Street Avenger 770 and 408W

[MikeStang 247]

Ok so I got my car up and going and I'm trying to tie up loose ends to get ready for Cruising the coast, anyhow let me give ya back ground.

 

Motor is 351 Roller Stroked to 408 Compression about 9.8 to 1 Alum heads etc... When I had the motor built we dynoed it with my HP 950 and it made 550hp at the crank...Fast forward about 3.5 years and its together, but I dont have the HP 950 however I bought a bunch of Ford Parts CHEAP and this Holley 770 Street Avenger with Vac advance and Electric choke was in the deal BRAND NEW never bolted on.

 

So I see its got some dirt dobber nests in the carb from sitting, so I take it down and clean them all out and bolt it back on and turn the key, the motor fires off instantly and Idles like a champ with the Electric choke.

However its running rich as hell at Idle and when I step off in it it has that standard Holley stumble then It takes off but with what I feel is far less than 550Hp, I attribute some of this to the fact that I have 3.25 gears "I was gonna build a C6" and a stock torque converter in the 4R70W... Trans is built but converter is stock and flashes to about 2800...

 

Now the motor has an MSD dillet Dist and I never touched the timing and the car starts instantly every time with no hesitation Hot OR Cold.

and no pinging at all.

 

I would love to get rid of that rich exhaust at idle and kill the flat spot, but im not sure where to begin with this Vac secondary carb.

I have adjusted the carb as I always do and set the idle and in gear it will Idle about 600 Rpms with a nice heavy lope.

 

Not sure if I should mess with the timing first or drop another spring in the Vac Secondaries, or Go up 3 sizes on squirters as holley states?

 

Whats yall's opinions

[barnett468 418]

hello,

 

 

what is your timing at idle with the dist vac connected AND disconnected?

 

are you using ported or manifold vac for adv?

 

you most likely can not totally eliminate a flat spot because your gears are far to tall. you must have at least 3.73 gears with that engine and 3.91 would be better.

 

i would run a 6.5 power valve and set the gas level in the carb so it is just below the inspection windows when the engine is running.

 

look in the carb and open the throttle just a tiny hair. if you do nit see a stream of gas squirt out your accelerator pump is dried out or the pump screw needs to be turned clockwise a little.

 

reduce and plug the dist vac advance and advance the timing until the rpm quits increasing by large amounts or until the engine stumbles. leave the vacuum off.

 

if need be put the stiffest spring inn the secondaries.

 

tell us the results.

 

.

[juit 34]

600 rpm wow that's kinda low, mine I only be able to do it streetable on 700

whats your vac readings?

a video will help a lot

 

also call holley they could give you a lot of pointers to tackle the altitude changes

Edited October 2, 2014 by juit

[1969Fstback 58]

I have a 570 on a mostly stock 302. As soon as I touched the pedal the seocondary would open and I could smell gas. Put the stiffer spring in and fixed that. I adjused the accelerator pump and got the dead spot gone. But my powertrain is much simpliar than yours. I am still rich at idle. Partly I think due to bad plugs. I have new ones but have not had time to change them. I figure they are done from the raw gas on them from the secondary. I am a little rich into it to, if plugs don't fix that, I am going down 2 jets sizes.

[MikeStang 247]

My Dist is an MSD Billet Non Vac Advance unit... I think the total timing was 34 degrees and that is where it made the most power at when we ran it on the dyno.

Seems like my secondaries open to slow in my opinion when I mash on it hard...

[MikeStang 247]

Barnette The pump shot is nuts on the money so far as timing is concerned, but may need bigger squirters...I believe the 770 comes stock with a 6.5 PV

[MikeStang 247]

And yeah I will be doing a gear swap, but when I built the rear end I had no intention of using a 4R70W....Which by the way works perfectly with the Bauman controller... and talk about a hard 1-2 shift OMG I LUVVV it......

