Carburetor Question

[JimNolan]

I've put some miles on the car now and I've brought it up to 4500 rpm a few times. When doing this I questioned if the secondary on my 750 was really kicking in. The acceleration was smooth but I didn't think the four barrel was kicking in. I put a quick change top on the Secondary Valve Housing and installed a lighter spring. NOW, the secondary kicking in is more pronounced, better acceleration at lower rpm's but you can feel a pulsating in the seat of your pants while it's screaming forward. It seems to be consistent also. HELP what do you guys think.

[machoneman]

Sometimes.....a bad spring will allow flutter (fluttering?) of the secondary throttle plate. Try the next stiffer spring (or ligther still) and see if it goes away.

[bartlett]

purple spring ...

[jayb]

I think you need a carb with mechanical secondaries   I know, I'm no help...

Here's a couple of ideas.  If you have access to a GoPro camera, mount it under the hood so that you can see the rod of the secondary diaphragm, and record what is happening.  Or, if you have access to a G-Tech meter, try the different springs and compare the 30-60 MPH times in one gear.  Go with the spring that gives you the fastest acceleration.  Or try the "old-timer's" method, which is to cut a "washer" out of an index card so that the hole in the middle fits tightly over the secondary diaphragm rod, and the outside is maybe 3/8" in diameter.  Unclip the secondary diaphragm rod and install the washer, pushing it all the way up the rod until it contacts the body of the secondary diaphragm housing, then go out and make a run.  If the secondaries are opening the washer will have been forced down the rod as the rod is pulled up into the secondary diaphragm housing.  You will be able to tell how far they are opening by the position of the washer.

[My427stang]

I think you need a carb with mechanical secondaries   I know, I'm no help...

I agree

However, as far as your issue, go one spring tighter.  Also make sure that when you changed the cover you didnt lose a corner of the diaphragm, sometimes they can slip when tightening

[JimNolan]

Thanks guys,
    I'm running an open air cleaner and Holley says that's the same thing as running a stiffer spring. I'll go another step lighter and see if that don't fix it. I was real careful to get the lid on straight as I've pinched the diaphragm before. Again, thanks.

[amdscooter]

Hey Jim, glad to hear you are up and running! Didya ever get a video posted? I'd be real interested to see/hear what she sounds like with our builds being so similar. What did you end up going with exhaust wise?

On a carburetor related note... (and hijacking your thread a bit.. sorry Jim  )  I was wondering what the "mechanical secondaries" crowd recommended? I'm thinking my 600cfm vacuum secondary Holley is a bit smallish even for my mild 390 build and have been shopping Holley 750 double pumpers. As an example, would something like this:

Holley 0-76750BL Model 4150 Ultra Carburetor Blue 750 CFM Double Pumper
http://www.ebay.com/itm/380405064705?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649

Be sufficient? If not what would be a suitable carburetor that you'd recommend? Mild 390 268H cam, Streetmaster, headers, C6 auto. Specific part numbers or linky dinks appreciated.   

[ScotiaFE]

It depends.
The DP's are great and really put the power down, but you can do the same with a vacuum carb.
And get a bit better milage. Also if your driving tends to be in town with heavy traffic then the DP is a bit much
for a mild 390.
Here's one with the same colour combo.
It is a Street Avenger and I've had great success with them.

http://www.summitracing.com/parts/hly-0-86770bl/media/images

[JimNolan]

Guys,
  Thanks everyone for responding. I'm happy, I'm happy, I'm happy. That sums it up. Went out a couple hours ago and changed my spring to the long yellow one. Reset the idle mixtures with a vacuum gauge. 15" is the most it'd give me. It idles smooth, real smooth. My initial timing is set to 14 degrees with the mechanical welded up for a 10L weight. While I was at it I reset the primary and secondary float bowls have the gas just at the bottom of each glass.
  I buttoned it all up and took it out on the highway. I rolled out in 1st, punched it and power shifted thru three gears to 4500rpm. There were no bogs, no nothing, it was just shit and get it. It's a lot more than what I had in my 300hp 390 but I still don't think it's 415hp like my calculators say. It's more than what my 66 Fairlane GT in 1966 had but I'd not think it's over 375hp. I know one thing. I wouldn't change any part of this build. It runs absolutely fine, cruising to the Drive-In Movie last night it showed how smooth it was on the highway. It'll still be a comfort and enjoyment to drive. I doubt I'll get 20 mpg like I did on the other engine, but I don't care. It'll take more than a paint job to run off and leave me.
  O yes, the sound. It sounds healthy with the exhaust maybe hinting of a large motor but when you turn it loose you'll hear it without any problems a mile away. Sounds good when you're on it. It's twice as loud idling as the other motor. Thanks, Jim  Machoneman,  you were spot on.

