[Custom10]

Originally Posted by hcompton

There is lockout for Mechanical advance and vacum advance. Both need to be set very restrictive and go for more initial.

You can close off the vacum line and setup with out vacum advance and see if it will help smooth things out better. Just to get the hole vac can system out of the equation and see if it needs to be tuned. It may turn out to not effect the lope at all.

yes agree, typically the mechanical adv and its curve are figured out first then the vacuum advance is worked on after that.

[bentwings]

I just got an Accel adjustable vac adv unit for GM points style dist.
I have not installed it yet but I have some numbers.

Acv atarts at about 4-5 in hg and is all in at 9-10 in hg. regardless of the adjusted travel. The adjuster limits travel or deg advance. Also becaust it compresses or relaxes the spring it changes the start and finish point.........however this is very small. Looks like about 1-2 in on either end of travel. I doesn't match the supplied instruction graph except for shap of the curve. In other words it is a bit faster adv at less vac.

For those that need vac at low speed with low manifold vac it seems to be the ticket.

Hopefully i can get this installed this week end and test it. I plan on starting at the halffway point and go from there.

$26 from Orelieys

[Custom10]

That is interesting, I always wondered if those were able to change the range and the travel with one screw, seems like it is mostly just the travel. That is a pretty good range though 4"hg to 9"hg

[hcompton]

grabbed a few links that may be helpful.

HEI Tuning Secrets

Ignition 2 HEI

Not my favorite rite up but these should help alot.

Hope this helps.

[PatM]

You might also be able to help yourself with a different vacuum advance mechanism. Lars over at Corvette-Restorations.com has put together a very large list of GM canisters and their intended performance parameters. I don't know how to copy and paste a link yet on my iPad, but if you'll try a Google search for "GM vacuum advance specifications" it'll lead you there.

PatM

[Zerocyde]

Originally Posted by PatM

You might also be able to help yourself with a different vacuum advance mechanism. Lars over at Corvette-Restorations.com has put together a very large list of GM canisters and their intended performance parameters. I don't know how to copy and paste a link yet on my iPad, but if you'll try a Google search for "GM vacuum advance specifications" it'll lead you there.

PatM

I found this on the list:

VC1868 AR35 1979 305 Chev Truck & El Camino 2-4 10 @ 6-9

Starts at 2-4hg.
All in at 6-9hg. with 10* advance.

Sounds good, and I found this...

But I'm not sure if it's the right canister for my dizzy. Should I go for that or just get an adjustable one from the parts store?

[oldbogie]

Originally Posted by Zerocyde

Long story short I had a long block 350 (sbc) built for me, rather stock rotating assembly and a "slightly better than average" set of aluminium heads and a thumpr cam (smallest). I put an edelbrock rpm air gap intake on it and a 650 proform holley double pumper.

I got it decently tuned for a beginner (buying a wideband o2 sensor soon to really dial in the jetting and accelerator pump cams) but I still get really REALLY bad bucking at cruise speeds. I know that's kind of to be expected with a thumpr cam, and also my rear-end is currently only running ~2.77 ratio. (going 4.11 in 3 months, can't wait!)

Anyway, my question is, can I cure a bit of that bucking with some fine tuning of my timing advance curve? I got a nice little msd billet style dizzy that is currently set to 18* initial and 18* mechanical for a total of 36* at about 3k.

The vacuum advance is currently hooked up to a ported source instead of full because letting my idle depend on my limited manifold vacuum (getting about 9psi) was causing the car to die all the time. Anyway it drives great other than the cruise surging\bucking, so I'm just hoping and I can smooth that a tiny bit with some knowledge on how the timing advance is affecting my cruising performance.

Thanks!

Where do you live? Airgaps and cold and/or damp weather don't mix real well.

9 inches of vacuum speaks to a really large cam which the smallest Thumper is at 279/297, it has nothing useful under 2000 RPM which your gearing doesn't help. It has a lot of overlap and pretty late closing intake. The late closing intake drives really hard on the compression ratio. This cam needs a pile of compression to pick up the bottom end, today's premium fuels have a hard time supporting this. Need to know the compression to say much more about this. On top of that aluminum heads want at least a full ratio higher compression because they pull the heat out of the burn so fast. A band aid to this would be to run the coolant temp up to 200-220.

A stock bottom end portends crappy piston design, before I comment, what's in there? This needs a lot of squish/quench that means a flat top or D dish piston that closes to .040 inch and maybe with this cam .035 inch of the head. So this brings in the next question of what is the deck clearance and how thick is the head gasket?

