Help with carb issues

[Broncojunkie]

I've been having problems with backfiring. I thought I had it figured out, but I still get an occasional backfire (actually, an "afterfire", as it is coming from the exhaust). I found that I can reproduce the backfire. It will happen at 90%-wot, at higher rpm...around 4k or so. I can reproduce it sitting in the driveway or while driving. Basically, it will happen when I stomp the skinny pedal. However, if I stomp the gas while in "too high" of a gear and rpm's don't increase, I don't get it. I haven't noticed a misfire. I've pulled plugs, checked wiring, checked timing (tried timing at various settings). I don't believe timing or ignition is the culprit.

I did have the idle mix set a little too rich. Plugs were sooty until I got that straightened out, so I didn't originally suspect a lean condition. After fixing the rich idle, I put a few miles on the car and checked plugs again. It looks like a lean condition, which might explain the backfire I'm getting. I'm just looking for advice here. I will be ordering a wideband gauge tonight so I can get a better idea of what's going on, but for now, I'd like to know what you think about my setup. Personally, it doesn't seem like I would be starving for fuel this much with the carb I have. I'm certainly no expert on this, so some input would be helpful.

Here's what I have:

Engine: 331 stroker (stock 302 block). Flotek heads (180cc intake port/58cc chambers), cam 560/560 lift, 280 duration, 110 lsa, around 10.5:1 compression ratio.

Carb: Quickfuel 650 mechanical secondary. Primary jet: 68, Secondary jet: 76, Primary/secondary air bleeds: 73, Primary power valve: 4.5 (no secondary pv).

Here's what the plug looks like. I was thrown off before, because the plugs were previously black/sooty with the rich idle mix. After leaning it out and going for a drive, this is what they look like.

[Walking-Tall]

First, which QFT 650 mechanical secondary? As a guess, by the jet/bleed/PV numbers you listed, I'd say the "Street-Q" 650... ?

Friendly suggestions at the end...

I don't know now how many stories like this I've heard about with new/newer or "professionally" modified "new and improved" clones or Holley carburetors. I do know that nobody who has shelled out big bucks for one of them ever enjoys hearing that job 1 out of the box should be to disassemble, measure and map out everything, and re-calibrate them... for the following reasons (some or most of which you'll see happening when you have your wide band O2 on 'er):

- IFR's (idle feed restrictions) cannot be sized for YOUR or anybody's unique combination (among other things, "ready to run" out of the box is marketing and a virtual impossibility (I suppose it depends on who's definition of "run" we are talking about) with the multitude of different combinations people put together that a carburetor will find itself bolted down onto), and located up high, where their proper function to meter idle/off-idle/cruise FUEL becomes dysfunction where they are easily and especially affected by their being well above the fuel level and by their close proximity to the idle air bleeds... most especially with larger than stock/mild camshaft idle/off-idle/cruise pulsations = idle air bleed air can/will intermittently get behind the idle feed restriction, causing a ratcheting, non-consistent, non-efficient, impossible to EVER find a sweet spot the engine wants/needs for steady, consistent, efficient idle/off-idle/cruised/light-part-throttle-acceleration air:fuel ratios...

- too much and too many "emulsion" bleeds into the main wells (more and bigger than the old school Holley 2 0.028" bleed arrangement per main well - see pic I created below to explain the modifications necessary for this point and for the above IFR point), coupled with big main air bleeds, makes for a fuel "curve", that begins early and rich, and then goes lean at upper rpm... a flat, consistent (never going lean - fully loaded danger zone) wide open throttle air:fuel ratio, from the instant the main circuit starts flowing to the red line rpm of your engine, is what's wanted/needed...

- in most of these, and in other special or billet replacement "upgrade" base plates... transfer slots that are too long/wide (rich idle/off-idle/cruise)... the only fix for this situation is to tap for and install 10-32x1/8" or 3/16" brass set screw TSR's (transfer slot restrictions) into the carburetor main body passages that lead to the transfer slots... with a restriction drilled into them of 0.081" for starters with a 4150 type carburetor... see pic below...

