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Discussion Starter · #1 ·
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!
 

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Can vacuum and mechanical advance curve tuning help my loopy cam bucking issue?

What kind of car do you have? what are the cam specs? do you have an overdrive transmission? Basically what it sounds like is the basic operating range of the camshaft is higher the cruise speed you are driving. Does the thumper camshaft recommend a torque converter stall speed? You may need to install a 2500 stall converter and increase the rear end gear ratio.
 

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my KARMA ran over my DOGMA
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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!
I agree the gears are not right,,,

For some timing help,,,First off the thumpers like manifold vacuum adv, 30+ deg at idle so if you have the vacuum adv at the desired 10-12 limit then you need a initial at 20-22. May need to limit the mech advance if you bump up the initial so you dont go past 36. However initial becomes much less significant if you run manifold vacuum adv but the vacuum advance cant be more than 14 deg of the equation so do the math. Also too high an initial and it will buck the starter when hot.

Check how much the vacuum adv is adding at idle with manifold vac, this will tell you where you are at. Let us know

Shot for no more than 48-50 full advance initial + mech + vacuum adv. "bucking" or surging can be caused by too much advance,

Also try and jet up, maybe the AF ratio is lean? get that AF meter yes good plan.
 

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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!
Since your a beginner at carb tuning your carb may be really messed up at cruise and you don't know it.

Secondly a few more degrees of initial timing probably wont hurt, start with maybe 20 degrees initial and 36 total. Put your vac advance back to manifold vacuum, not ported, and if you need to adjust the can so you get vacuum advance a little earlier.
 

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Discussion Starter · #5 ·
Thanks for the tips so far everyone!

I'll go get some numbers on my vacuum advance right now.

But I have an issue with everyone suggesting I put the vacuum advance on full manifold instead of ported source. I would love too, but what happens when I'm coming to a stop or something and the manifold vacuum drops, which causes the vacuum advance to drop, which causes the idle to drop and the car tries to die?

I've only got like 9psi manifold vacuum and I have power brakes. If I hook the vacuum advance up to full manifold then I have to turn the idle screw down, and then at that point, if I lose the vacuum advance the car will idle too low and die. That's why I went ported in the first place.
 

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I've only got like 9psi manifold vacuum and I have power brakes. If I hook the vacuum advance up to full manifold then I have to turn the idle screw down, and then at that point, if I lose the vacuum advance the car will idle too low and die. That's why I went ported in the first place.
so either your idle tune is wrong or your vacuum advance is wrong or more than likely a little of both.
 

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Discussion Starter · #7 · (Edited)
so either your idle tune is wrong or your vacuum advance is wrong or more than likely a little of both.
I'm waiting about an hour to go outside and fire it up to get the vacuum numbers, but as for idle tune, this is what I did...

I set the primary butterflies to show a perfect square of the transfer slot, then adjusted the rear butterflies to match the same amount of opening (by holding it up to the light). Then I set the initial and all-in timings to 18* and 36*, then I drilled the primaries ever so slightly until the idle held. At that point the mixture screws actually finally worked. (before drilling they didn't have any effect. Screwing them all the way in didn't even make the car die). So they are now set at about 1 and a quarter out. I can now set the idle as low as 750 and it will hold fine, but I've been keeping it at about 1k recently.

PS I know drilling is a no-no, but my butterflies are the screw on type and I figured if I ever figure out how to properly set the carb to allow enough air at idle I can just swap non-drilled butterflies from my old carb back on and make the changes.
 

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You minght want to try an adjustable vac adv.

I just got one but have not installed it yet.

It starts advancing at about 4-5 in all settings and at all settings it is in by 9-10 in.......this is less all around than the documentation but shoulod work for low vac motors.
 

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I'm waiting about an hour to go outside and fire it up to get the vacuum numbers, but as for idle tune, this is what I did...

I set the primary butterflies to show a perfect square of the transfer slot, then adjusted the rear butterflies to match the same amount of opening (by holding it up to the light). Then I set the initial and all-in timings to 18* and 36*, then I drilled the primaries ever so slightly until the idle held. At that point the mixture screws actually finally worked. (before drilling they didn't have any effect. Screwing them all the way in didn't even make the car die). So they are now set at about 1 and a quarter out. I can now set the idle as low as 750 and it will hold fine, but I've been keeping it at about 1k recently.

