Carb woes

Discussion in 'The Venerable Q-Jet' started by BYoung, Nov 5, 2018.

  1. BYoung

    BYoung Stage me

    Thank you for your response Cliff. It was definitely a lean condition and idle tubes have now been enlarged. It is responding better in the garage but due to the crappy weather down south I haven't had a chance to road test it. I'm also going to take another look at the distributor springs. I'll post again with my results.
  2. 72STAGE1

    72STAGE1 Runnin' with the Devil

    As in Cliff 's book, one of the primary problems in the past has been the secondary throttle shaft, they leak air , its a must when rebuilding any Qjet to drill the base plate out and install new 2 cents.
  3. BYoung

    BYoung Stage me

    This has been done.
  4. Cliff R

    Cliff R Well-Known Member

    Good plan. I mention distributor springs/weights because for some reason a high percentage of the Buick and Olds crowd have bought into the myth that ALL the timing needs to be in by 1500 RPM's or so. Many also block off, disable or don't use the vacuum advance as well.

    I've corrected "idle issues" with hundreds of these vehicles brought here (some from great distances) where no one involved could get them to idle clean in and out of gear simply because some of the advance curve is in well below 1000rpms and they are getting timing retard when the transmission is placed in gear causing too much drop in engine RPM's.

    The cure has always been the same, re-install the factory weights and stronger springs to make sure the ignition timing is ROCK SOLID at idle speed in and out of gear, then , if/as needed increase idle fuel delivery and bypass air so there is not a lean condition at idle. I'll add here that we've ran into a significant number of defective aftermarket HEI "conversions" and at this time are avoiding them and using points instead.

    What makes this scenario worse and I've seen this trend increasing in the past couple of decades due to the cam manufacturers pushing shorter seat timing/fast ramp camshafts on tight LSA's with early closing intake (advance ICL). They certainly don't do this to make these engines happier at idle, drive better, hit converters harder, and improved efficiency and engine characteristics in the "normal" driving range......they do it for "bling", or "attitude" at idle speed so you can have that lumpity-lumpity idle at the local Dairy Queen car show or cruise ins.

    At this time the biggest complaint we get to the shop is from the poor soul who just finished up their new "high performance" engine build and installed a Comp XE, or even worse a Thumper or Thump your Mutha camshaft. 110LSA is bad enough, 108 seldom works well at all and 107LSA has no business in a street driven engine, unless you are super quick and can back it out of the garage before it stinks you clean out of the house (wife woln't ride in the car either because it stinks her hair up...never a good thing!), and can deal with poor idle quality, throttle response, "reversion" and poor street manners and that's after you increased the ignition timing clear off the scale just to get it to stay running in and out of gear!

    Not trying to put a big black cloud over anybody's engine project, but with that said take a look at the dyno chart below.

    The first cam was recommended by Comp Cams for this 455 engine build. Nothing exotic, just a 455 with 250cfm iron heads and 9.3 to 1 compression, intended for strong low power and pump gas friendly. That did not happen and it actually "pinged" on the dyno and spun a couple of rod bearings. I got involved as the shop dynoing it was blaming EVERYTHING on the Q-jet. I told them to throw the cam up on Ebay and install the 2nd one I recommended, no other changes.

    Engine idles better, fine on pump gas and made just a tad bit more power at every RPM than the short seat timing cam on tighter LSA........Cliff

    PS: cam specs for Comps cam are 276/284 @ .006", 224/230/110LSA ICL at 106.

    My cam used much larger .381" lobes and 289/308 @ .006", 236/245 @ .050" and 114LSA with the ICL at 110 degrees.

    IMG_0223 - Copy.JPG
    Last edited: Dec 13, 2018
  5. johnriv67

    johnriv67 Well-Known Member

    Not to hijack, but you mentioned something important Cliff. Does a cam like this...

    276/295 219/238 @ .050 .490/.460 on a 109LSA 106ICL -(3* ground in)

    have a place in a mild 430?

    It seems quite similar to the Comp cam, but isn't every engine different and could mine handle it better with 9.7 static and 35* @ .050 Intake valve closing?
    Last edited: Dec 13, 2018
  6. Cliff R

    Cliff R Well-Known Member

    Correct, every engine is a little different. One thing we see with Olds and Pontiac builds is that they are under-square designs and very sensitive to the intake closing point and LSA. If you don't spread things out and get all that happening at the correct time then they don't scavenge well at low speeds and even exhibit some "reversion" below 2000rpms. Reversion is simply my term for "choppy" off idle until things are happening fast enough where it smooths out. During that period they typically require considerably more timing and fuel than an engine that doesn't exhibit those characteristics.

