Author Topic: Bog From Dead Stop  (Read 129 times)

Offline BanditRich

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Bog From Dead Stop
« on: August 12, 2019, 12:17:07 PM »
I have a 78 Trans Am, W72 400. When I bought this car 17 yrs ago it had recently been restored with  a stock rebuild with Edelbrock 600 cfm carb & Performer intake, factory exhaust manifolds with true dual exhaust no catalytic convertors and shift kit in the turbo 350 trans.
I pulled the exhaust manifolds & put in headers which really woke it up. My previous TA was a 77 same engine which I did the same thing to.
Both cars wouldn't spin the tires from a dead stop but after the headers went in they both would no problem & accelerate like a bat out of hell. But if I was driving along in the one I have now and hit it it wouldn't accelerate as good as the 77 I had with the quadrajet. If I adjusted the detent lockdown for the kickdown cable I could get it to accelerate like the 77 but then acceleration from a dead stop would suffer. I was forever trying to get it to have max acceleration from a dead stop AND when I was driving & hit it without success. So I figured it was just because it was a 600 cfm Edelbrock carb.
A couple years ago I was given the opportunity to grab the 78 W72 quadrajet which belongs on my car (17058266) fresh out of rebuild with Cliff's HP kit by a mechanic with decades experience with Pontiac quadrajets so I jumped on it.
It doesn't have idle bypass air and the plug over the APT is in place. Deviations from stock are #74 jets instead of 72's, and DA secondary metering rods instead of DB's. It also has .035 idle tubes and the accelerator pump rod is in the inner hole instead of the outer; and the choke pulloff release time is just about 2 seconds.
The throttle bracket had been modified to make it work with the Edelbrock carb & manifold which rendered it useless with the quadrajet so it has a Mr Gasket adjustable bracket in it now.
When the quadrajet was installed I took it for a spin and driving normally it behaved just fine but acceleration wasn't like it was on the 77. I adjusted the detent lockdown on the kickdown cable & have gotten it to have awesome acceleration from 20 mph up but no matter how I adjust it it won't make acceleartion from a dead stop better. From a dead stop it's like the car is bone stock won't spin the tires at all unless I hold my foot on the brake & push down on the gas pedal so it's at around 1200 rpms then take my foot off the brake and nail it. It has the factory rally shifter and if I put it in 2nd and do this it will spin the tires a little less than in drive but accelerates better; but still not like it did on my 77 or this one when the Edelbrock carb was on it.
What is causing it to not accelerate from a dead stop like I know it should; like the 77 I had? That carb had all stock parts in it. Could an air valve spring adjustment take care of this? I don't know what it is set at right now. Which way would turn it to make the secondaries open; which way makes them close; and how far should I turn it after it closes them?
Also like the Edelbrock carb the only things hooked up to the carb are the PCV valve & the distributor; and I had Cliff's electric choke put on this carb. It doesn't want to drop down from the cold fast idle though even if I wait til the engine reaches normal operating temp and tap the gas like when it had the Edelbrock carb on it.   :-\

Offline BanditRich

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Re: Bog From Dead Stop
« Reply #1 on: August 12, 2019, 02:07:13 PM »
I neglected to mention the hoodscoop is functional just like the 77 was and the snorkel that draws air from down the fender is still in place. I just found out I had the wrong air filter in there it is 3" tall and supposed to be 2 1/2". I had noticed that there was only about 1/8" space between the hoodscoop & aircleaner lid for air to get into the filter. Maybe that's why acceleration from a dead stop is lacking but great from 20 mph & up? Correct one's on order; I'll find out Friday that's the next chance I'll get to drive it.

Offline Cliff Ruggles

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Re: Bog From Dead Stop
« Reply #2 on: August 12, 2019, 03:44:08 PM »
The secondary air door spring tension is fully adjustable as is the release time of the electric choke.