PS I know drilling is a no-no, but my butterflies are the screw on type and I figured if I ever figure out how to properly set the carb to allow enough air at idle I can just swap non-drilled butterflies from my old carb back on and make the changes.
Why didn't you adjust the openings of the throttle plates instead of drilling? sounds like your throttle plates need to be repaired and you need to follow the instructions on the carb. You don't drill the plates unless you have a really big cam and even then it's not the correct way to adjust the idle.

Sounds like besides your vacuum timing possibly being wrong your carb is definitely out of wack. Was this a new carb or a rebuilt swap meet special?
 

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Discussion Starter · #10 ·
Why didn't you adjust the openings of the throttle plates instead of drilling? sounds like your throttle plates need to be repaired and you need to follow the instructions on the carb. You don't drill the plates unless you have a really big cam and even then it's not the correct way to adjust the idle.

Sounds like besides your vacuum timing possibly being wrong your carb is definitely out of wack. Was this a new carb or a rebuilt swap meet special?
It was brand new. I had the exact same issue with my last carb. My cam makes it so I need a crap load of air to maintain an idle, but opening the plates enough to give it the air it needs exposes WAY too much of the transfer slot. With my last carb I just cracked the rear plates open enough to give it the air it needs, but I felt that was a band-aid. So with the new carb I drilled. I knew drilling wasn't the proper way, but I couldn't find the correct info at the time and figured I would just do it as a temp fix since the plates where screw on and could be replaced very easily down the line.

I'm headed out now to see what my vacuum canister is doing.
 

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my KARMA ran over my DOGMA
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But I have an issue with everyone suggesting I put the vacuum advance on full manifold instead of ported source. I would love too, but what happens when I'm coming to a stop or something and the manifold vacuum drops, which causes the vacuum advance to drop, which causes the idle to drop and the car tries to die?
I've only got like 9psi manifold vacuum and I have power brakes. If I hook the vacuum advance up to full manifold then I have to turn the idle screw down, and then at that point, if I lose the vacuum advance the car will idle too low and die. That's why I went ported in the first place.
I dont the part where you say it dies at a light on manifold vacuum but runs ok on ported? you should have no vacuum advance at a light on ported? :confused:

Still no info on the vacuum advance starting vacuum point and its range? check this, there is something not right, it should improve your idle not cause the car to die. In your case you want the vacuum adv to be all in at idle, some will disagree however if it is floating around in the middle of the idle vacuum it will fluctuate too much and cause erratic idle. So if you have 9 " hg at idle then set it to be completely pulled in at say 7 or 8 " hg. That is to say if you even have an adjustable canister?

Dang thumper cams mess everything up, kidding, you will figure it out.
 

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I think he's saying he readjusts the carb when its on manifold vacuum and when he tips in it dies- which is understandable because those holes he drilled can make tip in a problem.

I think we all agree the plates need to be fixed, the carb should be set properly and if he needs to he may need to lower the vaccum required to get vacuum advance in. Luckily those are all relatively small things and can all be fixed in a few hours.
 

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Discussion Starter · #14 ·
Okay, I checked the vacuum can plugged into full manifold. It's adding 8* at idle and ~14* all in.

18* initial
36* total
WITH VACUUM
26* initial
~50* total

I thought the vacuum can should be giving all of it's advance at idle. Anyone else think it's weird that both distributors I bought for this engine came preset for 18* mechanical advance? And if it did, why isn't the vacuum can also preset for low vacuum situations?

Am I doing something completely wrong? Here is the dizzy I have now.
 

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Discussion Starter · #15 ·
I think he's saying he readjusts the carb when its on manifold vacuum and when he tips in it dies-
What I'm saying is, hooked up to full manifold, when the idle dips down real low it has a hard time bringing itself back up to regular idle.

Like when coming to a stop or something that can lower the idle, if the idle falls down to ~500, it struggles to bring itself back up to ~1000 (where I have it set to idle). It seems like, with full manifold on the vacuum can, the car is depending on that vacuum advance to stay alive, and when something (like brake booster) drains the limited vacuum that turns the vacuum advance to zero, causing the idle to struggle to come back up and turn the vacuum advance back on and then idle correctly.

