Low on power?

[patrickt]

Here are two pictures that show how I measure my vacuum secondaries opening. I put a zip tie around the secondary throttle shaft on the driver's side of the carb so that it is "handshake tight" and so that the shaft can rotate within the zip tie but tight enough so that the zip tie stays in the new position after the shaft has rotated. Here is the zip tie when the secondaries are closed and then the next pic shows the zip tie after a spirited run with the secondaries opening nicely.

[patrickt]

While I'm adding pics, if you suspect that your secondaries are not opening fully, then what you might consider (since it costs less than $50) is to replace your vacuum canister with a Quick Fuel Adjustable one, which is what I did. No more screwing around with those springs, which I always drop and step on, and adjusting your opening rate is done by just turning a screw driver. I circled it in yellow.

[cobragene]

Patrick I did rip my diaphragm on my second spring change so I changed it and added a quick change top with white spring. I learned from your post some time ago about the zip tie test that’s how I know it’s only opening about 80%. It is a 750 not a 780 that’s in my original post. I really appreciate you taking the time to help.

[patrickt]

The problem is one of the following:

1) There is a clog in the signal tap that comes off the primary venturi and connects over to the secondary venturi and both connect to the diaphragm housing;

2) There is a leak at the point where the vacuum signal tap goes in to the diaphragm housing, like maybe the little cork gasket;

3) There is a leak in the diaphragm in the housing;

4) There is a jam of some sort in the diaphragm;

5) The linkage on the driver's side is jamming up and preventing the full opening up of the secondaries;

6) The linkage directly below the diaphragm housing is jammed up or the secondary throttle shaft is somehow jammed up.

Let's test for a vacuum leak first. Borrow an extra hand from somebody and then you can wire your primary all the way open, then have your friend push the bottom of the secondary plunger up in the housing and thereby opening the secondaries, then you place one index finger over the signal tap hole in the primary side and the other index finger over the signal tap hole on the secondary side, then tell your friend to release the plunger. If the plunger doesn't stay put while you keep your fingers in place then you have a leak.

[davids2toys]

Awesome thread!

[patrickt]

Originally Posted by cobragene

It is a 750 not a 780 that’s in my original post. I really appreciate you taking the time to help.

This is a good thread that you might enjoy where we helped our favorite Texas gynecologist get his 4160 in order: Best Way to set the idle mix on a 4160 The big difference between a 4150 and a 4160 is that the former has a secondary metering block and the latter has a secondary metering plate. The vacuum secondary stuff is still the same. I like the 4160, with the right secondary plate, and with the mechanical choke stuff all removed, and it gives me extra room in the turkey pan, which is a custom two-piece removable job fabbed by Bob P. at ERA. I made a couple of other small mods to it, and do a light rebuilt on it every five or so years, and replaced the parts that didn't like ethanol and literally flaked like a snowstorm, and now have it exactly the way I want it (after a good 15 years of monkeying with it ).

[HighPlainsDrifter]

Hi,
pull plug #1 and stick a rod in on top of piston, better if you have a dial indicator. Rotate and see if timing marks on harmonic balancer are lined up to "0" , I have seen a couple slip and the balancer needs to be replaced. Anyway it could throw the timing way off.
Perry

[cobragene]

Your right this is an awesome thread, because of patrickt, I did print and use your carb adjustment procedure a while back, very informative. I haven’t had a chance to do your testing rundown but will soon, and will report. Highplainsdrifter another good point but already did that and it was spot on but I replaced the balancer because of age. I could be wrong but secondary not opening the last 20% wouldn’t seem to be a huge deficit.

[patrickt]

My advice is free, and worth every cent.

[cobragene]

Here, Here!! But will I get a consultation fee, or a Retainer fee?

[Dominik]

My supporting "like" to Patrick earned me $100!

That aside I had a car with tires made from wood the other day, which spun all the way to 140 mph..

[cobragene]

Patrick I completed all of your tests and they all passed, diaphragm held and no vacuum leaks, I am going to try the shorter more restrictive air cleaner tomorrow but that doesn’t seem very likely, I read somewhere to remove the diaphragm check ball and take a very small screwdriver and tap it on the ball seat to make a slot so more air can flow, does that make sense to you? I also read you can drill out the passage to the diaphragm, but I couldn’t see a way to do that. Again thank you all for your help.

