Hello everyone!

Finally that i fixed my oil and coolant onsumption issues on my 355, im thinking of trying some different jettings on my year 71 olds QJet (#7041250), which has still the factory jetting for year 71 (Prim. Jets: #7031970, Prim Rods #7034849 "49B", Sec. Rods 7038256 "AS".

I was told that the factory jetting in year 71 is really lean, because of lower compression ratio and the really mild stock cam in this year.

the engine has speed pro flat top Pistons with 0.040 fel pro head gaskets, so i assume somewhere around 9.5:1 comp ratio, 7a Heads with new valves and comp roller tip rockers, cam is a Lunati 272/276
217/221duration @ 0.050
lift in/exh is 0.485/0.485
Edelbrock performer Intake, RPM is already lying in my garage, Hooker long tube headers.

So i think, my engine is "somewhere" similar to a year 69 350

its clear for me, that every carb on every engine needs different jetting, so theres no way to tell that that my kind of setup need jets and rods #whatever.

My question is just if something like the factory jetting of a Year 1969 or 1970 olds 350 Qjet would be a good starting point?

I see from some research, that year 1969 uses the same primary jets and rods than year 1971, only different secondary rods ("AS" for year 71 and "AT" for year 69). Year 1970 uses the same Primary jets as 69 and 71 BUT "52C" rods.

I would be glad if some people with similar mild 350 builds can tell me, which jets/rods theyre running in their QJets.

Thanks for any thoughts!

 

I have to look up a similar build and I’ll edit in the basic carb configuration. Those jets and rods will be in the ballpark. You might want a slightly softer primary piston spring but all of this is tough to tell without baseline mixture numbers and the ability to see where changes take you. My suggestion would be to get a Lambda or Air/Fuel gauge to help you optimize the carburetor.

Do some searches on here “AFR” “A/F” “air/fuel” “mixture” ‘AF meter/gauge” “lean” ‘rich” “lambda”

I subscribe to disabling a Qjet’s secondaries (wire the air valve shut), doing all your tuning on primaries and once happy, then free up the secondaries and tune wide open throttle. It’s a trial and error process, Qjets are a bit of a pain as you have to pop the tops every time primaries need something changed but it becomes a 5 minute process once you do it a few times. Also, before starting the process, remove plug in center/front throttle plate and free up the primary piston part throttle adjustment screw, they tend to be pretty stuck, not worth wrecking it if you can’t break it loose but if you can, it provides another fine tune parameter for primaries.

Consider also, Cliff Ruggle’s book “How To Rebuild And Modify Rochester Quadrajet Carburetors.”

 

How is the idle quality with that bigger cam? The 71 carb is probably a lot more generous than the late 70's or the worst, the 81 to 85 Canadian/Global non US market non CCC carbs, super small passages. I would pick up Cliff Ruggles book, call him on the phone and order a High Performance kit from him along with the quick release pull off. He will send the idle tubes, primary piston spring, jet or rod change needed and look at bushing the primary throttle plates, he offers a kit as well. That will give you a starting point, most important, buy a wide band tuner to get everything where you need it, otherwise you are completely guessing. The book shows you mods like increasing the secondary air door opening and recipes for the idle circuits, jetting for particular bleed size. His HP kit includes things like the secondary air door cam and spring. You will also want to enlarge the idle channel restrictions and behind the idle mixture screws. What fixed the oil and coolant consumption?

 

Thanks a lot for your help! I think thats a good idea to disable the secondaries to get the primaries set up first 👍 for sure, i already welded in a thread bushing in my exhaust for the lambda gauge, a good mechanic i know told me to do this and i will do some test driving with the gauge attached to search for the best able adjustments.

I already have Cliff's Q-Jet book laying around, just havent found time to read myself through it.

I have the large adjustment kit from Quadrajetparts, with different jets, rods, power pistons etc. i hope this kit will have all the stuff that i nees for proper adjustment.

 

the idle quality is good with my cam just slightly rougher. just found that it makes less vacuum at idle than i had with my stock engine, but i think thats normal with a bigger cam. I found no vacuum leaks. Yeah, im sure that the 71 Qjet is a better choice than one of the lste 70's. I was lucky to find mine in the "german craigslist". Just spent 35 € for it.

I will call Cliff for his opinions about performance modifications, since i dont want to destroy my self rebuilt Qjet . I already did the rebushing of the throttle shafts when i rebuilt the carb 2 years ago by turning my own bronze bushings and reaming the baseplate to fit them in, the holes have been worn out pretty good. Never had any vacuum problems/leaks.

the fix of my oil consumption was to use my stock, higher valve covers with the better/larger oil baffle, so no oil is able to get sucked in by the PCV Valve.

The source of my coolant to oil problem was that the driver sides Head gasket was leaking from the water pocket to the Valley area. VERY mean, because it makes the same noise in the same spot when pressure testing as a leaking intake manifold gasket. So i junked 4 intake gaskets just to find out that its the head gasket. A real PITA, but it has one positive effect: learned some more again.

