So after figuring out all my timing and distributor issues, it is running real good. But I can't seem to get the carb adjusted right. I had it close but it was sputtering a bit on wot. So got a vacuum gauge to try this properly, but not getting best results, although much better. My carb is a holley brawler 650 double pumper. So it's a tiny bit large for my high output 350, stock except carb, distributor and intake. Hers what I've done and results. First, the carb has 3 vacuum ports. One metered up top, and 2 down below on front. 1 of the 2 has 0 vacuum unless you open the throttle. The other has full vacuum all the time except wot obviously. So I put gauge on the full vacuum one. (I also put the vacuum advance on that one when driving it) I get about 45 on the gauge. That's with all 4 screws 3/4 open. Adjusting I started on front passenger side, turned until gauge went as high as I could get it, then did same on other side. It's now at 50 inches. 2 screws are at 1/4. Had to drop back to idle at that point with idle screw. Then went to pass rear turned it to about 1/2 open and it went to 52 inches. Then other side same but gained barely an inch so about 53inches. Now it drives super nice and is real snappy, goes good. But if I run it up then let it off and coast in gear, it will pop a couple times out exhaust. So this means lean to me? Except if I turn any screws out, I lose some vacuum, instructions say max vacuum. So that's an issue, why does it do that. Also carburetor instructions say all 4 screws should be the same, mine are not, and anything I do causes them to get worse as well as vacuum loss. Fronts are 1/4 open, backs 1/2 turn open. So now I'm all confused. Seems to really run great and fast, it's really just the exhaust popping on hard decel that scares me. Could it be simple like my new 2.5inch flowmaster exhaust with cross over? Could it be the reason it pops? Actually sounds like those European race cars when they pop decellerating. Just no were near as bad. It pops maybe 2 - 3 times max. I think I'm close. Can anyone straighten me out?

 

Popping on decel is a lean condition.
something is up with your vacuum gauge, it should be closer to maybe 18-22 inches of vacuum. 45 and 53 is not achievable from an engine.

 

Ok your right. I was reading the wrong number on the gauge. If you look on the right of the gauge in the green area for tuning, that's the 50 I was reading. My mistake the inches are on the inside dial so it was about 22inches. The gauge us brand new first time used. Sorry about that.

 

Your original carb was a 750 CFM. So, now you are tiny bit smaller.

No expert on those Holley brawlers. But I suspect that is a 2 star rating out of 5. If you wanted a Holley and wanted a D.P. why didn't you go with the 4150 series?
You must have the spreadbore Intake or was it the price difference?

The ported vacuum or "timed" port should be capped. No need for it.

You likely have a vacuum leak somewhere causing the popping. However, could be the dizzy. Remove four plug wires off one side of the dist cap. Lay the cap on its side for an internal inspection. Using a Mity Vac tool, hook it up to the vacuum canister. Note how many inches of vac it takes to move the advance and max out.
Then release the tools vacuum and make sure the adv plate snaps back. Easily. That could be part of the problem.
Also note if any wires under the rotor are rubbing on the dist casing. Shorting out, chaffing, pop-pop-pop.

Next, reinstall cap and follow cyl numbers 5 & 7 plugwires along their entire route to the plugs. At no point should those two touch each other. That is known as Inductive Crossfire.

 

It was definitely the price. 300 VS 800. I'm on a disability pension. Lol. The distributer is brand new, probably 1000 miles on it. I had another thread when I got that, and guys here got me timed real good. Vacuum canister works right. Lightest springs installed. It didnt like the mid ad heavy springs at all. I think it was 55 total timing. Mopac were I got the carb said it came with a 600. It had a quad on it so I put an intake on it for the double pumper. When it was badly tuned when I first put it on it was real lean because I didn't check the screws cause it idled fine. Test drive it and it backfired alot. When I re did it was set almost closed, all 4 and manual said it arrived tuned to start well at 1.5 turns all 4. I should have checked. But at least I learned why it backfires. Now those fronts are almost closed and the backs barely a half turn. Any more and it gets rich for sure. So basically I'm real close, but the slightest movement of those adjuster messes it right up. Today it ran fast and harder than ever before. I just can't figure out which screw is likely lean. They are matched almost perfect side to side. But open more on the backs. I can't imagine having a 750 on there. I imagine those adjusters would be darn near closed being that big. It gets rich so easy. If I open the fronts any more, the vacuum drops off pretty fast. If I open the backs more it bogs down on hard accel so clearly floods. Spark plug wires are new as well, but I'll check to make sure they are seperate. Should be as I put long tube headers on with the flowmaster system, and wrapped them cause I read they melt wires, and did make sure they don't touch the wrap anywhere. I think its barely lean, just scared to make a move on any screws it goes so good now. Ill bet my mileage improves as well. It's been a bit low, but getting better as I figure it out. About 210 miles a tank, should get closer to 260 so I'm getting there.

