AFR spikes lean rolling to a stop ?

Hey Fellas, Anyone here experienced a condition where your AFR gauge dives lean rolling to a stop ? If the stop is abrupt enough, it can cause the car to die. The carb is an ATM brand 950 cfm E85 double pumper.4150 body. Qft 110 gph mechaical pump. 7 psi fuel pressure at idle. The strange part is, nowhere else in the RPM band does the AFR read lean. AFR is 12.5-13.5 in gear at idle.. cruises slightly richer.. and about 10.5- 12.5 at WOT. I can't figure out why the gauge spikes lean as I apply the brakes while rolling up to a stop sign, the engine will then stumble especially if the stop is abrupt. It will then recover if I ease up to the stop and show normal idle range 12.5-13.5 ? What gives ?. I spoke with ATM carburetors and their tech guy had no answers for me. Any ideas carb gurus ?.. thanks in advance.

You check the float levels?

Yes, the level shows half on both primary and secondary bowls. Glass sights.

hi

if you have power brakes you may have a vac leak ?

Try lowering them. IFR in the upper position may cause it.

Your rear float is too high.

Its showing lean on the gauge because a misfire always reads lean. The gauge may not be reacting fast enough. If you record it you might see it start to rear rich before it pegs lean and stalls.

Cool, I'll give that a shot and report back.

[quote=GTX MATT]Your rear float is too high.



Ok, lowered the rear float to where the fuel level was at the bottom of the sight Glas. Same condition.
So, for giggles I even lowered the primary float to the same level.. still spikes lean and stumbles ?.. any other suggestions?
Idle screws are out 1.5 turns.. only way to mask the condition slightly is to bump all four up to 2+ turns, then .. I'm idling at 11-11.5.. gurgly on take off.

i fix the same problem several times by riching up the idle fuel restriction

He didn't say it was misfiring, you lead him in the wrong direction.

To the OP, I always had problems with holleys and hard stops, I had the opposite problem tho with the carb flooding. It sounds like ur carb is setup too dependent on the front barrels, if it's a 4 corner idle carb try opening the rear blades a little more and getting your idle screws setup properly.

Well, I ordered an IFR kit from Summit. The carb is specd with 46's from the factory. Swapped them for 52's. No change. Swapped them again for the largest in the kit . 59's.. seems to be masking the condition slightly. The car overall is running excellent other than this lean condition when on the brakes. Is this just something most guys live with with these types of carbs ? My old 850 double pumper didn't seem to have this problem, but I never ran that combo with a wideband O2 sensor either. .. I don't recall it stumbling or stalling while braking but,, like I said... totally different combo.

Does your carb use jet extensions? If not, have you tried them?
https://www.holley.com/products/fue...ing_rods_bleeds/main_jets/parts/122-5000

Have you tried going in reverse & hitting the brakes if so does it still do it ?

Yes, factory specd in the secondary only.

No, but I'll give it a shot and report back.

How about timing? Maybe more advance?

When checked with a timing light ,I'm all in at 38° at 2,000 rpm. Msd Pro billet dist mech advance only...no vac advance, Msd Digital box.

But what is the timing at idle, you should be in the 20 to 25 deg , & about 10 to 15 deg of mechanical advance

He'll never get that with MSD bushings. 18 degree is the smallest. So he's probably @20 initial. I had to get bushings from FBO, they are 10 and 14. I think he needs to find out whether his IFR is in the high or low position. Run a hose between the vent tubes with a hole on top in case fuel is coming out on a hard stop spilling into carb.

I'm at about 20-22° at idle. 37-39° at 2k rpm. Ifr's are in the high position. Upper corners of the metering blocks. The condition will occur even coming to a stop rather gradually. I run a reverse manual valve body.. if I'm in high gear approaching a stop and down shift to second just before it trys to stumble, the rpm increse will mitigate the stumble if I slow down gradually enough. The gauge will still flutter towards 15-16.. but the car won't stall.

You need to move them [IFR] down to keep them covered in fuel. They may need to fattened up a bit especially with e85.

I've fattened them up to the largest IFR's I could find. It's a purpose built E85 carb, it came with size 46 IFR's factory. I now have size 59's installed which are the largest available in the kit I bought. Is there a brand of metering block you know of that has the IFR location located at the bottom ?

You don’t need new metering blocks. Drill and tap the lower position and use brass set screws from McMaster-Carr or similar and drill your own jets.

Yep it's easy lots of videos out there. The screws are 6/32. Drill slow with some oil.

I use a pin vice to drill my brass set screws, #6, #8, #10 or larger
No oil though, just hand pressure pressing and slow hand rotation speeds

Ok, I'll move forward with relocating them. Hopefully this does the trick. I'll post the results. Thanks for the input all.

a word of caution when drilling brass set screws, or enlarging holes that are already there. this applies to all methods, drill motor, drill press, or hand operated pin vise.
watch the "break through" at the end of the hole. this is where the majority of broken drills happen, because the flutes of the drills stick on the last couple of fines. this happens whether using lube or not, and is just the nature of the material.
it is also a good idea to replace the bit size often, so you have a sharp cutting edge.
most think brass is a soft material [and it relatively is], however, it is abrasive, and requires the sharpest tools you can use to successfully cut, machine, or drill it.
just speaking from a 45+yr working career as a machinist, [and still practicing] with lots of scrapped brass and tooling under my belt.

had this same issue, before you go nuts with drills and modifications try dropping your timing to 8 or 10 btdc and open up your primary throttle blades, turn your idle screw in.

Welp, after screwing with this carb for months dealing with this condition, I've just decided to cut my losses. Switch back to gas and put
my trusty Holley 9380 850 double Pumper back on. I tested it today.No more stumble while rolling to a stop !! HALLELUJAH!! .. Perhaps relocating the IFR's down low would have fixed it?.. but still , I didnt wanna go drilling away on the carb based on what is still just a guess in my opinion.
Wanted to say thanks to anyone who gave feed back or offered input. I'm gonna sell the carb, hopefully the guy who buys it can tune an E85 carb better than me.