99Hawk120

Here's a question that's been bugging me for some time. I've had a couple people try and give their best guess at it, but no one I've asked seems to have experience with running a stick car.

First, between the A/V spring and the vacuum break:

1) Which one prevents the initial "bog" when stepped on?
2) Which one governs the overall rate at which the secondaries open?

I've had an interesting problem with the secondaries. Sometimes it will bog when you first stomp on it, so I've been tightening the A/V spring trying to get rid of the bog (it's still much looser than stock). However, it seems to take a while for the secondaries to actually open--the power comes on too slowly for me. I do have an extremely slow vacuum can.

What's more interesting though, is the following behavior:

When you first go WOT, the car is fine 95% of the time (the other 5% it bogs very briefly then goes). However, when you wind it up to redline and shift, sometimes it will fall FLAT on its face when you stomp the pedal in the next gear. It doesn't always do it. The slower I shift, the less likely it is to happen. If I powershift, it NEVER happens. Often, when it does it, it basically won't recover, I have to get back out of the throttle and try again. Observers have said black smoke comes out of the tailpipe when it's doing this.

Are the secondaries supposed to close completely between shifts? It almost seems like I'm "out-shifting" the secondary Air Valve, such that it isn't closed (or not closed enough) by the time I get back into the gas. Either that or my Air Valve might be sticking open? Anyone have any suggestions?

I'm running a D hangar and DR rods on my basically stock ZZ4 crate motor with 1 3/4" headers, T56 trans, 9" rear with 3.89s.

(Forgot to mention: I'm running an intank electric pump with a pressure regulator keeping it to 5.5 PSI)

five7kid

iTrader: (14)

The choke pull-off prevents the initial bog. The tension spring controls the opening rate.

The pull-off is "orificed", meaning it doesn't release immediately when vacuum drops. That also means it doesn't pull on immediately when vacuum is applied. So, that is probably the issue you're facing. You may have to open up the opening in the vacuum nipple on the pull-off (which is how it is orificed), or get a pull-off that has a quicker rate - I forget which one that is, but I believe Edelbrock uses it on their Performer RPM q-jet.

You already know the other solution - powershifting - never let off the throttle between gears...

99Hawk120

If the pull off controls the initial bog, then WHY does the carb bog with no A/V tension on initial application? With such a slow vacuum can (2.5-3 seconds), why do I need any A/V tension at all? But I can tell you right now with too little A/V tension it bogs pretty bad even with the slow can.

Something's not adding up here.

Maybe it would help if I understood what exactly causes the A/V to open and what exactly causes it close? Can you explain it by telling me how it SHOULD work?

82camaro

Sucking power from the engine opens the upper valve. The spring shuts it. The vac can keeps the process smoother. Back when I was running a q-jet, I had the best results unhooking the link to the vac. can(disables the vac. can from the secondaries), and adjusting just the spring tension for the best results.

99Hawk120

I suppose I could try that. I don't think it will help much though.

Damon

The vacuum can takes less time to open up on the motor because there is very little vacuum to bleed out of it by the time the throttle is all the way on the floor. Basically, it starts to bleed off the instant you start sending the pedal to the carpet. Plus, it has air valves trying to pull it open as air tries to enter the secondary bores.

In actual operation, the vacuum break basically does a "hold....hold....hold.....open all the way quickly" kinda release. It's not as linear as it may appear on the bench.

The vacuum break DOES keep the initial bog from happening, but it will bog anyway if there is not sufficient air valve tension to allow the air doors to open smoothly and progressively as the engine's demand for air increases.

Why does your car's suspension have springs AND shocks? Because it needs both. Exactly the same reason for it on the QJet's secondary air valves.

99Hawk120

Hmm.

Well, that helps me understand how it works on the initial opening... but it doesn't help me to figure out why my car will bog HARD sometimes at WOT after a shift. Or why the secondaries seem to open so slow (which gets worse with more A/V) tension. I've got quite a bit less than stock tension on the A/V (never quite gotten the hang of measuring exactly zero tension though )

It's a bit frustrating. I ought to mount a mirror over the carb to see what the secondaries are doing, I guess.

five7kid

iTrader: (14)

When you say "why the secondaries seem open so slow", are you judging that by the SOTP feel you get when those big secondaries open?

I finally accepted the statement by Holley (for vacuum secondaries) that "if you can feel the secondaries opening, you're getting a bog". When I adjusted to eliminate that feeling, suddenly my 60' and 1320' times dropped. What you are calling "opening too slow" may very well be "opening too soon".

Also, I probably doomed my 1901 Edelbrock q-jet by removing the link between the pull-off and AV. When I started getting that initial bog after I started playing with the AV tension, I couldn't find that link again. I called Edelbrock, they sent me the wrong one. Oh, well...

99Hawk120

No, I'm not really "feeling" the secondaries open, I'm judging by how quickly the motor revs up. Especially if you are doing 2500 rpm and nail it, it seems to take a while for the car to get going. The power comes in smoothly, I don't feel anything out of the ordinary...

