod like 101 with a 70/42, you get a relative measurement of diameter differential. It is not indicative of real flow but it creates a number to justify a step of 1-3 numbers up or down. If you pair an odd stepped (one step only odd), you get a single number change which is what I am after.

Using a 101 jet, 70/42 rod gives two numbers. 31 and 59

Change jet to 100, now 30 and 58

Make a large step down to 98 jet
Make a large step down to a 67/37 rod
Produces a 31 and 61

So a smaller 98 jet with a given rod combo produces the same low speed step as a 101 jet but gives a flow increase of 2 numbers to the power step.
So depending on what pivot point you want, like your perfect at low speed but power step is lean, a smaller jet/much smaller rod fits the bill. Or vice versa. This also dictates some fuel change in the secondary behavior as the primary will influence the WOT final AFR.

I was trying to find combinations of available jet/rod that would produce (1) number changes only at ONE step value only. So a big matrix. There are few but they exist.
Some give a 1 number increase and a 2 number increase for the individual steps.

This is forcing me to pull the 40 year old German jet reamer set out and the pin gauges to make jets (volkswagen teenage roots) .001 UP. But then I found the uniformity issue with essentially all pairs but most drastic was the 91/92 set which is specified as 92. And 91 is a SMALL, MINUS 91 so more like 2 numbers different from its friend. I have not touched a carb jet since 1991! I have to relearn things....