Thread: 78 Trans Am getting some love

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Glenn, Yes, when I get frustrated I start jumping around too much. I'll start checking some of these things out this week, one at a time.
I've had trouble keeping this car running good since I've owned it, people wonder why I never drive it lol.

Originally Posted by 53 Chevy5

I've had trouble keeping this car running good since I've owned it, people wonder why I never drive it lol.

Now we're getting to root. Is this a motor you rebuilt?

Originally Posted by 36 sedan

Now we're getting to root. Is this a motor you rebuilt?

Yes it is, I thought I mentioned that earlier when we talked about the cam. I rebuilt it 23 years ago, but there is probably only 8,000 miles on it or so. That's why I started questioning the cam timing, because I know I did not degree the cam when I built the motor. From what I remember, it is bored 30 over with new pistons, and I believe the Pistons were domed a bit to bump up the compression to around 9.25 to 1 versus the factory 8.5 to 1. The heads were redone as well.

OK, I just scanned through seven pages, but I didn't see any motor specs other than above, must have missed them.

I found it, it was way earlier when I started asking about some exhaust advice.No matter, I'm just hoping we can get this thing figured out.


' I need some exhaust advice here. Trans am motor is hopped up a bit, exhaust says potato potato potato but I don't remember the details of what I've done besides a larger cam and higher compression pistons. My old boss helped me out with it and he does know his motor stuff , he wasn't just throwing arrows in the dark."

I was just thinking about my old boss and him building engines. The guy is an arrogant ass but he did know how to put stuff together. He had a '88 Thunderbird with a stroke 351 who ended up being a 377. Ran in the mid 12's at 112 on pump gas. He would drive to the track bolt the slicks on that he had the trunk. After his run he would roll up the windows turn the AC on and then drive through the pits and crack his window open and poke fun of all the guys in the Chevelles and Camaros or what not. They would be sweating their butts off with the windows open and he would just say Burr is cold in here. It would be more funny if he wasn't an ass lol. He didn't want the car to go any faster because then he would be required to install a roll cage.

Lets get back to your present situation.
Do a compression test and a fuel pressure test first, lets see what we're working with.
When you do the compression test, do the first test dry, then do it again wet (a small squirt of oil in the cylinder), doing it this way will tell us if there is any lose past the rings.

After the compression test, adjust the valves with the method I posted above, then test compression again to see if it increased any.

Let us know what you find.

I think we're getting somewhere now. I finally had a chance to go down the list, compression is 150-155 straight across the board, I did not do the oil squirt in the cylinders though. I figured if their that close there shouldn't be any surprises inside. I reset the valves and I was pretty sure they were going to be too tight vs how you wanted me to set them and I was right. I was taught to go 1 turn after 0 lash and that is how they were, so I reset them to 1/4. I checked the fuel pressure and it was at 6 which Holley said is good. The vacuum did seem to pick up to 10 but no real change. I decided to buy an AFR gauge, put it in and It read about 15 at an idle which is stupid rich. I messed with the timing, vacuum advance, and turned in the idle mixture screws and nothing changed, it and it was running like it was before. I shut it off and stared at it a bit and did some abstract thinking and thought gaining vacuum is my #1 goal right now so I started it up and turned the idle mixture screws out 1/2 a turn further than before and BAM, it purrs like a kitten, my exhaust lope is gone, the stink is gone and it doesn't diesel when shutting it off. Vacuum is now a steady 14 and the AFR is nice at around 12. Before I get beat up on not adjusting them right the first time I did it like I always do, turn them in till you hear a change and then back it out 1/2 a turn, it always worked before. So when it was all said and done I needed to give it more fuel to lean it out. Thanks so much for not letting me rip my motor apart right away at the first sign of trouble! Now it should be a breeze to tune the rest of this carb with the AFR guage!

15 is lean, 12 is a little rich but OK with a cam at idle, 13-13.5 idle may be better, but always give the motor what it wants. Shoot for 14-15 at cruise, 12.00 - 12.5 WOT. Hold WOT for at least 15-20 seconds to be sure, it takes a bit for the accelerator circuit to run out and the secondaries to become active (not unusual to be a little rich at the beginning of WOT). Get it dialed in an read the plugs, the plugs will tell you what the motor needs.

