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.