Took it for a ride for the 1st time and i kept waiting for it to shift and finally at 3k i eased off of it because I thought something was wrong and soon as I did it shifted...

so i played with the new shifter a bit and figured it out...yeah it shifts at almost 3k easy driving and it shifts hard..may have to work on that

[sportsroof69 13]

I'm just curious why you think the secondaries open too slow? The only reason I ask, is many people are under the impression that you should get a sudden power increase, or feel them open, which is totally untrue. With a properly tuned vacuum secondary carb, it should be a smooth transition that you never feel.

[barnett468 418]

Barnette The pump shot is nuts on the money so far as timing is concerned, but may need bigger squirters...I believe the 770 comes stock with a 6.5 PV

 

yes the 6.5 is what they come with but you implied it was used so if this is the case it might have been changed or dried up which can cause a hesitation under some conditions. if the valve is old and unknown you should change it before doing anything else.

[barnett468 418]

My Dist is an MSD Billet Non Vac Advance unit... I think the total timing was 34 degrees and that is where it made the most power at when we ran it on the dyno.

ok that is fine but if your timing at idle is too low it can have a hesitation off idle. i would run as much idle timing as i can. if it likes more idle timing than just reduce your mechanical advance.

 

 

Seems like my secondaries open to slow in my opinion when I mash on it hard...

easy deal. buy the optional spring kit and throw the white spring in it. if they open too soon then try heavier ones. if the still open too late with the white spring you are unfortunately sol.

[MikeStang 247]

Will do.

The Carb was brand new but just sat in the box open.

Ill swap the PV and get a 5.0 and 4.5 in case.

I have 2 extra springs that came with the carb as well as the directions and video etc...

Gonna play with it this weekend

[1969Fstback 58]

Will do.

The Carb was brand new but just sat in the box open.

Ill swap the PV and get a 5.0 and 4.5 in case.

I have 2 extra springs that came with the carb as well as the directions and video etc...

Gonna play with it this weekend

 

How much vacuum are you producing? The guidelines state take vacuum divide by half. So 10' would be a 5.0 PV. Anything above 13' would be 6.5.

[barnett468 418]

How much vacuum are you producing? The guidelines state take vacuum divide by half. So 10' would be a 5.0 PV. Anything above 13' would be 6.5.

actually this is not how it should be done and holley actually gives three titally different ways to do it so its easy to understand how people can get confused. 99.9% of the carbs run fine with a 6.5 not matter what the engine vac is. its a bit complicated..

[barnett468 418]

Will do.

The Carb was brand new but just sat in the box open.

Ill swap the PV and get a 5.0 and 4.5 in case.

I have 2 extra springs that came with the carb as well as the directions and video etc...

Gonna play with it this weekend

why do you want to run a leaner power valve?

 

also, those carbs are jetted a hair on the lean side. i would think that your jets should be at least around 72 front and 76 rear.

Edited October 3, 2014 by barnett468

[MikeStang 247]

I don't WANT to run a leaner one, just would like to have them just in case so I dont have to make another trip to the store HAHa.

I was under the same impression you take your vac and divide in half and that is the PV u use...If I recall that is what it even says in my carb tuning video.

I have done lots of carbs in my life but aside from Quadrajets I have never tried to use a Vac carb for a performance application.

[barnett468 418]

I don't WANT to run a leaner one, just would like to have them just in case so I dont have to make another trip to the store HAHa.

I was under the same impression you take your vac and divide in half and that is the PV u use...If I recall that is what it even says in my carb tuning video.

I have done lots of carbs in my life but aside from Quadrajets I have never tried to use a Vac carb for a performance application.

ok, as i said, its complicated. you can do what you like, trying it will not hurt but they are WRONG, lol. also the power valve has nothing to do with idle. it basically only opens when your foot is on the floor. if you have a vacuum gauge in your car you can watch exactly when it opens and possibly feel what it does.

[1969Fstback 58]

actually this is not how it should be done and holley actually gives three titally different ways to do it so its easy to understand how people can get confused. 99.9% of the carbs run fine with a 6.5 not matter what the engine vac is. its a bit complicated..

 

 

Vacuum holds the valve closed as idle when vacuum is at its max. If you are running 13' or more regardless if you go smaller it will remain closed at idle. When you accelerate vacuum goes down allowing the valve to open and fuel to go through.

