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post #1 of 33 (permalink) Old Apr 9th, 01, 01:28 PM Thread Starter
 
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Here is my problem. I finished installing the Edelbrock RPM cam and Heads onto my 9.5:1 .060 small block 350 with 750 carb and Air Gap manifold last month. After breaking it in and changing the oil, I took it for a drive , but it backfired a little so I started checking things. I found my timing was off a little and I only had 8 lbs of vacuum. I thought that I corrected this when I found a bad vacuum line on the vacuum advance.
Today, I had 6 steady pounds of vacuum, but the car ran ok. I checked the timing and it was at 14 BTDC. I thought I would see what what would happen if I turned the distributor. The vacuum increased to 11, but the timing is at 24 BTDC. The car seems to be running great, and the engine is not pinging. Something is not right here. Does anybody here have any ideas where the problem is?? Thanks in advance.
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post #2 of 33 (permalink) Old Apr 9th, 01, 01:59 PM
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are you useing manifole vacuum for the vacuum advance? you should be, and you might need to recuve your distributor. any time you change a cam the distributor curve need's to be looked at. i bet you need the advance sooner now.

use manifold vacuum for the vacuum advance, if you do not belive me do a search on it and you will find out why i say to use it.

base timing around 12* to 15* BTDC is good so try this i bet you will like it better and so will the motor.
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post #3 of 33 (permalink) Old Apr 9th, 01, 04:10 PM
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Not to change the subject, but what does one use to measure the vacuum? What is the unit of measurement? (lbs?) Thanks!
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post #4 of 33 (permalink) Old Apr 9th, 01, 05:23 PM
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Ok tell me why I have been hooking up my vacumn advance wrong for over 30 years

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post #5 of 33 (permalink) Old Apr 9th, 01, 06:19 PM
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ohayden,
you mesure vacuum in inches of water.

oger,
i am not saying you are doing it wrong, but on a performance motor manifold work's better IMHO.do a search well ok i will post what a person named I-man posted. and he is very right and on the ball on this. i have goten the same info from other's who build street rod's and distridutor people.

"I only have a business to keep going, and not all the time in the world to read magazines and articles by dyno kings, so if it comes to a little time in my response vs posting, I'll keep the time and the business going, thanks.
Now, for the rest of us: Vacuum advance.

Ported vacuum advance was, in fact, used by many makers of performance and stock engines in the past. Reason, the fuels were so good that the burn temperatures of the combustion chambers actually lowered as upper rpms were reached, and the added timing from ported was needed to heat the burn properly. In those days, the amount of degrees added in the cannister were fairly large, with some engines having 20+ degrees, but this number was usually not attained, as the total amount of vacuum added by the ported port was lower than that needed to get the vacuum advance to pull all the way in.

Today's pump vended fuels, and their additive packages, or lack thereof, have just the exact opposite of the earlier fuels, they always burn much too hot, even stuff like the Sunoco 280 have this problem. Add vacuum advance into the lace most succeptable to detonation, upper from mid to upper rpm ranges, and you can take detonation to the bank, you're gonna get it.

The only time today that ported timing is beneficial, to a point, is on Exhaust Gas Recirculation designed engines. These engines recirculate partially burned gasses from the exhaust system into the intake tract again to be re-burned and reduce emissions output, and the partially burned fuel needs the added combustion chamber heat to more completely burn the exhaust fuels recirculated.

In today's non-EGR performance engines, stock to performance, we do not require any upper rpm added timing for burn temperature control, in fact, it is detrimental to proper running.

So why use a vacuum cannister at all? Well, there is a simple silver lining to vacuum advance, engines as we are working with like timing at idle in the 18 to 24 degree range, but this isn't all that easy to do with some parts packages. How do we do this? Well, first, we need to think of the vacuum advance as not that, but an idle timing supplement to the initial timing.