An important caveat about AFR gauges, if the motor is missing it will read lean on the gauge.

Originally Posted by 53 Chevy5

I did it like I always do, turn them in till you hear a change and then back it out 1/2 a turn, it always worked before.

The ol' lean drop trick, works for stock motor but no so good with a cam. With a cam, go for highest idle vacuum then 1/8 - 1/4 turn out (richer).

Originally Posted by 53 Chevy5

Now it should be a breeze to tune the rest of this carb with the AFR guage!

AFR gauges are like a chassis dyno for carbs, and way cheaper than a day (or two) on a dyno!

I got a question on the secondary air flap. I know the primaries all have to be set first, but when does the air flap need to be set and the best way to do it? I know I'm getting a little ahead of myself, but I was wondering that for a long time already.

It comes factory set at I believe 1-1/2 turns which is good for most motors. Lets get everything else set before we worry about the this. Remember, tuning this carb is much like a Carter (Edelbrock), the metering rods and pull up springs are your friends.
https://documents.holley.com/lit731rev4.pdf

I think I may have over carbed the engine, I maybe should have gone with the 650 cfm vs the 750 cfm. I have the jetting down as far as I can go with both needle and seat and the best I can get my AFR at 55-60 mph on a flat road is in the low 12's . While I had the carb apart I swapped the secondary jets to the smallest size at the same time because I could see the writing on the wall. I got that from a solid 10 to about a 11.5 at WOT still plenty rich and it's got a noticeably better pull than before better but I can still tell it's wondering what to do with all the gas at that point. I messed around with the timing from 34 -39 and it doesn't make much difference with the AFR. I'll call Holley and see what my options are at this point which I'm sure will be a smaller carb. On a side not I got my idle vacuum up to 16 and an AFR gauge is the best investment to make tuning a carb!

Have a safe and fun 4th everyone, I think we're going to take Rita out and drive around for the rest of the afternoon and try to find something to do. Most towns around here don't have much going on this year but It's a nice day for a drive anyway.

Originally Posted by 53 Chevy5

I think I may have over carbed the engine, I maybe should have gone with the 650 cfm vs the 750 cfm.

750 is a good size for your motor. I have dual quads on my 383 (1000 cfm), it’s all in tuning.

Originally Posted by 53 Chevy5

I have the jetting down as far as I can go with both needle and seat and the best I can get my AFR at 55-60 mph on a flat road is in the low 12's .

What primary jets and rods are you using. Keep a log of all changes and what they do (you’ll thank me later)

Originally Posted by 53 Chevy5

While I had the carb apart I swapped the secondary jets to the smallest size at the same time because I could see the writing on the wall. I got that from a solid 10 to about a 11.5 at WOT still plenty rich and it's got a noticeably better pull than before better but I can still tell it's wondering what to do with all the gas at that point.

Wait on the secondary jets, most of WOT comes from the primary and the enrichment circuits. Secondaries effect on WOT is about 15-20 seconds out. Most of the rich your seeing is from the accelerator, primary and enrichment circuits, lets tune them first. Move your accelerator linkage to the furthest out hole and see how it performs.

Originally Posted by 53 Chevy5

I messed around with the timing from 34 -39 and it doesn't make much difference with the AFR.

Watch the timing for detonation at WOT, 39° is pushing it and may actually lose power.

Originally Posted by 53 Chevy5

I'll call Holley and see what my options are at this point which I'm sure will be a smaller carb.

I doubt they’ll recommend a smaller carb, again, tuning is the key.

Originally Posted by 53 Chevy5

On a side not I got my idle vacuum up to 16 and an AFR gauge is the best investment to make tuning a carb!

Good job on the vacuum. AFR gauges are great! Just remember a miss fire will read lean, always check and read the plugs.

Originally Posted by 53 Chevy5

Have a safe and fun 4th everyone, I think we're going to take Rita out and drive around for the rest of the afternoon and try to find something to do.

I’m changing oil, plugs, wires, cap and rotor on mine today. In the morning I’ll finish with cleaning the air bleeds and changing the coolant, then out I go! Lol!!!

Happy 4th to all!