i have a 71 chevelle with a 402 BB,everything is rebuilt or new about 3 years ago.Its basically stock,headers,bored 30 over,hei.My carb seems to bog at high rpms only,but not all the time,is that a common problem with that carb?

Might need a rebuild or someone that knows how to set it correctly for you, quadrajet is an excellent carb.

Quads have such a small float bowl , one of the first upgrades I do to them is the high flow needle and seat. The float level setting is one of the most critical settings on these carbs , and they are very picky with fuel pressure. No more than 7 psi. or they will flood over.

Dave

that "bog" at high rpm's could be the secondaries not opening sometimes. check the choke linkage to make sure it's opening all the way up and that the little linkage that stops the secondaries from opening when the choke is engaged is allowing the butterflies to open after the choke opens all the way.
properly adjusted, you won't even feel those big secondaries kick in- it will just be a nice continuous flow of power.

Check your fuel pressure, I hate to go against the grain, but my 383(QJET) will not even wake up until I have at least 7psi of fuel pressure, and still have a slight hesitation during high rpms at wot.. You possible might need a richer secondary rod, also. I used to hate it when I ask for advice on my qjet and other people would tell me to buy a book, but it did really help. Not so much that I can take one completely apart, and rebuild it, but after some studying I do understand a qjet much better than before.

First thing I would do is check to make sure those secondaries are opening all the way as Novaderrik suggested..... Check out those secondary rods, they are most likely something in the range of a CH, let us know.....

BRIAN,as stated above its correct that q-jets dont like over 7psi esp in hot weather. Your q-jet seems to do better with more fuel pressure when your getting on it because its very likely got the smaller needle & seat (.110 or .125 oppssed to a larger .130,.135,or .145 needle & seat) in it . Sometimes running a little more fuel pressure will overcome/force a little more fuel past the smaller needle & seat correcting a marginal fuel delivery/starvation issue when your getting into it. But if your running 9-10 psi fuel pressure you may run into problems in the futuer on hot days after the needle & seat get worn over time,it might take a few seasons before it gets worn enough to act up on you. So maybe for now dont fix what isnt broken and just keep an eye on it in the future for signs of this to avoid problems down the road.

Scott

SS1,i dont know if you mean the bog is after your into it as the motor is rpming through its power band of if its when you 1st hit it to the floor when the sec open up?

So if it when you 1st hit it to the floor when the bog happens (at times)then picks up a few seconds later you need to adj/tighten the upper sec air valve spring a little bit. This is because its opening too fast dumping a lot of air in carb leaning out mix untill fuel catehs up to seocndaires.

It could also be experiencing fuel delivery/starvation problem due to a smaller .110 or .125 needle & seat (very common to most rbld kits these days) in float bowl causing a fuel delivery issue under load while getting into it. In that case you would need to get a larger .130 or .135 needle & seat.

Also,Run a good carter street/strip 6-7 mech pump with your setup along with a lighter fuel pump rod to avoid pump arm float at higher rpms,and a good lrg high flow inline fuel filter & get rid of the smaller somewhat restrictive in carb filter.

The secondary meter rods can affect fuel metering but they will not usually cause a noticeable bog when you 1st hit it or are rpming up through the gears,thats fuel delivery & sec air valve adj issues.

But for your setup i would run AX or BG secondary metering rods.

scott

BRIAN,as stated above its correct that q-jets dont like over 7psi esp in hot weather. Your q-jet seems to do better with more fuel pressure when your getting on it because its very likely got the smaller needle & seat (.110 or .125 oppssed to a larger .130,.135,or .145 needle & seat) in it . Sometimes running a little more fuel pressure will overcome/force a little more fuel past the smaller needle & seat correcting a marginal fuel delivery/starvation issue when your getting into it. But if your running 9-10 psi fuel pressure you may run into problems in the futuer on hot days after the needle & seat get worn over time,it might take a few seasons before it gets worn enough to act up on you. So maybe for now dont fix what isnt broken and just keep an eye on it in the future for signs of this to avoid problems down the road.

Scott

I am still somewhat new to qjets, so I am just trying to learn as I go and I appreciate the sharing of knowledge....

This is now starting to make sense now, I know everyone tells me I am running too much fuel pressure, and I am willing to bet I do I have a smaller inlet valve in there. So once it gets worn, the fuel pressure will end up causing the inlet valve to be pushed off, when the float bowl is full and cause unwanted flooding, is this correct?

Scott, what needle and seat would you suggest for my setup? 383 stroker, dart pro 1 heads, crower cam .474/.498, duration @50 is 214/220. I know it is a pretty mild cam but definately not a stock one either.

Also I am going to the track this weekend, do you think it is worth trying out some different secondary rod and hanger combos. My qjet currently has a CH on it, which are the short tip, along with a G hanger. I have a pair of CK, and DA/CY and B hanger. I have a slight hesitation at about 1000 feet when I am shifting into 3rd gear.

P.S. When it is time for a qjet rebuild I will definately being sending it to SMI this time around!!!!!!!!!

Thanks.....

I used a stock one, drilled it to 9/64's which is .141 & used a accelerator pump checkball, give it a few light taps on the top of the drilled hole to form a new smooth seat. There ya go you have your big needle & seat. Not sure what my fuel press. is, I'm running a Carter 120 GPH pump.

I run 5.5 psi @ 3000 RPM at the carb for fuel pressure with the larger needle and seat assembly in my 406 sbc and have no issues. Q-jets in my opinion are very fuel pressure sensitive, along with float level settings. My q-jet runs as good as any fuel injected engines I have driven, one twisted of the key, even in 40 degree weather and then you can drive it right off with no stumbles.

I bet your bog is a secondary air door set way too loose, most think looser is better which is why they get called quadabogs.

The Q-jet is the best most complicated carburator ever designed, I run nothing else if I have a choice.

Here's how I do this!!

One nice Sat afternoon I start tweaking on the top butterfly's!!

I adjust them so they flop in when I want them and lo and behold, GREAT!!

My 406 SB's are set up so they stay shut up to 70 mph, but when I kick it, they come in at 35 mph in 2nd gear!!

Sucker take's off like a rabbit outta the gate!!

pdq67

The sec. air valve spring is not the only thing on a q jet that controls the secondary opening rate. The primary vacuum break[choke pull off] also is part of the system. Does it pull off when vacuum is applied? If the the pull off is bad the sec. will open to quickly causing a bog. Also the car will load up cold due to the choke not pulling off. Just another thing to check.