[patrickt]

Originally Posted by cobragene

... remove the diaphragm check ball and take a very small screwdriver and tap it on the ball seat to make a slot so more air can flow, does that make sense to you?

No.

Originally Posted by cobragene

I also read you can drill out the passage to the diaphragm, but I couldn’t see a way to do that.

No. But what I would do is remove the vacuum diaphragm entirely and while it's off blow compressed air through the signal taps so you know they're nice and clear -- maybe even run a thin wire through them. Then confirm your linkage is not bent or jammed and then replace your diaphragm with the Quick Fuel Adjustable Vacuum Secondary. I know it works and it's really easy to adjust. $38 at Amazon: https://www.amazon.com/Quick-Fuel-Te.../dp/B00ZCQ18R6

EDIT -- And you do not need to remove the carb to install it. Just watch the circlip and the little gasket and don't drop them down in the engine somewhere.

[cobragene]

Ok Boss that’s what I’ll do. You know it’s funny I can change almost anything on that carb ( PV, squirters, diaphragm springs, jets, except pump cam or it’s timing) to make it run poorly no bogs ,stumbles ,hesitation. Doing those things just doesn’t make a big difference. Maybe I should look into the pump cam or it’s timing? I know one thing at a time.

[patrickt]

Originally Posted by cobragene

Maybe I should look into the pump cam or it’s timing? I know one thing at a time.

No, let's just get the secondaries to open fully. If you have enough air rushing by the signal tap hole, and the linkage and vacuum diaphragm are both working properly, then the secondaries should open completely with a soft spring. It really is as simple as that. Maybe there's a little chunk of gas goo in the signal tap hole. Make sure it's good and clean when you have the diaphragm off.

[jhv48]

You can adjust and readjust the vacuum secondary carb over and over but if you want that immediate power, you’ll have to ditch the carb and put on a double pumper.
Just buy/borrow one and stick it on. You won’t regret it. Even on Holley’s web site they recommend mechanical secondary carbs instead of vacuum secondary carbs for our cars due to their light weight.
I’ve done it twice on two different cobras. Both came with vacuum secondary carbs. One a Holley street avenger the other an Edelbrock AVS. Fiddled with both until I decided to bite the bullet and buy a double pumper. Woke those engines right up and now I can break the tires loose in fourth gear if I’m not careful. And that’s with sticky 180 treadwear 315’s.
Just do it.

[patrickt]

Originally Posted by jhv48

You can adjust and readjust the vacuum secondary carb over and over but...

That's a fair comment. If we can't make him happy with our $38 toy, then that's his next step.

[patrickt]

Cobragene, I'm still suspicious that there is something that is just not working right on your vac secondaries. I'd like to see if you can "force" them open with a tremendous amount of venturi vacuum. So, get a wash cloth out of the bathroom and take your car out to a reasonably deserted straight away. Then remove your air cleaner and drape the wash cloth across the driver's side half of the primary side, so that the carb is only breathing through the passenger side of the primary. Your car is now essentially a one-barrel carburetor. Set your zip tie so you can measure secondary opening. Then put the air cleaner back on so it holds the wash cloth in place and run it down the highway at full throttle, stop, remove the wash cloth and examine the zip tie. Unless something is broken that is virtually guaranteed to open the secondaries fully.

[Tommy]

Cobragene - Please record your wash cloth run for posterity and TikTok.

[MKS427]

Originally Posted by cobragene

Good point Patrick and that was the case which I corrected about 6 months ago, it was opening 80-85% but not much improvement. I even wired the vac sec closed and not a great deal different, the most I can get them to open is about 85% with the lightest spring. It feels more like a potent small block.
Incoming I don’t really want to do burnouts, I am just using that as a gauge if you will, I run 25 psi.

Try 22 psi on the rears and 23-24 psi on the fronts with the FE. 22 psi all the way around for a small block car.
Tires of different compounds have been discussed here, as well as set up. Tire compound is the first variable I'd look at. Don't assume your car is down on power, because someone lights their tires easier than you. Their car may not be set up properly. Anyone could light up a set of BFGs at 35 psi.
If you have good tires, you don't want to do burnouts, you want to stick to the pavement.