Please Help Me Figure Out This Nasty Off Idle Stumble!

[blackgt85]

Ok, here we go... I have a pretty nasty off-idle stumble/bog accompanied with a backfire when mashing down the throttle from idle. It happens when idling in Park, and when taking off from a stop when in D or 1. I have tried increasing the discharge nozzle size with .035's, and that was no help. The idle air mixture screws are set for maximum manifold vacuum, which comes in at 8 inches when idling in gear at 750 rpm. The timing is set at 10* BTDC, which is the stock specification. This car doesn't seem to be anywhere near it's potential with this combo. (I do have a looser converter and a Silverfox VB waiting to go in.) Any help with figuring this out would be greatly appreciated! Specs are as follows...

Car: 1985 Mustang GT
Weight: 3450 lbs.

Transmission: PA AOD
Stall: 2400 rpm
Rear Gears: 4.10

Engine: 306 SBF

CR: 10.5:1

Heads: AFR 185 Renegades 58cc (p/n 1388 )

Cam: Custom grind from Ed Curtis, Hyd. Roller

Intake: Victor Jr. with 1" spacer

Carb: Holley 670 Aluminum Street Avenger with .035 Discharge Nozzle installed, Vac. Secondaries, Electric Choke

Exhaust System: FRPP Jet Hot Shorties, 2.5" Pypes off-road X-pipe, 2.5" SpinTech ProStreets dumped


Justin

[83gtstang]

Check your power valve in your Holley to be sure you have the correct one and to make sure it's not blown. Get a vacuum gauge while at idle, get the reading and divide by 2 or half of the reading. That is your power valve you need.

[83gtstang]

Could also be your accelerator pump diaphragm, could not be adjusted correctly or torn.

[blackgt85]

Check your power valve in your Holley to be sure you have the correct one and to make sure it's not blown. Get a vacuum gauge while at idle, get the reading and divide by 2 or half of the reading. That is your power valve you need.

I just grabbed a 4.5 power valve earlier tonight. I'll probably throw that in tomorrow (time permitting).

Could also be your accelerator pump diaphragm, could not be adjusted correctly or torn.

I do have that adjusted correctly and the diaphragm isn't torn. I am about ready to just get a Double Pumper and let this SA go to someone with a more mild setup. It worked perfectly on my GT40P/Stealth/TFS1 combo...

Justin

[gr79]

Stumble, backfire, bog, etc is ign timing related too, especially a pop, sneeze, or worse, backfire.
Backfire thru carb or exhaust?

Totally different setup, similar symptoms:
My Ranger (DIS dual plug) has a occasional start problem, likely in the old crank sensor wire harness.
Anyway it will backfire real good (exhaust) upon cold startup when scanning thru timing maps due to lost tach signal.
Or when tach jumps, time to pull over and let it have a fit. Once settled down, good for the rest of the day.
Never bogs/hesitates except when fuel level is real low in tank.

Nothing wrong with changing carbs. Lots of times have to experiment combos to hit the mark.
No matter what the books or specs say. They do help guide in direction to take.

10° may be stock and a place to start, but that engine is not stock anymore.
Test drive it while playing with advancing the base timing.
Actual roads driven. No timing light. Everything connected.
Dist clamp temp. friction loose enough to turn dist for adjustment, but tight enough to hold settings.
Give the engine what it wants, then mark and lock it down.

Total timing. Advance up to spark knock under load then back off a bit.
Adj carb idle and idle mix as necessary to keep it running with steadiest reading possible.
May take hours or weeks to finalize setting due to different driving conditions/weather changes.

Keep an eye on the idle vac reading. It should come up.
8 is too low. That is like a light throttle vac reading for acceleration.
Have had engines with vac leaks read higher.

I dunno.
If it were mine, max total timing, with no knock, with highest idle vac, is what i would be doing next to eliminate that factor.
You may have already done this. In that case, never mind.
Heck with that mod combo we both would expect car to run like a bat outta hell with some manners.

[jsfrv6]

10 degs at ldle is not enough... You should have 15+ at idle up to 35 - 38 deg total... What ignition system are you running?

[blackgt85]

The initial timing is at 18* BTDC now. Got rid of the backfire (carb), but still has the stumble. Ordered up the Mr. Gasket spring kit p/n 925D and gonna figure out which reluctor arm is in the distributor sometime this week. Hopefully one of these days I can get out there this coming weekend and start dialing it in better.

As for the ignition system, I am running an MSD 6AL2 (mounted in kick panel), a Mallory coil, and a remanufactured Duraspark II distributor.


Justin

[83gtstang]

What are the specs of your cam? Seems like you have horrible vacuum if your pulling 8hg at the manifold? Maybe your carb is too big for what your are pulling in vacuum if that makes sense. My 85 roller engine is all stock, it pulls 17.5hg. If have a Demon 625cfm vacuum secondary carb.

[Jerry peachuer]

Carb is a good off the shelf carb I had it on my 331 it's large enough for street and being auto that is ideal

sounds like it's kicking in to quick or not quick enough (secondaries) vac secondaries and low vaccum is tricky all that is doing is opening your power valve or not opening it and the car stumbles
You can easily check this by unplugging the secondaries and somehow plug your supply line (safely) I actually made a single supply line for the main circuit only and made sure I can get the primaries good (don't do full pulls) it will fall on its face around 2800 rpm

Also every time you touch the distributor advance or retard you must go back and tweek air fuel mix screws

Also why is your idle so low bump that to 900 rpm or 850 area (just try this)

You have a very snappy high flowing setup sounds like this carb works great once dialed in and holds the tune very well

Your vaccum is all in the cam and if it's that low your brakes maybe not ideal but custom cam I never owned
My Initial was not important to me as far as setting it here or there I set total timing locked it down 34 @3 k was for my set up

It hot started well and was no drama my point is don't get stuck on initial timing it's not quite as important as up top when your in it

Initial means nothing but starting your car (not quite this black n white) but why do you care what initial is other than starting your car and your advance comes in after idle or tad after idle so this is going to be corrected shortly I'm sure I'm not a mechanic by trade this was my way as far as getting my setup dialed in and seemed to work well

What jets do you have primaries and secondaries ?

The best thing you can spend money on is get it dyno tuned and exhaust sniffed to get max power and enjoy your heads and custom cam

When it stumbles try to jump up or down in size by a size able amount as far as jets 68 go to 72
Squirted 32 go to 35 vac advance spring from stock to 2 sizes from where your at but only change one thing at a time (very important) and
make sure car is at operating temp at all times even to change timing
Most people try to go bigger with sizes when it's actually getting too much already

I would try different spring in your advance first and put carb back to stock settings set timing to 10-12 and reset your idle mixture screws 1-1/2 turns out set idle to above 850 rpm and verify your ignition is advancing smoothly with increase of throttle then adjust from there 1 thing at a time
Don't get frustrated you will learn a a lot with what your car wants and also I would run single fuel line to the dual feed 670 and leave back bowl unplugged there's enough fuel in the bowl to make the car go down the road in fact to empty both fuel bowls on the road is really quite a chore you would most likely be going into 4th gear (even though they fill automatically as you use fuel)
Make sure fuel level is set properly in the beginning as well also make sure carb has gasket under it and is tightened down you may have small vac leak which will add more fuel if a higher number power valve is in there
After no stumble your setup sounds like a lot of fun good luck sorry for the scattered random sentences

[Jerry peachuer]

Low vac and low number power valve will give to much gas to quick car coughs with auto more pronounced than manual trans