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#1 |
Senior Member
Join Date: Mar 2003
Location: St robert Mo
Posts: 2,001
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vacume advance question
I have had several people tell me my truck would run better with the vacuum advance disconected. I have a mild 355 (330hp) quick fuel mech secondary carb. Truck is not daily driver but not a drag racer either when drive it is street aplication. Does the vacuum advance soley help fuel milage?
what are the pros and cons of this. And one last question is it proper to just pull the hose or get the lockout kit? Thanks for any insight.
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1968 c10 lowered 3" 4" 355/Th400 built by Hatfield racing in joplin MO Last edited by 76bonanza; 05-15-2007 at 11:48 AM. |
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#2 | |
Account Suspended
Join Date: Sep 2005
Location: Monroe, WA
Posts: 3,815
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Re: vacumm advance question
Quote:
![]() The quick and easy way to feel this is just to pull the hose and plug it - then go for a drive around town ![]() The mechanical secondary carb isn't helping fuel milage, either ![]() |
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#3 |
"Trucks with Class"
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Location: OLYMPIA, WA, USA
Posts: 8,132
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Re: vacumm advance question
It depends on how much total advance is in the dist. I find some HEIs from the 70s & 80s that have a total of 50 degrees or more...they were probably in real light vehicles.
If you set the initial up to 8-12, the bottom end is strong, but they ping like crazy at cruising speeds/loads. You need to get the total back down to around 32-36 by either blocking the vac advance or limiting it and still have good 8-12 initial. |
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#4 |
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Join Date: Aug 2006
Location: Bakersfield,CA
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Re: vacumm advance question
no it wont run better with the vacumm advance disconnected, SBC like 32-36* of total timing, and about 8-10* of initial timing
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-steven Alte Seele S.S. "Allison" '72 C/10 LWB 350SB/TH350 - DD PITBULL IS NOT A CRIME |
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#5 | |
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Join Date: Sep 2005
Location: Monroe, WA
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Re: vacumm advance question
Quote:
![]() Vacuum advance is in addition to mechanical advance, adding advance to 50+ degrees at idle and cruise. I usually get called BS on that one - but it's a fact ![]() |
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#6 | |
Parts and more parts
Join Date: Nov 2003
Location: Lebo, Kansas (middle of nowhere
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Re: vacumm advance question
Quote:
YEP, I agree and I won't wave any BS flags. My stroker is set to get 35 degrees tOTAL and the response is amazing. I keep my vacuum advance connected to get the distributor to control the rest of the load. I know a gentleman tha has his 440 Charger set up with the vacuum disconnected, the mileag is bad on these cars from the GET GO, but his is downright nasty. He can't run over a 100 miles on a tank of gas and his gearing is not that low. |
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#7 |
Registered User
Join Date: May 2003
Location: Shreveport LA
Posts: 3,170
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Re: vacumm advance question
Vacuum advance can help it run smoother at idle and at low rpm. Too much vacuum advance can be a bad thing, however, causing an annoying miss at cruising speed.
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#8 |
Senior Member
![]() Join Date: Jul 2006
Location: Leander, Texas
Posts: 850
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Re: vacumm advance question
Everything everyone is saying is right on the $$. Another reason people run vacuum advance is it allows you to not have a ton of initial timing at idle. Therefore saving gas and not making your motor run hot at idle.
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#9 |
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Join Date: May 2005
Location: PA
Posts: 2,696
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Re: vacumm advance question
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#10 |
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Location: Leander, Texas
Posts: 850
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Re: vacumm advance question
Say you're running 8 degrees of initial timing. That would typically not be enough timing at initial. You're truck would fall on its face when you tried to go. When using vacuum advance you should have a hose from your ported vacuum port to the vacuum diaphram on the distributor. That diaphram won't see vacuum until you crack the primary throttle blades thus allowing the diaphram to see vacuum and add timing.
In the distributor you still have to consider the weights and springs to control how much total advance and what RPM it comes in. |
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#11 |
Registered User
Join Date: May 2005
Location: PA
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Re: vacumm advance question
Ahhh - I see. I thought you were supposed to use full manifold vacuum for vac. advance.
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#12 |
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Join Date: Sep 2005
Location: Monroe, WA
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Re: vacumm advance question
Ummm...no. There is significant vacuum advance at idle; this improves idle quality. This is why vacuum advance must be disconnected to set initial timing.
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#13 |
"Trucks with Class"
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Re: vacumm advance question
I just went through this again today with a stock 71 3/4T w/402 & TH 400. I put in a used HEI...55degrees total from the mech & vac and 8 initial.
The thing pings BAD at crusing when you put a slight load on the engine...AGGH! It did not do this with the stock point dist & vac advance. I pulled off the vac hose and took it for a drive this afternoon. The ping is GONE! and it runs just as strong as before. I did not get the adjustable timing light hooked up, so I don't know what the total is now with just the mechanical advance. I would say if you are having this problem, try no vac and see if it works for you. |
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#14 | |
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Join Date: Sep 2005
Location: Monroe, WA
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Re: vacumm advance question
Quote:
Mechanical advance should be limited to 34-36 degrees by 3000 RPM, using whatever initial is required to get there. If you're getting more mechanical than that, then you need to bush the pivots. What curve are you seeing with *just* mechanical advance? This sounds a lot more like a stuck advance than anything else to me - have you verified the movement of the mechanical and vacuum advance? I'm not just disagreeing to disagree ![]() |
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#15 |
67-72 Addict!