As we now know, the idle timing can now be raised to a decent level at idle, to help with a stable idle, pull against a bigger than normal cam, pull against a converter and help cool the engine better from more complete idle fuel control. There is a catch, though, too many degrees of supplemental idle timing can be very counter-productive.

If we use the 24 degree figure, we can do things like set the initial timing at, say, 14 degrees, starts easily, then add the 10 extra with the cannister. OK, how do we get the 10 crankshaft degrees into the 20 degree cannister? We use either a Crane scroll plate to stop the advance pin that conects the diaphragm to the point/pickup plate to restrict plate movement down to the number of degrees we wish different from the static initial timing and the total idle timing.

OK, now that we have the number of degrees restricted, do we want to be able to change the vacuum level the canister comes in goes out with? Yes. We do this in two different ways, we look at the information of the stock cannister pull rate, and select the proper one for our vacuum levels, or we add the adjustable cannister from Crane, with the scroll plate that comes with it. Adjust away, you will find the correct amount of added timing for idle to make your engine run correctly.

Well, doesn't the idle added vacuum timing stay in the system when the gas pedal is hit? No, actually. It is immediately dropped as the off-idle vacuum goes away, and the centrifugal advance acts as usual . Only 180 on this is an engine that has no idle vacuum from a realy radical cam duration, and then, the vacuum actually rises after the engine is brought off idle. We just wouldn't use any kind of vacuum advance on an engine this radical anyway. Static idle timing would also be jumped up to 18 or so with the redical engine anyway, to try to stabilize the idle quality. Once again, adding initial timing helps the idle, just depends what you have available to help.

All HEI distributors manufactured for GM vehicles as production ignitions have way too much advance, mechanical degrees and vacuum degrees, and usually cause mucho problems in our performance and stock engines until they are set up correctly, and have full intake manifold vacuum sourced.

99 percent of all vacuum advances have way too many degrees of timing available and need to have the total number reduced to work with full manifold vacuum, as the manifold vacuum will pull all the degrees available into the cannister, not like the partial vacuum pull available with ported vacuum sources does at rpms.

Now, to dispell some myths about dyno tests and ported vacuum advance. Dynos always use more jetting to keep the chamber, engine and water temps cooled off, and ported timing just counteracts the rich mixtures of the rich jetting meeded for the dyno. In all the experiences I have had with crate and built performance engines, ported vacuum advance has been simply not productive, no matter what the engine did on the dyno.

If you are forced to use ported vacuum for your supplemental timing surce, then the curve and initial are not correct, and/or the total amount of vacuum supplemental timing is too large foruse as an idle supplemet from full intake manifold vacuum."


this is from a post he made last year. i hope it help's you to understand why you use manifold vacuum. as you noticed this apply's to non EGR and emision car's. in anthor post he sugjested you limit your vacuum advacne to 10* allso. a proplerly curved and adjusted distributor useing manifold vacuum will give better performance and milage then one useing ported. i know this from my own wxperiance and people that i know that have some prety fast car's. they all use manifold vacuum. i gained 4MPG and smother idle. also the motor has alot more power that i can feel and is faster off the line.


[This message has been edited by ilbl8 (edited 04-09-2001).]
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post #6 of 33 (permalink) Old Apr 9th, 01, 07:07 PM
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There is one giant flaw in your whole explanation ported vacuum as used on a Chev v8 is not true ported vacuum. It is tapped just above the throttle blades not below them as in manifold vacuum. The only difference is there is vacuum at idle with the manifold vacuum. If the throttle blades are open it doesn't matter if the tap is above or below the throttle blade there is no vacuum. I personally like ported vacuum because when you first accelerate there is an increase in advance to help the car move easier

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post #7 of 33 (permalink) Old Apr 9th, 01, 07:27 PM
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ok here is anthor post by I-Man. it explain's ported vacuum.

