General Tech Good at troubleshooting? Have a non specific issue? Discuss general tech topics here.

Carb. flat spot

Thread Tools
 
Search this Thread
 
  #11  
Old 09-25-2008 | 10:58 PM
t9ojetpilot's Avatar
2nd Gear member
Joined: Feb 2007
Posts: 518
Default RE: Carb. flat spot

oh and by the by nice looking motor in that camaro!! i built a similar set up years ago and they go like a raped ape. i think your air filters may be a bit restrictive though for the needs of that combo
 
  #12  
Old 09-28-2008 | 02:24 AM
Camaro 69's Avatar
Senior Moderator
January 2010 ROTM Winner
Joined: Oct 2007
Posts: 18,306
From: The 'Burbs of Chicago
Default RE: Carb. flat spot

1. Thanks.
2. I know, that is an old picture and I put those filters on only for a short time. Last summer I put on a bird-catcher (just to add a little more flair). Today I finally had the car, my camera, and the sun outside all at the same time so I could take some updated "under-hood" (and I use that term loosely) shots. Updated signature picture will be forthcoming for your perusing pleasure!
3. About the carb/distributor...check page 2: Edelbrock Carb Instructions
I find it strange that Edelbrock offers both ways to hook up the vacuum advance, and nowhere does it say that manifold vacuum is a no-no! It's also strange that the Edelbrock carb has a vacuum port on the carb (front passenger side-full vacuum) for the specific reason of hooking it up to the distributor as described in their instructions.
Another thing that's strange is, since the beginning of time (almost), the method of setting engine timing always required you (by instruction) to disconnect the vacuum hose first. Why was that required since there is Zero vacuum coming from the ported line on the carb at idle anyway? The reason is because those were pre-emission (mid-1960's and older) hand-me-down instructions that actually meant something when the distributor used to be hooked up to manifold vacuum.
To clarify some points that I made that didn't seem to come across...
If you hook up to full manifold vacuum, one of the things you might need to do (if you can't idle it down enough) would be to tune your vacuum canister, either with a different one or an adjustable one. And then tune the distributor to mechanically pick up where the vacuum leaves off.
But, you ask, what if anything would be the advantage of hooking up to manifold vacuum? (I knew you were going to ask)!
a) If you have a larger cam but still want a vacuum advance, manifold vacuum would most likely be the better way to go, both for getting a decent vacuum source to begin with (more than the ported carb fitting can supply), and to also smooth out the idle. Of course, if your cam is so wild as to produce no vacuum, then you would need to go the full mechanical route.
b) On a manual trans car for instance, manifold vacuum could be a benefit because as the rpm's drop between gears, the vacuum will hold the timing more advanced than if it was hooked up to ported vacuum which obviously drops the advance considerably. There would be less power lag between gears with manifold vacuum. Now, if you're real good at power shifting, that becomes less of an importance.
c) I never did say full manifold was The way to go. And I'm not saying you said I said it was either! For a "normal" street car, more than likely ported vacuum is the way to go. All I was saying to begin with is that there's another option, something else to experiment with and test out.
 
  #13  
Old 09-28-2008 | 03:59 PM
t9ojetpilot's Avatar
2nd Gear member
Joined: Feb 2007
Posts: 518
Default RE: Carb. flat spot

[&:] i guess im still missing or failing to comprehend the validity of your point. ill make a couple points and well go from there. first the edelbrock instructions: remember this is a universal carb designed to fit both stock and aftermarket applications and millions of them. when i read the instructions you have cited i get that they are instructing you on stock(emissions or non emissions options) replacement positioning of the dist line and not instructing you where to place it in an aftermarket application as no aftermarket(performance) application that i know of uses manifold for timing purposes( if you have knowledge of this please enlighten me) you have to have the full emissions package for this manifold vacuum set up to work right!

we both know that manifold vacuum goes away with engine load and is not good to begin with in engines that are much over stock. we need vacuum as load increases to advance the timing and since manifold vacuum is going away ported is the only option. now this will most likely only benefit near stock or moderate?? engine combos and anything with much cam will have to go mechanical.

I do remember and have seen many tune up instruction mention to remove and plug dist line but all that i can think of came off a ported source or was one of the emissions versions and they didnt want an elevated idle speed to produce ported vacuum and throw off your timing when you set it. ill look through some of my 50-60s service manuals and see what they say

i know you dont have any of the vacuum scenarios you mentioned on your car , especially not vacuum advance of any kind as this combo has no vacuum if set up right so i invite you to show me or tell me where to find examples of these possibilities as i have just never seen them. i cant seem to think of a scenario where it would work in a beneficial or power producing if not robbing manner. vacuum is highest at idle and in order to use manifold vacuum advance here you have to retard base timing to compensate and this will limit total advance and drop the vacuum advance that is used at idle at part throttle which is when you need it most. as you know chevrolet likes 36 degrees of advance total and i know of no distributors for gm that allow more than this unless modified. so you cant go after tdc( to compensate for the manifold advance) and get all the advance you need. please please find me some examples as im always learning and this just isnt making sense to me kurt
 
Related Topics
Thread
Thread Starter
Forum
Replies
Last Post
pezlow
2010+ General
6
07-26-2010 12:52 PM
blackbird22
93-02 V6 Tech
6
07-20-2007 02:09 PM
thebandit110
Detailing
3
05-02-2007 04:44 AM
mcmcmahon
70-81 General
5
01-22-2007 11:22 PM
PyroFox04
LS Series Tech
6
11-14-2006 06:58 PM



Quick Reply: Carb. flat spot



All times are GMT -5. The time now is 02:59 PM.