Of course, I can't get the damn thing to idle right either. I have been fighting with this stupid carb for over a month now and it's really beginning to make me angry. Even when I get the fuel mixture good, like I finally did today, it STILL won't idle smoothly, and the computer is acting stupid. Who knows, maybe that's part of the problem. I don't know any more. I do know I'm about five seconds from sacrificing the computer to the car gods and going with a non-computerized Qjet. I'm gonna make another post about the idle though.

Alex99

Sorry to dig out that thread again, but I experienced the same problem 99Hawk120 is describing.

I get a bog for a split second after the 1-2 shift with my AUTOMATIC!
When I go to WOT from a standstill I get no bog.
My sec. vacuum can is drilled for ~1 sec and the air valve is opened about 1/2 turn. I use DR rods.
This all is in a 327.

My feeling is that when it goes into 2nd gear it stumbles for 0.5 seconds and goes again. I thought it would be because the vacuum can releases too fast or the air valve tension is too low. But this would not explain the non-bogging at the start.
BTW the trans uses racing clutches and is shifting extremely fast and harsh.

Thanks
Alex

Damon

Alex, you're marginal for fuel delivery. It's not your carb.

When you're steady-state WOT with an automatic the carb is wide open all the time, including between shifts. The secondary throttles and air valve are wide open right through the shift and so, are already well past the point where their job of preventing a bog is done.

What's probably happening, and I've had this problem with many cars, is that you are outrunning your fuel supply under WOT in the lower gear(s) where acceleration is strongest. The fuel pump has to draw the fuel a LONG way from the tank, and under hard acceleration in 1st gear it's even more difficult since it's drawing against the G-forces of acceleration (which is why it happens in the lower gears and not the higher ones).

So why only on the 1-2 shift? Well, you actually were out-running the fuel supply on the top end of 1st but it didn't happen to start actually bogging until just about the time it hit 2nd gear. You might think the timing of something like that would be almost beyond belief, but it happens all the time. A bog as it hits 2nd means marginal fuel delivery. A bog as it reaches the top of 1st means you're definitely outrunning the fuel supply. A bog lower down in 1st that won't allow the engine to even reach 2nd gear means you're way WAY beyond the fuel system supply.

I'd start simple if I was you- new fuel filter in the nose of the carb and a fresh fuel pump down on the block. If you like you can upgrade the fuel pump to something with more volume like a Carter or Holley high volume mechanical pump. It's not a guaranteed cure as the restriction may be before the pump somewhere- like in the tank or the lines going up to the pump, but it's got a good shot at working if you're only marginally outrunning the existing fuel system.

Alex99

Hi Damon,

thanks for the reply. You've brought something up I've never thought of.

The engine is in a 1957 chevy with stock fuel delivery system with 5/16 tubing. The pump is a new stock replacement that put out 9+psi that cause flooding of the carb.
I bought a cheap pressure regulator (the ones with a turnable disc on top for 1-6 psi) and drive around with the disc on 3.5. I never checked the actual pressure at this settig but it is ok for driving around. At the dragstrip I turn it to 4.5, but sometimes it starts seeping again on top of the carb (at the acc. pump).
Thatswhy I turn it back to 3.5 after racing.

I will try a higher pressure setting and see if it still bogs.
Although i can't really believe that the pump can't deliver fast enough since its only a very mild engine.
1/8 mile time is 9.3 sec with 76-77 mph with 3800lbs.

Thanks for the hint,
Alex

five7kid

iTrader: (14)

Get a 3/8" line and pickup installed. It does make a difference.

Damon

Best way to diagnose fuel supply problems is with a cheap remote-mounted fuel pressure gague plumbed into the fuel line just before it goes into the carb. You can duck tape it to the windshield temporarily and make a few runs- that's what I do. If you're outrunning your fuel supply it will be very obvious- pressure will nose-dive at the top of each gear (or at least the lower gears). Then it's just a matter of figuring out what's holding up the works and fixing it.

I have found that this phenomenon can happen even at some very modest HP levels. I had an old Malibu with a very mild 400 under the hood that would outrun the fuel pump only running 13.9 @ 100 MPH. And that was even with the stock 3/8" fuel line from the tank and a Carter 110 GPH high volume mechanical fuel pump. It just couldn't keep up. I removed the "sock" on the in-tank pickup first- no improvement. Ran fresh 3/8" line all the way from the tank, fearing a partially blocked stock fuel line- no improvement. Tried a different fuel pump- no improvement. Finally I went with an auxiliary electric pump back by the tank (Carter 5 PSI street pump) and suddenly I had plenty of fuel. Enough to eventually run high 11s on nitrous.

My buddy has a 72 Chevelle with a stout 383 in it. Had the same problem, but even worse as the car only had 5/16" fuel line running from the tank (originally a low performance 307 car). Put and auxiliary electric pump back by the tank (Holley Red) and it now goes bottom-basement 12s on motor and the fuel pressure is rock-solid at 6 PSI under all conditions. And it's still using the 5/16" stock fuel line.

