Looking to get suggestions from some carb gurus with experience. Ive heard that 750 cfm is usually like a sweetspot cfm for 350s, so i think ill stick with that cfm. Next question is, which brand? Holley, demon, carter AFB, edelbrock, or good ole rochester Q-jet(rebuilt ofcourse)??

This will be on a 3970010 350 block. I beleive its all stock except for an edelbrock dual plane intake. The car will be backed by a 700R4 and 3.42 stock gears (non posi). Stock ignition except for super accel coil. The car will be fairly light, im only keeping two seat,s and no carpet or any other interior. The car will be a sleeper street pounder with occasional trips to the striip maybe once every couple months. So i was thinking mechanical secondaries for quick response on a lightweight vehicle. I have experience with Holley's, i love them, i have experience with Q-jets and i love them also. Except the Q-jets are a b*tch to tune if youre not experienced. Have a little experience with carter but i dont know a whole lot about them. SO, what will get me the most "pep" off the line??? Any input appreciated. Thanks

 

Fairly stock motor? Stock carb (q-jet) would probably work pretty well. You'd keep fuel economy as well.

Five7 kid says that a double pumper tends to pick up some ET, when combined with a standard trans, and obviously a light car. I don't think you'd be gaining a huge amount though.

I think a q-jet would probably be best due to the tuneability (since you say you're experienced with them), and the lower cost. That's always a nice perk. You can save upwards of $200, which you can put towards something that will actually give you $200 worth of speed.

 

Oops, i guess i didnt relay that right. I have experience with Q-Jets meaning ive used them on several motors before, heck i can even tear one down and put it back together with new wear and tear parts. Howver im not experienced in tuning them (tuning well anyways), so i dont like them in that sense. The motor is fairly stock at the moment, but a hotter cam, and a higher stall rated torque converter is in the works. Would the Q-jet still be the best bet for that? And how do u tune a Q-jet well anyways? Thansk for the input. The main reason im shying away from the Q-jet is because of the secondaries. On my brother's camaro, the carb wasnt quite tuned working nicely in the choke, and vaccum department, so youd REALLY have to punch it to get the secondaries to open. I didnt like that because in the end i was using more gas to get them to open. I think with mechanical secondaries id be able to save a bit of gas when it comes to spirited driving, because the secondaries will open way before i have to punch it! What do u think?

 

Well, first off, I can't say that I have experience with any carbs aside from a q-jet. So i'm not spoiled with "ease of use" of the holley. And I can't give any comparisons then.

However, once a q-jet is setup right, it's a superior carb. The jet/rod combo on the primaries, along with smaller primary venturis give better mileage. You can put on a different power piston spring to get onto the "power" section of the primaries earlier or later, and you can bend the tang that initiates the seconaries, to get them online earlier/later. You can also play with the secondary air valve as well. The choke is a very nifty design, which gives a fair bit of adjustability.

I posted my sig above so you can see that i'm running a q-jet on my motor. I've got a medium cam, stickshift, ported heads, and I live in calgary. Check an online weather source right now and you can see what i'm dealing with (it's snowing). I was just driving it, and it fires up perfectly fine in the cold, hot or cold starts etc.

A holley is easier to tune though, from what i've heard, so i'll give you that. Parts are easier to get for it as well.

Lots of ways to tune the q-jet. If you know how to disassemble one, and are fairly competent at that, you're set. The book by doug roe, is ok, but i've got a few online resources that are great as well. Like this;
https://webdisk.ucalgary.ca/~jmknopp...ning_Paper.doc
also, there are a few q-jet guru's lurking around these very boards, that can always lend a hand.

I went full out on tuning my q-jet, drilling here and there, etc. Just to induce more problems, since my combo isn't nearly wild enough to need that. It now runs best, closest to stock.

 

You could have fixed the secondaries on the Qjet by adjusting the air valve tension. The Qjet has a flap over the secondaries that doesn't open until there's enough demand for air. This keeps it from bogging when the secondaries are opened. A vacuum secondary holley behaves similarly, but does it a different way.