When correctly adjusted you should have smooth/strong acceleration anytime you go to full throttle, whether stopped or in any gear, any vehicle speed, any RPM.

The choke pull-off is a player, as is the adjustment of the link up from the pull-off.  It must be holding the flaps lightly closed when applied.

Hanger height plays a role hear as well, as does the POE system, but those carbs do NOT need any help with secondary POE.

If you "feel" a hesitation, stumble, or bog when going to full throttle from a standing start then some adjustments are in order.  It should take of strong and smooth, and the transition should be "seamless". 

A lot of folks think you should feel some big lunge, transition, etc.  If that's the case there is still work to be done.

Also consider that those later 400's are extremely "weak" for low end power, as they have pathetically "low" compression ratios and horrible camshafts in them.  I've been around them for at least 40 years, and it's not uncommon to find one that woln't spin a tire without power braking, or do a decent burn-out unless you drop it from a floor jack at full throttle!  I've lost count of how many have been brought to my shop for carb work, custom tuning, etc.  Haven't to date had any of the auto trans set-ups (stock) impress me in the least.

I'll add here that they get worse with time and miles as the timing set retards the cam and compression finds it's way past the rings.

On a good note you can take one of those engines, zero deck them, flat top pistons with 2 valve reliefs, cut the heads to get the compression up closer to 9 to 1, install a Crower 60240 cam, high ratio rockers, Rhoads lifters......and......you'll think you added 100 horsepower.  The same lackluster low compression 400 will literally tear your head off when you go to full throttle, and ROAST any sort of street radial from a standing start with ease!

I actually ran that exact same combo in my Ventura back in the 1980's, backed by a stock converter and 2.73 gears.  Burnouts were just a quick hit of the throttle and at the track it ran 14.0's at 99-100mph, with a best ever 13.87 @ 101mph in good air.......Cliff

Offline BanditRich

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Re: Bog From Dead Stop
« Reply #3 on: August 13, 2019, 03:58:57 AM »
OK, which way do I adjust the spring tension - does turning it clockwise open the secondary flaps or close them? And how far should I turn it after it closes them? And how do I adjust the electric choke?

Offline Cliff Ruggles

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Re: Bog From Dead Stop
« Reply #4 on: August 13, 2019, 05:21:49 AM »
The basic setting for the choke is done with everything dead cold.  Loosen the screws, turn it clockwise till the choke flap is fully open, the CCW until is just barely closes.  That will be the leanest setting.  If you need it on longer after testing turn it CCW, and make very small adjustments, a little goes a long ways.

For the secondaries adjust the spring so it's loose, then tighten until it just touches the pin, then another 3/4 turn. 

Put a mighty vac or fully compress the pull-off, the link should be free in the slot but just barely holding the flaps closed.

Do some full throttle testing from a dead stop.  It should come in clean without any stumble/hesitation or bog.  In most cases when it does those things a little more spring tension is required.  If it seems to "lag", try a little less spring tension.

The best adjustments will be found when you have good traction.  If the tires spin the engine revs right thru any potential "transition" so it's difficult to tell if it's right or not.

If the tires slip/spin you can always unhook the kick-down cable and coast along around 30-40mph and stick the chit out of it.  Loaded hard against 3rd gear it will tell you if you have the secondaries set up correctly or not.......Cliff

Offline BanditRich

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Re: Bog From Dead Stop
« Reply #5 on: August 13, 2019, 10:51:41 AM »
I am inclined to say it is more of a lag than a bog; it doesn't fall on its face from a dead stop it just is slow for the rpms to rise and slow to accelerate. Like I said from a dead stop it feels just like the car is bone stock with all the emissions crap, closed hoodscoop, unmodified carb and restrictive factory exhaust system. I've had 5 late 70s TAs the 1st was in 1984 and couldn't change anything on them due to emissions testing aside from opening the hoodscoop; and it accelerates from a dead stop like the 1st few I had.
 