 

Sounds like you’ve got a good handle on things. Here’s the most basic specs from engine I was thinking of. Unfortunately I can’t remember if we did air bleeds, etc on that carb. I think we did which may account for the slightly screwy Rod/Jet combo, may also have just used what I had to attain an appropriate metering area. Tuned it 12 years ago so memory fuzzy and not a lot written down though I’ll keep looking.

355, 9:1 (measured), 202/212 @.050, 510ish lift Hyd Roller, RPM (started w/ Performer, RPM didn’t hurt it), small tube headers. 3.08, 200-4R.

HEI 18* initial, 36* total, 10* or so vac advance (ported)

7042250
Pri Jets 64
Pri Rods 39
Sec Rods DA .443
B Hanger
Primary metering area is .0020224 with 64/39 combo (richer than the stock combos you mentioned)

Pri Metering area 69/49 is .00185354 (69 jet w/46 or 47 rods gets near the.00202 area)
Pri Metering area 70/49 is .00196271 (70 jet w/48 rod gets near .00202 area)

Of course this may not be entirely relevant to your engine, it’s certainly gonna be in the ballpark. This combo, on that engine produced right about 14.7 idle, average 15.5 (15-16.5) cruise and mid/upper 12’s WOT.

Reminder - Don’t forget the primary piston spring with your potentially decreased idle vacuum.

Here’s a link with some helpful info and the metering area spreadsheet.

http://corvette-restoration.com/wp-c...arb_Tuning.pdf

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Obviously the 71 carb is much more generous in its idle calibration. But the late 70's carb are superior in every way once probably calibrated including an extra 50 cfm on the primary side. The DA and B hanger is very rich, it might be too rich. Get a wideband, without, you are using an educated guess at best.

 

I have that same cam in my engine; it’s the early version that was made by UltraDyne before their designs were sold to Lunati.
Also using a Performer RPM intake manifold and long tube headers.

The engine runs well with the original ‘71 metering rods and jets, with the secondary air door adjusted properly, but there may be some improvement from fine tuning with an A-F gauge.

The one big difference is the HEI distributor.
I spent a lot of time experimenting with initial and total advance and found 18* initial, with the vacuum advance connected to manifold vacuum, gave a nearly smooth idle with a steady 16” vacuum. Any less vacuum than that and the power brakes were marginal. The distributor has 18* mechanical advance for 36* total; the vacuum advance is limited to 10* with a home made stop.

 

Fun 71, you gained quite a bit of vacuum by bumping the initial, correct? Another thing to remember is HEI distributors gain timing higher up in the rpm range. I made a positive stop with a threaded 1/4" rod and tap the advance base. I still say the B hanger and DA rods will be too rich especially if the primary jetting is increased. Felpro head gaskets are .042" compressed FYI.

 

Yes, it went form a bouncy 14-ish to a steady 16+, which made the brakes work a lot better, and the RPM didn't drop when I put the trans in gear or turned the AC on.

 

I still have the vacuum advance of my MSD ready to run hooked to the "ported vacuum" port on the Qjet.

Whats the reason to hook it to full manifold vacuum?


My idle drops about 100 rpm when i put the trans in gear.

 

It adds timing right at idle, which helps part throttle response. Also potentially a smoother idle, slightly better economy and cooling.

 

Ok, good to know 👍 so basically, it sounds like theres no reason to hook the distributor anywhere else than on full manifold vacuum?

 

Try it, it depends on octane being used, compression etc and how much your vacuum advance, advances. Most of the aftermarket ones are 20 degrees of advance, at full advance. Some don't like it, most do.

 

Here are some news:

Today i was able to do some carb work.

I pulled the qjet off the engine and swapped the primary jets from the stock "70" to "72", primary rods are still the stock "49B", swapped the power piston spring to a weaker one from my quadrajet-parts tuning kit and installed the shorter-rod accelerator pump from the kit.

it was an easy job, because i applied a thin coat of grease on the air horn gasket when i rebuilt the Carb, gasket went off perfect, so i was able to reuse it.

Wow, what a difference! low end and part throttle is A LOT crispier! This has been good ~60 minutes of work in case of performance👍.

Im sure that the carb still needs some tuning with a lambda tool to see the results during driving, but now i know that the stock jetting was too lean for my engine.

 

This is why I prefer the later carbs, the adjustable APT makes a big difference in mileage turning in and power turning it out, takes 30 seconds. Glad the jetting made a difference.

 

That 72 carb has adjustable APT once you pull the plug & free up the screw which tends to be sticky, I think they used thread locker on them. APT is simply in the throttle plate instead of the choke horn.

71 - That 72/49 combo puts your metering area at .002185. Might be a tad fat but you may find that’s where your engine wants to be. I tend to set them up on the lean side. Time to get a gauge on there, tie off the secondaries and do some test driving. Takes some trial and error. Good move greasing the gasket, anti seize or grease the base gasket too so you can remove the carb a few times without needing replacement. Once the primaries are set, then do the secondaries, they’re much easier and more fun.

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