A mechanical secondary uses a second accelerator pump on the secondaries (hence the name "double pumper"), squirting a shot of fuel into the secondaries as they open to prevent bogging. Mechanical secondaries tend to use more fuel during spirited driving for this reason.

 

Ah ok, Sonix you just mentioned a ton of stuff i had never thought to do. Im gonna try it on the q-jet for my 350 in my camaro. See what i find. Maybe i will skip over the holley after all. Apeiron, how would i go about adjusting that tension on the secondary air valve? What i was getting at, is that although the holley puts an extra squirt of gas in there, its still at part throttle, so the cumulitive gas that goes down is close to, or still less than a Q-Jet at WOT. In any case, another thing that bothered me about the Q-jet (on my brother's camaro) was it was a later model where the idle mixture screws were capped. On mine they are not, but i still dont know what tool to use to get at them. Another question is, what effect does the float level have on the carb? The prob with my bro's camaro was, starting it was a big prob untill the car was cool. It had a Hot air choke, which i dont think was functioning correctly. If the car did not start after two cranks, it would flood, and wouldnt start unless you forced the choke open and cranked. Once it burned off the flooded fuel ( a few seconds) it would run beautifully. I have seen other people use Q-jets and they function beautifully, the choke IS a wonderful thing when its cold. I think the prob in my bros camaro, was a mixture of poor timing with an after market cam, bad float level and bad choke plate adjustment made it a prob. the one on my car functions beautifully for start-up so ill play around with the secondaries and see what happens.

 

This is for CCC Qjets, but the air valve adjustment is the same. https://www.thirdgen.org/quadrajet

The Qjet and the Holley, and any other carb for that matter are going to put the same amount of gas into the engine at the same RPM and load, as long as they're properly tuned. It's while the throttles are opening that there'll be a difference.

 

hmm, you make a good point Apeiron, a carb giving the, say 15:1 AFR, no matter if it's a q-jet or double pumper, will be using the same quantity of fuel, regardless of if it's got smaller primaries or larger... Or would the venturi effect give better atomization in smaller primaries? I figured a well tuned q-jet would get better mileage than a well tuned double pumper, both carbs running side by side on the highway, for example..?

Online - ugh, hot air choke? hooboy, that's oldschool alright. Not really a late model carb then is it? If it has plugs over the mixture screws, remove the plugs, then you can adjust the screws. On all non-cc q-jets you'll need this. I like to drill out the mixture screw holes to 3/32", so you have lots of adjustment, if you need it.
The only problem I have with my choke, is in the meatware. If I cold start it, one pump to flick on the choke, and crank it, it fires, and i'm done. If it's a hot start, no throttle needed, just crank it over. But inbetween, I have a tough time discerning if it's cold enough to give it that pump or not, and if I do, it can flood it, and i'm just cranking it over with my foot on the floor until it fires and blows a whole bunch of black smoke out. woo hoo. I think I may have leaking well plugs though as well.

I just re-read that air valve adjustment, as I was going to link it to another member - but it recommends removing ALL TENSION from the air valve, and trying that. Then doing a spark tune up, etc... I think that's a sure fire way to bog. 3/4 turn, and lock it, is a pretty universal "good" way to do it.

Let us know how you make out with it. If you get "hot and heavy" into the mods, i'll give you a few more little mods you can do to your q-jet, if you want to try 'em out.

 

Removing all of the AV tension isn't an automatic bog situation, since the choke pull-off slowly releases the AV as well.

I'd go q-jet if "normal" driving economy is of interest to you, the DP if it isn't. Q-jets will suck down a lot of fuel when the secondaries are opened as well. The DP is the better performance carb all-around.

Of course, you could get a 6210 Holley spread-bore double pumper. Only 650 CFM, but would give better economy than the standard Holley, better performance than the q-jet.

 

Differences in atomization might have a small effect on cruising mileage, but they'll have a more significant impact on throttle response again.

