Since I built this motor, I have been chasing, a fuel, ignition, timing, or vac. problem. When I floor the gas, right about 4,000rpm, the engine starts popping, HARD. If I step into it slow, it won't pop as much, but it almost always starts above 4,000 rpm. One person had suggest that I have a manifold leak. Another suggested that the engine is going lean when the secondaries are opening. I also suspected the ignition. It's a used points dizzy with pertronix I in it. I have great oil pressure (60 cold/45 hot) and the engine runs around 170-180 degrees degrees in this weather (20-40 degrees). 306ci KB flat top pistons eagle i-beam rods stock crank tfs stage 1 cam (.499/.510 w/1.6's) 1.7 roller rockers (.538/.542) hooker 6901 headers e7te heads, shaved ~10:1 comp weiand 4011 intake holley 600 carb vac 2ndary. #78 jets (primary) mechanical fuel pump ('66 GT-500 289 oversize with filter base) pertronix in-dizzy ignition running ahead of a c4 and 3.00 open rear w/ a 3k stall from edge
i had a similar problem once with a 750 vacume carb,i tried a 750 dp carb and it cured it.Then i put the vacume carb on my pick up and it worked fine.,so there was nothing wrong with the carb just the engine didn't like the vacume secondarys.Just something to try.
I have experienced those problems also Bruce. If you have not replaced the dist cap and rotor, do so. Resistance is a bitch. Plug wires good? I had a 600 holley do the same thing also. I believe you are running to small a carb, and I dont care what anyone else says, when I put a 700 double pumper on mine it really woke it up and runs better than ever. I ran 2 different new 600's on mine for twenty some odd years. Thought I was satisfied with them, until I put the 700 on and realized the motor just needed more cfm. Our motors are nearly identical. You might consider swapping to a larger carb. Dan
Sure sounds lean to me! Have you tried a spring that opens the secondary later? You don't have your secondary linkage rigged do you? (I am sure you know better, but a lot of folks try it anyway) These things could account for Derek's problem too. If a DP clears it up, then it is running lean for that engine's needs when the secondarys kick in. It may have run fine on another engine because that particular engine like the spring on the secondary or had more signal to flow the secondary circuit sooner. Dave
Have to agree,,, lean or low voltage,,spark. make sure you got 12+volts to coil when running. I'm leaning more towards lean fuel.
Few things to look at....I didn't go look at my Holley book, but isn't that a little big on the jets for a 600? Also, you can try to wire the secondaries shut, so they won't open, and just work the primaries. How about wires/cap rotor, big cap or little, and what shape? Plug condition and gap? And the dist is used, any end play in the top bushing, and how does the gear look? Where is the timing?
The carb does seem a bit mellow for his combo, but that won't make it lean. The 78s up front are a bit big, but they just might be saving him from total melt-down when the rear goes lean... The rear doesn't have jets, it has a metering plate. They make those in different sizes just like the jets. Have you tried a 'richer' metering plate? Dave Personally I have had very good luck with 650-750 DPs on similar SBFs.
I ran 78's up front on my 600 too. That carb is now on my cyclone spoiler, 429cj,, that motor likes that carb,, go figure. Dan
You need to re-jet the carb first. I just looked, it should have 66's in it. 78's are not just a little big, but way too big. You probably fouled the plugs, and that's why it misses. Unleaded fuel does not mark plugs very well, so you can't just think they are ok. All of my Holleys are within a couple of sizes from stock. Usually a size smaller, makes for more consistent ET's that way, rich will pick up when the evening cools down. I would still keep the secondaries from opening at all, tune the primaries until they are right, then open the rear up. Check (or replace) the power valve too.
accelerator pump working? I agree its probably a carb issue. I'm no carb expert but I've had my share of little problems. Most recently I relised I had the incorrect base gasket on my carb, that sure fixed my idling problems (and some other little stuff that was going on) when I corrected that mistake!
Bad pump will cause a stumble (or worse) off the line. Once you reach 4k rpms and have an issue, you are in secondary territory. Accel pump only shows when you stab the gas low rpm or come off idle. When he stabs the gas at high rpm and it stumbles, he is either does not have enough metering signal to start the secondary circuit, or the secondary jets are too lean. (no jets in this case, metering plate performs that function, but they are available in different sizes like jets) One could get a secondary block and use jets out back too. Easy mod. Dave
sounds to me like you are pinging. if thats the "popping" sound. richen it up a bit or mess with the timing
What about checking the spark plugs, maybe a different heat range, checking the gap? You should be able to get an idea of the leanness etc. but looking at the carbon, pits, and such with a magnifying glass.
Lean pop and ping are 2 different things and are distinct sounding. Lean pop is like a muffled backfire, the best I can describe it. The best discription of ping I have heard is 'marbles shaking in a tin can'.
