Which is a better pickup carb?

[rockman20]

Well, I have my choice down to 2. The Holley Truck Avenger which I do not remember the part# right now or the Edelbrock AVS with part # 1825. Here are my specs.

82 Chev Scottsdale with a SBC 355. (I believe that is a 350 bored 30 over.......) It has an Extreme 4X4 cam, around 9.5:1 compression, headers, accel super coil, Edelbrock Performer intake, and the rest is stock. It currently has a Rochester on it.

My reason for changing.

Right now under part throttle while pulling, the vehicle tends to shutter every now and again. If you nail the throttle to the floor, it smooths out. Basically as soon as you get the secondaries open, the shutter disappears. So I have kind of ruled out a fuel delivery problem cuz I would assume that would get worse at WOT. I have ruled out timing or ignition issues because I would suspect that to get worse at WOT as well. If I can get the pickup moving while it is cold yet (choke on) you don't notice the shutter near as bad as when the choke is off. So I am guessing that this needs to be a problem with the fuel flow on the primary side of things.

I could re-jet the primaries but I really hate that damn air horn gasket that they put on those. They uaually tear on me every time I remove them.

The pickup does see off road use. And it sees a lot of pulling. Which carb do you think would best fit my needs? I am kind of leaning towards the Edelbrock because I know them........but I am not going to rule out the Holley.

[1FaastC10]

neither, put a QJet on it.

[RockHQ]

Originally Posted by rockman20

It currently has a Rochester on it.

Originally Posted by 1FaastC10

neither, put a QJet on it.

You think he has a Dualjet with an adapter plate?

I'd start with cheap rebuild kit and see if it fixes the problem. A thorought cleaning with new gaskets and setting simple things like float level, air valve tension, front and rear vacuum break rod play and release time does wonders. The fact that your problem is not so pronounced with the choke on (and the secondarys closed) leads me to believe there could be a vacuum leak or just a lean condition on the primary side.

[Destructo]

I like my 4160 600cfm holley, only problem I ever had with a holley was a sticky throttle sometimes. Never figured out why that happens, I know I need a stronger spring on the one I have now.

[rockman20]

Here is the low down on the Q-Jet. I believe that it needs to be jetted up just a bit on the primary side. I have done this on another Q-Jet that I use to have. A friend now has it on his Buick and it runs like a shaved cat!

The Q that I have now I went through about 5 months ago. Rebuild kit installed, new float, set float level, new needle and seat assembly, new gaskets, new vacuum choke pull off, and I even had the base plate trued up and had new bushings pressed in because the old ones were getting a little sloppy.

The engine only has around 6 or 7,000 miles on it but I will go through and check for vacuum leaks. Maybe that is all it is. We will see.

[1FaastC10]

Originally Posted by RockHQ

You think he has a Dualjet with an adapter plate?

he didnt specify. if i said i had a Holley on my ruck, would you assume it was an 80452 600 vac secondary carb? NO.

i have seen 73-87 Chevy trucks with both QJets and DualJets, as well as owning a couple that had 2Jets on them. 3 different possibilities, all from one manufacturer.

regardless of you one might assume, there is obviously at least suspicion of a problem with the existing carb. rather than ruining a good running truck with a Holley or Edelbrock, stick something on it that is proven to be all around better. oh yeah... keep your comments to yourself the next time.

[1974ChevyC10]

i say try a rebuild on your existing carb and see if that cures the problem then if you dont like it,it doesnt cure the problem,etc then get a new one.its your truck so what you get is up to you so go with what you like.i personally have a carter afb on mine-same thing pretty much as an edelbrock and my truck runs better with the carter than it ever has with a qjet.just my 2 cents

[rockman20]

I've been doing some investigating here and not knowing a whole lot about why engines do strange things, I want to throw this out to see if anyone knows or has experienced this. (I am guessing the vacuum leak theory might be the ticket here)

Okay, the truck shudders on pulling acceleration and just the primaries open. Kick in the secondaries and it smooths out.

If I up the initial timing which makes my full timing higher, the shudder isn't as bad.