Taking out the air valve tension shouldn't cause much of a bog, the dashpot is still controlling the opening rate. Whatever works, though.

 

Interesting.... always learning here, always.

 

Actually it was very late model carb, during the install, my freind put the wrench to the fuel filter area too quick, and it stripped that fuel line section by about 1 thread.... Of course fuel wont get stopped by anything so sealants didnt work. This was on a GOOD q-jet thats i rebuilt (early model) hot air choke, nice idle mixture, perfect start-up and good running with lots of performance. But since it stripped, i opened her up, and swapped all the parts to my brothers late model Q-jet that originally had an electric choke. Neither choke co-operated with us, so ultimately we went to manual choke, even that was tricky. My car isnt actually running yet, but ive heard the engine. The freind who stripped my carb, bought a 78 monte thinking it was a 305 for a derby race, turns out it was actually a 3970010 350! Not sure if its 4 bolt but u never know. That engine is now going into my berlinetta, but the guy i gave it to finish is taking his sweet *** time with it, so ill check back when its running. For now alot of what you have said makes sense so ill give it a shot and see what happens. Fuel mileage is not of concern to me, i just want alot of GO off the line. Quick response for when a civic pulls up kind of deal.

 

I have a carb in my sig for sale if your are interested.

 

Yes ive seen it a few times, i would have contacted you right away if it had mechanical secondaries.

 

Alright, ive been playing with my stripped q-jet and trying the things you guys mentioned, and im getting real excited! So Q-JET IT IS!!!

A few more questions though. Before i read the AV adjustment carefully i just started screwing with the carb, and i found the screw on my own. Unfortunately i turned it to omuch and i think i warped the spring, in anycase, i wont make that mistake with the good carb.

The other thing is, i put some new tangs/or prongs/or metal tab things in, to get the secondaries to open ALL the way. I noticed the stock ones open most of the way, but not all.

Secondly, i bent the tab to make the secondaries open EARLIER. But how early is too early???? Meaning, how far should my primaries be open before the secondaries open up?

This is, keeping in mind i have a 700R4, with lockup and 3.42 gears. So i prolly wont be going past 2500-3000 rpms under normal driving conditions. But if i see that civic at the stop light, i wanna GO!!!. Assuming, that when the primaries, and secondaries are closed they are at 0 degrees, and 90 when fully open, i noticed the secondaries started to open when the primaries were about 70 to 80 degrees.

SO i made the secondaries to open when the primaries were 45 degrees. Is that too "greedy"? Id be at roughly half throttle with primaries at 45, and secondaries just opening, and full throttle with both primaries and secondaires at 90. What is yours set at sonix?

And FINALLY, is there a place that sells a "kit" for a collection of metering rods? It would be nice to have a set to play with when i get to fine tuning, and incase i run into the "bog".

I also have not taken into account that the primaries will be slightly open to allow the car to idle. Waiting to hear. And sonix, id love to hear any tuning advice you can give me on these babies, i know they are amazing carbs if tuned correctly. Thanks again!

 

If I had a digital camera, i'd let you know what I did for the secondary air valve. I ground off the nub that limits the air valve from opening all the way. But I had a WOT bog, and one thing that was suggested, was to undo this mod, so I drilled the casting (the air valve), and put a #4-40 tap through. This hole is right where the nub used to hit on the air valve. Know what I mean?
Then I put a small machine screw and nut through, to keep the air valve from opening all the way. Now I can adjust it as needed.

I also removed the dashpot/vacuum break/front vacuum break, and drilled a hole into it, so I could access the time delay hole. This vacuum break prevents use of the secondaries until you've hit 0 vacuum (WOT) for at least 2.5 seconds or so. This usually keeps you out of the secondaries in 1st gear. My relatively light stick shift car, I figured could use the mod - I drilled it to .025" or so, and came up with 3/4 second opening time. Not sure if I'd recommend you do this, but at least be sure your vacuum can will hold vacuum and not leak. let me know if you want more info on this.

