(Expert?) Carb Advice Needed

[Bigprairie1]

Ok, I have a '72 Chev 1/2 ton with the original Rochester Quadrajet on it and I have started doing some tuning on the carb and I'm curious if anybody has any good tips on setting this carb up to best running.
First: Jet adjustment? I've heard that most of these always use...2? turns out from lightly seated as the initial adjustment (do I have that right?)
Second: The truck came from and spend pretty much all its life in Auburn, California (pretty close to Reno and at about an Elevation of approx. 1500-2000 feet above sea level). Would this elevation have required changing the jets on this carb from a sea-level application? (I don't know when this issue starts to kick in on these things). I'm at sea level here so although the truck runs well I want to make sure I have everything at its best.

Anyways the backstory is that a few of you may remember I bought a classic truck a couple of months ago out of California. The truck is a '72 Chev 1/2ton. Well I finally got it home (late) mostly due to business commitments.
It's a really nice original truck.....tight, dry, straight and a really good driver. It has a 350/4bbl-TH350 in it and I'm looking forward to doing my own tune on it and sprucing up a couple (very) minor things then using it as a reasonably regular driver.
If any of you guys (gals?) can offer some insight on tuning up this puppy a little further It would be much appreciated.
Many Thanks!!
BP

 

[swl]

Re: (Expert?) Carb Advice Needed

I'm pretty sure those screws you turn out are the idle mix. Set base timing, no advance. set idle speed. then turn in til it starts to stumble. then back out til you get the highest idle speed or vaccum. then turn the idle speed back down, kinda back and forth between idle mix and idle speed. been a long time, but thats the way I remember it(though my instructor would probably slap me for gettin it wrong) cant remember squat about the secondaries though. Im sure someone more knowledgable will be along shortly

 

[j_martin]

Re: (Expert?) Carb Advice Needed

Sometime about that time they put a stiffer spring on the power jet. Later they put a solenoid on it and modulated it. Anyway, a lighter spring will make it stay lean a little higher, and usually greatly improve mileage at normal driving.

I had a converted small bus with a 350 and quadrajet in it. Ran like chit, and couldn't get into high gear in a stiff head wind. Got tired of it and clipped a turn off said spring. Changed the needle for one for an older carb. Got into the distributor and ground the cams for an even advance to about 3200 rpms, and set the timing by ear. That helped a lot. Then I set up a solenoid and spring so the spring held the secondary lockout in place and the distributor advanced about 10?. Even at that, it'd go a lot better than before, and with much better mileage. If I flipped the toggle switch for the solenoid, which retarded the timing and let the secondaries go, everything in the van not tied down would be against the back door.

overall, the weak point in that carb is the float and needle valve.

 

[puddle jumper]

Re: (Expert?) Carb Advice Needed

Before you start trying to set the carb up make sure you have no vacuum leeks. Very important and check your brake booster and carb throttle plate shaft for play. You will never get it right if there's play on that shaft. Keep the idle below 800rpm so it stays on the idle cercut. Turn both screws in all the way and turn one out 1.5 turns. Run the engine with a vacuum gauge hooked up and turn in or out on till you get highest vacuum then turn the second out on till you get the highest vacuum again. Then its set. Vacuum leeks will make your tune up no good.

Set your timing at 8deg with the vacuum on the distributor with the idle below 800rpm. If you have a advance timing light set it to 0 ad rev your engine up to 3000rpm and make sure you timing does not go past 32deg advance. If it does retard the timing on till it does.

There you go a perfomance tune up on a stock engine.

 

[puddle jumper]

Re: (Expert?) Carb Advice Needed

If you want to check your timing setting go down by the Greyhound in the inner harbor heading out of town. Now pull that hill as hard as you can. If your engine pings or knocks back the timing off on till it doesn't.

 

[fat fanny]

Re: (Expert?) Carb Advice Needed

The onlt thing I can remember about the rochester on my ole chev's (85's)was that when the d9 canister went bad it was inevitable that he charcoal in the canister would contaminte the fuel system until it was removed from the system. You fortunatly are not gonna have that issue with that ole beast (nice) A full rebuild is in order first off and then trhe tuning is all by ear and elevation in my case wasn't a factor some rochesters needed a special tool I got from snap on to ajust but when I did it was easy and only took a few miles on the road to get it tuned usually turn them both al the way in and back-em off 11/2 turns and then fine tune from there in my case the 1 1/2 turn was dead nut's hope this helps and good luck!!!!!!

 

[Alpheus]

Re: (Expert?) Carb Advice Needed

Read this thread post # 7
http://forums.iboats.com/showthread.php?t=388794

Then watch these videos. You will be an expert before you know it...
http://forums.iboats.com/showthread.php?t=362542&highlight=QUADRAJET+VIDEO

 

[Cofe]

Re: (Expert?) Carb Advice Needed

Sometime about that time they put a stiffer spring on the power jet. Later they put a solenoid on it and modulated it. Anyway, a lighter spring will make it stay lean a little higher, and usually greatly improve mileage at normal driving.

I had a converted small bus with a 350 and quadrajet in it. Ran like chit, and couldn't get into high gear in a stiff head wind. Got tired of it and clipped a turn off said spring. Changed the needle for one for an older carb. Got into the distributor and ground the cams for an even advance to about 3200 rpms, and set the timing by ear. That helped a lot. Then I set up a solenoid and spring so the spring held the secondary lockout in place and the distributor advanced about 10?. Even at that, it'd go a lot better than before, and with much better mileage. If I flipped the toggle switch for the solenoid, which retarded the timing and let the secondaries go, everything in the van not tied down would be against the back door.

overall, the weak point in that carb is the float and needle valve.

LoL Wow did this bring back memories.
Had a 68 Camaro that I rebuilt a 400 small block 4bolt main for. Installed a Cannon Ball cam with solid lifters that went to work at 2,100 rpm. Then installed advance timing weights in the distributor with a light spring on the Rochester secondarys.......When the hammer was dropped, it was hard to keep it on the road for the first two gears.

Thanks Alpheus for those videos. I wish that I had them back in the 70's when the library and Hot Rod Mag. was the only info for Rochesters.

 

[Bigprairie1]

Re: (Expert?) Carb Advice Needed

Wow, I'm sitting here smiling at the response above and thinking.....what a great forum and collection of technical knowledge. All of the responses bring a lot to my info base and I'm very appreciative guys!!
I do like using the vacuum gauge extensively during the tune up stage and I had it on their briefly the other day. It was pretty steady but not 'rock steady' and was reading something in the 16"-18" range to my recollection.
Overall, the truck is a very nice runner but I'm born to tinker on certain levels and having just got it I of course...want to play around with it a bit and put my own 'spin' on it. I did find (2) loose spark plugs and 3-4 loose header bolts.
I will be checking for vacuum leaks as recommended to see if there is anything amiss in that department.
If the weather holds up today I'll be putting in an electronic ignition and dumping those points.
I'll post some results over the next couple of weeks for an update on tune up and the safety inspection which has to be done before I can register the truck here. Currently the horn, washer pump and the (former?) exhaust leak might be barriers but I'll try to take care of those in the next day or two.
All Good and again...many thanks guys!! You indeed know your stuff!
BP
Oh, one last thing: Any ideas on that altitude/jetting business? given the differences that the carb might see now with the truck at sea level instead of 1500-2000 ft? or is negligible?

 

[rbh]

Re: (Expert?) Carb Advice Needed

Sounds like you should just tune it and keep the jets it has in it if you ever drive over the Malahat or bring it into the interier of BC the altitude is alot higher than 2000 feet on the passes.

By the way "NICE TRUCK BUD", great score!!