PY Online Forums - Bringing the Pontiac Hobby Together - Major lean AFR spike when secondaries open after stabbing throttle.

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- - Major lean AFR spike when secondaries open after stabbing throttle. (https://forums.maxperformanceinc.com/forums/showthread.php?t=833807)

Quote:

Originally Posted by Formula jg (Post 6060612)

Replies to latest questions.
Tom,
a. Current float setting is as you described in your post #13.
b. I will check vent whistle dimensions this weekend.
c. Vent tube extension holes have a greater area than total combined original vent tube(s) area.
d. PCV had been checked and functioning as it should.
e. 32 IFR relocated to low side of metering block.

johnta1,
I replaced complete brake system a few years ago with Kore3 Z51 4-wheel disc and DSE dual 9"/master cyclinder setup designed for this type of brake conversion. But as most of us know just because its a new part doesn't mean there isn't a defect. I like your suggestion to isolate a potential vacuum issue at the booster, will test it this weekend.

Formulajones,
So there should be no vacuum drop during braking other than the initial high vacuum reading that drops down to the idle vacuum reading as the car comes to a complete stop, correct?

Scott65,
Yes there is an Idle Ease feature however I don't need to enable it with the way things are setup it idles great.

I have to mention that not withstanding the hard brake stall issue and the recent secondary opening stumble the carb just feels perfect in all circuits and this is continuously backed up by the data log numbers and spk plug color, all looks really good.

We have focused a lot on the brake stall problem in the above posts which I'm happy to continue but have to ask the following child like question. Is there a potential that both the brake issue and stumble issue might be related or caused by the same problem?

"Typical" 850 cfm calibrations, be it Holley, Barry Grant, Quick Fuel, etc HISTORICALLY have had the IDLE FEED RESTRICTION in the lower position and with a .037 or .038 drilled opening, NOT .032.

Where did that IFR number come from?

Tom V.

Tom,
I made my own IFR’s along with HSAB, IAB, etc. from brass set screws.

What area of the annular should the squirter be contacting?

If I am reading your question correctly, the fuel spray should contact the lower 1/2 of the booster on the side. People have tried using extended tip shooters but sometimes they can drip or syphon fuel from the Pump Circuit.
Some metering blocks have a small hole drilled where the fuel in the metering block makes a right hand turn in the metering block before entering the main body.
This feature kills the pull-over issue so then you can use the extended tip shooters.

Tom V.

Quote:

Originally Posted by Formula jg (Post 6060641)

Tom,
I made my own IFR’s along with HSAB, IAB, etc. from brass set screws.

What area of the annular should the squirter be contacting?

Yup I've got my IFR at .031 and it runs great, no pig rich cruise anymore. I want to try 029 and see if it likes it

Do both of you have a 4 corner idle carb?

Tom V.

Yes my 850 MD has 4 corner idle.

That would explain part of the reduction in the idle orifice size as now the idle mixture flow is divided up by 4 IFRs, vs 2 for a typical 4781-2 carb.

In the old days the idle screws were turned out 1-1/2 to 2 turns out on the Primary Metering Block. With the 4 corner idle stuff, the normal setting is 3/4 turn to one turn out. So how far out are your idle mixture screws?

I believe I have a theory on why you are using .031 IFRs.

Tom V.

Tom,
All 4 idle screws are approx. 7/8 of a full turn out.

johnmachota78,
I was reviewing the countless past data log graphs I have from tuning this carb and found I went as low as .022 IFR with no real ill affect on the fuel curve, ended up at .032 IFR for 2 reasons:

a. Smoothest AFR
b. Gave me the widest IAB dial in range.

4781 Holley Carbs came with (as I posted). TWO .038" IFRs in the low position

With 19 cfm of normal airflow going thru the engine at idle they were able to control the proper Idle air/fuel mixture to the engine.

Enter 4 corner Idle Systems. It would make sense that the engine still wants the same amount of idle air/fuel mixture but now you have 4 sources for it and you don't need 4 .038" IFRs in the metering blocks. You need a reduced area amount of fuel or smaller IFR orifices.

Tom V.

The 850 MD Holley version specs show 36 IFR.

Ok so explain something to me.

The 850 Mighty Demon/Holley specs are 36 IFRs, not that too different from the .038" Holley specs. On post number 28 you said, you went down to .022 IFRs without any ill effects. Yet on the same post you said that .032 gave you them smoothest idle.

So you do not consider a difference in the idle behavior an effect?

I am getting a lot of different stories on this thread.

Tom V.

Most times carb calibrators compromise on the posted "as built Carb Specs" vs the specs that the individual engine likes for best drive.

Tom,
As i mentioned in an earlier post I’m not an engineer so I’m not qualified to explain how or why. Also I said ‘no real ill affect’ I did not say it was the same as the 32 IFR. Just meant the 22 IFR wasnt radically different than the 32 IFR. Sorry for any confusion.

No Worries. Most Idle Quality is subjective anyway, nasty idle to one person is mild to another.

Tom V.

Update.:
Secondary Opening Stumble

Replace Sec. MJ from 76+6.5PV to 86.
Checked vent whistle length OK at 1 5/16".
Remove Pri. MJ extensions.

No change to secondary opening stumble when stabbing throttle from cruise, AFR goes of the chart lean spike (engine will cut out if I don't let up on throttle). Also, I now have a bad stutter/stumble when staying on the primaries and lightly stabbing throttle (not enough to open secondaries) from a light cruise.

Is it possible that a spark related issue would cause this type of problem?
It's HEI cap and rotor are good (checked them today).
Can someone tell me how to test the coil and module, I have a multimeter but would need step by step instructions.

Update:
Hard Braking Stall

Removed and plug brake booster vacuum source.

Engine continues to stall with hard braking. Does this rule out the booster as a possible contributing factor?
When I reviewed the data logger chart for the brake stall and compared it to last week I noticed as vacuum and RPM drop the AFR doesn't really go crazy rich. For example last week setup with sec. PV in the hard brake AFR was 11 and this week with sec. PV plug and 86 jets braking AFR was down to only 12.

I'm starting to think something else is going on here to.

Might need to check the two small wires going to the pickup coil.

Vacuum drops when you mash on the loud pedal and when the vac advance lets off it wiggles the wires to the pickup. May just be insulation holding broke wires butted together.

Clay

Carb always gets blamed first, That is a fact.

Tom V.

It all looks and feels intact.

Using an aftermarket module?

Tom V.

Why are numbers up. Shouldn’t it be numbers down on center cam.

ICM is a GM 990 and coil is an Accel at least 20 yrs old.

As for the vac. adv. center cam numbers up I have to dig up my notes when I dialed in the timing. Went through a lot of different weight/cam/spring combinations and the engine seemed to like this setup.