DIES at idle. 455 Stage 1, C118 cam, 10 to 1, Removed 72 Buick 455 Q-jet for this 1979 Buick 350 carb project which hauls ass but WON'T IDLE. Both carbs got same recipe shown below. The 72 idled great. 1979 Buick 800 building it same as my 72 carb, using RUGGLES 2ND RECIPE, mostly. Just will not idle so far. Idle bypass- drilled .093 Did not help! Plugged back shut for now. Idle screw holes .093 Idle tubes .039 Down channels .055 Idle bleeds- upper .070 Idle bleeds- lower .070 Main air bleeds (2) .070 Main jets .077 (from 72 carb) Rods .048K (tapered) replaced old style .048B rods Power spring #7 Edelbrock Atp setting-- about 4 turns up got idle down to 1000 rpm. Float level .250 Fuel Pressure 4 ? Stock pump Ign Timing 14 to 34 +Ported vacuum with 10 max Plugged every vacuum tube, gaskets are all good, new diaphram pull-off, no tunning changes, ported vac advance (no need to use full advance to patch a problem). all throttle valves close good, new shaft bushings, and the off-idle ports set to a pinpoint exposed below primary valves. I found two holes inward from idle slots that get vacuum, now i'm bewildered.(Have now plugged these EGR passages.) Laying one finger across the rear of choke horn allows good idle, in & out of gear. THIS STORY WILL NOT GO AWAY UNTIL IT IS RESOLVED. STAY TUNED.
CHANGES TODAY; 1. Repened .096 idle air bypasses. 2. Enlarged idle screw hole to .096. 3. Enlarged idle down channel from .055 to .058. 3. Pluged EGR holes in baseplate to gaurantee no interferance with idle. 4. Lowered new float from 1/4 to 5/16 to stop nozzle drip. (didn't work) Returned to 1/4 and will leave it. 5. Torqued carb to 80 inch lbs to insure against leaks & warping. After warmup, some improvement but had to turn idle speed screw in one turn to maintain idle, and still noticed nozzle drip. After setting ATP 4-5 turns up and idle screws to best vacuum, idle was still weak and died in DRIVE. Reset to 2 turns up. During idle, I covered the idle air slots in the air horn with my fingers and repeatedly closed and opened the holes. Very strong idle IN DRIVE when held totally closed!! (This is crazy. My 72 carb had .125 holes and perfect idle.) NEXT STEPS: plug idle bypass holes, reset idle speed screw to expose pinpoint slots below the throttle plates and recheck for nozzle drip and idle performance. I am reluctant to lower the float or enlarge the idle channel and screw holes again, until all else has been tried. This 79 carb runs like fuel injection, just has idle problems. Something is different from the old style carbs.
I Would Call Cliff Up And Discuss The Problem With Him. After Talking With Him On The Phone I Was Very Impressed With His Knowledge And Willingness To Discuss Q-jets. I Do Not Know A Better Source For Q-jet Help. Good Luck P.s. His Number Is On His Website.
Ruggles didn't really troubleshoot my carb, just gave advice about jetting and said he could find the problem. I'm trying to solve the problem without sending it away and spending 100's on it. The slots on the underside of the airhorn allow air into the side passages to the air bypass holes under the carb. The suction in the airhorn slots was very strong making me suspect an internal vacuum leak. This carb has been unwarped by my "Bakers Garage" unwarping tools and a 250 degree oven (done several of these).This carb is vary flat and seems unlikely to have a leak but I will be looking into that. I AM REALLY HOPING TO ATTRACT SOME OF OUR EXPERIENCED GUYS TO OFFER HELP ON THIS. I'm still getting nozzle drip idle or no idle.
What I have learned about unwarping qjets: 1. The Bakers Garage tool works, but do NOT torque to 25 FT.LBS per instructions.!!! You will break the small bolt and damage the air cleaner bolt boss in the air horn. You may also crack the secondary divider wall in the float bowl as I did on my 72 carb. 2. Torque air horn to 150 INCH lbs, bowl to 200 INCH lbs. and heat one hour. 3. Retorque, heat another hour. 4. Retorque again as it cools. If you don't retorque, it won't be flat enough and may leak air, fuel, or both, and cause tuning problems. I will continue trying to solve my idle problem after reheating my 79 carb.
I had a carb with upper idle air bleeds (top cover) and main body air bleeds (holes just below the top into the airstream). It should have one or the other I believe. It was probably a rebuild that was assembled with the wrong parts. I fought with nozzle drip for a long time before discovering it. I switched top plates and solved the problem. Now I just need to drill and tap the base plate to take a metering jet so I can fine tune the bypass air that I drilled out to big!
FIXED: I love it!!! IDLES GREAT NO MORE NOZZLE DRIZZLE Way better than my 72 carb that always had some stumble and drizzle. The fix went like this: I removed my open plywood spacer and added a 4 hole aluminum spacer. I used an upper and lower 4 hole gasket. I cut the rear barrels of each peice into a large oval to open up the rear barrels, but left the front as is. The spacer and gaskets supported the area between the front two barrels from the manifold, up to the carb. I had decided that the open spacer was allowing the throttle plate to sag, especially across the center, letting air pull through the gasket and dissable the idle circuit. Sandwiching the front half of the q-jet by squeezing things together, solved the problems. This recipe allowed me to back off the idle speed screw until the idle slots were tiny as a pinpoint. Now there is no drizzel, stumble or sag. It hits like fuel injection at any speed.