The 9 inches of vacuum makes the use of vacuum advance more useless than not. You're either going to have to pick up the static to 25-28 degrees or get the centrifugal to come in a lot sooner. Maybe all in by 1500-1800 RPM. What about an MSD multi strike control box, this is way more important than a billet distributor unless you’re going to use the distributor to hold the garage roof up or something like that. back when we had points pushing on the shaft with several pounds of force we needed billet distributors and everything was a casting with bushings, today the thing spins a wheel to make micro gauss magnetic fields in a transistor and suddenly we need billet aluminum housings and roller bearings to operate what that uses the energy of an iPod.

Advancing the cam another 4 degrees may help this a lot.

The Thumper has a radical split between the intake and exhaust durations, it's like it’s designed for an engine with cast iron exhaust manifolds that connect to a Y pipe and factory single exhaust. With headers and good flowing duals this is way too much exhaust timing especially if using a decent aftermarket or ported head. If you have a decent exhaust I can guarantee this thing is over scavenging the cylinders and you'll go broke before you can put enough gasoline in to overcome this bad habit.

Bogie

[Zerocyde]

@oldbogie, Yea, I've about had it with this thumpr. If I knew what it was I probably would have said no when my builder offered it to me.

I've been considering swapping cams this income tax, but I have no clue how to pic a cam that's gonna make it so I gain as much NORMAL drivability as possible without losing any power, because as annoying as that thumpr is, the car does get up and go.

[hcompton]

Originally Posted by Zerocyde

@oldbogie, Yea, I've about had it with this thumpr. If I knew what it was I probably would have said no when my builder offered it to me.

I've been considering swapping cams this income tax, but I have no clue how to pic a cam that's gonna make it so I gain as much NORMAL drivability as possible without losing any power, because as annoying as that thumpr is, the car does get up and go.

Thumpr cam also make big power. They make the sound but they fully take advantage of the lobe design to make some big numbers. They are hard to replace without going to roller or solid and not loose any power.

I think the gears will sort this stuff out. 3.73 will be plenty and it will no longer bump in gear. If you had the bmt you would need 410. 410/373 will make your car jump and run. Off idle issues will not be a big problem.

Also most of the dizzy recuruve kits come with new moduale and adjustable vac can and new springs for the mechanical advance.

This shows the key parts of any recurve kit. Notice the black plate that is very important its set to stop the movement of the mechanicsl adv. Should be set to the max for big cam.
Crane Cams 99600 1 Distributor Advance Vacuum Adjustable GM Hei Kit 021174007299 | eBay

This is a complete kit and should get you set. I understand that your dist. Came setup with all these parts so maybe not needed but wanted to show full setup. Call jegs to get the one with low HG vac can.
Pertronix Hei Distributor Upgrade Kit Flame Thrower Coil Cap Rotor Module GM 694342006440 | eBay

Sorry about the ebay links. Just trying to keeo it simple. Jegs is good place to buy from they usally have everything and are always cheaper than summit.

[danzigs]

I had a very similar problem with a lopey cam, low vacuum, stalling and surging issues. Played around with different mech springs and ported to full manifold vacuum, different initial timing etc. etc. and never got rid of my headaches until locking out my distributor at 36 degrees which was suggested to me on these forums. Thank you Fbird! My cam is not very streetable but definitaly liveable now! I run my vacuum advance on ported and use a kill switch to help wing my hot motor over with no kickback issues. Now that my initial timing is higher it made dialing my carb in a lot easier and gave me better vacuum for my power brakes. Maybe this can help.

[F-BIRD'88]

You need to recurve the distributor to allow more initial base timing at idle
that does not decrease when rpm drops.

The thumpr cam wants more BASE timing at idle.
Shorten the mechanical advance curve to 10deg.
Now you can run 26deg base timing and 36deg max mechanical timing.

Then use ported vacuum for the vac advance and limit the travel of that to
10-12deg max. SO that the most it will add at highest manifold vacuum is 10-12deg vac advance.

Do not use the lightest tension, fastest mech advance springs
as they tend to be inconsistant and the timng jumps around too much, at idle.
1 light and 1 medium spring usually works best.
Make sure the adv system works consistant, smooth and returns to base timing at idle.

Make ure you are on the base of the advance curve when setting idle timing.
(slow the idle speed down to set base timing)

You need a minimum of 26deg at idle that does not vary at idle and should not increase until 1100+rpm.

