I am having an issue with "rough running" at part throttle. When cruising around 2500 rpm it sounds like it has a bit of a miss. Also when letting off the throttle then easing back int it it stumbles a bit..WOT seems ok and vacuum is good on idle at around 14. The problem started last fall. I decided to call Cliff Ruggles. Carb Q-jet 800 7043240 LA Old setup: 73 main jets drilled out to 76 (drilled out to 76 by Jim W when he assembled my motor on my budget build) 44B metering rods Stock power piston CT secondary rods K hanger Cliff's accelerator pump The old setup worked good, but ran quite rich. New setup (recommended and bought through Cliff): 74 main jets 43B metering rods Orange power piston DA secondary rods K hanger Added APT screw Motor: Buick 455 1970 small valve heads mild port job TA 413 cam Edelbrock dual plane alum intake 1" phenolic spacer Headers and 3" exhaust with H pipe Timing: HEI with some mods 21* At 900 rpm 34* all in at 2500 I am still having the same issues after changing the carb parts, new plugs, cap, rotor and verifying timing and accelerator pump function. I am running a choke pull off from TA. Please help. I have a weekend long car cruise in a couple weeks and want to get this figured out so the old Buick can represent well. Thank you, Matt
I would start with distrib, what gear is it, bronze ?does the advance stick? Are the bushings in housing worn? Also the coil under the star wheele that u have to remove the shaft to change the 2 wires tend to break inside the insulation so u won't see the broke wire. Is the problem only when driving ? Need more imput
Cliff, there is 4 degrees added with the vacuum advance. I believe the distributor is 1975 and seems to be in good order. I replaced the weights and springs about 2000 miles ago and just verified that they are not sticking. The timing moves with the RPMs according to the timing light. Thanks, Matt Sent from my iPhone using Tapatalk
It would be way more than that unless you limited it. Is the engine surging at cruise? Most stock canisters add between 14 and 20* at 16" of vacuum. How about the APT screw, did you set it up with the tip in method described in Cliff's book?
I limited the vac adv to 4. I don't remember exactly why, but it worked well....I just advanced a bit more it did not make a difference. I messed with the APT screw. According to Cliff's book, you cover the choke a bit and the RPM should go up about 100 RPM. My car does this whether it's all the way in or 2 turns out. Am I correct to assume that all the way in lifts the rods allowing the most fuel, or do I have it backwards? Thanks for the help! Sent from my iPad using Tapatalk
8-10* of vacuum advance usually works the best. 40-45* of cruise timing gives you the best MPG. If you have too much the engine will surge at times. I believe the APT sets the lowest point for the primary rods. The lower the rod, the leaner the part throttle light load mixture will be. Do you have a picture of where the APT adjustment is on that carburetor? I believe CW raises the primary rods, not sure.
It really seems like a carb issue the more I drive it. Could the float be set wrong? Choke pull off be out of adjustment or sticking? Power piston spring installed wrong? You can kind of see the screw in the 2nd pic on my original post. It's right under the fuel inlet- right below where the fuel filter is. Thanks, Matt Sent from my iPhone using Tapatalk
It makes 14 at idle so I haven't checked any farther. I thought this was pretty good for my combo so I left it alone. Sent from my iPhone using Tapatalk
I didn't try removing the spacer. From what Jim told me he has seen them help, but never hurt. I will probably try that tomorrow. Any other ideas?...... Sent from my iPhone using Tapatalk
Yeah I have a great absolute best super terrific idear, since u have Cliffs book , read that thing cover to cover 2 times ,or at least chapters 2 & 6. Grab a can of carb cleaner , with ur air cleaner off give a quick squirt holding can a few feet from carb aiming at primary side . You should immediately stumble . If it idels up ur lean or leaking vacume someware. You can check u have power piston with no what looks like a cut nail sticking out the bottom, make sure the clip that the float rides on slightly pokes up so when air horn is installed u have a tight fit, if ur pwr pis spng is too weak if won't lift it up or over come the part throttle vacume signal
Greetings, I have re-read the book and tried some more things: -New Accel pickup and 140005 coil -Disassemble and check float levels-set to 1/4" -Reassembled carb and adjusted APT screw using Tip-in method -Rechecked timing and adjusted vacuum advance to 10 degrees -Used the carb cleaner trick to check for vacuum leaks It runs great at WOT, I Still has the same issue at part throttle. One thing that I never thought about-Last year I put a TA street/strip 1534C fuel pump in with no return line. Could I have too much fuel pressure causing a rich condition at part throttle? Can anyone recommend a descent fuel pressure regulator with a gauge? From what I read, 6 psi seems optimal but the TA puts out 8. Thanks, Matt
Only the early Q-jets were vulnerable to higher fuel pressure. My 1972 800 CFM 7042240 easily handles 8-9 psi with occasional readings of 10 psi. Can you replicate the rough running in Neutral/Park at cruise RPM?
Larry, Yes, the problem is there whether I am driving or just cranking up the idle RPM in the garage Thanks, Matt
I'd start by adding about 10 degrees timing via the vacuum advance. This will increase engine efficiency at part throttle/light load. Otherwise you will need to run a much richer mixture at part throttle. I run into this sort of thing a lot. These engines REQUIRE vacuum advance at light load. For very well thought out engine combinations usually about 10-12 degrees is plenty. For lower compression engines, lots of quench area, too much cam, cams on tight LSA's we often have to add a bit more. In any and all cases, give the engine the timing and fuel it wants to be happy. I'd also make sure the power piston hanger arms are level and exactly even. 9 out of 10 carbs that come in here with troubles in the area you are tuning in have bent hanger arms from folks prying out the PP or when they locate the tongue of the gasket under the PP. The arms bend very easily, and on APT carbs with tapered rods they MUST be locating the metering rods correctly in the jets for best results. I'd also add here that the idles system adds fuel thru transition and to light part throttle. If it is not opened up enough for the application, it will also cause tuning issues in that area. I've cured a LOT of similar issues adding little idle fuel, usually with slightly larger idle tube/DCR's, or even closing down one or both pairs of idle airbleeds slightly........Cliff
Cliff, I have 34 degrees of mech advance and 10 degrees of vacuum advance for a total of 44 while cruising. Do you think I need more? The TA 413 cam has a 113 lobe center. The spec says it wants at least 9.5:1 compression. Mine is at 9.73:1. I will look in to your other suggestions. Thanks, Matt
What does the vacuum do at the RPM this problem manifests itself? Could the valves be too tight? Do you have any adjust ability in the valve train?
Did you degree the 413 cam when it was installed? How much quench does the engine have (piston to head distance at TDC). I worked on a 455 recently that used that cam and it was not degree'd in and found the engine to be a tad "lazy" at idle and low rpms, and it wanted a lot of timing and fuel to be happy......Cliff