Thanks for letting us know what you found to be the problem . Very interesting . I run mine flat on the intake with a thick gasket and after many mods it works great , very responsive .
One more thing...... Air flap is set at 80-81 degrees opening max. Secondary throttle is 91-92 degrees. Air flap spring is at 3/4 and will be experimenting with less. Air valve is set at .015" rod free play with vacuum diaphram sucked down. (MUST BE TIGHT OR YOU MAY GET BOG OR HESITATION AT WOT.) Diaphram is a little slow at about 2 or 2 1/2 seconds to fully release. I can't make this carb bog at any speed. It hammers when my foot hits the pedal. I may carefully experiment with some quicker opening of the flap though.
Thanks for this information, I'm going to try one of the later Q jets at some point. it looks like the APT feature could be an asset in tuning!
Gary, Vacumm leaks will definetly affect all aspects of carb operation, but are most pronounced at idle. The plywood spacer may of also allowed some air to leak thru unless it was sealed. Glad you got it sorted out.
UPDATE .... These APT late model carbs use tapered "K" rods with .026 tips, OR tapered "M" rods with .036 tips. (The big end of the taperd diameters range from .045 to .054). REMEMBER THIS when working on these APT carbs. These rods tip diameters .026 for the K, & .036 for the M, are equal to a difference about 4 jet sizes!!!! THATS F O U R,... 4 . So what I did...I pulled the .0.048 "K" rods (with.026 tips) and install an .052"M" rods (.036 tip) into my same .077 jets and my GS loves it. APT 800 cfm carbs respond well to leaner jetting than the 1971-1974 Q-jets. NOTE: As to flow area, for what that is worth, this rod change is equivalent to having pulled my .077 jets and installing 73 jets with the .048 K rods (with .026 tips). Yes, I dropped 4 jet sizes equivelent by installing an M rod. I could have used .073 jets and .045k rods for the same flow area. Swaping K and M rods may add or subtract the equivelent of FOUR JET SIZES from where you THINK you are so be sure you KNOW what your doing here. Best use O2 meter to be sure all your tuning is matched with proper jetting. Hi Sean, Gary B
Just wanted to point out that the early units have APT as well, located in the baseplate front under the fuel inlet. They can be a REAL pain to get the screw to loosen up after being stationary for 40 years. PB blaster and heat works ok. If anyone has other tips to free these up Id love to hear them. Once they are loose, the adjustability produces outstanding tuning results in my experience.
The old carbs can be adjusted with a little work down in the basement. Just be sure to use "B" metering rods, not "k" or "m" rods. The really good rods are the RARE tapered Oldsmobile rods. Makes adjustment easy though. Cliff Ruggles may have, or make, some tapered rods. The later K and M rods are shorter and cannot be used in older carbs. ALL LATER CARBS HAVE A DOME ON THE PASSENGER SIDE FRONT CORNER. That is the aneroid dome. They are adjusted through a hole in front of the choke tower. These are all great 800cfm carbs to modify. A Buick 350 or 455 carb is ALL THE SAME after the recipe modifications are done. A 350 carb will go right on a 700 horse BBB, no problem.
Gary, I think that your issue was one of the open plenum spacer. You may recall that any open spacer dilutes the signal to the primary throttles, making them slow to respond, or weak, due to lack of signal. The four hole spacer amplified the signal to the primaries, allowing the carb to respond to the signal. Though you did not mention whether any cam or compression changes were made, I suspect that this was the issue.
I used the plywood open spacer for a few years along with my 1972 Q-jet. I began using it because my engine was hard to start when hot, and it kept the carb cooler than the old aluminum 4 hole spacer. When I installed the 1979 Q-jet, I could hear air rushing into the bypass entry slots at the upper end of the primaries and closing them with my fingers stabilized the idle considerably. That is when I came up with the idea that the throttle plate may need to be squeezed to seal tighter. The 4 hole spacer and gaskets worked. Now having thought on this subject many times, I think that the plywood open spacer CAUSED THE Q-JET TO TWIST (and suck air) because my caliper showed one corner mashed several thousanths below the other 3 corners. I will use phenaulic only from now on. OH, the 4 hole runs better too. Ken, I don't know the EZ method of adjusting the old style APT. I want to thank everyone for the input. This is such a great site!! I have learned so much and it's entertaining
Gary, This method has worked very well for me in the past. Try setting the fast idle to about 2200-2400 rpm, in neutral or park, and adjust the apt for a slight rise in rpm when the incoming air is slightly restricted. You can do this by slightly closing the choke, or blocking the air with your hand. If the rpm rises about 50-100 rpm, try driving the vehicle with that adjustment. If the vehicle surges slightly, it may be acceptable; if not try going slightly richer to eliminate the surge. A very slight surge will maximize the economy, however, this is your call. Let us know...