- "4-corner idle" carburetors do not require constant idle feed holes below the secondary transfer slots (rich/touchy idle mixture)... so if yours has them, block them with whichever method you're comfortable with... jb-weld to block works for me...


Here's one I remedied like I'm talking about above earlier this year... now workin' well... that was so damn rich all over the place like I'm talking about above, and lean upper rpm too, that in fact the plug-fouling and the engine oil dilution with gasoline was so bad that the gentleman's still fairly freshly built engine ended up damaged too... anybody figure Holley or QF "warranty" covers that kinda stuff?...









Metering block/s remedy:




TSR's (transfer slot restrictions) addition locations (#8 & #17 in image):



Friendly suggestions:
- if your camshaft has ~230-degrees of 0.050" intake duration, install 0.033" IFR's, in the down low location...
- 0.073" idle air bleeds should be fine, and after remedy you should be able to more easy/predictable/comfortably adjust idle mixture...
- 0.028" or 0.026" main air bleeds should go in it, coupled with reducing all of the original "emulsion" - two 0.028" bleeds per barrel...
- if "4-corner idle", block secondary constant idle feed holes below secondary transfer slots...
- if the transfer slots measure similar to above, install 0.081" TSR's...
- PVCR's (power valve channel restrictions) should be 0.040"+...
- replace the silly 4.5"Hg PV (that doesn't belong on much of anything - they'll have no doubt installed those due to over-emulsion, the early and rich main circuits discussion above), with at least a 6.5"Hg, or probably more likely with your combination, an 8.5"Hg... for whichever vacuum level your combination indicates (and therefore requires) the extra enrichment to be there...
- after all of the above, jetting for full load and upper rpm should be safe rich - carefully investigate with the wide band for ~12.5:1...

Once all of the a$$-backwards "technology" is remedied out of them, they work fine, predictably, safely, and efficiently. If all of this is clear as mud, lol, lemme know, and I'll try to better explain things. Good luck with it

[Broncojunkie]

Walking Tall,
Thanks for the thorough response! Yes, most of what you said is way above my pay grade lol! But I'll have some time to sort through it here in the next few days. I don't mind reading up on this stuff and now I know what I need to research.

The carb I'm using is a QF Brawler Race Series BR-67199. It is equipped with 4-corner idle screw adjustment.

[Broncojunkie]

I should also mention that I have a couple older Holley 4150 carbs lying around, including a 650 and 750. I was having problems with the 650 idle mix adjustment for some reason and couldn't get it figured out. The 750 is due for a rebuild, since it has been sitting for a while. I figured it may be a little too much carb, but I could be wrong.

[Walking-Tall]

The best place for learning about Holley carburetors (and others, and other topics) on the planet in my opinion... below is a link to the Holley stickies board at RFS (Racing Fuel Systems) for the meat and potatoes of how and why of the modifications for improvement that I mentioned above... or for getting a good grasp and understanding the what's and the why's, and then having somebody do the modifications for you...

https://www.tapatalk.com/groups/raci.../stickies-f31/

Okay, QF Brawler Race Series BR-67199... all the same checks and changes still apply.

It's been said by a handful of people, and I say it: if you want a good one, get an old one. The primary metering blocks with mixture screws on your double pumpers may well already have 0.033" idle feed restrictions, and down low where they belong... not as pretty as the QFT's, but they'll far-and-away better get things done. There's a freshened LIST-3310-2 750 vacuum secondary on my 302, with a speedy secondary opening spring, and I've no worries whatsoever about it. IMHO there's no such thing as "too much carb"... good function and razor-sharp response is all about getting all the wee metering orifices inside them and up top of them the right sizes... the results are surprising fuel efficiency, and the unhindered airflow race is on, with no penalty... a 750 on a 331 should work real well, with guarantee of increased horsepower and average and peak torque and with your cylinder heads and camshaft etc.

Best of luck with it