"If the vacuum cannister is left connected to the ported source, vacuum at the venturi source will always pull the cannister timing into the total timing equasion, even at WOT. The pull sourceport for ported vacuum stays high off idle, and doesn't drop vacuumm pull at 7/8 to full throttle plate openings, so the port retains it's pull.

If the vacuum cannister is left on full intake manifold vacuum sources, the vacuum cannister will remain dropped off as long as the vacuum levels under the throttle plates stay low from the plates being wide open, and no vacuum timing is supplied to the engine. As the vacuum levels off within the intake manifold during cruise and set throttle plate operation, say at 3,600, the intake manifold vacuum will come back to somewhat normal levels, and pull some or all of my supplemental vacuum timing back into the total timing equasion, making the mechanical total, say, 30 degrees, with the supplemental vacuum timing contributiing another, say 6 to 8 degrees, well, heck, there's 36 to 38 degrees right there.

All this and smooth idle AND acceleration"

and here is anthor one from him.

"Doesn't matter what maker or type ignition used, matters what you do with the timing and curves.

From the user feedback and hands-on fiddling on distributors I have set up for these engines, over 40 of them, the GM timing specs for idle are way too conservative, the total is too much, and the vacuum advance is connected to the wrong source with too many degrees of added timing.

I make the curve so the initial timing is set to 8 degrees/BTDC, and the vacuum advance adds 6 degrees at idle (full manifold vacuum), for a total of 14 idle degrees timing. Also, initial timing isn't just for starting, it is for stabile idle speeds and setting the start point for the mechanical curve to begin. Too far back (retarded), and the engine runs hotter, doesn't accelerate well on pick-up off idle and won't idle well at all. 8 degrees BTDC is not a problem with starting on any 502 engine.

I set mechanical curves to give 20 to 22 crankshaft degrees, mated with the 8 initial gives 28 to 30 degrees overall. I start the mechanical curve between 800/1000 rpms, depends on just which comp ratio, cam, etc., limit at 2400 rpms max. I have never seen the need for 36 degrees in one of these engines.

These timing numbers may sound very conservative to people used to shorter stroke cranks, but they are correct when the length of piston movement to degrees of crankshaft are matched to the max pressure area needed for best piston push. These numbers are close to where all engines need to be, just sound different because of mountain motor stroke lengths. Same thing happens with a stroker 350, the 383 engines, numbers change, curves are relative.

The full manifold vacuum advance allows the engine to pull against a converter much better and with a smoother idle than GM recommended specs. You also want the supplemental idle timing to fall off, as it does on full manifold vacuum, as the engine accelerates, allowing the mechanical advance to do it's job properly, advance the revs smoothly.

Ported supplemental vacuum timing adds too much timing in the mid and top rpm ranges, leaning the engine out and causing detonation and tuning/jetting issues.

As far as where the timing should light the mixture off is concerned, the whole function of spark advance is to get the maximum burn in the combustion chamber to happen off-idle at an optimim point between 10 and 15 degrees after TDC. Moving the timing is the only way to keep the proper expansion within this degree range as engine firing events narrow in time effectiveness (rpm increase). This area of crankshaft degrees is where the most expansion of fuel/air mixture benefits the push against the piston's downward force (burn rate). As the piston procedes down the cylinder, actual compression ratios decrease, but pressure expansion compression ratios (burning, expanding mixture) increase, causing more pressure on the poston, and making power.

The effective power production range of crankshaft degrees in one cylinder are just over 65 degrees. After that, the crankshaft has to move in a different direction (end of downward/outward movement) and starts to lose push effectiveness during this directional change. Since this rotational direction change happens near and through 90 degrees ATDC, the next cylinder in the firing order picks up where the last one diminishes, and power carry-over production and smoothness isn't diminished much.

Think of how the overlap of power pulses is in a 12 cylinder engine, now you know why those things are so smooth."

i could not post it better my self. he sayd it all. and the man know's hi stuff. he build's distributor's for a liveing. his bizz name is if i rember "DAVE's small HEI" or something like that. he make's an HEI that is mounted in a point's distributor so you can retro it and look like you still have point's.


