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78 Firebird

I have a 78 Firebird with the 400/4 barrel. The engine is stock, as it came from the factory. I've tuned the car up (plugs, wires, cap, rotor; carb was rebuilt a while back). Normally, the car runs quite well. However, if I drive it, let it sit for an hour or two and let the engine cool off, and then start it, it idles poorly. Then, when I go to accelerate, it runs rough, misfires, smokes, etc. for maybe two miles of driving until it fully warms up again. Then after that, it's fine again.

I know this is engine temperature related, and I'm thinking it's some vacuum controlled emissions component that is not working right, or something like that.

Is there anyone on the list who is familiar with these cars or related vehicles from this time period, who might have an idea of what I should look at as possible causes of this problem?

Thanks.

Reply to
njot
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Check the operation of the choke and choke pull-off. Make sure everything is adjusted to spec and working properly.

Reply to
Mike

After you shut the engine down, remove the air cleaner, look down the carb with a flashlight and see if you see fuel bleeding down the intake. If so, it's time for a carb rebuild/replacement.

Reply to
Larry W

Thanks. Both of these are good ideas. I'll check these things.

Luckily I have the factory service manual for the car, also, and I did some reading.

Apparently, the way the vacuum lines are hooked up to the distributor vacuum advance, as well as to the secondary choke pull off, is very important. Sometimes they want manifold vacuum applied, other times they want ported vacuum applied, other times they want no vacuum applied, all depending on these thermal vacuum switches threaded into the cylinder head. If you don't have things hooked up just right,the car won't run right. I'm thinking this is a possible cause.

It's amazing the way they got some of these emissions control devices to work in the years before emission control computers became available on cars in the 80s. Miles of vacuum line all over the place that have to be hooked up a specific way. Make one little mistake and it doesn't work right. No wonder so many cars from this time period didn't always run right.

Reply to
njot

If the smoke is black, then likely the choke is closing too much. When you start the car from dead cold, does it do similar things? Does it act similarly at similar engine temperatures?

Choke. Other temperature-controlled items are the air cleaner and exhaust heat riser valve, but a malfunction in either generally won't produce the symptoms you're getting. I can't recall offhand if the EGR is controlled via the TVS. If it's a manual transmission car, the TVS down by the oil filter won't have any effect on anything you're experiencing.

That's the real Pontiac service manual, right? It will have lots and lots of information. One drawback is that it assumes everything is there and hooked up. After thirty years, that's not all that likely, especially with the mindset of "remove all the emissions junk" that held in the '70s and '80s.

Yeah, assuming that you have the original carb and distributor, hooking everything up as per factory is the place to start. If you still have running issues, then it's time to determine if any of the emission components have gone south.

Hmm, hook up a few wires wrong in an EFI system and things won't run right either. Or, ask yourself how well an EFI system with 30 years of aging will run without maintenance.

In my experience, as past owner of a '79 Firebird 301, and current owner of a '78 Trans Am 400, the driveability of a properly-maintained system is not bad--certainly not like what I've heard of the early '70s issues.

The 400 actually has more emissions-control items hooked up and nominally- working: heat riser valve, EGR, air cleaner--than the '79 did!

Reply to
Ed Treijs

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