    I really have ZERO experience with the 430's, but have spent some time with Pontiac 428's and Olds 425's. Both of those are shorter stroke engines, oversquare and very efficient compared to the bigger 455 engines. They fair much better with narrower LSA and we typically use 110 or 112LSA in them here. I also put pretty "big" cams in them and higher compression to compensate as they love to rev and quickly crippled if you don't cam them accordingly.

    For the big 455's they are pretty much big "tractor" engines and going to try to throw a LOT of power at you early. They also see a cam with 230 @ .050" about like a 350 sees a cam with 214 @ .050", so we NEVER put cams in them smaller than that for any reason.

    I'll attach a dyno sheet to show how a 455 can "gobble up" a roller cam and have idle and driving characteristics like a stock 350 engine.

    455, 8.8 to 1 compression 26 0cfm heads, stock intake, stock exhaust manifolds, 282/287 @ .006", 230/236 @ .050" and 112 LSA with the ICL at 108. Note on the dyno sheet a big fat power curve but done just past 5000rpms. This engine also idled dead smooth with no lobe all the way down to 600rpms. Even with that said we see folks using smaller flat cams in 455's all the time, then wondering where all the power is?........Cliff

    johnriv67 likes this.
  7. johnriv67

    johnriv67 Well-Known Member

    And Cliff, I expect you mean 455 Olds/Pontiac, not Buick, with that dyno sheet? I know a 455 P is 4.150 x 4.210 and Olds is 4.126 x 4.250, both undersquare.

    I know this is a carb thread, so maybe I'll take my "engine theory" elsewhere
  8. Donuts & Peelouts

    Donuts & Peelouts Life's 2 Short. Live like it.

    Why is'nt good to flip a carb on its back?
  9. Donuts & Peelouts

    Donuts & Peelouts Life's 2 Short. Live like it.

  10. Cliff R

    Cliff R Well-Known Member

    I was just providing a dyno sheet to show how a 455 CID engine makes power when you put a short seat timing cam in it on a tight LSA vs much larger camshaft on a wider LSA.

    Throwing in bore to stroke ratios, rod length to stroke, etc quickly becomes a topic of much controversy, and to this day even the "experts" can't agree on any of that stuff. Some will tell you it's a BIG deal, others will simply say the rod connects the piston to the crank, etc.

    Of course engine design plays a role here, and there are many variables. The trend I've seen since I work with ALL designs that came out of Detroit in these vehicles is what I talk about. I leave the theory and conjecture for the "Trolls", resident experts, and guru's to squabble about.

    I put up useable and accurate information as I know it to be from direct testing, nothing more, nothing less. I will have also worked with many thousands more of these engine/drivetrain/vehicle combinations that most who will read this.

    Two things I have found here as common denominators are that lowering compression hurts power (torque production). It's not going to come back by installing some fancy lobed camshaft with shorter seat timing on a tighter LSA. That seems to be more the trend these days and promoted constantly on the Forum's. I've also found that shorter seat timing and much more "aggressive" opening/closing specs can create issues with high RPM operation and typically require much stronger springs, and even then I've seen problems on the dyno with those set-ups when the springs loose control of the valve action even when lightening up some of the parts in the assembly on either side of the rocker arms.

    For my own engines and the ones we prepare here we use higher compression, which opens up the door for larger camshafts and offsets negatives from additional overlap. We tend to go wider with LSA and later with intake closing and avoid camshaft profiles with "aggressive" valve opening/closing specs. This lowers dynamic compression/cranking compression a bit but improves scavenging and intake charge/cylinder filling at speed. It also spreads out the power curve, which lowers dynamic (decreases octane requirements) and pushes power and peak VE higher in the RPM range, all else being equal.

    My own engine is a good example of what can be done with a well thought out combination and still effectively manage currently available pump gas.

    455 cid, 11.3 to 1 compression, 295cfm heads, aluminum construction with fast-burn chambers, 289/308@ .006" roller cam with .381" lobes, 112LSA and ICL at 109.5 degrees. That engine EASILY manages any grade of pump fuel currently available and has lived fine since 2009 on a steady diet of 87 octane. For race days I'll mix in about half a tank of 92-93 octane but it probably doesn't need it. It just gives me a "warm and fuzzy" as most outings we find ourselves running in the heat of summer and "hot lapping" back to staging in final rounds with things pretty well heat-soaked.

