Stumble off idle

[Abbottracingheads]

You can run36 total timing locked out with the vacuum advance. Just leave the vacuum advance on manifold vacuum, not ported vacuum. When you get on it the ported vacuum will put you at 46 total. You lose the manifold vacuum at WOT which will leave you with 36 total under full power.

[levisnteeshirt]

I recently fixed a carb that is sold with a billet base plate ,,, that's all good , but they made the throttle plates wrong , not long enough , they are oval , not a circle , the blade swung away from the slot very quickly ,, I put Holley blades in it ,, waa laaa, fixed ,,, I did a couple other things to it as well ,, guy loves it now , 540 BBC with a Pro systems Hp 1000 style carb ,,, he told me it had zero off idle response and is now crisp

[Steve Zicht]

I recently fixed a carb that is sold with a billet base plate ,,, that's all good , but they made the throttle plates wrong , not long enough , they are oval , not a circle , the blade swung away from the slot very quickly ,, I put Holley blades in it ,, waa laaa, fixed ,,, I did a couple other things to it as well ,, guy loves it now , 540 BBC with a Pro systems Hp 1000 style carb ,,, he told me it had zero off idle response and is now crisp

The Barry Grant carbs were basically a good carb. Problem was that the QC wasnt. I pulled more than a few apart to fix things that were manufacturing flaws. Buts thats another story...................

[F-BIRD'88]

You can run36 total timing locked out with the vacuum advance. Just leave the vacuum advance on manifold vacuum, not ported vacuum. When you get on it the ported vacuum will put you at 46 total. You lose the manifold vacuum at WOT which will leave you with 36 total under full power.

This is not true. The only real difference between ported and full man vac signal is w/ported vac you get no added timing when at idle. The vac adv starts as you open the throttle a bit from idle. But when you open the throttle wide open full throttle there is no added vacuum advance timing at all on BOTH. Because the manifold vacuum remains very low when at WOT on both ported and full man vac sources. Neither add timing when the throttle is wide open. Timing will be 36deg at WOT the whole time you keep it rugged WOT. The ported and full manifold vacuum add timing only when at PART throttle.

The difference is added vacuum at idle or no added vacuum at idle.

[F-BIRD'88]

The barry grant carbs have a IDLE EZE- auxillary idle air path to add extra idle air flow
for big cams. This allows adjsutment of the idle air flow at idle ,independent of the throttle blades position at idle. You need to get the throttle blade position correct (T slot exposure at idle ) and the IDLE EZE adjustment correct also. the IDLE eze circuit added rpm at idle but also added air so it also LEANS the whole idle circuit with raw air that is NOT MIXED WITH ANY GASOLINE. So that air is non combustable and does degrade the idle fuel overall mix a bit, Especialy if the IDLE EZE is too far open at idle. TOO MUCH added raw air.
So a little goes a long way.

less ILDE EZE and MORE intitial base timing is needed. Close the idle EZE. Lock out the MECH advance, set the locked out timing at 36deg BTDC. And reset-pre-set the T slot exposure on both pri and sec .

Now fire it up and let it warm. Reset the idle mix screws best idle. If it does need more rpm at idle, then add a little bit of IDLE EZE air flow. it should idle at about 800rpm neutral and drop very little in gear.
Keep the throttles in the sweet spot ( T slot exposure at idle) only add a little IDLE EZE if it needs any at all to get the required idle rpm. AS you do add IDLE EZE flow you also to also add idle mixture screw. To restore the idle fuel overall mixture balance.
This motor should need little or NO idle EZE flow at idle. But it does need plenty idle timing.
Then use PORTED vac advance.

I do the same thing by tweeking the PCV valves' air flow rate +/- at idle.
Drilling the primary throttle blades for more air flow serves the same purpose
as the BG carbs IDLE EZE or my tweeking the PCV valves flow at idle.
But reguardless, too much added auxilary idle (raw) air flow does lean and degrade the idle air fuel mix and QUALITY. You can trim the whole idle circuit up or down depending on how much added ax air flow. But remember it is RAW air so a LITTLE goes a long way of you need more idle speed and or need to trim the whole idle circuit a bit leaner.
( it is like making the IFR smaller BUT the added air is RAW air, not more or less MIXED air fuel.)