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post #8 of 33 (permalink) Old Apr 9th, 01, 07:48 PM
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You still don't understand true ported vacuum is tapped in the venturi like an old VW or an early Chev 6. Chev v8 carbs don't do that. During cruise and wide open throttle they act the same whether the tap is above or below the throttle blade.

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post #9 of 33 (permalink) Old Apr 9th, 01, 08:41 PM
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Heres all the proof I need...

ported [email protected]

manifold [email protected]

I have always prefered ported vaccuum...very few engines I have built or tuned run have ran better with manifold. I am not an expert on the subject, but my belief is that with manifold vac, you have to set the initial timing lower to keep a reasonable idle speed...when you nail it, you lose the manifold vac and your engine is then running off of the initial timing alone...there isnt enough rpm yet for mechanical advance to come in to play yet. I may be way off base with this theory, but I know what works best for me.

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post #10 of 33 (permalink) Old Apr 9th, 01, 09:57 PM
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oger,

that depend's on where the orafice is located above the throtle plate. the closer to the venturie the more the signal, but when it come's in is diferant. the closer to the throtel plate the less signale but come's in sooner till you are in cruse mode, then the closer to the throtle plat the more like manifold vacuum you get in cruse(stronger signal). just look into a V-8 carb, the port on most hi-pro carb's like a demon or a holley are closer to the venturie. the port is not just next to the throtle plate. on a edelbrock/carter it is just above the throtle plate. on a Q-jet it depend's on the intended use of the carb. it only come's into play when you have the throtle's open at a fixed point and then get's stronger after that. it work's basicaly the same way the port for vacuum secondary's on a holley carb work's. if you mesure it at cruse on the high way it is less then manifold vacuum. i have done that on a demon, edelbrock and a q-jet. it very's depending on carb size, and make, but average i got was almost 6" less vacuum from ported verses manifold.except the edelbrock. there i only got about 2" less then manifold. the carb desiner places the ported port where it will do what they want when they want. you have no control over that.

travis,

the curve on your distributor has to be set up for manifold vacuum. if your vacuum pot is a OEM emision type they are set for ported, they start out at around 2" to 5" of vacuum and peak out at around 8" to 10" with between 20* to 40* at the crank. if you put it on manifold you have too much advance comeing in too quiley so you will see a drop in performance. and one run dose not prove the idea.

on my vortec motor with 9.1:1 CR it made me faster and quiker off the line, but only after i set it up for it. when is was set up for ported it was ping citey on manifold vacuum and a dog! i have 32* totale timeing at 3200RPM. my vacuum advacne is limited to 15*. my base timing is at 12*BTDC. my motor realy came alive after i set it up this way. so have a lot of other people's.

just switching to manifold with out seting your curve for it will give you what you got. it is not an apple to apple compairison.

i bet if you set your curve up for manifold vacuum you would get better off idle performance and mid and top end too. also your plug's would stay cleaner and you would get better gas milage.

i used a crane kit on my HEI folowed the direction's that came with it. a vortec motor perform's with less totale timing than other head's too. this has been proven on the dyno and the street by other's. that include's racer's.

i do not want to start a pissing match i just want people to know that there are other way's of doing thing's. if you feel ported is for you that is fine by me. i just feel it is not the way to go in a non-emision street motor. so do a lot of other people and some are very big name's in the performance bizz. but there are some that like ported too. i guess it is up to you.

i have alway's thought places like this where for info exchange and to give you other idea's. that is what i am doing. all so look at alot of sucesfull performance motor's i bet you will find most use manifold vacuum, unless you have such a big cam that your base timeing is in the 18* to 24* BTDC just to run and your idle is at 1000+ RPM and you are only pulling around 4" or 5" of vacuum at idle, then a vacuum advance is a way to get detonation, you use only mechanical. just look a all out drag motor, no vacuum advance. oh well i guess we have talked this to death, if you try it the right way cool if not cool too. me i will allways use manifold vacuum from now on. oh and my motor run's cooler at highway speed's with it too. and i gained gas mialge to the tune of 4MPG. i also tow much better now. it was a win situation for me.

