Carburetor Idle is too high



Carburetor Idle is too high

Postby grumpyvette » April 13th, 2014, 5:27 pm

one of the more common problems I see is guys who have issues with performance engine builds , guys who install performance cams with significantly more duration than the original stock cam and guys that probably added a few other performance mods to the build who still think that the car should sound like it now "has a set" , SO IT IDLES WITH THAT CHARACTERISTIC RUMBLING/LOPING IDLE THAT MOST GUYS SEEM TO WANT, but they don,t seem to grasp that the reason the engine sounds like that is the increased duration and usually tighter LSA in the cam timing is causing reversion pulses in the intake and exhaust that the stock parts never allowed.
you should start by disconnecting the throttle petal linkage, and any transmission linkage, from the carburetor, as having those linkage,s bind, at times and are a frequent issue causing the engine to idle higher than desired, also check clearances so you have enough room to go from idle to fully open without binding
viewtopic.php?f=55&t=861&p=33717&hilit=carb+linkage#p33717

viewtopic.php?f=55&t=109&p=6685&hilit=carb+linkage#p6685

viewtopic.php?f=44&t=621&p=920&hilit=carb+linkage#p920

viewtopic.php?f=39&t=617&p=2569&hilit=+linkage#p2569

viewtopic.php?f=55&t=444&p=547&hilit=+linkage#p547
The very fact that youve added additional duration in the cam timing tends to make the off idle, and low rpm response less crisp and it usually requires you to let the engine idle slightly higher in the rpm range to allow it to compensate, thus the engine that idles fine at 750rpm-800rpm with a stock cam , once modified will frequently require you to set the idle at 950rpm-1050 rpm to function correctly with the new performance ,longer duration cam, and any attempt to force it to idle at the original 750 rpm are doomed to failure with the new components installed.
your engine with the newer performance components is by design going to idle a bit higher in the rpm band but its your responsibility to determine if theres a part leaking , ms-adjusted or defective,and yes youll be forced to learn how to tune under a new set of condition's the stock engine never presented you with.
then of course theres the very common vacuum leaks, loose vacuum hoses, getting the ignition timing curve set up incorrectly or having the various idle adjustment screws, or jetting set up wrong, having the wrong accelerator pumps,accelerator pump cams, squirters, linkage or linkage adjustments, but in most cases these are minor or major contributing factors while the major cause is a vacuum leak, now this can be a miss adjusted throttle blade, very easily, or an air bleed jet or other factor but the fact remains that carburetors control engine rpms mostly by limiting or introducing additional air flow rates, and its usually UN-controlled air entering the engine that allows the engine to idle higher in the rpm range than you want it too operate., sources for UN-controlled or UN-metered air flow are commonly warped carburetor base plates, improperly installed or incorrect carburetor base gaskets or overly tight carburetor mount studs.If you are sure you have no vaccum leaks and have checked everything else. You may have a throttle blade that is not centered in the bore.or you could easily have throttle linkage binding, Id suggest temporarily disconnecting the throttle linkage and try turning in both air bleed screws to just the point they bottom out this should effectively kill the engine and indicate no major vacuum leaks, then turn both out one and 1/2 full turns as a start point before proceeding, as the engine should start there.
You would have to loosen the throttle blade screws a little and close the throttles all the way backing out the idle screw. Then re tighten the throttle blade screws and re set idle.
you might be amazed at the number of times that simply disassembling a carb, inspecting its components, cleaning and reinstalling them, and then adjusting them has cured MYSTERIOUS problems, this of course requires a good understanding of what each component and sub-system does and how its properly adjusted.
This is one reason many experienced performance guys prefer the HOLLEY 4 barrel carbs as components are easily available , and the basic concepts of operation easily understood with minimal training


related info, with a great deal more in sub links
IF you don,t like reading links and learning your in the WRONG FORUM


viewtopic.php?f=55&t=1037&p=26532&hilit=propane+leaks#p26532

viewtopic.php?f=55&t=1115

http://www.chevyhiperformance.com/howto/45798/

http://www.junkyardgenius.com/holley/tune01.html

http://www.holley.com/data/Products/Tec ... -3rev2.pdf

http://www.holley.com/TechService/FAQ.asp

http://gasalley.thetumbleweeds.net/tech ... onIdle.htm

http://www.barrygrant.com/Assets/docs/P ... 0Demon.pdf



viewtopic.php?f=55&t=7763&p=37227&hilit=warped+base#p37227

viewtopic.php?f=32&t=6011&p=18597&hilit=meter+rods#p18597

viewtopic.php?f=55&t=1144&p=13054&hilit=meter+rods#p13054

viewtopic.php?f=44&t=5575&p=16927&hilit=propane+leaks#p16927

viewtopic.php?f=55&t=1820&p=4706&hilit=meter+rods#p4706

viewtopic.php?f=70&t=4683

viewtopic.php?f=55&t=109

viewtopic.php?f=55&t=1790

viewtopic.php?f=55&t=1639

viewtopic.php?f=70&t=3949

viewtopic.php?f=55&t=8126

viewtopic.php?f=70&t=1015

viewtopic.php?f=70&t=2798

viewtopic.php?f=55&t=8126&p=28205#p28205

viewtopic.php?f=70&t=202

viewtopic.php?f=44&t=773&p=1309&hilit=meter+rods#p1309
IF YOU CAN,T SMOKE THE TIRES AT WILL,FROM A 60 MPH ROLLING START YOUR ENGINE NEEDS MORE WORK!!"!
IF YOU CAN , YOU NEED BETTER TIRES AND YOUR SUSPENSION NEEDS MORE WORK!!
grumpyvette

User avatar
Site Admin
Site Admin
 
Posts: 14105
Joined: September 14th, 2008, 1:40 pm
Location: florida

Return to Intake Systems , fuel systems and related

Who is online

Users browsing this forum: No registered users and 5 guests

cron