I didn't modify when the secondaries kicked in, as far as linkage adjustments. Generally you won't need to do that unless you've got a LOT of cubes, and need the huge airflow of the secondaries, for driving around on mid throttle, ie, before 75% throttle when the secondaries normally kick in. I don't know what your mod there will do for you, it might give a huge bog when you go from say 45% throttle upwards, or it might just suck back mega fuel. I'm not totally sure, but that's a pretty radical mod.


You should be able to beat that civic on the primaries, at about 60% throttle. Say it's turbo charged, then you can punch it to the floor and snap open the big barrels.

Edelbrock sells some q-jet parts. They don't really have much though, I think they only have the primary rods for the pre '74 castings, not the post '75 units. They do have some primary jets though, and a few hangars and secondary rods. They also have primary power piston springs, I bought that kit. You can buy a full kit with a whole bunch of bits from them, but it's pricey. I just go to the JY, and find all the beat up q-jets laying in the mud, and sit in a nice cadillac, and tear them apart, and take all the rods and jets with my 5 finger discount. It's gotten me most of the parts I need, except for 44-48K rods. I'll be damned if I can find those little bad boys.

When you have the carb upside down on your work bench, use a wire style spark plug gapper, and set your hot idle screw, such that it holds the primary blades open .030-.040" from the bottom of the vertical slot. You'll see what I mean when you try it. This prevents any tip in bogs.

Your mild setup should work quite well with a very stock q-jet.
Don't get carried away with any carb mods just yet. The doug Roe book has a handful of neat mods in it, but just keep in mind, these are for the super stock racers that are going low 10 second quarter miles, or running BIG blocks, again, running about the same speed. A high 12-13 second car shouldn't need much mods at all.

 

Sounds good! I did do the secondary AV limiter adjustment, but instead of drilling a hole, i just bent the little tab back, so its almost fully open. I will probably do that mod on the good carb with the screw, you can maintain near stock and all out operation that way. I would like some more info on the vacuum break you were mentioning. I know that there is a plunger type vacuum thing that controls the secondary AV but im not sure where you're drilling. I got the part where you are trying to reduce the delay of the secondary AV opening though.

So in short, you are saying i DID get a little greedy? lol. Good thing this is a practice carb. Theoretically speaking, how much of the carb will the car run off of at full scream, WOT? In my bros 305 camaro, it had a cam, and for a while we didnt even know it was running completely on primaries (no wonder it sucked). I found out when i was racing an M5, i had my foot to the floor, and he was getting ahead of me, so i put my foot to the floor even further. It moved about 1/4in or so, barely noticeable, that is if it werent for the HUGE change in engine sound, and me speeding up a shitload! I didnt beat him, but still i got my secondaries to work! In anycase, im sure the secondaries were only about 20-30 degrees open or so. So i dunno a if a 350 will take much more, but whats the max they will open in that case?

I suppose ill make it so my secondaries will open at maybe 65-75 degrees of primaries. I know the secondairies arent needed just yet, but i would still like to have some control over them. I will definately waste a bit of gas experimenting with this, but its only a summer car,so i wont feel so bad.

As well, could you elaborate a bit more on the power spring you keep talking about? I think you mentioned its a good way to tune the primaries, except i dont know where its located. Like i said ive taken these things apart plenty, so a small description will help. Thanks again, im really learning alot.

 

ok, here's a good thread on the choke pulloff
https://www.thirdgen.org/forums/carb...ighlight=drill

When you hammer it to WOT, the vacuum break loses it's vacuum, so it slowly releases. If it wasn't there, you'd hammer the pedal to the floor, the secondaries would flop open, and it'd bog. So the vac break is a happy medium. But a 2.5s delay is fine for grandpas deville when he's merging onto the highway, but my TRANS-AM wants the snap sooner So in that thread, Lo-tec's picture shows that he tore it open, I didn't do that, I just drilled the same hole he did. Then using a teeny drill, felt around in there for the orifice, and opened it a bit more. Then plugged the hole with my finger and tried it, just under a second. So I epoxied over my big access hole and called it done.