 

If you have a large cam and only runnin 9' of vacuum and have a 6.5 valve then the motor will be flooded at idle do to the valve not being closed.

 

6.5 works for most engines due to most having vacuum over 13'.

 

it's not that complicated or confusing.

[barnett468 418]

If you have a large cam and only runnin 9' of vacuum and have a 6.5 valve then the motor will be flooded at idle do to the valve not being closed.

actually this is incorrect. the vacuum must get down to 6.5 to open a 6.5 valve.

[Max Power 74]

You need to measure vacuum at idle in gear. Whatever your vacuum reading is, your power valve should be rated at half. 10" at idle means you need a PV rating of 5, but they don't make a 5, so a 4.5 PV is what you need. Otherwise you get too much enrichment.

 

Your rear gears are fine, they are not the cause of any of your problems. Deep gears help mask a bog, but they aren't the cause of it.

 

It is also rare that your secondaries are opening too early, although getting accurate vacuum readings would help you know that. Usually factory Holley setting for secondaries are pretty conservative, so I doubt you are tapping the secondaries at anything close to idle.

 

Also, nobody here asked about your cam, which surprises me. You mention you have a strong lope at idle (and enough cam to make 550hp), which suggests to me that you had to set your idle up enough that you are exposing the transfer slots. (The transfer slots are what transitions the idle circuits to the cruise circuits) You can verify by looking down the throat of the carb. If you are into the transfer slots just to get it to idle, there is little that the idle circuit can do for you. You are pulling largely from a nonadjustable fuel circuit, and the is just a byproduct if big cam cars, especially with automatics. You just have to live a little rich with big cams in these things.

 

If you aren't into the transfer slots, adjust your idle circuit in gear to the highest vacuum reading.

 

Edit: I guess I posted this without seeing page 2. Sorry for the redundancies.

[Max Power 74]

actually this is incorrect. the vacuum must get down to 6.5 to open a 6.5 valve.

 

This is straight from Holley:

 

https://ac17cb7e3cc3506f0d23-7386afb7d61f5e5af0e5a817d2877bfe.ssl.cf1.rackcdn.com/techlibrary_power_valves.pdf

[Max Power 74]

 

also call holley they could give you a lot of pointers to tackle the altitude changes

 

What?

 

OP lives in Louisiana.

[sportsroof69 13]

This is straight from Holley:

 

https://ac17cb7e3cc3506f0d23-7386afb7d61f5e5af0e5a817d2877bfe.ssl.cf1.rackcdn.com/techlibrary_power_valves.pdf

 

You're wasting your time with this one......

[Max Power 74]

Lol....ok.

[buening 63]

I've found the following beneficial in tuning Holleys: http://www.nastyz28.com/~ericf/tech/htune.pdf

 

Have you pulled the bowls to see if it has stock jets? I'd put larger secondary jets in to see if it gets rid of the bog. That bog is a classic sign of being lean. My 670 Street Avenger had a bad bog and the larger jets fixed it. Larger squirters are typically the last thing to mess with.

[MikeStang 247]

Well I made a few changes last night.

When we dynoed the motor It was running NO PCV System, just baffeled covers with one breather in each valve cover.

 

I pulled a breather and popped in a PCV valve and hooked it to the carb location for PCV.

I then checked the timing and it was 10 degrees so I bumped it up to 12 degrees and re-adjusted the carb and idled it down some more.

I was not in the transfer slots, I ALWAYS turn screw all the way out and then work my way up and pay close attention to transfers...usually you can tell when your into them because when you kill it the motor will want to run on.

 

Anyhow after doing that its not near as rich as it was but still noticeable.

I checked Vac in park and it was about 11.5 - 12" needle wont stay still, it just vibrates really fast LOL.....1/2 degree or so.

 

I did pop the vac secondary spring out only to find it HAD been swapped to the lightest one, so I put the factory one in and need to go try it out...

 

If that dont help and I have issues I will pull it back off and double check the jets etc to be sure they have not been changed.