but before you condem it try it the right way is all i ask.

oh and travis, after i did the move to manifold vacuum all my carb problem's went by-by. because my throtel plate's are now in what i call the sweet spot with just the right amount of tranfer slot showing and that is between .020" and .030". no more bog or stumble during transition when you punch it or roll it up slow under a load.

[This message has been edited by ilbl8 (edited 04-10-2001).]
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post #11 of 33 (permalink) Old Apr 10th, 01, 10:30 AM
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I decided to check just to make sure I wasn't nuts or something. Took the line off the vacuum guage on my car and hooked it to the ported source that my advance was hooked to. Idle 0 part throttle 18in or so same as manifold tap wide open throttle 0 just like manifold vacuum. The vacuum secondaries source on a Holley is in the booster area and does see high vacuum at wide open throttle.

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post #12 of 33 (permalink) Old Apr 10th, 01, 11:04 AM
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did you use 2 vacuum gages and see the diferance in the vacuum curve, when it start's with ported and manifold and compaire the too? just wondering because i did and it shocked me at how much advance i was getting from a ported vacuum advance when i was under a high load, and knock too from it.less power too then the way i have it now. i did this towing a 5000 pound travle trailer up a very steep hill at highway speed's. i had more power and ran cooler with manifold vacuum.

i bet your timed port is just above the B-fly. the higher up you go the less the signal. also the later it will come in. this is all relavant to the B-fly position at cruse too. in a larger carb, say a 750 or 850 the B=fly is not going to open as much as in a 600 or 650 at cruse. this is because of the larger bore and smaller venturie. oh and rember ported vacuum is there under hi-load but low manifold vacuum runing. this can lead to knock, there for i bet you use 92 or 93 octain fuel to help stop it. me i am knock free on 87 octain and i have lot's of power and i save at the pump. thuis is a a 9.1:1 CR with iron head's

most, i am not saying your motor, are over carbed. the larger carb is better at WOT because of the less resistance to flow and that give you more air into the motor with les efort on the motor to get it. a smaler carb will give you better performance in general because for a carb to work best it need's 2 thing's, flow and vacuum. that is how it meter's fuel. but oh well, like i said it depend's on some variable's, like the mechanical curve and vacuum pot you use.

manifold vacuum set is a suplement to and to enhance the faster mechanical advance curve. ported, as use by the car maker's since emision's is part of the curve, not a enhancement it. this is because of the way a emision system work's it is not there to make more power but to give you a cleaner burn only. on the low CR stock motor's.

as we all know a cleaner burn is not alway's the most power, in fact a slightly lean mix with the spark advance fairly hi give's the best power, you realy have to push the line on detonation to do this.

all i am saying is useing manifold vacuum and seting your cure up for it will give you more power across the whole RPM band, and better fuel burn too. it has been proven by alot of people over and over again. this is only on a non emision motor. if you have a cat, EGR and a air pump you need the lazey mechanical curve and ported vacuum to make it all work right. also rember most emision carb's are tuned lean and the ported vacuum give's it a big help at makeing some power.

i personaly dought i will change your mind, all i ask is look at it, heck maybe try it. you might be suprised by"thinking out of the box" that the EPA and other organization's have forced on us.

ported was used in the 60's too on performance motor's but, the fuel was of a higer quality and more stable, it also resisted pinging much better so the added advance at higher load's was a good thing. not with the fuel we have to use now, it burn's hotter and is les stabile. all so you need more of it to get the same power then you used to need as gas has a lot more lighter chemical's in it to help it burn hotter and cleaner then the gas in the 60's.

oh well. just look at the bigger picture, is all i ask. and do the #'s you might be suprised.

