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Old 03-03-2006, 04:57 PM
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What's a good timing curve for a 4100lb car?

Hey all, apologies for the long post. I have a '73 Chvelle Malibu Sedan with a 355sbc, HEI ignition, edelbrock perf. intake, edelbrock Q-jet 675CFM carb with mech. secondaries, comp 270H Magnum cam, 1500stall and a 700R4 with 2.73's out back.

When we first put on this carb it had 69 size jets in it and it would surge on the highway when you were doing 60 indicating that our primaries were too lean. So we took it on a short trip and kept changing jet sizes until we got it so it didn't surge, and drove great, but now we had (and still have) and pop at certain times when you engage the secondaries and they don't pull as strong as I think they should pull.

Our first trip that we did was with the .69 jets in it because we were just trying to get the 500 miles on it for the 1st oil change, and my dad said that if you run an engine a little on the lean side when you're breaking it in, it helps the rings seat quicker (don't know how true that is). So anyway, we managed to get 15MPG highway when it had those jets in the primary side.

Then we started increasing the jet sizes on another trip to the same place, and we took the primary jets all the way up to 75's. We jetted down quick because cruising down the highway at 55MPH netted us 11MPG, we were literally dumping fuel out the exhaust. The reason for all this jet size change was to try and figure out what the stupid pop was sometimes when the car switched to the secondaries. We're now on the way back down and now I think we have 74's or 73's in there. We also messed with the secondary needles and the primary needles and now we have a really good primary needle in there, it's real fat up top for part throttle but gets real skinny near the bottom for when you really need the power, but it's still drinking a little too much gas and still has great throttle response but a stumble upon leaving a stop. So we still need to drop the jet size because it's still rich, that and I can smell gas when I drive it.

Now I've been searching through old threads for popping through the carb and most of the threads people suggested the same thing. The main thing I want to talk about here is timing. My dad bought a distributor recurve kit for our GM HEI. He has it set so it's 'all in' buy 2000RPM's. The timing is set at 38 degrees with 24 degrees of dist. advance. His way of thinking is, get your timing set to what you want it, then jet around that.

One thing I've heard people say is that a lighter weight car can have a quicker timing curve, but a heavier weight car takes a slower one. Also that popping through the carb indicates a lean spot in your timing or something like that. The car only pops through the carb when you're climbing a hill and it's under load, or the RPM's are low. HOWEVER! This can be cured by shifting down at the exact moment you punch the gas, it won't pop at all when you do that. But if you let it do the shifting, then in most cases it will. Except if you're on flat road.

There is also a stumble when the car is warmed up, if I'm sitting at a stop and push the gas, most of the time the car will kind of stumble a little bit then go. It does NOT do this when it's cold. Which also leads me to believe two things: The rpms are higher when it's cold because, well, it's cold. 2. When the cars cold it requires a richer mixture to run, so the primary jets being too rich, may make it run well cold, but when it's warm, it's too rich. The inside of my tailpipes are also black, but the car never smokes, except in the morning when it's warming up, but that's cold steam smoke cause it's cold outside.

Oh! another thing. One of my dad's old friends told him about putting a washer in the dist. to limit the amount of advance it could make, he did this and the car didn't like it, it took a lot more pedal to get it to move. The total timing was around the same, maybe more. I think it was around 40 degrees total, maybe a touch more. But we were running more inital and it didn't drive as good.

What I want to know out of all this is:

1. Is the timing curve that we have set right now too quick/slow? Would have it 'all in' by say 2500, or 3000 get rid of the popping?

2. My dad is trying to get MPG out of this car (hence the 700R4 and the Qjet carb), that's part of his reason for the timing curve that he picked out in the first place. Is he on the right track?

The car drives pretty darn great right now, the only thing are the smell of gas when I'm driving, the gas mileage (about 10.5 city), the popping through the carb, the stumble sitting at a stop, and when I have the RPM's high on the road and I engage the secondaries, it goes from pulling to pulling a little more, it's like opening up the secondaries doesn't really do a whole lot more aside from making more noise (and what a sweet noise it is ).

Are all these things I'm describing adding up to poor jetting/timing issues? If they are, could you please advise me on what I should do to correct them ?

My dad and I are going to be jetting down a size this weekend, so maybe we can make other adjustments too! Thanks for spending time reading over my problems, I just want to make this car run better.

Sorry for the long post!!

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Last edited by Malibu73; 03-03-2006 at 05:07 PM.
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Old 03-03-2006, 05:18 PM
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Just to clear a few things up, there are no vaccum leaks, the car idles/runs good aside from what I've already said. This motor has over 2000 miles on it, it's brand new! It's got Crane Roller Rockers, new stainless steel valves, comp springs, hydraulic lifters. The valves have been lashed.
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Old 03-03-2006, 06:57 PM
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diagnosis aside, I might shoot for "all in" by 2800 or 3000 and then use an adjustable vacuum canister to add an aggressive advance during part throttle cruise. That low stall speed converter (although OK for the combo) will maybe like the advance to start a little later. With that cam there is plenty of low end cylinder pressure so you don't need advance that early. The 2.73s are also a good reason to keep the advance curve later. You'll be spending a lot of time with your foot in it in the sub-3000 RPM range.

I don't think that the advance curve is too aggressive that it would be causing a lean enough condition to pop through the carb. You would have crippling detonation long before it got that lean. It may be a combination of lean mixture and too much advance, but advance alone isn't the problem there.

My suggestion is to readjust the secondary air doors. They have a spring and two set screws on the air horn. It sounds like they are slamming open causing enough of a lean bog that its popping back. My Qjet has no secondary lockout cam for cold operation and if I open it up too soon before its warm, look out hoodliner. There is a flathead set screw on the side of the airhorn near the pivot shaft of the secondary air doors. Then up underneath that set screw is an allen head screw. Loosen the flathead set screw and turn the allen screw in by half-turn increments and see if that helps. Over time the spring in there that resists the secondary doors gets weak and lets it slam open. Tightening that allen screw puts more pressure on the spring.

You can also double check the vacuum brake for the secondaries. If its not operating properly it won't allow smooth operation of the secondary doors. Just push it in by hand, stick your finger over the hole and see if it stays there. If it moves, replace it.
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Old 03-04-2006, 12:51 AM
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Well, my dad suprised me tonight by wanting to work on the car.

So, we made a jet and rod change from a 72 jet/46 rod, to a 68 jet with a 32 rod. We've also switched out the secondary needles for fatter ones.

This had great response in the car, it pulled harder in the secondaries but is still a little to pokey around town.

We're going to fix that with another primary needle/jet change to lean out the primary side somemore. Because it used to be very peppy in town, but still popped.

We also adjusted the secondary butterfly door spring that you mentioned, that helped as well.

After we're done jetting, we might mess with the timing but so far, everything's looking up.
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