Also, let me re-state to be clear:

WITHOUT VACUUM: 18* initial, 36* total.
WITH VACUUM: 26* initial, ~50* total.

That means the vacuum is adding ~8* at idle, and ~14* over 3k rpm. Isn't a vacuum advance can supposed to be giving it's full advance at idle?
 

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What I'm saying is, hooked up to full manifold, when the idle dips down real low it has a hard time bringing itself back up to regular idle.

Like when coming to a stop or something that can lower the idle, if the idle falls down to ~500, it struggles to bring itself back up to ~1000 (where I have it set to idle). It seems like, with full manifold on the vacuum can, the car is depending on that vacuum advance to stay alive, and when something (like brake booster) drains the limited vacuum that turns the vacuum advance to zero, causing the idle to struggle to come back up and turn the vacuum advance back on and then idle correctly.

Also, let me re-state to be clear:

WITHOUT VACUUM: 18* initial, 36* total.
WITH VACUUM: 26* initial, ~50* total.

That means the vacuum is adding ~8* at idle, and ~14* over 3k rpm. Isn't a vacuum advance can supposed to be giving it's full advance at idle?
If your idle is dropping that much at a stop light you obviously need a higher stall in your transmission. Getting it tuned right will help some but you will always have that problem until you get a decent converter. If you do some research you may be able to use a higher stall from a different car that you can pick up from a junk yard for cheap.

you never did answer to what stall you have meaning its likely stock and likely (but not necessarily) really low. NOT ALL FACTORY TC's ARE THE SAME!!!
 

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Discussion Starter · #17 ·
If your idle is dropping that much at a stop light you obviously need a higher stall in your transmission. Getting it tuned right will help some but you will always have that problem until you get a decent converter. If you do some research you may be able to use a higher stall from a different car that you can pick up from a junk yard for cheap.

you never did answer to what stall you have meaning its likely stock and likely (but not necessarily) really low. NOT ALL FACTORY TC's ARE THE SAME!!!
Sorry, I have a manual. Any advice about my vacuum can findings?
 

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my KARMA ran over my DOGMA
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What I'm saying is, hooked up to full manifold, when the idle dips down real low it has a hard time bringing itself back up to regular idle.

Like when coming to a stop or something that can lower the idle, if the idle falls down to ~500, it struggles to bring itself back up to ~1000 (where I have it set to idle). It seems like, with full manifold on the vacuum can, the car is depending on that vacuum advance to stay alive, and when something (like brake booster) drains the limited vacuum that turns the vacuum advance to zero, causing the idle to struggle to come back up and turn the vacuum advance back on and then idle correctly.

Also, let me re-state to be clear:

WITHOUT VACUUM: 18* initial, 36* total.
WITH VACUUM: 26* initial, ~50* total.

That means the vacuum is adding ~8* at idle, and ~14* over 3k rpm. Isn't a vacuum advance can supposed to be giving it's full advance at idle?
You need a canister that will be fully pulled in by 7-8 "hg your idle vacuum is 9 "hg correct?

What is the duration of the cam?, what is the vacuum level at crusie speed? get a gage for the inside of the car, extend the hose or what ever, what is the starting point of movement of rod travel in "hg vacuum of the cansiter? what is the end point of rod travel in "hg?

Set it to start to move at 3-4 "hg, get it to stop moving at 7-8 "hg, limit the added advance to 12-14 at the crank. If it is not adjustable get one that is, crane or accel maybe MSD.
 

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Zerocyde,
Just my two cents. Try disconnecting the vacuum adv. and plugging the carb port, and take it for a short test drive. Keep it short because it will be retarded somewhat. Make sure its allready hot before the test.
If the bucking/surging stops you know its probably too much advance from vacuum can. Then get a vacuum limiter plate that Crane sells and install it, and try limiting your vac. adv. as someone else suggested. Its a pretty cheap
test that you can get rid of if you don't like it. You'll also be able to use it later after you change your gears to optimize your mileage. You can also use it with a adj. vac. can.
Usually a good sign of too much vac. adv. is surging when deccelarating. Forgive the spelling. What happened to the spell checker?
I'd leave it connected to ported vac in you current tune IMO.
FWIW
ssmonty
 
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