You got too much IDLE EZE air flow The idle EZE is too far open . it is covering up the lack of idle timing but is killing the idle fuel mix quality. Thus the FLAT SPOT.
Get all this stuff just right and BALANCED before you even think about drilling IFR's ETC ETC
99% of the whole setup is done without drilling anything on the carb 99.5% of the time.
Put the drills away.

[F-BIRD'88]

If you are using a PCV valve you do not need any IDLE EZE flow. CLose the IDLE EZE shut.
Your cam isn't that big. But it will want plenty timing at idle that is dead stable in neutral and idle in gear. So lock it out again.
IDLE EZe air flow +PCV valve idle air flow combined is TOO MUCH RAW air flow added at idle.
Some PCV valves also just flow too much air at idle. They are not all the same. Some generic pretty chrome-billet aftermarket ones are terrible and really can screw up the idle.

You may have the wrong PCV valve.

[Steve Zicht]

If you are using a PCV valve you do not need any IDLE EZE flow. CLose the IDLE EZE shut.
Your cam isn't that big. But it will want plenty timing at idle that is dead stable in neutral and idle in gear. So lock it out again.
IDLE EZe air flow +PCV valve idle air flow combined is TOO MUCH RAW air flow added at idle.
Some PCV valves also just flow too much air at idle. They are not all the same. Some generic pretty chrome-billet aftermarket ones are terrible and really can screw up the idle.

You may have the wrong PCV valve.

This is assuming this BG had the IDLE EZe. Not sure which BG carbs did. If IDLE EZe equipped , it was like carburetor tuning on some of the old Carter 4V's. I seem to recall that was called an AIR BYPASS VALVE or sometimes referred to a ' GULP' valve.

The original idea was to keep the primary throttle blades closed as far as possible. It had what looked like an idle screw on the primary side but it was only to serve as a throttle stop to keep primary blades from sticking in the bores. The idle speed was adjusted with a large screw that was between the mixture screws. If memory serves, many early GM cars were adjusted this way. This was an excellent design but not many mechanics knew how to adjust it. aaaaaaaaahhhhhhh the good ole days !

I tried to submit a very similar idea to Mr.Gasket years ago in the form of a carburetor spacer plate. The air bypass channel and adjustment screw were made into the spacer plate. The idea was no eliminate air bypass holes that had to be drilled into the throttle blades for big cams mods on the carbs.
I thought of this idea after seeing a few too many ' do it your selfers ' bring me carbs with *ucked up throttle blades where bypass holes were drilled into them incorrectly and bent them up pretty good.

NOPE ! MR GASKET rejected my brainstorm of coarse ! LOL

[73c34me]

No idle eze adjuster on this mighty demon. Left everything the same, but installed a 8.5 PV from a 6.5. Significant improvement; mild stumble off idle now. Perhaps a different pump cam OR shooter size? Would like to tune this out if possible, before returning to locked out timing. Thnx. Jim

[Steve Zicht]

No idle eze adjuster on this mighty demon. Left everything the same, but installed a 8.5 PV from a 6.5. Significant improvement; mild stumble off idle now. Perhaps a different pump cam OR shooter size? Would like to tune this out if possible, before returning to locked out timing. Thnx. Jim

Not sure the 8.5 Power Valve was the right move but it gives an important clue. Its either a bit lean on jets or simply wants more squirt for a longer length of time.

What size primary accelerator pump system ? 30cc or 50cc ? Sounds like the pump shot needs a longer duration. If your BG carb has the smaller 30 cc pump, get the larger one. That larger pump assembly , pump arm and pump cam should help alot. Your gettin close. One step at a time.

As a matter of coincidence, I was just looking a a 68 440 wedge Roadrunner last night and it has a rather large runner Indy intake manifold on it and it has a serious stumble off idle. It was just changed from a six pack set up and never had any stumble at all before. Along with the intake theres a Holley 850 on top that had the small pump on the front that somebody switched before. I told my buddy to track down the big pump and related hardware and to put it back on. I'll bet that larger pump shot is what that motor wants.

