Hot Rod Forum banner
1 - 15 of 15 Posts

·
Registered
Joined
·
7,345 Posts
Advance

Doc here::pimp:

It might, but on a carb engine, I would suspect a primary pump more than advance....

One way to test your advance, and the easiest is, find a full manifold source of vacuum on your engine manifold, and route it to your advance while running the engine.

If it is working, the engine will speed up when you hook it up, If it isn't nothing will change except for the idle slow down when you hook up the vacuum to the advance(no vacuum leak).

If the Diaphragm is blown on the advance (vacuum leak), then there will be no speed change at all with it hooked up or with the open vacuum line.

Another way to test it if you have a hand vacuum pump/gauge, is pull the cap, hook the pump up to the advance unit and watch the advance plate, It should start to pull around 5 in hg, and fully deflect somewhere around 10 in hg. If it does, it's good.

Doc :pimp:
 

·
Registered
Joined
·
302 Posts
Discussion Starter · #6 ·
Ok I did that and my tongue stayed on the vacume line just fine. When I sucked on it I could hear the diaphram going in and out. Here's the thing that puzzles me. I have a brand new Holley 670 Street Avenger. Under an idle when I pull the vacume line off the carb (where Holley says to put it) there is no change to the rpm of the engine. If I put my finger over the outlet on the carb and increase the rpm there is very little vacume. There reason I asked is I get a stumble when taking off and when I'm driving down the road and give it some throttle it stumbles then catches and goes fine. It drives me to drinking. I've checked everything. The weird part is the Edelbrock carb I took off also stumbled from a stop but once you got going it was fine and even had good throttle response when you jumped on it. I don't have a lot of hair to pull out so it would be nice after spending the money to have it work right. I would have thought a brand new carb would work great.
 

·
Registered
Joined
·
7,345 Posts
ported Vacuum source

Doc here:pimp:

The reason there is no Vacuum at idle at that port on the carb is, it is a ported vacuum source.

It will not create any vacuum until air velocity is built up by passing through the throttle body (partially open butterflys), and past the ports.

If you want to eliminate the possibility all together, plug the ported vacuum source and run a full manifold vacuum source (it responds better anyway)

If it runs better, stay with it.

If not, I'd still look at the fuel system, pressure, pin hole leaks in the line, fuel pump failing, clogged filters and primary pump on the carb.

Doc :pimp:
 

·
Registered
Joined
·
302 Posts
Discussion Starter · #8 ·
Thanks Doc. The filters are new, fuel lines are new, I run a Carter P4070 electric fuel pump that is about 5 years old I figured if an electric fuel pump was wearing out it would result in poor performance at higher RPMs. I'll try your suggestion in relocating a new vacume soarce. I was told by many not to plumb into my intake manifold but there was no reason given.
 

·
Premium Member
Joined
·
3,519 Posts
Vacuum advance is only activated at part throttle and really only exists to keep your plugs clean. I doubt it adds any power, at WOT your motor has 0 advance from vacuum, just initial and mechanical.

Motor should be 8-12 degrees initial, 22-24 mechanical (for a total of 30-36 or 38 degrees). Vacuum advance actually pushes your part throttle advance in to the 40 degree range (like about 44 degrees of total advance or so).

K
 

·
Registered
Joined
·
159 Posts
[email][email protected][/email] said:
You can check it easily by taking the distributor cap off and sucking on the vac advance hose. It is fairly easy to move it and if the diaphragm is busted, you will suck a lot of air.
I Agree with that method ,,if you can sudk vac advance by mouth
you should get low vac advance.,

as i have found after blaming carbs is ,, .. Fast vac advance at low vac depending,,.. on cams etd is inportant ,,
 

·
Registered
Joined
·
302 Posts
Discussion Starter · #12 ·
Thanks all I will be spending today with your advice. This is starting to make sense as my anitial shot of throttle at any speed creates a stumble then catches and pulls fine. I just figured that this area should be right on if the carb came right out of the box new. Maybe I'll phone and get the CEO of Holley over to correct this :p
 

·
Move It On Over!
Joined
·
227 Posts
Might want to try advancing your timing a little more. I had the same kind of problem - it would stumble a WOT from a stop for about 1/2 a second and then get going. Advancing a couple more degrees did the trick - no hesitation at all.
 

·
Registered
Joined
·
302 Posts
Discussion Starter · #15 ·
Pas55 I did everything with the vacuum advance answers and found out that was all fine. I even went to the manifold for direct vacuum like someone else suggested (thank you for that) and the engine ran terrible. I took your suggestion and increased the pump shot from .031 which is stock for the 670 Street Avenger to .035 and although I don't want to try it on the road (street rod) as the weather has really turned, one thing I noticed right away was when I rev the engine from an idle I seem to have a lot better response. It was sort of soft and slow responding. Hope this works and the next dry day I will try it out. Let you all know and thanks.
 
1 - 15 of 15 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top