350 timing how much

[kevindtimm]

I'll be the first to admit that I could be wrong, but my understanding of vacuum advance is that it is NOT additive of the final advance number. I realize the OP has measured 35/54 and 40/60 numbers, but that leads me to believe that he has a failed vacuum advance (and therefore FAR too much total timing)

[Mertz]

Thanks everyone. I will change the bushing and get an adjustible vacuum advance

[Billla]

Originally Posted by kevindtimm

I'll be the first to admit that I could be wrong, but my understanding of vacuum advance is that it is NOT additive of the final advance number. I realize the OP has measured 35/54 and 40/60 numbers, but that leads me to believe that he has a failed vacuum advance (and therefore FAR too much total timing)

Vacuum advance is additive - grab a distributor and push the weights out and see how much the points plate moves..then apply vacuum to the can and see it move even more.

I know it sounds weird , but it is correct - and around 50 degrees is about right.

[LONGHAIR]

Hey Billla, wouldn't the vacuum level being higher above the throttle blades indicate a restriction at that point?
All of those numbers seem a bit odd to me though. They have to be "drive way" testing....As in "no load", so 3000RPM would take very little throttle opening? In a driving/towing situation, there would be far more throttle opening to achieve 3000RPM. The centrifugal advance would be the same of course, but the vacuum advance should be less, since the vacuum in the entire intake would be less.

Vacuum advance is rather a mis-nomer. If it is connected to "full vacuum" and the idle vacuum is enough to pull it all in, you have all that you are going to get from it. You don't get more as you accelerate, you get less. The vacuum cannister "releases" it's advance. All the while the centrifugal advance is coming in to compensate. They don't really accumulate, unless you have the revs high enough and still have the vacuum high at the same time. Really the only time you would see that "on the street" is when "coasting" ....off the throttle at high RPM.

[Billla]

Well, there's a couple of things that come to mind - basically it's an indication that the carb may be too small...but I'm still thinkin'

Agreed that the numbers are "driveway" - but he's still seeing 20 degrees of advance and too much cetrifugal.


Well...kinda The advance will drop as the butterflies open further, but then the manifold vacuum will increase with load...which is at cruise.

[LONGHAIR]

Well, there's a couple of things that come to mind - basically it's an indication that the carb may be too small...but I'm still thinkin'

That's what I was thinking, either carb that is too small, a restrictive air filter or duct?

[Mertz]

The carb is an Edelbrock 1406 and has a K&N top filter and a standard 4" high round filter so there is no restriction and the carb can't be too small for the stock 350. My tests were done with the filters off.

I am going to try blue springs and a blue bushing. That should give me 21*cent coming all in at 3000. I will set the initial to 14 to get 35 total at 3000. BTW cruise at 70mph is at 3000rpm.

Good point about the amount of vacuum under load. I probably won't see the full 20 degrees advance like I did in the driveway. I should then be between 51TT and 55TT which I think is acceptible.

I hope that this setup will give me good torque off the line and hopefully help my mpg.

[Mertz]

I have the springs and bushing changed to the blue spring and blue bushing. I set the initial to 12 and am getting 34 cent and 54TT. All in at 3000. I don't feel to much difference in torque but it goes like a bat ot of **** from 45 to 70. Too early to tell if my gas mileage has changed. I used about 1/4 tank with all the tuning.

[Billla]

So if I read this right, you're seeing a performance difference from the change? The numbers look a LOT better! I suspect you'll see better milage as well. When's the next time you tow?

I agree the carb's not too small, but I have been thinkin' My thought is potentially the vacuum advance spring is too stiff, and this is causing the secondaries to open late and/or not open enough. If you drop me an email/PM, I'll send you a set of Edelbrock tuning springs and documentation - do some testing and find out what works, then send the ones you don't need back. It's a really easy swap to do on the vehicle. I've GENERALLY found that our trucks like one spring lighter than what comes standard in the 1406.

Timing (which you've addressed) and then carb. This is really great work you've done - and it's cool that folks can see what a little investment in tuning can do! I harp on it a lot - maybe too much - just because it really is the key to achieving the best power, driveability and economy the engine can deliver.

Never done one of these tunes remote before - kinda cool

[Mertz]

Thanks for the offer. I can get a set of springs here in town. I should be able to lighten them up since I have such high vacuum. The ones in it now look silver but they could have lost their color. What color would they have started with?

[Billla]

Cool - the tuning kit (P/N 1487) is a good buy.

The stock spring is orange - 5" Hg. The color holds up pretty well. If they're plain (no color), then they're WAY too stiff for sure - did you get the carb used?

IMHO, I'd go back to the stock spring and see - then go from there.

The Edelbrock 1406 Owner's Manual is a great reference.

[Mertz]

The carb came with the truck but am not sure if the guy knew enough to change it. It does not have any color. I will get the kit and give it a try. I did mention that I have the .073/.042 rods in it now didn't I?