Join Date: Feb 2002
Location: The TAX State!
Posts: 7,857
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Re: vacumm advance question
I run 19 degrees initial and 42 total on my truck.
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Jim 1970 C/10 Fleetside w/Ghost Flames Lowered 4.5" front and 4" rear (Raked) 355/350 Turbo w/shift kit 10" Redneck Performance Verter w/2500 stall Hooker Super Comps part#2808-1 Performer RPM Air-Gap 12 Bolt w/3:73 gears- Eaton Posi Comp Cams XE262 with 1.6 Crane Energizers, Road Demon 625 and Brodix IK 180 heads |
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#16 | |
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Re: vacumm advance question
Posted by PICKMUP;
Quote:
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'69 GMC C2500 Custom Camper, 8 1/2' bed, New GM 350, NP 435 Close Ratio 4spd. Trans., 3.73 Dana-60 open.Camper and Trailer wiring, PS, PB, AC, tach , three gas tanks, stereo speakers, 2nd owner, Work-Truck supreme. |
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#17 |
"Trucks with Class"
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Re: vacumm advance question
All HEIs are not created equal. Most drop in, set up pretty close, and run good...but some of these have just way too much dist advance built in (vac & mech combined) and have to be "fixed" to make them compatible with our heavy trucks.
This is one of those. I don't have separate vac & mech figures on this one yet, but total is over 50. My 68 GM serv manual (396) calls for mech: 28 @ 4200rpm (only 15 @ 2000) and vac: 15 @ 15"+ with 6 initial setting. My 79 GM serv manual (454) call for even less with mech: 20 @ 4200 (only 14 @ 2800) and vac: 10 @ 13"+ with 4 initial setting. Sorry, I don't have a '71 manual close, but it looks like GM set these heavy trucks with BB engines to run around 30 degrees total advance for running in mid-range cruising rpms at part throttle. That seems to agree with what has worked best in the past. So, until I can get an adj vac adv pot or limit the mech adv to shorten up the curve...I will have to leave the vac adj off. Not best, but the pinging will burn a hole in the pistons over time...and it drives me crazy. |
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#18 |
Registered User
Join Date: Aug 2006
Location: Bakersfield,CA
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Re: vacumm advance question
i have a stock 350 with the points dizzy and i have the VA hose connected to the carb (qjet) and its set at 8* initial, i have no idea what the total is
what would you all say to that? right/wrong, what? all the talk thats gone on so far is giving me a headache ![]()
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-steven Alte Seele S.S. "Allison" '72 C/10 LWB 350SB/TH350 - DD PITBULL IS NOT A CRIME |
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#19 |
Registered User
Join Date: May 2003
Location: Shreveport LA
Posts: 3,170
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Re: vacumm advance question
Some race motors can benefit from vacuum advance. It won't do squat at full-throttle on the track, but it can help it run smoother and cooler in the pits.
I had a 292 with a 310* cam in a Nova for a short time, and hooking up the vacuum advance (manifold vacuum, please) made it run smoother at low rpm, such as in residential areas & school zones. Ported vacuum was introduced for smog devices, and the HEI distributor was designed as a smog device. Recalibrate it like Crazy said and it should work much better. Just don't tell the smog police. Last edited by jimfulco; 05-17-2007 at 10:59 PM. |
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#20 |
Senior Member
Join Date: Mar 2003
Location: St robert Mo
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Re: vacumm advance question
One other question, I am from the camp of full vacuum at idle also. But I have always wondered about tuning the carb. Do you tune the carb idle screws with advance plug in or not. Because if you tune it with advance disconnected then when you connect it to full manifold vac the idle tends to go up also?
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1968 c10 lowered 3" 4" 355/Th400 built by Hatfield racing in joplin MO |
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#21 |
Registered User
Join Date: Apr 2003
Location: NB
Posts: 3,367
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Re: vacumm advance question
Man I put a edel 1406 on my truck a few months ago and I can't get it to stop pinging and poping.
This brings more light no the subject 4 sure I think I am going to try turning the dizzy to full advance and see what happens with out the VA....... Good thread keep er going
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1968 Chevy C10 307 3ott fleetside 1967 Chevy C/10 V8, 3spd, fleetside lwb.Sold 1967 Chev C/10 step, 383, M21. SOLD |
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#22 | |
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Join Date: Sep 2005
Location: Monroe, WA
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Re: vacumm advance question
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#23 |
Senior Member
Join Date: Mar 2003
Location: St robert Mo
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Re: vacumm advance question
What does pinging sound like exactly? I have what sounds like an exhaust leak louder under truck and very metalic sounding, but maybe it is pinging it does it idle.
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1968 c10 lowered 3" 4" 355/Th400 built by Hatfield racing in joplin MO |
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#24 |
Parts and more parts
Join Date: Nov 2003
Location: Lebo, Kansas (middle of nowhere
Posts: 6,821
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Re: vacumm advance question
In most cases the so called "Pinging", otherwise known as spark knock happens under load or acceleration. If you have a metallic noise at idle, that would be another thing. I would try driving with the window down and listen as you accelerate your truck to get out onto a street or highway. If you here something that sounds like a light rod knock, then you are experiencing the "pinging" we have referenced to.
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#25 |
Account Suspended
Join Date: Mar 2003
Location: ** THE FALL GUY **CHICAGO IL
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Re: vacumm advance question
sounds like a bag of marbles in a tin can
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