which carb to buy

[Clone TIE Pilot]

really.... q-jet... man the one i had on my 307 was from 81cutlass and sucked always had problems.... choke never worked right.. seemed like it would never stayed tuned trying to get something good to were i dont have to try and tune it to run like it should... ii didnt grown up with these carbs so i'm not an expert on tuning these things out.. , the motor just rebuilt to stock, 68 block, #8 heads i bought the motor 2 years ago, and never heard it run so i'm hoping all is good,but is there a difference from early 70's quadrajet

Qjets are great carbs but there are some pitfalls to look out for. First you need a Qjet that is matched to your engine's specs. Qjets are very application specific carbs, they aren't a generic one size fits all carb like most aftermarket carbs. Then there are the "remanufactured" Qjets ( not the same as rebuilt) which are ruined carbs that you should stay away from as they have cheap, sub standerd, generic spec parts among other problems. Then there are also the junk, cheap rebuild kits that the auto chain stores sell, again keep away from those.

 

[Clone TIE Pilot]

If you can find one that actually came from a 350 the jetting should be right on. A BOP version might be tougher to find than a Chevy but the only real difference is the angle of the fuel line inlet. You should be able to find a good core and buy a kit for less than $100. I'd probably look for one with a 70XXXXX over a 170XXXXX number.

Olds and Chevy Qjets have more differences than the fuel inlet. Olds Qjets use 2 idle bypass air holes on the throttle plate while Chevy Qjets only have 1 idle bypass air hole. Olds Qjets use restrictor inserts in the main air bleeds in the air horn, Chevy Qjets don't use restrictor inserts, etc.

 

[DRIVEN]

All true^^. I was trying to keep it simple and was referring mainly to bolt-on interchange differences. It's unfortunate when people have negative Q-jet experiences and I was trying to minimize "fear of the unknown". I've found that they are extremely trouble free if properly matched by application. Generally speaking a pre-smog 350 Chevy carb will run well on a pre-smog 350 Olds, etc. Not so much when using one from a 472 Cadillac or smog 305.

 

[dogsht]

"Qjets are very application specific carbs, they aren't a generic one size fits all carb like most aftermarket carbs."

This may be true in regards to jets & hangers but jetting should be application specific for any carb. As far as the carb itself they are more one size fits all then any other carb especially any square bore carb. The triple venturi system to hold vacuum & for smooth instant throttle response regardless of motor displacement is part of why this is.

If you are going to run a cam in excess of 240@50 duration then a square bore becomes a better choice. IMO

 

[Clone TIE Pilot]

"Qjets are very application specific carbs, they aren't a generic one size fits all carb like most aftermarket carbs."

This may be true in regards to jets & hangers but jetting should be application specific for any carb. As far as the carb itself they are more one size fits all then any other carb especially any square bore carb. The triple venturi system to hold vacuum & for smooth instant throttle response regardless of motor displacement is part of why this is.

If you are going to run a cam in excess of 240@50 duration then a square bore becomes a better choice. IMO

It's more than just jets and hangers, there's needle seat size, to different float sizes, to idle air bleed hole size, to idle tube size, to accelerating discharge hole size, to accelerator pump shaft size, to differnt types of idle bypass air system setups if it's even used, to size of main air bleed restrictor inserts if used, etc. There's just alot of little things set to match the engine the carb is designed for, and all those little things add up. While the basic casting is one size fits all, however they were machined and assembled into 100s of different application specific factory configurations by GM. Usually the newer a Qjet is, the leaner it's setup and thus has a narrower application range. Older Qjets were setup richer and have a wider application range, though too rich can cause gas washing.

This is why the adjustments on a Qjets are only for fine tuning, they were built to be close to what the matching engine needs. Aftermarket carbs have coarse adjustments to make up for not being built to application specific configurations. This is also why reman Qjets are awlful, they replace application specific parts with generic parts which will never work right.

In short, no generic aftermarket carb can beat an application specific Qjet that matches your engine's needs.

 

[beermonkey9417]

heres a good cfm article i found:

 

[dogsht]

Parts stores etc Re-man Q-jets are awful. And in many cases if you have a decent rebuild-able core having a small shop pro that specializes in Q-jets do it specific for your application doesn't even cost a lot more. But the quality difference is undeniable.

Clone I liked your recent post on the LS thread as well. Lots of incite learned by experience over time and paying attention. Some objectivity that the average white knuckle enthusiast / internet hypothetical hot rodder and magazine believer never gets. No guru here but I to like helping the others and especially newbs (where I can) who are trying to do something cool and are here to learn.

Beermonkey my good man I used to love those charts and that one in particular but have lost my enthusiasm for them. Their is some good generic in the ballpark sense to them that can be helpful especially for a newb and especially if you are using a squarebore carb. But guesing at efficiency is a stab. Now I am not a carb dyno tuner guru but I guarantee none of the engine masters builders for example use them. When it comes to the carb or various carbs when comparing & optimizing, Motor or chassy dyno operator tuners want to see in conjunction with other things how much vacuum from idle all the way through the pull that the carb is pulling. If the engine has to suck too hard to get the air and fuel through it then its too small for the ideal or max power. Often you see people come in and they proudly have their small cfm square bore working great and they have optimized it at the track then are shocked when the shop pulls a bigger carb off the shelf and without optimization immediately pulls another 20+ hp out. The base of the crystal ball was the the motor pulling too much vacuum. ehh regardless of what a chart may suggest.

 

[Cutty1]

ok well i'm not sure what my specs are on my motor supposely built to stock and the block is 68 so which carb should i get that has those specs.. and i contacted personal http://www.customrebuiltcarbs.com/ from this place and they said they have one in stock for my motor for 275 to 20 shipping kinda pricey i think but not sure when it comes to custom carb and what custom means

 

[Clone TIE Pilot]

ok well i'm not sure what my specs are on my motor supposely built to stock and the block is 68 so which carb should i get that has those specs.. and i contacted personal http://www.customrebuiltcarbs.com/ from this place and they said they have one in stock for my motor for 275 to 20 shipping kinda pricey i think but not sure when it comes to custom carb and what custom means

That is a pretty low price, too low to be true. Looking around on their site they are a remanufacturer, so keep away from them. You really should know your engine's specs since you are working blind without knowing them, not good. Year of the block really doesn't matter, you should know what type of heads, valve size, how many CCs, etc. You should also know the camshafts specs, CID, the compression ratio, stroke, quench, etc. If this motor is rebuilt to stock 68 specs. Then what 68 specs? What model and submodel 68 car? For a rough guess, I would agree with Driven's suggestion and go with a Qjet off of a late 60s Olds 350. Just not sure about using a Chevy Qjet since they set their carbs up alot differently than Olds did. Thank you for the props Dogshit, I am not a top expert either but I try to share what I know too.

 

[bill]

With my engine, a stock 350sbo, #6 heads..it runs great with the 500cfm 2bbl rochester. Excellent throttle response and cranks with just a touch of the pedal and a bump of the starter. Unless I go with a lot more performance upgrades, or plan to drive across the country, I wont be removing it to add a 4bbl intake and carb. Just seems to me to be a waste of expensive gas....jmo of course..