[Billla]

Page 9 of the manual I linked to has the standard configuration; I really would take the thing back to the stock calibration at this point and work from there.

I converted to Edelbrock about 7-8 months ago and have now installed a bunch of them. I buy quite a few 1406's off Craigslist for $100 or thereabouts, and every single one of them is because they "didn't work right"...and every one has had a completely screwed up config.

[hotrod 80]

The Edelbrock/afb is very tunable . It also seems very misunderstood . I also buy the ones that don't run right . People force the rods down without having them in the jet and bend them up . A 350 should call for around 611 CFM .

[Mertz]

The springs were definitely silver (Plain) so I switched to the orange ones and it seems to have a little more torque. Sure wish I could check all this out on the dyno. I used another 1/8 of a tank of gas hotrodding around the country side. I guess that should be expected when the foot is to the floor. I was reading about 5 to 6 hg under acceleration so it appears the orange ones are the right way to go. Some have said I should be on the silver springs. Which are going to give me the best mpg?

I won't be towing the trailer until August but I can do some mpg checks in the mean time.

Thanks everyone for the help.

[dznucks]

i recently installed a crane adjustable can and think i may be having problems with excessive vacuum advance. does anyone have pic of how the little round restrictor plate, that came with the kit, works? i have an idea but i am not positive.

[Billla]

Do you have the instructions and are trying to follow them, or do you need the instructions? I can scan/send the instructions if needed, or talk you through the install with some pics.

Do you know your timing curve? What's giving you the impression that there are problems?

[dznucks]

I am pretty sure that i do not have the right curve.
I have found that one combo of the springs runs better than the rest and that is where i left it. When testing each combo of springs, following the instructions, i cannot hear any signs of detonation. therefore i cannot tell which curve is the right curve. I have been reading this post and the link from the GM eng, as well as other resources to try and understand exactly what is going on.

This weekend i am giong to get the data like Mertz provided earlier, so i can see exactly whats going on. I also just bought a timing light with the timing adjustment on the back so i can take these readings.

as for the round restrictor plate, the instruction sheet that came with the kit doesnt mention the restrictor nor does it show pics of how it is installed. during heavy accel, i can feel the motor run rough at first, then it runs like a raped ape until it hits a certain rpm and goes back to crap again.

i know timing is my issue and i am just now trying to actually learn it.
its a jasper 350 mild cam, edlebrock 1406, nothing speacial.

[Billla]

It's definitely something where "seat of the pants" tuning doesn't work well - your approach to getting the timing set is right on.

We can tackle the plate once we know what's going on

[dznucks]

preciate the help,

i am going to return to the first spring combo suggested on the Crane install instructions to get the timing data, blue/silver i think. i'll use that as baseline.
i will post the data this weekend.

thinking about the no pinging thing. I wonder if i am not loading the engine enough to reach detonation.

yes "seat of the pants" tuning can get very misleading.

[dznucks]

I returned to the blue/silver springs and set my total timing to 36 at 3000rpm, this gave me a idle timing of roughly 16 deg.

1000
Ported Vac - 0
Manifold Vac - 19
Timing no Vac - 30
Timing w/Vac - 31

1500
Ported Vac - 2
Manifold Vac - 21
Timing no Vac - 35
Timing w/Vac - 35

2000
Ported Vac - 15
Manifold Vac - 21
Timing no Vac - 36
Timing w/Vac - 62

2500
Ported Vac - 22
Manifold Vac - 21
Timing no Vac - 36
Timing w/Vac - 62

3000
Ported Vac - 23
Manifold Vac - 21
Timing no Vac - 36
Timing w/Vac - 62


it runs much better, but there it still feels as though its missing something at low rpm and on the top end.

looking at the numbers, the timing with no vacumm seems to high and the max timing of 62 sounds too high. Vacuum seems good.

[Billla]

The springs seem a LITTLE light to me - but the total mechanical is correct so as long as you're SURE you're getting no detonation with them, we'll stick - 36 is the right number.

The vacuum advance is providing too much advance for sure - definitely need to install the Crane limit fitting.

I wonder if your secondary spring is a little too stiff as well - that may be where your bottom end is missing.

[dznucks]

Do you have any pics of the limit fitting installed? How do i go about setting that up?

Which secondary springs are you talking about?

[Billla]

I don't, but I can see if I still have the instructions and do a mock-up for you. It's pretty straight-forward, and it's indexed by the number of degrees to be limited. You're getting 26 degrees of advance and should be getting around 15.

Secondary springs in the carb - or the air valve spring if it's a Q-Jet.

[dznucks]

i am not 100% on the detonation. i rarely have the truck running at WOT under a heavy load. When i test for the detonation i am usually on a slight grade and motor is at 2000-3000 rpm on city streets. Thats the easiest way for me to but the engine under heavy load. Is there a better way?

i agree that the timing is coming in too soon. i may switch to the heavier springs and try it out. I would appreciate any instruction you can give me on the limit fitting.

the carb (edelbrock) is the next thing i am tuning once i get the timing worked out.