Offline BanditRich

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Re: Bog From Dead Stop
« Reply #6 on: August 13, 2019, 02:06:30 PM »
I'm going to get the electric choke to work like it should before I adjust the air valve spring and see how it is after that. As it is right now I start it up and it idles in park around 675 rpms no matter how long it sits and as soon as I put it in drive or reverse it immediately drops to around 450.
I think it should have better acceleration if I get the choke working correctly so I can turn the idle up so as soon as I tap the gas when it's warms up it will drop to around 675. I think I have the idle speed screw set too low due to the electric choke not working correctly.
As far as tire spin goes I just put brand new tires on when I got it out of storage. It has the WS6 package with the 8" rims and I went from the stock P22570R15's to P25560R15's. Now my car is a mirror of the Macho TAs it has every mod those did and it also has subframe connectors.
The old tires were OLD. They were the same ones that were on it when I bought it in 2002 with a date code of November 2000.
My goal isn't to get the tires to spin it is maximum acceleration  from a dead stop & at any speed.
Once I do the suggested adjustments I'm sure I will be a happy camper.
« Last Edit: August 13, 2019, 02:19:32 PM by BanditRich »

Offline Cliff Ruggles

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Re: Bog From Dead Stop
« Reply #7 on: August 14, 2019, 01:29:29 AM »
The most idea scenario is to launch without any spin at all, so tune for the smoothest transition and quickest acceleration even if it's not spinning the tires. 

If you put big enough or good enough tires on it then no doubt it's going to be hard pressed to spin them w/o "power braking" it.  Those engines don't have much going on for low end torque, especially the ones with the bigger chamber heads and 7.7 to 1 compression ratios.

Even the "HO" 400's topped with 6X-4 heads were barely over 8 to 1 compression. 

Compression is power (torque).  Low compression engines, especially smaller CID never make much of it, at least at lower RPM's.....that's just the way it is with these things.......Cliff

Offline BanditRich

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Re: Bog From Dead Stop
« Reply #8 on: August 15, 2019, 04:29:56 AM »

"Do some full throttle testing from a dead stop.  It should come in clean without any stumble/hesitation or bog.  In most cases when it does those things a little more spring tension is required.  If it seems to "lag", try a little less spring tension. "

[/quote]
Cliff are you referring to the air valve spring or the choke pulloff screw? And what you said about getting the electric choke to work properly  -  I just loosen the screws and turn it?
« Last Edit: August 15, 2019, 04:33:19 AM by BanditRich »

Offline BanditRich

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Re: Bog From Dead Stop
« Reply #9 on: August 16, 2019, 08:02:47 PM »
I adjusted the electric choke but it still won't kick down. And I must be blind as a bat. I see the air valve spring in the diagram in the pamphlet of carb adjustments you sent me with the HP kit but I can't locate it on the carb. Where exactly is it? Could you or someone else please post up a pic?

Offline Cliff Ruggles

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Re: Bog From Dead Stop
« Reply #10 on: August 17, 2019, 04:08:31 AM »
Did you verify voltage to the choke?

Make sure that you didn't put a gasket under it, it needs to ground on the housing.

The secondary spring is easy to access with a tiny flat tip screwdriver.  You'll need to loosen the allen set screw under it to make adjustments......Cliff

Offline BanditRich

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Re: Bog From Dead Stop
« Reply #11 on: August 17, 2019, 05:49:47 AM »
Cliff - where exactly on the carb is the secondary spring?
« Last Edit: August 17, 2019, 06:13:31 AM by BanditRich »

Offline BanditRich

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Re: Bog From Dead Stop
« Reply #12 on: August 17, 2019, 01:06:50 PM »
OK I just watched a youtube video I know where it is. I thought I took the gasket out of the choke housing I will check next time I get under the hood & check the voltage if there isn't one in there.
I can't wait to adjust the secondary spring tension so my TA launches the way I know it should.