 

If the fuel mixture screws are truly that finicky about adjustment, I would suspect the gasket (seal) under the screw to be the culprit.
I too had issues with sloppy mixture screws to the point where just barely touching the screw altered the vac reading.
I have tried black plastic gaskets, white plastic and of course, original Holley cork. I think the cork have the best seal but not much longevity in the heat of a C3 engine bay.
The more you fiddle with those mixture screws the more the gasket tends to back off from the Metering block. The gaskets tend to follow the threads of the screw rather than staying close to the M block. Look closely. I bet there is a gap there. I have had to force the gasket with a small screwdriver, back to where it is supposed to be seated in the M. block.

 

wide band o2 systems have become good value.

 

You have 3 "cruise" circuits. Idle, transition, and main jets.

• Idle screws work primarily at idle but have some effect thru 2000.
• Transition circuit works from just above idle thru 2000. Not easily adjustable.
• Main jets typically kick in around 2000 and control mixture after that.

Your idle screws are awfully far in, so I suspect you are having rich idle problems.

What rpm do you suspect your are lean and are having your lean decel popping at? Is it above 2000? or below 2000?

 

Not sure of rpm. I drove it 2 hrs today it only popped once after I ran it up to 100mph then let off. Popped 3 times. So probably 3k ish in rpm. I have to decel hard to make it happen. Rest of time around town it didn't pop at all. And it's driving real well. Used to vibrate at 110mph, now 125mph smooth as silk. Scared to touch it it's so good right now. Only thing other than the pop is a slight hesitation 4th gear when I punch it, bogs a bit then lights up. Since I'm lean obviously I'm figuring I might turn the fronts out a hair. There only open a quarter turn, backs just over a half turn. Holley said when timed they should all be the same. If I drive hard but run threw the gears smooth, not drag racing shifts but run right up change threw all gears, it goes like a scalded dog and rips. Man it's faster now. But if I drive normal into 4th gear, then pumch it, say 2000rpm, that's when it bogs a second then goes. Not really even a bog down it kind of goes flat then lights up. But only if I push the pedal real fast. If I'm smooth it rips right up. Idk, I'm afraid to touch it. It's nice like this, just that pop if I'm aggressive on the gear down. 70mph, let gas off, no pops at all. Has to be over 90mph to make it pop. Checked screws, they are tight and dont seem to have an issue. Might drive it awhile see how it does. Then check plugs see what's going on. When rich it was carboning the plugs a bit. Hoping leaned out they clean up a bit. Carb only has about 1000 miles on it so pretty new. As for the quad that was on there. I threw that pos as far as I could throw it. What junk. Anything you have dismantle to tune is not for me. Measure rods etc, what a pain. It ran crappy needed tuning bad. After reading tuning manual, that's when I threw it across the yard. What a crazy carb those are. Some day if I have more money I'll get a good holley. But my pension doesn't allow for that ah this time.

 

90+ mph would be 4000+ rpm so definately main jet there.

A little bog at 2000 could easily be just the accel pump(s) arm adjustment.
You need to make sure they move instantly. The way you do that is move the throttle to wide open, then hold it there, and adjust the accel pump screw until you have exactly .015 on a feeler gauge.