18 is not enough.

Do you use the PCV valve? If not this screws up the carb idle circuit calibration.
You don't need drilled throttles. (once you correct the timing curve)

Check the power brake vac diaphram for leaks. Fix all vacuum leaks.

The proform carb idle circuit may need fine tuning to ge the idle AFR just right.
But you want the throttles/idle fuel tansfer slot exposure just right at idle.
remove the carb , flip it over and reset the PRI and SEC throttles opening at idle .020-.030" slot exposure even on BOTH. Install a 4.5" power valve and block the holes you drilled in the pri throttles. (or replace)
Make sure th PCV is hooked up to the base of the carb and functional.

The bigger thumprs need so much base timing at idle you might as well kist lock out the mechanical advance curve and run full 36deg fixed timng all the time..

+ some vacuum advance for part throttle cruising)

Get some gears for that thing. If yoj want to cruise at a very low rpm
get a smaller cam with less overlap (wider LSA)

[F-BIRD'88]

Bucking at cruise speeds..... Check the distributor cap/rotor phasing alignment. This is adjusted by adjusting the at rest install position of the mag pick up relative to the cap/rotor.
the vacuum advance moves the mag pick up. thus it effects the cap/rotor phasing.
Excessive vac advance travel,combined with poor phasing causes distributor cross fire, engine misfire and bucking.

The cure is to improve-adjust cap-rotor-mag pick up phasing using a holed-windowed old dist cap and a timing light to watch the phasing as the vac advance works, and limit vac advance max travel to around 10-12deg .

many of the aftermarket distributors have poor phasing and excessive vac advance travel.

[Zerocyde]

Originally Posted by F-BIRD'88

Do not use the lightest tension, fastest mech advance springs
as they tend to be inconsistant and the timng jumps around too much, at idle.
1 light and 1 medium spring usually works best.
Make sure the adv system works consistant, smooth and returns to base timing at idle.

Make ure you are on the base of the advance curve when setting idle timing.
(slow the idle speed down to set base timing)

You need a minimum of 26deg at idle that does not vary at idle and should not increase until 1100+rpm.

Okay, so here is my plan.

1. Limit the mechanical advance to 10*. (which means I need to find and buy 10* bushings because the lowest mine came with are the 18* bushings that are already in.)

2. Find a spring combo (maybe 1 light and 1 medium) that will make the mechanical advance not even start until 1100, and advance fully by 2500?

2. Replace my primary plates with non-drilled ones.

3. Set initial timing to 26*.

4. Do a mixture screw adjustment and get the car to idle at about 900.

5. Hook the vacuum advance back up to ported and adjust the can to not give more than 10-12 degrees.

One question though. At 18* advanced my vacuum can is ALMOST touching my intake manifold. I don't think I will be able to turn the dizzy any more advanced. I'm not an expert on distributors, but can I just move each spark plug wire counter-clockwise?

[danzigs]

Pull the cap and mark the rotor in relation with the distributor housing. Pull the distributor out, and restab it in a different position. Just be sure the rotor doesnt come out of adjustment from your mark on the distributor housing. It will slightly when you put it back in but then turn it till your mark lines up again. Your timing will remain the same.

[F-BIRD'88]

Ya just yank the distributor and re stab it in a better position.

To see the effect of the increased idle timing and help you determine the exact amount of idle timng your motor
likes, now , before you get and install the 10deg bushing, you can remove the distributor and temp lock out the mechanical advance system so its fixed. (read the directions) Set the now fixed timing at 34-36deg.
and get the carb all reset. Now move the timing around and find what the motor likes at idle. (best idle manifold vacuum) best throttle response and idle quailty.
( between 24 and full 36deg timing at idle). If it likes the full 36deg locked timing, and does not protest at low rpm full throttle ( lugging the motor) then go with it. If it does ping a bit if when you lug the motor down
then go with the short timing curve.

You can check and determine what the motor likes by driving in first gear with no throttle input
idling along with clutch out in first gear. ( this is similar to what the motor sees on a automatic trans, while idling in gear.

Again as long as the motor does not ping when lugged, while using locked timing, you re good to go.

Locked timing ( no mechanical advance) + 10deg of vacuum advance.
Install a ignition power interupt switch if its hard to start up when hot with locked timng.

BE SURE the GM starter motor brace/bracket is installed.

try the locked timing. Those thumpr cams need lots of timing at idle. ( EGR effect at idle of the narrow LSA, extended ex duration and high intake/exhaust overlap)