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Old 10-25-2012, 02:15 AM
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Can vacuum and mechanical advance curve tuning help my lopey cam bucking issue?

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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Old 10-25-2012, 05:59 AM
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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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Old 10-25-2012, 07:27 AM
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Nope you need gears.

But you can run thru the setup steps for setup. Most of the time with large cam and compression: initial timing is high with as little as can be gotten from the vacum advance and weights.

But its still going to lope in gear with 273's. stall speed or manual trans wont fix it.

Hope this helps!

Last edited by hcompton; 10-25-2012 at 07:34 AM.
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Old 10-25-2012, 10:01 AM
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Quote:
Originally Posted by Zerocyde View Post
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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Old 10-25-2012, 10:17 AM
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Quote:
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.

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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Old 10-25-2012, 12:49 PM
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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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Old 10-25-2012, 01:13 PM
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Quote:
Originally Posted by Zerocyde View Post
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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Old 10-25-2012, 01:21 PM
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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.

Last edited by Zerocyde; 10-25-2012 at 01:30 PM.
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Old 10-25-2012, 01:29 PM
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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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Old 10-25-2012, 01:55 PM
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Quote:
Originally Posted by Zerocyde View Post
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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Old 10-25-2012, 03:04 PM
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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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Old 10-25-2012, 03:07 PM
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Quote:
Originally Posted by Zerocyde View Post

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?

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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Old 10-25-2012, 03:18 PM
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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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Old 10-25-2012, 03:23 PM
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Roger that AP
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Old 10-25-2012, 03:37 PM
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Did you make the changes for your carb to run at low vacum. Most carbs need updates to run that low vacum. Holley can help with that if your not sure.

Yeah you needed to open the rear plates to get it to idle. No need to drill but it should work as long as the holes are small. I prefer to buy notched plates.

You need to buy a vacum can that can work at low vacum. Stock style does not work right. Then move source. Also change mech advance to be all in at 2000- 2500 and more intial usally. But it depends on the setup.

Still need gears to fix your issue. 4.10 can be hard to live with 3.73 might work better. The smallest thumpr should be ok with smaller gears but not sure of your total build out.
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