I thing your small problem is related but thats what tuning is all about. PATIENCE !

[73c34me]

Factory stock pump; 30cc i believe. The stumble is almost gone, so close to being where it needs to be. Idle feed restrictors have been reduced. Would have to look up my notes to see what size they are. Idle seems clean, but spark plug reading shows a little rich, but not bad.

[F-BIRD'88]

Idle feed restrictors have been reduced

Why did you do that? you don;t need to change the IFR size. it is probabily got a flat spot just offidle because the idle feed restriction is just a hair too small now.
Put it back.
Lock out the disz. It wants to be locked out.
If anything at all once you do this it might need very minor very small idle side air bleed size tweeking.
You cannot determine the idle or off idle by looking at the spark plugs.
Set the idle with manifold vacuum gauge. The priority is idle quality, vauum gauge reading at idle and idle rpm. NOT AFR at idle. Do not go by the AFR gauge at idle. it wants what it wants at idle.

Then once the idle is all set up,,,, test the off idle-throttle tip in transition AFR using a tach, manifold gauge and a AFR meter. It need only stay near the 14.7:1 AFR ZONE +/-. some shift is fine. If it goes-shifts too lean off idle it will bog (15.5:1 AFR or leaner)

You are just like most street guys: highly resistant to giving the motor the timing it really wants at idle to idle correctly.
You can play with that carb all you want but basic lack of idle spark timing will still result in crappy throttle response. and eventual spark plug fouling over time.
You want to burn the fuel quickly at idle. The cam overlap slows the fuel burn time at idle.
needing more basic initial timing at idle.
Thats the nature of increased cam-valve overlap.

[F-BIRD'88]

You do now that the IDLE EZE on a BG demon carb is down in the center hole where the air cleaner stud goes. You adjust it with a long screwdriver. I believe all the BG demon series carbs have it.
If some do not have a idle EZE its new to me.
What is the carb model number?

[73c34me]

No idle eze. Don't have the carb here so can't check, carb is an earlier version Had locked out timing with previous cam, so definitely open to it. Idle is set with vacuum gauge to highest reading. Previous tune, IFR re-sizing was necessary to eliminate the pig rich idle. So I left them in as I retune the new set up. No AFR gauge, old school tuning

[F-BIRD'88]

Does this carb have a 3/8" vacuum port on its base? If so, this carb was setup-calibrated to allow for the PCV flow at idle. The PCV flow effects the overall idle AFR. it is part of what tunes the idle circuit.
Did you use it with or without a PCV valve? If you eliminated the PCV them the WHOLE idle circuit air fuel curve calibration gets shifted Richer. ( can make it "pig rich")
Also requires more throttle opening at idle, with cams, Wrong T slot exposure at idle.

What makes you think it is "pig rich at idle"? exhaust smell at idle?
idle AFR? off idle AFR shifting richer?

[F-BIRD'88]

Does this carb have a 3/8" vacuum port on its base? If so, this carb was setup-calibrated to allow for the PCV flow at idle. The PCV flow effects the overall idle AFR. it is part of what tunes the idle circuit.
Did you use it with or without a PCV valve? If you eliminated the PCV them the WHOLE idle circuit air fuel curve calibration gets shifted Richer. ( can make it "pig rich")
Also requires more throttle opening at idle, with cams, Wrong T slot exposure at idle.

What makes you think it is "pig rich at idle"? exhaust smell at idle?
idle AFR? off idle AFR shifting richer?

"previous cam" what was it?

What cam now?

"old school tuning" Well you can shorten this exersise a lot by adding a bit of new school to that.
A low cost simple heated O2 sensor type narrow band AFR gauge or a good wide band gauge really helps with dialing in the off idle transition and the main jet at cruise and part throttle.
You can get the carb tune and drivability very sharp. Using these new school tools, in addition to the old school stuff.

Wide band: I like the NGK Powerdex afr gauge.