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Old 06-16-2013, 04:30 PM
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Poor performing '76 400

Hello. I have a 76 grand prix with a 400, 106k miles, and it is performing poorly by my seat of pants dyno. I know they are rated low from the factory, but it seems excessively poor.

I rebuilt my carb, no tuning just factory spec. I blocked off EGR. Plugs look ok, wires look ok too. Have not done compression test. Idles and runs nice and smooth - just low on power. Have not checked timing.

Question 1. Ignition - I'm guessing this has HEI? Should I keep this or upgrade to something else? 600 dollar MSD out of question. What should timing be set at? It does have vacuum advance, but have not verified that it works properly

2. Cat looks original. Plugged possibly? The rest of the exhaust looks pretty small too, RA manifolds (don't want to deal with headers) and duals will happen eventually... for now, bypass cat?

3. Hard starting when cold - don't believe the carb is leaking out, and I changed fuel filter. Do I need special filter with check valve? New / better fuel pump? I remember reading about some tube by the gas tank that may be responsible as well

Thanks!

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Old 06-16-2013, 05:13 PM
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Quote:
Originally Posted by spawn_x View Post
Hello. I have a 76 grand prix with a 400, 106k miles, and it is performing poorly by my seat of pants dyno. I know they are rated low from the factory, but it seems excessively poor.

I rebuilt my carb, no tuning just factory spec. I blocked off EGR. Plugs look ok, wires look ok too. Have not done compression test. Idles and runs nice and smooth - just low on power. Have not checked timing.

Question 1. Ignition - I'm guessing this has HEI? Should I keep this or upgrade to something else? 600 dollar MSD out of question. What should timing be set at? It does have vacuum advance, but have not verified that it works properly

2. Cat looks original. Plugged possibly? The rest of the exhaust looks pretty small too, RA manifolds (don't want to deal with headers) and duals will happen eventually... for now, bypass cat?

3. Hard starting when cold - don't believe the carb is leaking out, and I changed fuel filter. Do I need special filter with check valve? New / better fuel pump? I remember reading about some tube by the gas tank that may be responsible as well

Thanks!
I would check the cat to see if it is plugged. Those old pellet style cats were real turds.They plugged easily.
What you can do is lower the exhaust at the manifolds , just loosen the nuts and drop the pipes. Then go drive it. If the performance improves greatly then you know the exhaust is plugged. Change The cat first would be my first choice.
I dont justify removing a catalytic converter...but do what you have to do .
Cold hard start could be Carb siphoning back as well as evaporating dry.
Be sure that your choke works/closes operates correctly.

Try using the little filter with the rubber seal and spring check valve system. It keeps siphon back in check.
Set your base timing to 12 degrees. Make sure you have a ported source for the vacuum advance, bypass any thermal vacuum switches for the distributor signal, to help take out some of the guess work. Also be certain that the mechanical advance is working.Physically check it by moving the ignition rotor by hand , see if it springs back when you let go.If not , pull the rotor and lube the shaft and get it loosened up.
Check for the amount of advance with a timing light while watching the RPMs on a tach. You should have at least 20 aditional degrees mechanical by the time you hit 3000-3200 rpm.If not ...change the springs to some lighter ones . Get a kit for the dizzy springs. Use stock weights.
Check the exhaust first my man.I think you will likely find that to be the biggest trouble.
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Old 06-16-2013, 05:23 PM
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much appreciated. Thanks
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Old 06-16-2013, 08:16 PM
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I agree with getting all the general tune up items done but these mid to late 70's Pontiac 400 were dogs when they were brand new. Some of those engines only had 7.6 to 1 compression and those were some of the heaviest cars of their model ever made. My friend had a 76 Firebird with a 400 that only ran 18+ second 1/4 miles before were rebuilt it. So don't expect much unless you're goiung to do some major work on it.
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Old 06-16-2013, 08:33 PM
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Ive done my homework - it is a 557 block with 6x-8 heads so neither those are in my favor. At the moment I just want to get it running decent while I save up for a rebuild.

Rebuild will be ~350hp with either machine working heads/block/intake or getting 6x-4 heads, RA headers, mild cam, and try to reuse block/crank/rods, depending on how they check out.

If I win the lottery.. different story!