by the way if you use a hotter therm, like a 195* therm, manifold vacuum will give you a better chance ot detonation too, i say use a 185* or a 165* high flow therm. manifold vacuum tend's to run hotter then ported, and heat=power in a gas motor. in my mind our goal is to build a powerfull motor and to run it on pump gas and be knock free while doing it. i feel manifold vacuum and the faster mechanical curve is the right way to go.

you are free to disagree with me. but do not condem me, untell you try it and set it up right. is all i ask. this will usaly mean recurving your mechanical advacne to come in faster and to have less vacuum advance and to come in at a higher vacuum leval then ported. mine come's in at around 8" and peak's at around 13". my motor under high load, like pulling up a hill in 3rd gear has about 10" of vacuum, i do not ping this way, but with a faster pot and a higher vacuum advacne i would unless i am useing ported vacuum with it and a lazyer mechanical advance. but i get more power cause my motor is not depending on the vacuum advance as much. it is allready there via the mechanical.

oh well happy motoring!

[This message has been edited by ilbl8 (edited 04-10-2001).]

[This message has been edited by ilbl8 (edited 04-10-2001).]
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post #13 of 33 (permalink) Old Apr 10th, 01, 11:35 AM
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The vacuum that operates the secondaries in a holley comes from the pri and secondary throttle bores. There are venturies (passages) that feed the diaphram.

The ported/non-ported issue has always been a hot topic around here and our sister site www.chevelles.com . Remember factory spec's call for ported to aid in keeping polution numbers down, They also kept the initial timing settings very low compared to what most of us use today.

Non-ported will assist starting, over heating at idle and help with a performance engine's pulling against the torque converter do to higher idle. As I see it non-ported vacuum allows the performance oriented a best of both worlds advance system. Full mechanical advance is prefered on the track but doesn't provide daily driver properties on the street.

Using a non-ported vac sourse allows for a lower inital timing for ease of starting but once the engine is at idle the the vac brings the inital up to a level that provides optimal cooling and aids the pull against the torque converter, hense smoother idle/off idle street performance. When you jump on it, it's more likke full mechanical.

What I think has been overlooked and will have a big impact is the vac advance must be reduced to about 8deg's. If your curve is setup properly and comes on about 800-1000rpm and is all in by 2600-3000rpm you won't be seeing the timing drop before climbing again when the vac goes away with full throttle.

Taking a stock dist curve with a stock vac can and applying non-ported vacuum may indeed provide less than desireable results. It's a package deal. Non-ported vac, 20-22deg mechanical before 3000rpm, 12deg inital and 8deg of vacuum advance that goes away at WOT. The results are 20deg at idle,(without hard starts)34deg total in before 3000rpm.

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post #14 of 33 (permalink) Old Apr 10th, 01, 09:38 PM
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I am not saying that your way is wrong...but believe me, I have tried everything possible to get my truck into the 14's on tuning alone...I have put over 200 passes on this thing in the last 2 years...different advance curves, initial/total timing settings, jetting, metering rods, step up springs, spark plugs, etc...no matter what I did, the truck flat did not like manifold vaccuum, period. My truck is gear limited...no doubt about it, and could use a little more stall. I can use 87 octane if needed (I use 89 for saftey margin) with no ping, and I get as high as 15.5 mpg at 65-70 mph on the highway (2600-2800 rpms). I have no smog devices of any kind...not even egr...and even on long hard pulls with my camping trailer this thing doesnt come close to overheating or pinging. I am going to have to change something to get 14's...but for now, I am not convinced that manifold vaccum is best. Noone can make a blanket statement saying that manifold or ported is best because there are too many variables invoved.

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post #15 of 33 (permalink) Old Apr 11th, 01, 12:36 PM
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I think that people are using manifold vacuum to cover a lean problem in the idle and off idle circuits. If it works sure beats messing with the carb untill you get it right if you ever do.

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