Quad Jets are only hated by those who don't understand them.There are tons of them holding stock and super stock records.My money is its a 396 that has been bored to 402 so nothing we have said matters until you get casting numbers for the block and heads. LOL! Be sure to tell us which Quadra-Junk is on it because I hate those!
70/71/72 402
- Thread starter Tim
- Start date
I've had a few run in or 2-3 with Quadra Jets. I've just never been shown how to get them right. Still have several setting in my carb library but I dare not try to use one.
I've only seen one year that I couldn't get right and that was the 66 [1st.year] with that funky [f'd up design] needle and seat.The rest are very tunable and reliable once adjusted right.Their not complicated just different.One of my favorite was the 85 and later light truck carb.
As long as some caveman hasn't had his hands on it, there great carburetors.
Skip FIx
Well Known Member
My little Pontiac 400 in a 3900 lb car ran 12 teens @ 111 with a Q jet through mufflers. They are just a little harder to tune having to pull the top off to swap primary jets and rods. A 74 jet and 44 rod are good starting points for most but using a wide band O2 helps dial them in as it does using a Holley.
I'm glad a few folks finally chimed in in defense of the Q-jet. They really can be made to work very well. The only problem I ever have on my 74 Nova is that CA gas kills the accelerator pump, despite what all of the ads say. You can do a lot from the top with rods and hangers.
Another carb is the funny looking Holley/Summit with the removable top?? Watch the "cheap carb" episode on engine masters!
Another carb is the funny looking Holley/Summit with the removable top?? Watch the "cheap carb" episode on engine masters!
True, but they're on Super Stock and Stock NHRA cars not because they're liked but because they have to run the OEM carb
The Qjet is great once you get it dialed, and it doesn't fluctuate once you do.
I put an LS-5 in my 71 K10 years ago. It came from a 1970 full size. Check the specs from GM Heritage. Scroll half way down for engine. LINK 454, 390 hp, 10.25:1 CR, forged steel crank. The LS-4 also had forged steel crank. Both are 2 bolt main.
Greg Reimer
Well Known Member
I've been around, done this stuff probably for too long, but I remember this stuff from when it was a lot newer. Good questions that come to mind ARE:
1. There were three bore size production BBC motors:,vintage '65-'72.
a.4.094-396 from '65-'69.
b.4.125-'70-72 402. This engine used the exact same piston ring as the 348 W motor and the 400 small block. 5'32" top and second ring, 3/16" oil ring.
c.4.250"-66-69 427, and the '70 on up 454.
2. There were two crankshaft strokes available on BBC's in those years:
a.3.760- '65-72 396,402 and 427 inch motors. They were steel until the '68 325 horse motor came out, then '68-72 the two bolt main standard performance motors had cast cranks, and the performance motors, 375,385,425 and 450 horse engines all had steel cranks.
b.4.00"-70 and newer 454's. Most standard performance passenger car and truck engines had cast cranks, the LS-6 had a steel crank,'70-'71. The LS-6 wasn't offered in '72.
3. Rochester Quadrajets came on everything except the '69 only 396/265 horse engine which had a big 2 barrel, and the big port headed hi po motors from '65-71. They got a big Holley unless you had a tri power Vette. Those were big Holley 2 barrels.
4. Much conjecture on Quadrajets. Many people trying to make that carb work right and not having good results need to consider a few things.
a. They have a very small float bowl. A car with a noticeable big bog right after a full throttle launch are probably running out of fuel in the float bowl. A mechanical fuel pump has a problem keeping up with demand because it is engine RPM driven, so it takes a few seconds to start delivering adequate fuel. In NHRA Stock Eliminator and Super Stock, we run an electric fuel pump that is regulated to a constant pressure of around 6-7 pounds. If that doesn't seem like enough, it pays to remember that that is a constant pressure rather than a variable or a maximum pressure that changes with engine load or engine RPM. This carb works best by maintaining a ready supply of fuel volume rather than fuel pressure. That keeps the float bowl full.Usage of a needle and seat with the largest fuel inlet like the 454 carbs has is essential.
b. The secondary air valve is air velocity controlled, with an adjustable spring that keeps it closed. A black and white vacuum actuated pull-off keeps it shut. Hard acceleration drops the intake vacuum and then the difference in pressure below the air valve allows the atmospherical air above the valve to push it open. In doing so, there is a hangar on top of the valve that rotates with it and lifts two metering rods out of two wells in the bottom of the bowl casting, allowing fuel to enter two discharge nozzles in the secondary well right below the two air valve butterflies. Inadequate spring tension on the two secondary air valve butterflies causes them to open too quickly and the engine bogs pretty badly. Proper spring tension adjustments and proper vacuum break operation is critical on these carbs.
c. Proper part selection is critical. Needle and seat assemblies, carb floats, and accelerator pump piston material is critical, especially in locations with fuel additives and compounds added to gasoline that tend to dissolve regular conventional materials can be a problem.
d.Use a good supplier of parts. I recommend Quadrajet Parts, the Carb Shop, both accessible online, also talk to fellow Stock and SS racers and go to some of their carb guys. Most of them forgot more during their last nap than I will ever know. Q-J's can work so well that they will surprise you.
I have raced in NHRA Stock Eliminator since 1983, most of it in a '68 Chevelle 2 door hardtop with both a 327/250 horse or a 327/275 horse Powerglide trans combination. Never had issues with carburetors on these cars, the 275 horse car has gone high 11's, the 250 horse has gone 12.20's. I also have a big Chevelle wagon that at 3800+ pounds has gotten into the 12's with the 250 horse engine.
Hope all this helps!!