    The engine makes enough power to easily push my car to 7.18 at 97 mph (last outing at our local 1/8th mile track at 3500DA) in full street trim. I use a factory intake, factory Q-jet, stock HEI (recurved only), tight converter and 3.42 gears. Running full quiet exhaust and 8" wide DOT tires 26" tall with "stock" suspension. I do have welded in sub frame connectors and adjustable pinion snubber plus axle tubes welded to the carrier. No bars of any kind used.

    We set up a similar car for a customer recently that went 11.80's at 124mph on the first outing in the heat of August. The only difference is a tad more compression and slightly larger solid roller cam, higher flash stall converter and 4.10 gears with 30" tall tires.

    For the most part I like to focus on "stock appearing" set-ups, and always try to use as many stock parts as possible but will typically work with each one to get the very most out of them. Nothing more fun in this hobby to drive to a local track and click off some strong runs with a car that looks like it would have trouble getting out of the 14's. The best part of all is when folks wonder over after you've clicked off a few runs and see the stock carb/intake/distributor/valve covers in place.....the looks you get at that point can be "priceless"!........Cliff
    Lucy Fair, johnriv67 and pbr400 like this.
  11. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    DCR- Dynamic Compression Ratio

    LSA- Lobe Separation Angle

    ICL- Intake Center Line.
  12. Donuts & Peelouts

    Donuts & Peelouts Life's 2 Short. Live like it.

    Thanks Larry
  13. BYoung

    BYoung Stage me

    Thanks to Ken and Mark the carb is finally performing as it should. Both gurus provided input and their expertise to narrow it down and it turns out the carb had a lean condition. Richening it up did the trick and the car is fun to drive again. Thanks V8Buick for all the help.
    Last edited: Dec 24, 2018
    techg8, johnriv67 and Brett Slater like this.
  14. BQUICK

    BQUICK Well-Known Member

    "For the big 455's they are pretty much big "tractor" engines and going to try to throw a LOT of power at you early."

    Cliff, you must be referring to Pontiac or Olds tractor motors in the 455 Buick camp.....big valve, short stroke that like to rev (relatively speaking) ;)

    Also, I've never seen a 11.3 comp Buick 455 that will run and race on 87 octane.....
  15. Cliff R

    Cliff R Well-Known Member

    I'm intimately familiar with Buick 455's, have tuned, driven, worked on and raced a good many of them over the years. With the big 455's they make great power just like the Pontiac and Old's when done right, and GOBS off upper mid-range and top end power. They don't need quite as much camshaft much like the Big Block Chevy's we've done over the years. This comes from better flowing heads and improved bore to stroke ratios.

    The big "undersquare" Olds/Pontiac 455's will still make great power, but you really need to cam them to push power up in the RPM range otherwise they'll dump it all on you right off idle. The smallest cam I'd ever put in a 455 Olds or Pontiac build with a little compression in it would 286 @ .006" and 230 @ .050", and nothing tighter than 112 LSA, at least for well thought out street/strip engine build on pump gas.

    As far as running higher compression on pump gas, it's a "recipe" just like anything else. There are many "players", including quench distance, combustion chamber shape, intake closing point/camshaft specs (LSA, seat timing), cylinder head flow ratios, close control of timing/fuel curves, controlling engine temps, how hard it is loaded (torque converter efficiency, vehicle weight, gear ratios, etc) and many other things that have some effect on the final outcome.

    Of all those items mentioned the camshaft is one of the biggest players, as it will have a MAJOR impact on the cylinder filling abilities and when/where in the RPM range it will be the most efficient. Too small, LSA too tight, intake closes too early and you can pound the rod bearings out of one of these engines at or below the Internet's proverbial "brick wall" of 9.5 to 1 compression!.....FWIW.

    There's hardly a single day goes by I'm not contacted by someone looking for some carb help with their "fresh" engine build. Just had an email over the weekend from a potential customer who put a 262 duration camshaft it a 455 Olds' build with 10 to 1 compression.....and wonders why it pings at light throttle unless the vacuum advance is not used, and everywhere else if the total timing is greater than 29 degrees. Unfortunately a custom carb build isn't going to be the cure for those issues.........Cliff
  16. HotRodRivi

    HotRodRivi Tomahawks sighted overseas

    I know that im late to the party here but that base gasget looks thin and possibly incorrect.

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