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AFR and VACUUM values
After several works and test drives I try to solve some carburator problems regarding the engine.
SPF 306 ALU Heads Ford J302 Intakes to heads portet Comp Cam 288R @050 246/246. Dur. 288/288 Holly 600 vacuum CR 10.5:1 Idling at 900 +/-: Vacuum idling 7.0 - 7.5 hg AFR idling 13+ / 14+. Between these values the AFR leaps up to 20 and then back to 13/14. Sometimes there are longer, sometimes shorter intervals between the leaps. The engine sound on idle is like a hard working truck. Once it reaches the 1500/2000 RPMs, AFR is at 12+/13+ and vacuum +/-10, engine sounds very smooth. Ignition timing is 12° at idling, 32° at 2700 RPM and 34° at 3000 RPM. Is there probably a spark missing, which causes the leaps regarding the AFR figures? Or what ...........? |
I`m not very familiar with the small block engines , but here are some generalities to check . Always start by making sure the ignition is working correctly and timing is right before working on the carb .
A large portion of carb " problems " are actually ignition problems . To me , your vacuum at idle seems low , as does your timing ( I have a big block 482 engine ) and idle speed . I run 18 to 20 degrees at idle ( 1100 rpm ) and a max of 37 degrees on my engine . Check your plug gaps also ... and install the spark plugs with a torque wrench . I found that getting good A/F readings at idle can be difficult because of reversion due to cam overlap , so I don`t pay much attention to any readings until I get to the 2000 rpm range . Also , check for vacuum leaks around the base of the carb and intake manifold . Good luck , Bob |
Not knowing what you have for a distributor that timing does seem awful low. Does your distributor have vacuum advance, mechanical advance or both? Small block likes about 30 degrees max depending on the fuel your running. A lower octane fuel will not let you get away with more than about 27 or 28 degrees max.
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Ignition parts are all from MSD. Distributer with mech. advance.
Will check for infos regarding the ignition and advance timing. |
I built a dry sumped 358ci chevy engine one time with 12.5-1 compression and about that sized cam. Idled great at 1600rpm and and ran like a cat on fire. I.E., your cam may be a little on the big side. Otherwise, about what I would expect with a 306, good job getting it to idle at 900rpm.
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Once it reaches the 1500/2000 RPMs, AFR is at 12+/13+ and vacuum +/-10, engine sounds very smooth.
Sounds like your power valve (if you have one) is coming in at 1500 which is dropping the AFR and you stated motor sounds good. If you have 7.0 vacuum then the power valve needs to be a 2.5 or 3.0 max. If it doesn't sound good at idle maybe it is too lean and the spiking might be an intake, carb vacuum leak or ignition related as others have mentioned. I'm running a 496FE with big solid roller and my vacuum is only 7.5-8.0. |
That is a pretty radical cam for a 302. Not a street cam that's for sure. You need to resolve your timing issues first. If you have vacuum advance hook the distributor to manifold vacuum, then drop you idle back down. That will improve the idle. If no vacuum advance them you need more initial timing like was said. Something in the range of 16-18 degrees. Depending on your compression your total timing should be around 32-35 degrees, not including vacuum advance. Generally the power valve rating should be 1/2 of manifold vacuum at idle or steady cruise. So for 7 in vacuum the power valve would be 3.5. Stock Holley power valves are usually around 6.5. So you would think when you open the throttle and the vacuum drops 1/2" it would go way rich. If your power valve is too low you can have a part throttle flat spot. With Holley's sometimes the power valve restrictor jets are not right and can make it too lean or rich, but 20 is way off. So the power valve could be right but the restrictor jets are not. Summit Racing has a YouTube video on selecting the right power valve.
I say get the timing right first and then check out the power valve. |
New Adjustments
2 Attachment(s)
- cam was installed at engine (comp cam *cam quest online* recommended)
- engine no vacuum ignition advance - PV 4.5 at the Moment New experiences Ignition 18° total 34° / springs 1x silver H, 1x silver L Idling still too lean. Afterwards I restricted IAB with a wire of 0.041". At idle 850/900 the AFR drops to 11.5. Once the revs moves to 2000, AFR up to 12+, accelerating AFR 12.5. Engine sounds better than before. On the test drive the feeling has been better than before. Sec. vacuum valve working with purple colour spring, opens at approx. 4000 RPMs. I replaced the wires on both IAB with the wire tubes. Hole Diameter 0.067" (Foto).Idling at 850-900 AFR is 13-14. Vacuum 6-6.5, idle mix screw 1 3/4 turns open. How can I adjust the idle mixing without the plastic tubes on the IAB? Opening the IFR? SBF 302 - YouTube |
If your AFR is rising to 20, that is a lean condition, not rich. It would not be caused by a missed spark or a power valve opening early. Those would cause rich conditions. Brent tells me sometimes you get a quick transient lean spike, and that may be ok. But if it is more than that, you could cause some damage running for extended periods at a lean AFR.
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Back from test drive:)
Wat a difference!!!! Runs from idling to 1800 / 2000 rpm without stumbling. Smouth and with good acceleration. Roll start in 2. gear, reving up the engine and change to 3. gear, the rear wheels are spining:3DSMILE: A totaly new feeling:D. Cruising at AFR 13. I have to Change the jets for cruising and maybe WOT. The spikes up to AFR 20 are now on intervals betwen 1 min to 2.5 min. |
A misfire DOES show up as lean, high oxygen from no combustion.
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