Experiences in Carburetor Tuning (was: Question...) - Pontiac GTO Forum
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post #1 of 35 (permalink) Old 10-02-2018, 09:39 AM Thread Starter
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Experiences in Carburetor Tuning (was: Question...)

I'm in the process of tuning my new carburetor after my recent rebuild, and am working through some issues. Would folks here be interested in a running commentary of what I'm working on and how I'm doing it? I've got at least one problem that I'm still trying to figure out - I think I'm "on its trail" but it's still not fixed.

In the past I've been very vocal about how much I like Quadrajets. That hasn't changed, it's just that my engine I believe is to the point where my QJet isn't big enough to feed it so I've told myself I'm being 'forced' down a different path.

Up to ya'll - let me know - and oh, if anyone has already dealt with and solved the problems I'm hitting, myself being a first-time Holley newbie, I'd sure love to hear the solution.

Thanks,
Bear
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post #2 of 35 (permalink) Old 10-02-2018, 01:21 PM
 
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I seem to learn a ton of good info from guys like you Bear!


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post #3 of 35 (permalink) Old 10-02-2018, 08:20 PM
 
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+1 for your troubleshooting story. I have a very old Carter 9635 and a newer 9635SA that both need rebuilding. The more I read about what others are doing makes it easier for me to take the plunge and try the rebuild myself...
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post #4 of 35 (permalink) Old 10-03-2018, 01:07 AM
 
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"...I like Quadrajets. That hasn't changed, it's just that my engine I believe is to the point where my QJet isn't big enough to feed it so I've told myself I'm being 'forced' down a different path..."



Just curious. There are several Pontiac Super Stockers running 9's, with 750 Q-jets. Guessing that these engines are making around 600hp. So, exactly what would be the limit of power that a correctly built 800 cfm Q-jet could handle ? As you probably well know, SMI even makes a 900 cfm Quadzilla, tho I don't know how streetable it would be.

https://cliffshighperformance.com/si...hp?topic=598.0

But, my engine guy won't use anything but a 1050 or bigger Dominator on his 495 & bigger engines.
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post #5 of 35 (permalink) Old 10-03-2018, 09:01 PM Thread Starter
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Originally Posted by bigD View Post
Just curious. There are several Pontiac Super Stockers running 9's, with 750 Q-jets. Guessing that these engines are making around 600hp. So, exactly what would be the limit of power that a correctly built 800 cfm Q-jet could handle ?
Dunno... I think it takes more than 600 hp to get into the 9's unless the cars are super light. But anyway, honest - I really do like that 455 SD Qjet I have and according to Cliff, it flows more than 800. I started down the road I'm on now for two main reasons:
  • CFM. Everyone has an opinion of course and I've researched a zillion of them. What I finally decided was close to the truth was that my 461 needs about 870 CFM at 6500 rpm. It's not going to see 6500 maybe a couple times a year, if that, but I wanted to plan for that plus some headroom.
  • Intake Manifold. I wish to goodness I could use my stock cast iron intake, but it's just not possible. When I set it on top of the engine there's about 1/8" of open intake port showing *above* the edge of the cast iron. These days no one that I know of makes a decent spreadbore intake that will cover these ports. I've got an old original Torker 1 that I was running on the car that will fit these ports, and I did back to back dyno tests comparing it to my stock factory intake back when I first built the engine in 2010 with #722 heads at that time. It lost quite a bit of bottom end compared to the factory intake and even though it made some up after 5000 RPM, I don't think it was enough to make up for what it lost down low. Plus throttle response with that manifold isn't great. A friend of mine who has a 72 convertible with a mild 455 (not nearly as stout as my car) is running a Performer RPM with a 750 Demon and that thing has INSTANT throttle response everywhere. It's impressive - every bit as snappy as any fuel injection system if not better. Part of that is going to be due to idle vacuum but my goodness - even with my previous cam that made 14-15 inches at idle - running the SD Qjet on the Torker1, it wasn't even in the same county as his car in terms of responsiveness.

And I guess, part of me was tired of arguing with all the brand H guys. I figure at least I'm going to learn every bit of what these things are about while I'm at this so the next time I get into it with one of them I can speak their language. I've learned a lot already... I'm still a big fan of Rochester and wish I could keep running mine.


