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Quadrajet Carb Talk and Tips / Re: Corvette Carb Rebuilt By Clifford Not Idling Correctly... Help
« Last post by Cliff Ruggles on Today at 02:56:17 PM »
What are the engine specs?

You've mentioned "bone stock build", but no details.

What cam was used in the engine (specs). 

Overbore, dished pistons, flat tops, and what head gaskets?.....
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Quadrajet Carb Talk and Tips / Re: Corvette Carb Rebuilt By Clifford Not Idling Correctly... Help
« Last post by jberenyi on Today at 02:43:54 PM »
Quote from: Kenth on Today at 12:59:43 PM
In 1974 Corvette uses a std performance Qjet or a high performance Qjet.
The std 7044206/7 units uses 46B pri rods #75 jets .034" idle tubes .100" idle bypass air and CH secondary rods.
The HP 7044210/1units uses 43B pri rods .037" idle tubes .120" idle bypass air and DA secondary rods.
In any case i would open the idle needle holes to .095" and idle tubes .002" to start with.

Good info.  One thing to note, I know this carb's casting number is not the one I listed above.  Also, my baseplate is missing a port for the vent hose on my charcoal cannister!  After paying about $800 for this carb I'm disappointed its not working 100% on a spec build bone stock 1974 350. 
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Quadrajet Carb Talk and Tips / Re: Corvette Carb Rebuilt By Clifford Not Idling Correctly... Help
« Last post by Kenth on Today at 12:59:43 PM »
In 1974 Corvette uses a std performance Qjet or a high performance Qjet.
The std 7044206/7 units uses 46B pri rods #75 jets .034" idle tubes .100" idle bypass air and CH secondary rods.
The HP 7044210/1units uses 43B pri rods .037" idle tubes .120" idle bypass air and DA secondary rods.
In any case i would open the idle needle holes to .095" and idle tubes .002" to start with.
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Quadrajet Carb Talk and Tips / Re: Corvette Carb Rebuilt By Clifford Not Idling Correctly... Help
« Last post by jberenyi on Today at 11:06:38 AM »
The newly rebuilt engine is a matching number 350 bone stock except for .030 over on bores.  Clifford built this carb from scratch because I did not have a carb for him to build.  This Vette uses  7044206.  When I got it back it ran just fine with the tired engine but now its a completely different animal.  I also had Cliff rebuild the original distributor and he re-curved it to be in at 36 deg at 2800 rpm which I have confirmed it is.  Initial timing is set to about 12 deg at 700 rpm.  Vacuum is about 15Hg at 1000 rpm and it will get better as she breaks in.  I confirmed there are zero vacuum leaks.  When I last spoke to you all I noticed that with 5 turns out on the mixture screws and cupping the airhorn with my hand the rpm would rise signifying a lean condition.  I turned the idle screws out to about 8 turns and and it got rid of the lean condition about 90%.  If she idles at 750 rpm and then cup with the hand it will go up to 800 rpm but that's it.  The carb was provided with 75 jets and a 44 primary which is concerning because the specs say it should a 46.  But me being at 4500 ft. above sea level maybe its okay. Anyhow, at 8 turns out she's running better but not perfect with respect to idle only in drive or park.  All other things like cruising, acceleration, etc. she's perfect.  I really think we have an IFR that needs attention on sizing.  Thought's anyone?
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Dialing in your rebuilt Quadrajet carburetor / Re: 1903 Won't Respond to Tip-In Procedure
« Last post by Cliff Ruggles on Today at 03:12:06 AM »
Thanks.  Correct about timing, and fuel requirements follow that as well.

The better you do with the engine build in terms of compression, quench distance and cam choice the LESS timing and fuel the engine will need to be happy.

If you were doing this thru the "smog" years you found out first hand how these things work.

Back in those days I owned and drove a 1970 Chevy Impala with a 350/300hp engine.  It was powerful, idled smooth, great throttle response and nailed down 20-22mpg's on the highway w/o overdrive.  It was stone stock with the only modification being dual exhaust, H pipe and a set of free flowing mufflers. 

Those engines had flat top pistons, small cam, and excellent flowing small combustion chamber 041 heads.

I also owned and drove a 1970 GMC 3/4 ton truck with a 350 in it.  Even with 4.10 gears it got pretty decent fuel economy and made great power.  The engine started using a lot of oil and needed valve guides and seals.  Back then I had no funds for this sort of thing.  I was offered a set of finished heads from a local hot-rodder in trade for the heads from my 350 engine.

He just told me there were 1.94's and would work fine.  Well I did the head swap one weekend using parts store "rebuilder" thick head gaskets.  That move knocked what felt like about 100 horsepower out of my EXCELLENT running 350 engine.  I couldn't believe it.  What a "turd" it was with the new heads in place.  Then I started looking at the heads I removed and noticed smaller chambers and much larger ports.  I pulled the heads I was given back off the truck and compared them closer.  The heads he had given me in trade were "882" castings. 

I gave him back that junk and did a few side-jobs and got my 1970 heads rebuilt and put them back on the truck.  I also used .020" thick steel head gaskets just like came off of it because I wanted everything right back the way it was.  Power was restored and I logged that experience deep in my memory banks. 

LOW compression, thick head gaskets and chitty flowing heads KILL power in these engines.  The smog years were not good for the industry and they didn't get it figured out till almost 2 decades later........
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Quadrajet Carb Talk and Tips / Re: Corvette Carb Rebuilt By Clifford Not Idling Correctly... Help
« Last post by Cliff Ruggles on Today at 02:55:13 AM »
I looked in my QuickBooks program and it looks like that job was done back in 2017.  No details on the engine build and I don't remember the job.  It's lost in the shuffle as I've done close to 15,000 of them since I started keeping records.  We'll just have to wait for details.......
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Quadrajet Carb Talk and Tips / Re: Corvette Carb Rebuilt By Clifford Not Idling Correctly... Help
« Last post by Mudsport96 on July 31, 2021, 01:57:59 PM »
Please elaborate on newly rebuilt.