Yea, that's the thing, you have to really put your foot to the floor to get into the secondaries, then when you do, it's a small amount of foot to get a large range of flow. That's why you can bend that hanger. It'd take a fair bit of bending before the rod snaps, so you can play with it. One of these days i'm going to try it, just not high on my list right now. Try small changes here.


Power piston spring - you know holleys, so you know what a power valve is? Same sorta deal. More fuel enrichment at low vacuum.
So when you're cruising around with high vacuum, the primary rods are sucked down into the primary jets via vacuum. So your 73 jets are plugged with 43 rods, so you get the difference in flow. But now you roll into the throttle a bit, and your 18" of vacuum dips to 12" as you accelerate, now the POWER PISTON SPRING (which happens to be rated at say 8-14) is stronger than the vacuum, so it starts to push up the primary rods. Now the flow area is about 73-36 - more fuel, more power. Once your vacuum drops below the bottom #, say 8", the rods are all the way out, and you have 73-26. A q-jet rods have the same power piston tip size, .026".

So if you have an "early" power piston spring, it'll be higher in number, it'll "come off" earlier, and you get the primary enrichment earlier. If it's a later one (usually used on big cam cars), then it'll come off later. So if your motor is stock, you get 19" vac idling, then a 10-14" might be what you want. You don't want to be on primary enrichment while granny is driving to church. Or while you're idling.
My car pulls 13" vac idling. I run a 6-9" spring I think. big cams- lower vac, and it drops to even lower vac earlier.

 

Oh THAT POWER SPRING! i know it better as the one that makes assembly of the carb impossible because it keeps popping up while your trying to put the top part on! In any-case, much better explained, ill see if i can tinker with that a bit. As for the choke pull-off, ill see if my car bogs first, if it does ill go that route, otherwise ill keep it as is for now. Its kinda complicated, and i dont wanna mess anything up. Went to visit my car today, the guy had incorrect motor-mount bolts, so ill get those for hima nd maybe i can go out for a test drive next week.

 

? You shouldn't be fighting that spring to assemble the carb. The spring you fight when assembling it is the accelerator pump return spring. The power piston should be held down by a nylon collar, so it's out of the way.

The choke pulloff won't cause a bog as is, if you overdrill it, it can cause a bog. But stock it may not be allowing you to use the secondaries soon enough, so you can get faster secondary use if it pulls off sooner. But yea, don't worry about that yet.

 

I would use a 1406 elderbrock just my 2 cents -- easy to tune, but you might have to get a fuel presure regulator to turn the fuel presure down to 3.5 psi
later
rick

 

ur right, it the nylon collar helps, too bad i leanerd that very very very late, so i had been fighting with it quite a bit. Rick, what kind of performance would you expect out of the 1406??? Ive heard alot of ppl use these, and ive also seen quite a few in cars. Seems to me its the cheapest of the performance carbs, but im not very fond of them. Mainly because my buick 350 had a carter, which is basically an old edelbrock, and it was on a very weak engine. But my opinion can be swayed depending on pros and cons.

 

I had one on a 327 that pulled 11.95's in the quarter in an elky--I blew the motor,
I put that same carb on a roller 305 from an 87 formula with an lt1 cam and it works well. easily tunned and I run e-85 with it. i get about 14 mpg with a turbo 350 and 373 rear gears.
for street the perforamce is great, you can tune them like any other carb for the strip if needed. I have great off the line and a pretty decient top end rpm of 6200 rpms. no flaws from the carb. Any carb can be good for any type of driving if you tune it for it. I have seen rochesters on 9 second bigblock chevys
just my rant and rave

GB

rk

 

Sounds good. If i stumble across one i wont hesitate to pick it up. But since $ is always a concern, ill polish up the q-jet i have for now and see how i fare with that.