[bartlett]

4,500 rpm ?  ....Let the big dog eat ... 6,000 is more like it 

just givin ya a hard time ... sounds like your getting it dialed in .

[JimNolan]

On the way to dinner this afternoon I was doing about 35mph in 4th gear and kicked it to the floor. It accelerated without hesitation and I'd say at about 55mph you could feel the secondary kick in, it wasn't a recognizably drastic change but one that you could feel and it continued to accelerate very well. I'm done. Nothing gets changed. I thought maybe the 750 was going to be a little much for my engine but I think it fits it very well. I think the vacuum secondary is a wise choice if you spend most of your time traveling or cruising. I've never had a mechanical advance before but I can see how accelerating the car at slower speeds could cause problems with a mechanical secondary. It seems to be doing everything it suppose to without any sort of hesitations or burps. Thanks again. Another satisfied customer. See you guys April 27th at Beaver Springs.

[Monzy]

I'd say if you've got mechanical secondaries you need to know when you can hit the gas and when you shouldn't.  4th or 5th seeing as you have a tko wouldn't go over very well if you hit the gas to hard at low speeds.  You've got to down shift or flood it out.  I like my mechanical secondaries because I know exactly when they're on or off.

[jayb]

I wish I knew why mechanical secondaries on an automatic car get such a bad rap.  The old rule of thumb that you should use vacuum secondaries on an automatic and mechanical secondaries on a stick car is a really gross oversimplification, IMO.  I like the positive action of mechanical secondaries, and I've never had a problem tuning a mechanical secondary carb to work on an automatic car.  Jim's combination is pretty mild, and the vacuum secondaries obviously work well, but mechanical secondaries would work fine too.  And I've always found that while vacuum secondary carbs need some signficant tuning to work on most combinations, mechanical secondary carbs are pretty much a bolt on and go proposition.The only thing they really give up is mileage (and who cares about that LOL!).

[amdscooter]

Guys,
  Thanks everyone for responding. I'm happy, I'm happy, I'm happy. That sums it up. Went out a couple hours ago and changed my spring to the long yellow one. Reset the idle mixtures with a vacuum gauge. 15" is the most it'd give me. It idles smooth, real smooth. My initial timing is set to 14 degrees with the mechanical welded up for a 10L weight. While I was at it I reset the primary and secondary float bowls have the gas just at the bottom of each glass.
  I buttoned it all up and took it out on the highway. I rolled out in 1st, punched it and power shifted thru three gears to 4500rpm. There were no bogs, no nothing, it was just shit and get it. It's a lot more than what I had in my 300hp 390 but I still don't think it's 415hp like my calculators say. It's more than what my 66 Fairlane GT in 1966 had but I'd not think it's over 375hp. I know one thing. I wouldn't change any part of this build. It runs absolutely fine, cruising to the Drive-In Movie last night it showed how smooth it was on the highway. It'll still be a comfort and enjoyment to drive. I doubt I'll get 20 mpg like I did on the other engine, but I don't care. It'll take more than a paint job to run off and leave me.
  O yes, the sound. It sounds healthy with the exhaust maybe hinting of a large motor but when you turn it loose you'll hear it without any problems a mile away. Sounds good when you're on it. It's twice as loud idling as the other motor. Thanks, Jim  Machoneman,  you were spot on.

Nice Jim! About your timing though, am I mis-reading you here? 14 initial and a 10L weight = 24 degrees total advance?  I'm rolling over to the parts store today to pick up a vacuum gauge as I still cannot locate mine. Sounds like you've got her dialed in pretty nicely. I still have a bit of hesitation coming off the line, sometimes I need to feather the throttle a bit to get her rolling. But much like yours, once she gets rolling the acceleration is pretty good. I think the larger double pumper would go a long way to fix most of that issue. 

Had a guy pull up next to me in a "70" 440 Roadrunner last weekend on the El Camino. He wanted to go... I dropped it into 2nd and mashed the pedal for about 3 seconds before better judgement took over. But for those 3 seconds... 