Bear

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post #6 of 35 (permalink) Old 10-03-2018, 10:04 PM Thread Starter
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Getting into the story now I guess...

My engine and car at the present time:
  • 461 cubic inches (4.25 stroke x 4.155 bore)
  • Heads: 72cc Edelbrock round port. "Entry level" port work by Dave Wilcox. 2.190 intake valves, 1.77 exhaust valves.
  • Cam: Bullet solid roller. 251/257 @ 0.050, 110 LSA, installed at 106 ICL. .625/.623 net valve lift after allowing for valve lash
  • Rockers: Crower stainless, full roller, 1.5:1 ratio
  • Lifters: Crower solid roller, HIPPO oiling, needle bearings, standard height, cutaway
  • Valves: Ferrea - Ram Air IV length
  • Springs: PAC double. Installed height 1.910. Seat pressure 230, open pressure 570
  • Retainers: PAC Titanium
  • Locks: Isky 7-degree super locks
  • Timing set: Rollmaster double roller (.005 short due to align hone)
  • Balancer: Romac - SFI rated
  • Compression: 10.3:1
  • Crank: Eagle forged
  • Rods: Eagle H-beam forged 6.700 length
  • Pistons: Ross forged, full floating pins (double spirolocks) D- shaped dish
  • Windage Tray and Crank scraper: Spotts performance
  • Rotating assembly neutral balanced by Butler performance
  • Oil pump: Luhn Performance 80 psi
  • Intake: Northwind - single plane
  • Carburetor: AED Performance 850 HO Annular
  • Fuel pump: RobbMc 1100 mechanical, set to 6 PSI output pressure, RobbMc regulator at carb inlet (set to 5 psi)
  • Flex plate: TCI SFI rated - neutral balance
  • Converter: Tri Shield Performance 9.5" (I'm REALLY liking this! Drives like a stocker until you hang your foot in it)
  • Transmission: TH400
  • Rear: Moser 9" - Wavetrac diff - 3.50 gears
  • Instrumentation: Factory Rallye gauges supplemented by AEM Digital X-series Coolant Temp, Trans Temp, Oil Pressure, Wide band UEGO Air/Fuel meter
  • Ignition: HEI with Davis "Dyna Mod" module

First challenge after getting it running and driving has been trying to balance the settings for idle and low speed part throttle cruise. When I first got it running, I was able to accomplish a reasonable idle "in gear" at about 750-800 RPM, A/F ratio in the 13 to 14 range which produced an idle vacuum of around 7-8 inches Hg. The idle was fine. However cruise was not. Even the slightest pressure on the throttle, enough to barely start the car moving and send the rpm up to about 1500 would send the A/F mixture plummeting into the mid 9's, even high 8's. There it would stay pretty much 'everywhere' at a steady state cruise. Getting up to around 60mph would see it creep up into the high 9's, low 10's but this was still WAY too rich. Getting dangerously into fuel wash territory - so rich that raw fuel could be getting into the cylinders and diluting the oil.

I tried jetting the carb down which did nothing really. I tried fiddling with idle mixture to get it as lean an idle as I possibly could. Right away I got into a Catch-22 situation. If I adjusted for a lean idle in neutral at around 900 to 1000 rpm, say mid to upper 14's on the A/F meter, as soon as I put it into gear it would shoot up WAY lean - 16's, 17's, 18's - and higher. A few times I saw it go so lean that it was above the gauge's ability to register it. I wasn't able to find an idle mixture setting that would let it idle at 800 rpm in gear without it going super rich driving. I couldn't figure out what was going on. I bought Dave Vizard's book on tuning Holley's but didn't find anything that addressed this specific problem. I started reading everything I could find on the net. That's when I found this thread on a Mopar forum:

Holley Rich at Light Throttle | Moparts Question and Answer | Moparts Forums

It seemed to fit with what I was seeing. The thread also mentioned getting into a situation of not being able to get both idle and cruise into an acceptable mixture range at the same time, and why.