This is a cruiser / daily driver, nor do I have the money for anything fancy

Getting a timing gun tomorrow and will go from there. Thanks!
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Old 06-17-2013, 12:37 PM
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Quote:
Originally Posted by spawn_x View Post
Hello. I have a 76 grand prix with a 400, 106k miles, and it is performing poorly by my seat of pants dyno. I know they are rated low from the factory, but it seems excessively poor.

I rebuilt my carb, no tuning just factory spec. I blocked off EGR. Plugs look ok, wires look ok too. Have not done compression test. Idles and runs nice and smooth - just low on power. Have not checked timing.

Question 1. Ignition - I'm guessing this has HEI? Should I keep this or upgrade to something else? 600 dollar MSD out of question. What should timing be set at? It does have vacuum advance, but have not verified that it works properly

2. Cat looks original. Plugged possibly? The rest of the exhaust looks pretty small too, RA manifolds (don't want to deal with headers) and duals will happen eventually... for now, bypass cat?

3. Hard starting when cold - don't believe the carb is leaking out, and I changed fuel filter. Do I need special filter with check valve? New / better fuel pump? I remember reading about some tube by the gas tank that may be responsible as well

Thanks!
With that kind of mileage I'd start with the timing set and assume it's worn out till proven otherwise because it it fails the price of fixing the result is very high.

Conditions that indicate it's time for a new set of gears and chain.

- If you feel a loss of power through out the range.

- It becomes hard to start

- You find you constantly have to fiddle the distributor to keep it on time with the crankshaft.

- Test you can do once the engine is running is with a timing light connected run the RPM's up a couple, three thousand then quickly close the throttle while observing the timing marks. If they come back to a steady base reading the timing set is acceptable, however, if the timing marks wander up and down it's a good indicator that the timing gears and chain need replacement.

Bogie
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Old 06-17-2013, 01:32 PM
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I feel for you. I bought a '76 Trans Am in about '78. I was just coming out of a period of early Corvettes and 409s. What a pig that 400 was! I had it tuned by a shop a couple times... but still a pig!

Emissions tests were even worse! To get it to pass I had to unhook the vacuum advance, reset the idle screws, and change the timing. Then it would pass... but was almost totally undrivable! Of course I would go home and undo all of the changes after the testing. }:-)
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Old 06-17-2013, 03:29 PM
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Have you verified that at WOT the secondaries are fully opening?
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Old 06-17-2013, 04:27 PM
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Quote:
Originally Posted by spawn_x View Post
Ive done my homework - it is a 557 block with 6x-8 heads so neither those are in my favor. At the moment I just want to get it running decent while I save up for a rebuild.

Rebuild will be ~350hp with either machine working heads/block/intake or getting 6x-4 heads, RA headers, mild cam, and try to reuse block/crank/rods, depending on how they check out.

If I win the lottery.. different story!

This is a cruiser / daily driver, nor do I have the money for anything fancy

Getting a timing gun tomorrow and will go from there. Thanks!
Once you determine the timing set isn't worn (or you replace it), set the advance curve up for performance. The info here should get you in the ballpark.

I've found you might not need 36 degrees of total timing- the few 400s I've tuned all liked 34 degrees, but that remains to be seen if that holds true on yours.

One thing that's also working against you is the rear gear ratio. Stepping up to a lower (higher numerically) ratio will wake it up once everything else's taken care of.
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Old 06-17-2013, 08:56 PM
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Originally Posted by Silver Surfer View Post
Have you verified that at WOT the secondaries are fully opening?
Still on my to do list..

I checked the timing today and base timing was spot on 12 degrees and varied nice and stable through RPM range, no jumping or anything odd.

I don't want to change rear end because my freeway rpm would go up (3 speed)


Quote:
Originally Posted by TucsonJay View Post
I feel for you. I bought a '76 Trans Am in about '78. I was just coming out of a period of early Corvettes and 409s. What a pig that 400 was! I had it tuned by a shop a couple times... but still a pig!
I still have to check / replace the cat, but it appears that this what the factory performance was.. I was expecting to roast tires being a 2wd with 400ci, but doesn't look like it will happen stock

Thanks for the tips!
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Old 06-17-2013, 09:02 PM
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Still on my to do list..

I checked the timing today and base timing was spot on 12 degrees and varied nice and stable through RPM range, no jumping or anything odd.
I am running 26 initial with no vacuum advance! You need a lot more timing with this cam! I thought it ran like a dog with 20 initial.
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