[amdscooter]

I wish I knew why mechanical secondaries on an automatic car get such a bad rap.  The old rule of thumb that you should use vacuum secondaries on an automatic and mechanical secondaries on a stick car is a really gross oversimplification, IMO.  I like the positive action of mechanical secondaries, and I've never had a problem tuning a mechanical secondary carb to work on an automatic car.  Jim's combination is pretty mild, and the vacuum secondaries obviously work well, but mechanical secondaries would work fine too.  And I've always found that while vacuum secondary carbs need some signficant tuning to work on most combinations, mechanical secondary carbs are pretty much a bolt on and go proposition.The only thing they really give up is mileage (and who cares about that LOL!).

I've come around to that same mindset after reading about the last few weeks. I know a lot of folks say the vacuum secondary carbs can be dialed in nicely.. and I believe them. But the time I'm spending under the hood trying to get my current 600cfm Holley vacuum secondary carb dialed in is time I feel would be better spent just driving the car. I'm probably going to order a 750~800cfm 4150 this week. If I lose a few mpg in the proposition in exchange for some really good off the line performance and a whole lot less tweaking it's a fair trade to me. To be honest I could not tell you what kind of MPG I get. My mindset when I get into the Fairlane is when it gets low on fuel.. I put more in. 

[jayb]

If your build is similar to Jim's, I'd get the 750, or maybe even a 700.  I think an 800 may be a little big...

[JimNolan]

I wish I knew why mechanical secondaries on an automatic car get such a bad rap.  The old rule of thumb that you should use vacuum secondaries on an automatic and mechanical secondaries on a stick car is a really gross oversimplification, IMO.  I like the positive action of mechanical secondaries, and I've never had a problem tuning a mechanical secondary carb to work on an automatic car.  Jim's combination is pretty mild, and the vacuum secondaries obviously work well, but mechanical secondaries would work fine too.  And I've always found that while vacuum secondary carbs need some signficant tuning to work on most combinations, mechanical secondary carbs are pretty much a bolt on and go proposition.The only thing they really give up is mileage (and who cares about that LOL!).

Jay,
   You're right I'm sure. The only thing I know about vacuum secondary's or vacuum advance for that matter is what I've been told. The only thing that has led me to my understanding carburetors is word of mouth and the factory installing these items even on their performance cars. I've fought these carbs for years trying to figure out why they stumble, making my own accelerator pump cams and changing nozzles or power valves so I could have performance as well as drivability. I should read your book more often.

AMDSCOOTER.  A 10L weight is equal to 20 degrees of advance. I'm used to welding up my own using a predetermined limit opening to get what I want, so I'm just used to signifying the results in what you find imprinted on the weight. If I ran a serious performance car just for drag racing etc. I'd get a MSD distributor or equivalent to have better control. As it is, my car winning on the drag strip is TOTALLY dependent on my ability to repeat my dial-in, no matter how fast or slow it is.

PS. This 750 is probably the best "out of the box" carburetor I've ever bought. I set the float levels, idle adjustments and secondary spring. I am well pleased with the carb. Thanks My427stang for telling me what to get.

[amdscooter]

^^^ Duh..  me<== smacks head. 10L = 20 degrees. I should have remembered that from when I pulled my stock dizzy and the options were 13~18L. I'm really glad to hear you have yours dialed in.  I'm going to take the lazy route and get a 700~750 double pumper with an electric choke and hopefully be able to call it good under the hood for now. I need to get my buckets redone so I have somewhere to sit this summer as I'm driving it around. There is a spring poking out the front on the drivers side keeps scratching the back of my leg.   

[Rory428]

First off, a properly working Holley with vacuum secondaries will not produce a "kicking in" feel. Since they open smoothly, under load, as the airflow is needed, they will offer a smooth, linear surge as they open. Also, there is no reason why a vac sec Holley carb will not make the same power as a equally sized double pumper. My 454 Fairmont has gone high 9`s with an ancient 3310-1 Holley 780 vac sec carb that has NOT been heavily modified. Hell, it still has the choke tower in place, the only "mods" are a bit of experimenting with sec diagphram springs, accelerator pump cams and squiters, and jetting, as well as having a broken off  baseplate mounting ear welded back on, hardly anything trick. I have tried several other carbs over the years, including 750,800,and 850 DPs, a 830 annular DP, a 950HP series, a 1050 Dominator, as well as 960 and 1050 "3 barrels". None of these carbs were any quicker at the dragstrip than the 780 vac. Also the 780 had the best idle quality and drivability in the pits and while staging the car. So the old rumor that a double pumper is always better for a manual trans car seems to be a myth as well.