383 SBC Holley 750 eye burning stinky idle

[HQM383]

Well said brother. I’ll wait for the AFR.


What’s the difference between reducing the size of the IFR vs increasing the size of the IAB?

The IFR regulates how much fuel enters the idle circuit, the IAB is how much signal to the IFR is bled off. Change of IFR has more impact on AFR. A change of .001” in IFR diameter can have the effect of .005”+ iab change. The smaller the iab the stronger the pull on the IFR from the t-slot and that can deliver fuel further into throttle position. The opposite being true of larger iab - weakened signal tapers off sooner. So while different combinations of iab and IFR can yield the same steady state AFR the combination that is right needs to work in conjunction with the mains system of emulsion bleeds, main air bleeds and even booster leg dimensions. IFR is always the place to start. Use a default iab (.070” is commonly used) and trial IFR sizes until the AFR in the cruise range where mains would become active, is close to what is wanted. Then fine tune with iab up or down if necessary. Then it’s time to move onto the mains circuit.

[rgalajda]

Good explanation from HQM383

" trial IFR sizes until the AFR in the cruise range where mains would become active, is close to what is wanted."


Sometimes with tight LSA cams and/or long duration we end up with a larger IFR and are stuck with that to get a good idle. At least in a Holley carb.

[PRNDL]

You guys are awesome. Thank you. This is great info. I have the carb leaned pretty good. I just don’t want to go too lean. I was able to bring down the initial idle a little.

[PRNDL]

What are your thoughts on the 4 corner idle screws being even? I tuned and tuned this thing over and over to the highest vacuum and rpm. The left wants 1 turn front and back and the right wants 1-1/4. If I try to trick it and do 1-1/8 all around vacuum drops. Any ideas?

The car runs way better. Stumble is gone and misfire seems to be gone. Exhaust still smells, but smells different. Again, very little practical experience here. Just theory on things I read.

[PRNDL]

Good explanation from HQM383

" trial IFR sizes until the AFR in the cruise range where mains would become active, is close to what is wanted."


Sometimes with tight LSA cams and/or long duration we end up with a larger IFR and are stuck with that to get a good idle. At least in a Holley carb.

This makes sense. Which is why I’m leaving the IFR’s and IAB’s alone until I get my AFR. I feel if I can just tune this idle, it may be ok.

My mixture screws are at 1 to 1-1/4 turn. If my IFR’s were too big, wouldn’t I. Be at only 1/2-3/4 turns?

[PRNDL]

...and that's why you want manifold vacuum.

I have a similar unit like Performance Ignition in my 261 sixbanger - it's called 123Ignition.
It's around since many years and pretty popular here in Europe.
I also ended up at 38 degrees for idling and 50ish at cruising speed - something I couldn't do with the mechanical advances (let alone the rest of my curves).
I love that thing.

It's a pity that Performance has no offerings for our inline 6 engines though.

Frank

Yeah, I read about 123 ignition and that’s how I stumbled on progression ignition. This engine loves a lot of initial timing but starts to hunt around. Now with the carb set a little better, I was able to lower it down a little and the idle if far more stable. Had it at 42, now I’m at 36

[rgalajda]

"What are your thoughts on the 4 corner idle screws being even? I tuned and tuned this thing over and over to the highest vacuum and rpm. The left wants 1 turn front and back and the right wants 1-1/4. If I try to trick it and do 1-1/8 all around vacuum drops. Any ideas?"

Sometimes being their in person and relating information over internet doesn't get related the same .
I'm having trouble relating to that. Don't worry about that 1 turn versus 1 1/4 left to right.

How much vacuum did you end up with, at idle in gear and in park?

[PRNDL]

12 in park and 9 in gear. Any turn in any direction even just 1/16 drops or down to 8hg

[skinny z]

Sometimes being their in person and relating information over internet doesn't get related the same .
I'm having trouble relating to that.

Very true.

[PRNDL]

I get it. In a nut shell, the right side wants to be 1/4 richer than the left. I read somewhere it’s because the floor heights are different on dual plane intakes. Driver side primary and secondary 1 turn out. Passenger side primary and secondary 1-1/4 turns out.

[skinny z]

I get it. In a nut shell, the right side wants to be 1/4 richer than the left. I read somewhere it’s because the floor heights are different on dual plane intakes. Driver side primary and secondary 1 turn out. Passenger side primary and secondary 1-1/4 turns out.

Pretty much.
Or any other number of distribution issues.
But from where you've started to where you're at are at least tangible results.
From here on in, it's fine tuning. Which, by the way, never ends.
At one time I was involved with a very finicky large cammed 350. Changes in the weather would have the owner getting under the hood and tweaking this or that. Driving from Chicago to the Grand Canyon presented some very interesting tuning challenges.

[PRNDL]

Thank you guys. It’s 90% better now. You have been a big help. The stumble is even gone. Radiator is done. I did it last night. Need to do my plugs, oil, and filter. I’ll report back.

I’m going to leave my IFR’s alone due to my mixture screws being around one turn out. I figured if the IFR’s were too big, I would be at 1/2 turn or something.

[steve cowan]

A lot of off idle stumble can be to rich on the transfer slot,I always fit adjustable jets in main body.
I also don't think the rule is all 4 idle screws have to be out 1 1/2 turns or there about.
I have found on milder street deals the secondry idle screw can be leaner if required, the engine is only idling.
We set all 4 transfer slots for 20 thou or so but most have progressive secondry linkage ?
I have also seen where engines are set with lower initial mechanical timing because there is to much vacuum advance at idle,you go to accelerate away from low speed and have a stumble because timing retards .
Not saying you have this issue just thinking out loud.

[skinny z]

I have also seen where engines are set with lower initial mechanical timing because there is to much vacuum advance at idle,you go to accelerate away from low speed and have a stumble because timing retards .

You're right on that one Steve. I've heard that particular phenomenon described as "torque sag" and tends to be more prevalent on stick cars than automatics.
That said, it's that tedious tuning task of springs, advance cans and limiter plates along with your trusty mighty-vac to get it right.
Through luck or determination I somehow have managed to get my heap as smooth as EFI in the idle, off idle area. I'm thinking the higher stall converter helps a lot here too.
A programmable distributor such as PRNDL's would certainly make life easier in this regard.

[1980RS]

I had a Holley 1000hp with A/D booster that was a big hesitating carburetor off Idle, played with a bunch of crap and no help and the problem continued. It wasn't until we got it on the dyno that we found it was super lean with the factory jetting. Went up almost 9 sizes to get the fuel curve right on the engine that day and when I put the 1000 back on my engine, guess what? no more hesitation. Found that one kind of funny.