Other than that you could get a Holley book, and an AFR gauge, and teach yourself how to tune it. It's not that hard on a Holley.
Holley Accel pump adjust

 

apparently these brawler are lower end carbs. What ever it's what I can afford. The pumps I did check and they are set no gap. I do have instructions, and they are easy. But the issue is it's extremely sensitive to change. But I can move the screws half a turn before vacuum moves. But if you move it a fraction of that, then drive it, it either right or messed right up. But as said I can move them alot without noticing a change until driving. I can actually close one off and still drive, just shitty. So right now it very close because it drives real good. So I'm afraid to move it and mess it all up. It's up down up down up down. Can't get it bang on. The instructions said if it bogs or is flat on the hit then the secondary jets are too big. Wonder if I should drop the size on the back ones. Fronts seem ok. Looking in there, the backs certainly give a big gulp when you hit it. I think it floods then cleans out and goes. That's how it feels. My temp went down too. Was around 200 now it's 180-190. Just filled up but I seem to be getting way better mileage too. I'll figure it out eventually. I have 2 fuel injection systems, pumps lines fittings everything. A fitech and a holley sniper. If I get too frustrated I'm put one of them on. Lol. I'm not a fan of them tho. Supposed to self tune but you still need a tuner to tune them. The self tune is very basic. I have 2 because I removed them from other cars and went back to the carbs. Efi is actually harder to tune I'd say. But, may have to go that way. Another said I should have a 750 on there. Mopac told me 600 was stock, havnt a clue what the quad is. Anyway, I figured this carb was too rich or big so figured the 750 would be worse wouldn't it? Much bigger more fuel? Figure those screw would be almost closed with that one? I'm gonna drive it a tank and check the plugs see what's happening. It's close.

 

CK for exhaust leaks too. Even a small one at the header or downstream can cause a pretty good backfire on deceleration.

JIM

 

Automatic or manual?

My 80 4-speed did this. The lean popping is "afterfire". I haven't 100% fixed it (waiting on an engine swap and carb rebuild), but three things helped:

Shift into neutral instead of engine braking
Make sure your timing is perfect, and make sure your vac advance is on manifold vacuum
Adjust the idle mixture (it seems you are already doing this).

I'll rejet the carb, if I don't switch to EFI.

 

First: If your accelerator arm has "no gap" at WOT you need to adjust it. .015" at WOT is the spec. A little clearance at WOT means you have "zero gap" at low throttle openings. The pump needs to work instantly from cruise. Anything else can cause a bog. Most common issue with Holleys.

Second: Your 650 DP is fine for that motor. Don't let anyone talk you out of it. But almost all DPs need a little "tuning" to get them spot on. Your engine combo has 23 inches of vac at idle. That is actually a lot, especially for a DP, and means the cam is fairly mild. That pulls more fuel from the carb. The transition circuit on a DP by design is set a little on the rich side for moderate street to race builds, with bigger cams and lower vac than yours. The extra vac pulls extra fuel, making it rich. That is why the idle mixture screws are almost all the way in vs 3/4 to 1 turn out. On vac sec carbs the transition circuit is designed a little leaner, for cams that typically have a little more vac. But either can be changed.

To fix this it is not about jet size, it is about the transition circuit. That model has adjustable air bleeds, and possibly adjustable idle restrictions also. If you are willing to experiment, you can reduce the idle feed restrictions a little. You need to remove the float bowl, and the metering block. A lot of people add a very small diameter wire into the orifice, or change to a smaller idle restriction if they are removeable. Then you will find you have to back out the idle screws a little to get a clean idle, and the idle screws will not be so sensitive. The idle screw should be able to give you both a lean roll-off and a rich roll-off if it and the transition circuit are in the correct fuel flow range. Holley say anywhere from 1/2 to 1-1/2 turns out is the correct range for a Holley. With yours liking to be set at 1/4 turn, is why I make this suggestion.

I gotta admit Holleys are a lot more fun and a lot easier to tune than a QJet.

All of this carb tuning must be done after the timing curve is set. Please confirm you have about: 12-15 degrees at idle (no vac). Another 10-12 degrees at idle with vac can connected to manifold vac. And exactly 36 degrees total (no vac & no variation) coming all-in somewhere around 3000 rpm, and it does not go higher. Please get all that set first before trying to set-up the carb. If any of those numbers are off, you can get bogs that no carb will fix. All the necessary carb tuning adjustments will shift if you change the timing.