I found threads that mentioned "transfer slot restrictors" that seemed to make sense. The transfer slots provide fuel to the engine during the 'transition period' when the throttle has moved off idle but hasn't yet been opened enough to cause the main circuit/boosters to come online and start feeding fuel. This engine makes so much torque that the majority of the time it's running on the transfer slots, even at 50-60 mph. The force that causes fuel to feed through the idle circuit and also through the transfer slots is primarily engine manifold vacuum - not air flow through the main venturis. The problem is, at an 800 rpm idle manifold vacuum is less than 10, but as soon as rpm starts to come up manifold vacuum comes up too, and fast. I hooked my gauge up so I could read it in the car, and once the car gets rolling and it's running at 1500 rpm or better, manifold vacuum is at 20 or better so it's "sucking" a lot harder on both the idle circuit and the transfer slot circuit. This explained how it could be 'right' at idle but sloppy rich at cruise. The problem then was how to get fuel flow to even out over such a wide range of manifold vacuum.

.... to be continued

Bear

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post #7 of 35 (permalink) Old 10-04-2018, 11:54 AM
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Bear, great stuff and thanks for posting, but please clarify why you can't run the GM intake on your heads. I could of sworn that a lot of people run the stock Pontiac Q-jet on E-heads and KRE heads, etc. What does Cliff say? I think you are giving up a lot in the 'real' rpm ranges running a sub-par intake. I know that if I had a 461, I'd want to run it at 1800-3000 rpm 99% of the time and over 5000 rpm once a year. Low end torque and crisp throttle response are what make a big heavy GTO move out, as you well know!! BTW, have run Holley carbs on Pontiacs, Fords, Chevys, and Mopars I have owned....and they always seem to work well to make excellent power, but are always a bit on the thirsty side due to their coarser calibrations, in my experience. They always drove smooth and surge-free.
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post #8 of 35 (permalink) Old 10-04-2018, 02:03 PM Thread Starter
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Originally Posted by geeteeohguy View Post
Bear, great stuff and thanks for posting, but please clarify why you can't run the GM intake on your heads.
I tried, believe me. Back a few years ago when I first got these heads, to replace the #722 's, the first thing I did was to sit my factory intake up on them. It was a big surprise when I could see 1/8" of exposed open port ABOVE the edge of the intake manifold. I think it's not so much from the overall port height as it is where they're located in the heads. I was running my factory iron intake with the #722 's and I had to work it over pretty good to get it port matched which left only 1/8" thick gasket surfaces at the top and bottom of the ports, but it worked. A Ram Air IV intake or an HO/SD might work because they're taller than the regular ones. I looked into that and got the impression that all the reproduction ones are pretty poorly made, and genuine ones are out of sight cost wise, that is when you can find them. I just checked Ebay and there are some there - the cheapest one is $1,276.00 and that's with no heat crossover.

Anyway - right now I'm getting pretty frustrated and discouraged with the whole thing - again. I've got some things that I'm worried about from having it running so rich for awhile.

Bear

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post #9 of 35 (permalink) Old 10-04-2018, 03:58 PM
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That last sentence is a cliff-hanger, Bear. If your oil didn't get too diluted, you'll be fine. I've seen a lot of this over the years, and it has to be a chronic issue to wash the rings....very hard to do in a drive or two. I don't analyze things as much as you do....due to lack of brain power. I go simple and basic on this type of stuff. Think about it: every carb ever made has to idle, transition from idle, cruise, and go WOT. Fuel ratios will change from around 15:1 at idle, should be at 13-14.7 at cruise, and will drop into the single digits when you open it up. I know with different carbs and different power valves, lower engine vacuum can cause the power valve to be fooled into opening too early, causing over-rich conditions. I wonder if there are any aftermarket Q-jet manifolds out there that work with E-heads? I've toyed with the idea of E-heads or KRE's for my '65, but if I can't run the stock intake, forget about it! I wish I could be of some help here....you surely don't need to go down the rabbit hole chasing ghosts, at this point. I would step back, take a breath, and then start making one single change at a time. If I had an aluminum Q-jet manifold, I'd send it to you. Hang in there, bro'.
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post #10 of 35 (permalink) Old 10-04-2018, 04:54 PM
 
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