1. There were three bore size production BBC motors:,vintage '65-'72.
a.4.094-396 from '65-'69.
b.4.125-'70-72 402. This engine used the exact same piston ring as the 348 W motor and the 400 small block. 5'32" top and second ring, 3/16" oil ring.
c.4.250"-66-69 427, and the '70 on up 454.
2. There were two crankshaft strokes available on BBC's in those years:
a.3.760- '65-72 396,402 and 427 inch motors. They were steel until the '68 325 horse motor came out, then '68-72 the two bolt main standard performance motors had cast cranks, and the performance motors, 375,385,425 and 450 horse engines all had steel cranks.
b.4.00"-70 and newer 454's. Most standard performance passenger car and truck engines had cast cranks, the LS-6 had a steel crank,'70-'71. The LS-6 wasn't offered in '72.
3. Rochester Quadrajets came on everything except the '69 only 396/265 horse engine which had a big 2 barrel, and the big port headed hi po motors from '65-71. They got a big Holley unless you had a tri power Vette. Those were big Holley 2 barrels.
4. Much conjecture on Quadrajets. Many people trying to make that carb work right and not having good results need to consider a few things.
a. They have a very small float bowl. A car with a noticeable big bog right after a full throttle launch are probably running out of fuel in the float bowl. A mechanical fuel pump has a problem keeping up with demand because it is engine RPM driven, so it takes a few seconds to start delivering adequate fuel. In NHRA Stock Eliminator and Super Stock, we run an electric fuel pump that is regulated to a constant pressure of around 6-7 pounds. If that doesn't seem like enough, it pays to remember that that is a constant pressure rather than a variable or a maximum pressure that changes with engine load or engine RPM. This carb works best by maintaining a ready supply of fuel volume rather than fuel pressure. That keeps the float bowl full.Usage of a needle and seat with the largest fuel inlet like the 454 carbs has is essential.
b. The secondary air valve is air velocity controlled, with an adjustable spring that keeps it closed. A black and white vacuum actuated pull-off keeps it shut. Hard acceleration drops the intake vacuum and then the difference in pressure below the air valve allows the atmospherical air above the valve to push it open. In doing so, there is a hangar on top of the valve that rotates with it and lifts two metering rods out of two wells in the bottom of the bowl casting, allowing fuel to enter two discharge nozzles in the secondary well right below the two air valve butterflies. Inadequate spring tension on the two secondary air valve butterflies causes them to open too quickly and the engine bogs pretty badly. Proper spring tension adjustments and proper vacuum break operation is critical on these carbs.
c. Proper part selection is critical. Needle and seat assemblies, carb floats, and accelerator pump piston material is critical, especially in locations with fuel additives and compounds added to gasoline that tend to dissolve regular conventional materials can be a problem.
d.Use a good supplier of parts. I recommend Quadrajet Parts, the Carb Shop, both accessible online, also talk to fellow Stock and SS racers and go to some of their carb guys. Most of them forgot more during their last nap than I will ever know. Q-J's can work so well that they will surprise you.
I have raced in NHRA Stock Eliminator since 1983, most of it in a '68 Chevelle 2 door hardtop with both a 327/250 horse or a 327/275 horse Powerglide trans combination. Never had issues with carburetors on these cars, the 275 horse car has gone high 11's, the 250 horse has gone 12.20's. I also have a big Chevelle wagon that at 3800+ pounds has gotten into the 12's with the 250 horse engine.
Hope all this helps!!
ls-5 FROM FACTORY HAD A CAST CRANK.
Greg Reimer
Well Known Member
Not exactly. For whatever reason, the 402 came out in '70 and was put in Chevelles,Novas, Camaro, Monte Carlos and El Caminos.They continued to refer to it in these car lines except the Monte Carlo as a 396, fender emblems all said 396. If you had an SS 454 Chevelle, it had 454 emblems with large red numbers on it. Kind of commanded attention. The 70 Monte Carlo had either a 402 or a 454 big block. The SS454 Monte Carlo, a rather rare beast, had small emblems at the front of the rocker panels right behind the front wheel opening that simply said SS 454. The rear panel between the trunk lid and the bumper was painted black, and it had SS 454 emblems on it. If you had a Monte Carlo with a 402,it had an air cleaner with a Turbo Jet 400 red and silver sticker on it. There were supposedly a few 400 small block Monte Carlos made that year with the 265 horse 2 barrel motor, and the funny thing, if you went to your local parts house to buy a water pump, a fuel pump, fan belts, rad hoses, or anything like that, they would look in the catalog and ask," Is it a 4 barrel or a 2 barrel?" That's a strange question to be asked if you need any of those parts, but when you consider that it refers to the difference between a big block or a small block, it suddenly makes sense. Another interesting oddity, the 400 small block mainly came out in Impalas and Caprices, as well as the Monte Carlo. That replaced the 396 as an Impala motor. If you got a 70-72 Impala with a big block, it had to be a 454. That meant, also, you could buy a Chevy with a 400 and it could have been a 402 and the factory would have called it a 396, a 400 or a 402. Think of the confusion ordering parts this must have caused!!
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My brother has a 70 malibu 402 4 sd, documented car. 400 emblems on fenders.
Greg Reimer
Well Known Member
That's an interesting car. It must have had the 4 barrel big block but been a Malibu, not an SS. Maybe that's what I saw and didn't know it since the car was driving down the freeway, not parked somewhere. I went to the NHRA Classification Guide for 1970 Chevelles, and lo and behold, the Guide doesn't show a 2 barrel 265 horse 400 in the Chevelles. It was an Impala motor that first year. An Impala 2 door HT with a turbo 350 and a 400 SB was a definite high school parking lot burnout contest contender.
Car came from an auction near Minot ND a few years ago. Complete except carburetor. Canadian build so build docs were avalible.
None like that from US. cars had 396 emblems and the truck said 400
The big confusion back then was the trucks also had the small block 400