Cam part number
Heads
Compression
Piston part number if available
Head gasket
Etc.

Could be in the boat i am. Not quite enough compression to get a solid idle but just enough to run fine everywhere else.
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Dialing in your rebuilt Quadrajet carburetor / Re: 1903 Won't Respond to Tip-In Procedure
« Last post by old cars on July 31, 2021, 01:37:04 PM »
Wow that was a long read. Cliff was right when he said the carb should have been installed with stock calibration. Even if changes to calibration were wanted/needed you at least no where you started.

Unless you are as experienced as someone like Cliff, making more than one change at a time can be counter productive. Making single changes enables you to learn how each circuit works/reacts to changes.

.040" in the hole pistons and thick? head gaskets is a disaster for detonation/timing issues. Not sure I ever saw what the total ignition timing was. That led me to wondering why it wasn't backed off a bit.
Vortec heads do not require or like as much total timing.
Cliff's comments on ignition timing/curve, vacuum advance were on the money.
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Dialing in your rebuilt Quadrajet carburetor / Re: 1903 Won't Respond to Tip-In Procedure
« Last post by Cliff Ruggles on July 31, 2021, 04:28:46 AM »
I was expecting MAJOR improvements moving up to the roller.  We did end up making a little more power with the HR cam, after spending the entire weekend on this testing it netted 3hp/4ft lbs better than the Crower RAIV cam we started out with.

Wow, I got a lousy 3hp/4ft lbs improvement spending well over $1000 on a cam swap...bummer.

Even though the HR cam made a few more HP it actually made it at 200rpm less than the flat cam.  All of this baffled me for a while until I thought about what the engineers were doing with cams back then.  They were using longer seat timing and wider LSA but not much lift.  They must have known that the time the valves spend off the seat can very quickly trump shoving them around with greater authority and much further off the seats. 

I suppose it all makes sense if you sit back and think about it.  When we have 30-40 or even more cycles per second happening longer seat timing provides a considerable more amount of time for cylinder filling even if we aren't shoving the valve open as far or opening/closing them more aggressively.

Just some food for thought here as it's pretty rare to see anyone actually doing any back to back cam testing with no other changes.  The folks selling cams ALWAYS change several items in the mix so their product wins the prize.......FWIW.......Cliff
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Dialing in your rebuilt Quadrajet carburetor / Re: 1903 Won't Respond to Tip-In Procedure
« Last post by Cliff Ruggles on July 31, 2021, 04:28:33 AM »
Good news.  You really didn't give up any power, it's making more power.  Wider LSA simply spreads it out some and doesn't spike it nearly as high.  This reduces octane requirements at the same time.

I'm NOT surprised idle quality is improved with more seat timing.  The longer seat timing and wider LSA will also make for a nice improvement in upper mid-range and top end power.

The engine may not "feel" quite as strong IF you are judging improvements by the "seat of your pants".  Do NOT be fooled by this.  Smooth/strong power over a broad RPM range will trump throwing all the power at you early and in a narrow RPM range. 

This simply happens because you are applying more power to the pavement on a full throttle run vs throwing ALL the power at you quick and early.  When this happens the quick blast of power "feels" like a shot of nitrous and we evaluate it as an improvement over the "locomotive" power from a smooth/flat/strong power curve.

I've tested camshafts on the dyno and it also reflects the same thing.  Tight LSA and shorter seat timing spike peak torque high and it happens early. 

One test I did back to back a few years ago was testing a Pontiac RAIV cam against a custom ground Comp cam with their "best lobes" (at that time). 

The factory cam was 304/316, 231/240 @ .050" and 113 LSA.

The Comp cam was 284/296, 240/248 @ .050" and 112LSA.  We had it ground to replace the Crower RAIV clone cam thinking the bigger Comp cam would make more power and improve both ET and MPH at the track.  The Comp cam also had a LOT more lift than the short lobed RAIV cam.  Even with high ratio rockers the RAIV cam was only .516" lift.  The larger lobe Comp cam was over .560" lift. 

EVERYONE involved in the swap was betting the "modern" Comp cam would outrun the old RAIV grind everywhere.

Well, NOTHING went well that day on the dyno.  We made great power with the RAIV cam, right at 500hp/570tq.  Once we dialed in the best numbers I installed the custom ground Comp cam and we fired up the engine.  Immediately I noticed that the engine idled a little better and it was really "snappy" right off idle whacking the throttle quickly.

I checked the timing and we started making pulls on it.    My heart sank as it printed out the first dyno sheet.   The larger Comp cam was down nearly 15hp and 25ft lbs torque....WTF?

I made several adjustments, timing, fuel, and it just got worse.  I even went in and move the cam ahead 2 degrees and then retarded it 2 degrees and things just got worse with everything I did.

I finally returned everything to the initial settings and fine tuned it to perfection and we were still off 10hp/22ft lbs torque that we just were NOT going to get back.

The engine also made peak power 400rpm's LESS in the RPM range with the larger cam, or done at 5200rpm's vs 5600rpm's with the RAIV clone.  I just couldn't figure it out?

So I changed directions and installed a HR cam (we had one ground just in case this testing didn't go well). 

The HR cam was a LOT bigger than in lobe lift than either of the flat cams.  It used .361" Comp XFI lobes, 284/296 @ .006, 230/242 @ .050" and 112LSA.


........Continued
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