Here is the kicker. I am up to about 12* initial which isn't too bad, but my vacuum advance adds another 36* advance which makes a total of 48* which is higher then I like to see. (But I do not hear a ping at all!) But with it at this setting, I am seeing better fuel economy as well. I am sitting at around 12MPG which is better then the 9 I was seeing with my timing set at 8* initial and 44* total.

I haven't pulled plugs yet to see if they are white or brown or black or what they are but I am guessing they will be white. I am still guessing that this is a lean condition, but it might not be coming from carb. (Like I said, I just got done spending some time and money on this Q-Jet not too long ago) I am going to disable all of my vacuum devices (heater crap) and test my vacuum choke pull off on the carb and my vacuum advance on the distributer. If they check out good, I will then take the truck and drive it for a while and see if I notice the shudder without the heater stuff hooked up. If this is true, I know there is a leak somewhere there. If not, I will get out the carb cleaner and start spraying all of the seals where a leak might be.

If I can't find it, I guess I just need to put in the 454 I have always dreamed about! Seriously though, has anyone experienced this problem before? The thing idles nice and it runs nice to. It is just when you are under power like a load in the bed or pulling a trailer that it shudders like it is starving for gas. Hit the secondaries and it cleans up and pulls good. Move the timing back and shudder gets worse. It doesn't make sense to me.

[RockHQ]

Originally Posted by 1FaastC10

he didnt specify. if i said i had a Holley on my ruck, would you assume it was an 80452 600 vac secondary carb? NO.

Well Rockman20 did mention secondaries and Rochester in his initial post which is pretty self-explanatory isn't it. If you had paid more attention initially and provided an answer that's actually useful in solving the problem instead of a blank statement like this:

Originally Posted by 1FaastC10

neither, put a Qjet on it.

we wouldn't need to fill this thread with useless crap like this.

Back to the topic,

Lean mixtures need more spark advance to burn efficiently than rich mixtures, which may be the reason why you saw an improvement in mileage and the problem was less pronounced after you upped the total timing. A few other things that come to my mind are:
-Accelerator pump
-Advance curve (vacuum, centrifugal)
-primary rod hanger spring

The easiest way to diagnose ignition and fuel delivery is by analyzing exhaust gas (primarily unburnt hydrocarbons, excess O2, CO2, CO and so on), it is a dead giveaway.

[rockman20]

So RockHQ......you got a sniffer I can borrow for the weekend? :-)

If the Rochester "Q-JET" was easier to re-jet, I would not hesitate to do this. But I can not seem to find that air horn gasket without buying an entire rebuild kit again. I don't really see the benefit in that. (I do have different sized jets for the Q and different sized metering rods and secondary hangers and secondary rods as well)

But it doesn't explain why I am lean. Usually a Rochester is pretty good in many different cubic inch variables. Knowing that lean mixtures need more timing to burn more effectively is leading me to believe more and more that I must have a vacuum leak somewhere.

I guess that I will have to start plugging holes........and then work on my truck and find out where my leak is! Yea, bad I know.

Engine temps remain pretty steady and right at the 196* mark. But, then again, this thing does have the diesel radiator in it so even if it is lean, I don't think it could heat it up. (I know that lean conditions cause a hotter engine......learned this on my Olds)

[RockHQ]

Originally Posted by rockman20

So RockHQ......you got a sniffer I can borrow for the weekend? :-)

Naw i'm not fortunate enough to have that kind of money ($10k+) to spend on something that's not needed weekly But you could take the truck to a shop that specializes in modern electrical systems, fuel injection and custom setups. Those places often have the most knowledgeable techs and the best equipment. A 10 minute sniffer test will reveal so much more than any home tool test can. During a 1-hour session you can do a lot to improve your setup, so the cost shouldn't be unbearable.

[flip66]

put an adjustable vacuum advance on it A 355 motor should not need any more then 600cfm carb.

[MylilBowTie]

Vacuum leaks usually cause the problems at idle and low rpm and go away at WOT. Have you changed anything lately? Intake manifold bolts tight? Do you have a vacuum gauge to use?