 

Wow, that's interesting info. Gonna have to read up on those adjustments, sounds complicated. Timing should be good, but I'll check again. I spent 2 weeks on here installing the new electronic distributor and figuring out timing. I did total time it and what an improvement. But total with vacuum was a bit high, but that said I had a real tough time reading it. If you saw my thread, previous owner put a short water pump on and my timing marks are right at 12 o'clock. So very hard to read. So checking total time was hard since I had to hold throttle at 3k with one hand, and check timing by looking under alternator and timing gun under alternator. Almost lost my face several times checking. I got it perfect as I could, but total with vacuum was a bit high, around 55-57. I'll try again with a helper. One note is I did all that Ignition work to try and stop pinging. Quite a bit, could hear it with windows down in all gears. Never did improve much, so I figured tired engine and probably timing chain rattle since it wasn't valves. Adjusted those twice no noise from them. So yesterday booting around testing my carb settings. It not only runs real well, but it no longer pings. When I noticed I was amazed. Didn't think carb tuning could fix that. Also used to vibrate a bit on hwy. Just enough it made the shifter reverse lock puller rattle a bit. Used to put my finger on it to stop rattle. Now it's so smooth there is no ping or rattle. Also throughout tuning carb I also use the full range to test. Used to get 110 mph, on a good setting 120mph but would vibrate enough to scare me. No last night I hit 130mph and it was incredibly smooth the whole way. Also at idle it's alot smoother. Used to cough ever so often. The v8 cough. But now it's dead smooth. I sure must be close just that popping. Bog is small so doesn't bother me, I rarely smack it like that except testing. Regular driving it's fine. I rarely go over 75mph too so popping doesn't happen much. As said, think I'll drive a tank or 2 then check plugs. Plugs tell all lol. I'll have a better idea what's up then I think. I am getting Bette mileage too. I'll read up on those adjustments. Thank you

 

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LOL, ya I hate them. Find them gutless too. Raced one side by side once. My double pumper opens up right away, and I was already 3 cars out on him. I could hear his quad slowly opening the secondary. By the time it was wide open I was 4 cars out. We had equal power as he didn't gain or lose after that. But it took forever for him to get there. Mine lit right up. Sounds stupid too in my opinion. But ya, just too much work and special tools required to tune one. Couldn't believe I had to take it apart, measure rod lengths, test spring tensions. Change accereatir pump stuff. Golly never seen a more complicated carb. Ya hate them. There's 3 out there in the back 40 now. Special manifold too. What a stupid idea. It's in the back 40 too. Lol

 

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Could be, have no idea what has been done to it. But it was un drivable with it. And nothing I could do to it, too complicated, and can't afford a mechanic on disability. That said I doubt a mechanic now a days knows what a carburetor is. Also, dismantling the holley to tune us not normal procedure, I'm just having issues learning to tune. It was a suggestion by another with knowledge. But tuning a quad is not possible for regular people. Holley anyone can tune with a bit of knowledge. But calculating rod lengths and spring tensions etc is way above a regular guys knowledge. If I didn't live in Canada and could afford this lars guy, I would give it a try. But having to do that every time I do a tune up is costly. I think you got lucky too, hopefully it never needs tuning again. Because your paying some one else to do it or your heading to university to learn how to dismantle and fix a quad. That said. I'm just not a fan of monopolies. Thats what a quad is. Your dependent on them for the rest of its life. If it breaks, you have to buy another because there special and nobody else's stuff fits. Need a special manifold, parts etc. If the holley breaks, I can put anything on there including a TBI. Need parts, 100s of aftermarket support. The quad, better call quad cause nobody makes anything to help except for manifolds. I know Vette guys love them. Expected flack for stating my opinion. I like the power of the holley too. I actually bought one too small. Apparently I should have gotten a 750 not 650. So the quad was likely a 750 and was gutless compared to the holley. So, vettes came with holley too, it was an option. So not worried about purist opinions. I decide by performance. Let's face it, nobody in the racing community uses a quad. Not 1. Not that I race, but I respect the reasoning they don't use them. No performance, that's why. I think if you ever tried a holley you would crap your pants when you see the performance upgrade.

 

I actually think the brawler series is pretty decent, especially at the price point.