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Thread: Can vacuum and mechanical advance curve tuning help my lopey cam bucking issue? Reply to Thread
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Topic Review (Newest First)
11-03-2012 10:00 PM
bentwings Well It was only 30 deg today when I left the shop. The car started right maybe 2-3 rev of the starter and it was running. Gotta love the mini starter.

Once the motor was warmed up I noted that the AFR was about .5 richer than it had been. I think what is happening is that the vac adv is setting the mixture off sooner and more optimum. This allows less throttle pedal (opening) and puts the throttle blades in a different location on the transfer slot...and a different AFR. What ever ...the car ran just great. I drove about 30 miles the long way to the breakfast and there was no lean surge like there was a couple weeks ago. After breakfast..couple hours it still was only 30 deg but the car started right up. I drove around a little then stopped and slowed the idle down a couple hundred rpm. Now the blades are tight in the bores so that's as far as I can go.

It's amazing that a simple change like this can smooth things out. I'm glad I elected to only change the vac adv then go after other items.

Next on the list is to richen up the idle feed restrictions one step (4 jets) to tweek the just off idle mixture. I think it needs to be a little closer to 14.7 rather than the 15.6-15.8 that it is right now. I'll try and do that tomorrow then later in the week I have a good 100 mile trip planned to check mpg for the last time this year.
11-02-2012 09:49 PM
bentwings Interesting read. thanks

Well I got the adjustable vac adv installed today. No fiasco like last time. I dropped the dist right in and hit the original timing right on the money. BTW I don't care what the initial is as long as it doesn't kick back against the starter. I just set the timing at about 2500 rpm and let initial fall where it may.

I preadjusted the vac adv to what I thought might be about 12 degrees. Again right guess.

So I have 34 deg at 1900 rpm centrifugal plus 12 deg vac at 10 inches vac. I measured the vac adv with the motor running and used a small hand vac pump. works nice.

The idle speed picked up 200 rpm so I need to slow this down tomorrow. The idle vac went from 9-10 in to 14-15 in and it is very smooth. In gear it went from 750-800 to almost 1000 so again I'll slow it down a little tomorrow after I take it for a nice cold weather cruise. The AFR did not change but I don't count this untill I get a good 1/2 hour drive. We'll see the final results tomorrow but it sure looks encouraging.

I have very lean jets and transition mix so we'll see how just changing the vac adv affects running and the tune up since I changed nothing else.

I also noted that the exhaust smell was much less. At least it didn't stink up the shop like it did before the change.
10-29-2012 10:33 AM
Originally Posted by Zerocyde View Post
@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.
Go here and read what I wrote on Friday the 24th. It shows what can be done with modern parts selction. Depends on whether you want to make noise or make power. See the attached file in that post.

10-27-2012 03:09 PM
Originally Posted by F-BIRD'88 View Post
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)
This is a great idea, I'm gonna go try this.
10-27-2012 02:09 PM
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.
10-27-2012 01:11 PM
Originally Posted by F-BIRD'88 View Post
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?
10-27-2012 05:46 AM
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.
10-26-2012 11:44 PM
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.
10-26-2012 05:06 PM
Originally Posted by Zerocyde View Post
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.

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 youíre 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 itís 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.

10-26-2012 04:22 PM
Originally Posted by PatM View Post
You might also be able to help yourself with a different vacuum advance mechanism. Lars over at 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.

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?
10-26-2012 02:46 PM
PatM You might also be able to help yourself with a different vacuum advance mechanism. Lars over at 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.

10-25-2012 09:03 PM
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
10-25-2012 08:53 PM
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
10-25-2012 08:41 PM
Originally Posted by hcompton View Post
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.
10-25-2012 08:38 PM
Originally Posted by Zerocyde View Post
No, only when doing something that uses up the limited vacuum like braking after engine braking or something like that.

I know it has a vac lockout piece, but the canister itself isn't adjustable I don't think.

So, if mine is adding 8* at idle, and 14* at 3k, then does that mean it's starting to advance under 9hg and finishing sometime after? Okay, so how do I adjust where it starts to advance and where it is fully advanced? Is that the alan wrench down the tube adjustment?
yes possibly it needs more than 9 to finish advancing, that is why I suggested to first see what the motor pulls under different conditons and rpm then see what the range and starting/stopping points are on the can. You can then decide what you need to do with it.

yes the range is set with an allen key via a set screw through the hose nipple opening, some have it some dont, I think the size is 3/32 or there abouts.
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