[cliffsta]

well based on what i read, edelbrock theoretically works better than a holley at odd angles, like in a 4 W D vehicle, but i personally with my 305 with a 2 barrel, im gonna get the holley rebuilt OE replacement. IF i had 400 bucks i would get another OE carb, but i don't have 400$. I think all this about people saying 1 carb is better than another is smoke and fog. Pretty much a carb is a carb if its the same design. Crossing manufacture's special carbs is different, but when it comes to direct OE replacements and remanufactured units, they are pretty much the sam.e

[MylilBowTie]

http://www.carburetion.com/quadrajet.asp

You can get just the airhorn gasket(s) $2.49

[rockman20]

I like the link! I am going to pick up a few of them just to have around. I have another Quad that I would like to use on a friends car but I know that I jetted the crap out of that one. (Before I really knew what I was doing)

I haven't done much to the truck lately. I actually just started noticing this shudder a while ago. (Mainly because I have not towed with it at all until now.)

The engine is pretty fresh so no new parts on that. (about 6 or 7,000 miles I would say) I did notice this a while back so I changed out my plug wires and I put a hotter coil on it. It seemed to go away but now it is back. Last night I used the pickup a lot. This is one of those annoying "sometimes" problems. Towing empty horse trailer I thought that I was riding the bull I was going to pick up! Shuddered pretty bad when accelerating. Once you got moving it was fine. Got to where I was going, got the bull, headed back. No shudder at all. Nothing. Acceleration was good.

Dropped the bull off and the trailer and thought, what the heck. I stepped on it just to where the secondaries were not on. Shuddered a bit. Drove 75MPH down the interstate for about 25 minutes, got home, tried it again and nothing. It accelerated good.

So whatever this dang problem is, it doesn't happen all the time. And this is a base 355 with aluminum intake, Q-Jet, TH700-R4 transmission, a vacuum for the choke pull off, a vacuum for the distributer, and a vacuum for the heater controls.

It has got to be vacuum somewhere. So I will work on it this weekend

[MylilBowTie]

Check the base carb gasket that goes to the manifold. Sometimes its the vacuum for the breaks check the line, rubber hose, and the check valve that goes into the brake booster.

[rockman20]

No brake booster on this. Pickup was originally a diesel so it has the brakes that are power assisted with the power steering pump rather then vacuum.

Literally there are only 3 things on the pickup that use vacuum. The choke pulloff, the heater controls, and the vacuum advance. It is really a simple setup. I love the power steering assisted brakes. It is kind of nice not to have that giant vacuum booster in there.

[screwballl]

i tend to prefer to match intake with carbs since thats what theyre best designed for.. i may not have it on my current truck but i do have plans on replacing my stock Rochester intake with a edelbrock performer intake to match my Performer 600cfm... also it all depends on how the truck will be used.. for best fuel economy plus torque, i found the edelbrocks tend to do better in that category...
more power would be the holley or good all around usage the Qjet

[rockman20]

Well the weekend came and went and boy did I learn a lesson. (but not all bad I guess) I went and picked up a Holley rebuilt Rochester Q-Jet. 250 at the local speed shop. Not bad I guess, but more then I wanted to spend.

(This is after hand testing all heater controls and the vacuum advance and spray testing all visible seals on the engine.)

Put the carb on and it still acted up under load. I was getting pissed. Finally took a wrench to the intake bolts. Hmmmm.......valuable lesson #1. When doing an engine swap DO NOT have 6 people helping you. It is impossible to tell what anyone has done. The 8 bolts on each end of the intake were torqued down. But the center 4 which are hard to get at were not. DOH!

So I tear everything back down and put some new gaskets in. VOILA, it works again! And not a minute too late. Finished it Saturday about 8PM and had to use it Sunday at noon to tow a stripping chute back for our bull riding show we are putting on during the 16th of October. This chute was pretty heavy plus going into a 30MPH head wind on the way home, and 4 guys that weigh around 200-280 pounds a piece in the cab.

Oh how I wish I would of went with a 383 around that time! With the 3.40 gears and the 355, it was pulling pretty hard, but never once chugged at all.

The good news is, I already sold my old Q-Jet. $150. I could of gotten more, but I am selling it to a friend. The carb has very few miles on it from rebuild.

[MylilBowTie]

I guess my bill will be on the way glad to see you got it working