Alex99

Thanks for the help so far guys,

Before I can make the pressure test I need to get a gauge. But its a good idea beforè I throw money at the problem.

I was wondering if a 3/8 line will be sufficient since its only 1/16 bigger that 5/16 and I definetly will upgrade the motor to a 383 or Turbo. So I thought about going to 1/2" lines instead. Is this overkill? Is ist possible to run taht size without using adapters, that would become the bottlenecks in the system?

Thanks
Alex

Damon

Problem with 1/2 inch line is adapting it to the smaller fittings on the tank pickup with almost certainly won't be that large. Ditto most fuel pumps won't take a line that big directly.

Even with adapters it would probably still be a good idea- maybe even better than straight 3/8" line. It's the "friction" of pulling the fuel through a LONG length of fuel line that is the problem, not a short, modest restriction. Grab a 20' length of 5/16" aluminum tubing and try to blow through it. Your face will turn red with the strain. You really can't beleive it would be that difficult. Then blow through a 6" length of that same 5/16" tubing and notice the difference. It's a "holy cow! I never though it could be so different!" kinda difference. It's really eye-opening. You'll never look at your fuel lines in the quite the same way again.

Alex99

Thats good news, I guess.

You`re right with the tubing. Although it would still be better to change the complete system to the same bigger size, it will probably help if I change the pickup tube to 3/8 (they are available) and the long tube from the tank to the pump to 1/2 for less restriction.
Thats a cheap way trying to solve the problem, though a high volume pump can`t be wrong either.

Thanks
Alex

five7kid

iTrader: (14)

3/8" pick-up, 3/8" aluminum tubing (easy to flare and bend). That's what I did, and it's working fine. ~400 HP being supported.

Alex99

I want to give an update regarding my problem.

I changed the long tank-pump line to a 1/2" line. It didn't changed the bog after the first gear.
I still have the small pickup tube but the main restriction is gone.
I thought it could be the weak stock pump, but today I made a test and changed the secondary rod back to what I used before the DR rods. These have no letters on them but are close to AY or AZ rods. The biggest differece is that they have a long tip.

A small bog is still there but definetely way smaller than before.

Now I'm asking myself if the air valve ever opens up the whole 90°. Snce I have only 327 cui the engine never needs the whole 750cfm and will never pull the DR rods to the thin tip.

I also upped the fuel pressure to 5.5 on that el cheapo regulator which seemed to help.

What dio you think about this?

Alex

Alex99

ttt

five7kid

iTrader: (14)

If you didn't change the pick-up, you didn't change the restriction.

The 1/2" line really doesn't help any over a 3/8" line.

If the rods need to be pulled out more at a given AV opening, put a smaller letter hanger on.

Damon

The DR rods are on the metering tip at 70* of air valve opening, depending on what hanger you're using. That's not the problem.

The reason the bog got smaller but didn't go away is that the new rods you put in there are LEANER than the DRs, but probably about the same on the metering tip. Less fuel being fed, acceleration not quite as strong, and you're putting less demand on the fuel system. Hence, the bog gets smaller. You're making MARGINAL improvements, plus one tuning "head fake," but the basic problem is still there.

Get a remote fuel pressure gague kit and you won't have to guess if you have a problem any more. Or yank the secondary rods completely (then you KNOW you're running rich) and see what happens to the bog. I'll bet it doesn't get any better, or maybe worse.

Alex99

Yes, I still need to change the pickup, but since I live in Germany I will first place an order if I need more parts due to high shipping costs.


Damon, the DR rods are these:
DR 1323 1250 1093 882 746 581 570 M

The AY rods:
AY 1352 1294 1165 932 567 567 567 L

At 70° AV opening the DRs are at 746 and the AYs are at 567.
I can`t believe they are leaner at WOT.

They are marginally leaner at partthrottle though.

I`m still looking for a good pressure gauge to check the actual pressure.
The acceleration SEEMS to be better than with the DRs.

I really appreciate the help of you guys. I made these changes to see what happens, and this is it. I know I have to upgrade the fuel delivery still, and I will, but the results I got with the different rods were surprising.
To see what you guys say to this, I posted them.

Thanks Alex

Alex99

Hi,

I wanted to update the thread and let the interested know what solved my problem.

The problem was probably a combination of a too small fuel line and a weak fuel pump. A couple of weeks ago I had access to a fuel pressue gauge and observed that the pressure went to 0 at WOT.
I changed the stock pump to a Holley 110GPH pump and the pressure stayed at 5 psi all the time.
I also changed the pickup tube in the tank to 1/2".

Now the car pulls much better in the higher rpms. I was on a dyno 2 weeks ago and got 255 rhp. We'll see what it does on the strip.
Thanks to Damon and five7kid for your help.

Alex