Hi everyone. I ended up putting a 1968 Pontiac GTO 400 engine in my 1968 Pontiac Firebird. I rebuilt the entire engine and reused the original cam and heads. All new bearings and piston rings. It's got a 4 barrel Carter carburetor, Hooker headers. Rebuilt the Saginaw 4 speed transmission and has a new clutch. The rear end is original as well. That being said the car and motor run really well. The only problem I am having is I think I should have more acceleration then I do. From the stop position I punch on the gas and it doesn't have a real fast acceleration. It sounds mean and tough but no get up and go. The car shifts fine and runs well at high speeds. I need to know what the steps are in finding out why I am not getting a lot of initial acceleration in first gear. The initial punch is good but dies off quickly and the rpm's go really high so I have to shift into 2nd and then 3rd. The engine has been running for about 2 months driving on the weekends. I'm trying to figure if this is just the nature of the car with low acceleration or if I am missing something. I don't even know what information you need so let me know. I am hoping it is a simple fix or if I need to change/upgrade equipment? Thanks in advance for your help.

Welcome to our site.

You used the original cam and lifters? How many miles on cam? I would have put a new cam and lifters.

What is the rear end ratio?

What carter carb did you use? Themoquad 650 ?? Is it bogging when you slap on it or do you feel your running out of gas go up in the gears with the accelerator floored?

[The only problem I am having is I think I should have more acceleration then I do. From the stop position I punch on the gas and it doesn't have a real fast acceleration. It sounds mean and tough but no get up and go. The car shifts fine and runs well at high speeds. I need to know what the steps are in finding out why I am not getting a lot of initial acceleration in first gear. The initial punch is good but dies off quickly and the rpm's go really high so I have to shift into 2nd and then 3rd.
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This is a little confusing. You state the inititial punch is not very fast, but then say that the punch is good. Do you have good acceleration only at high speed in upper gears? Is it good out of the hole and then nothing or the opposite? If you are pairing a close ratio tranny with a lower rear end, you will not have much punch out of the hole, and will probably have to rev it up quite a bit to get out of first gear.

I had a carter carburetor once...junk in my opinion (for our applications anyway) get yourself a good quadrajet.

And I'd question reusing an original camshaft. Also, do you know what rear end ratio you have?

I'm going to go a bit simpler... Had a similar throttle response issue. While internal engine or carb issues are common, did you check the throttle linkage at the pedal? Sounds a bit cheesy but I had a ruptured (blown out) plastic bushing on my accelerator pedal that prevented the car from going wide open throttle. It's a simple check... Have someone depress the gas pedal fully with the engine off and check the linkage on the carb... If she's got about 1/4 inch of play is say you found the issue, secondaries are not opening. This would cause the car to feel fine at highway speeds but lack on the low end... Just a thought

I agree in principal but I had a 850 ThermoQuad in my 350HO /w Thunderball cam and supper high 1st gear (3+) in a Super-T10 and it worked great!

I think goose may be onto something. I have had my car run a bit like you describe and it turned out that the secondaries weren't opening because something was in the way of the plates. I have no idea of the configuration of your Carter carb but I would check that first as well. Push the throttle linkage open and look down the throat to see if all the plates in the bottom are open fully. It very well could be an ignition issue too. Bad points or cap.

I'm not sure of the mileage of the cam and lifters. I bought the engine on Craigslist. The guy had no idea the milage of the car it cam out of. The lobes looked fine when I took the cam out.
I don't know the rear end gear ratio and not sure how to figure that out?
The carter is an AFB but there are no numbers on it to figure out what it is?
The best description I can say is that at the stop position in first gear I stomp on it and it accelerates but about 2000 rpm it doesnt continue to really put you back in your seat. Once I get to 3000 rpm I shift to 2nd and the rpm's are still really high into the 2000/3000 rpm so I shift into 3rd, But if I'm next to a regular car they are accelerating way faster than me with me stomping on it.
The points and cap are brand new. I will check if if all the plates are open on the carb this weekend. I didn't rebuild the carb but that maybe an easy fix if that's all it is.
I'm going to post a pic of the carb to see if you guys know what type it is.
Thanks.

Maybe I am scared of getting over 3500 rpm's as well since I just rebuilt it? What is the max I should take the rpm's to?

Okay I have no idea how to post pictures.

Second picture of the front of the carb.

did you check linkage?

The throttle opens all they way so I think the linkage is fine, but I'm not sure if the plates are opening all the way. I'll check that tomorrow.

I would follow the suggestions above and definitely figure out what you rear axle ratio is first. If your original rear axle is still in the car, you can try and locate the two letter code that should be stamped into the axle on the drivers side axle tube near the welded on bracket for the brake lines. Or you could get all the factory documentation on your car from Pontiac Historical Services.

Here's the Firebird rear axle chart:

Firebird Rear Axles

Strange things can happen over the now 50 year history of these cars. My California spec 400 coupe was totally intact including the smog devices yet somebody in its past had swapped out my entire factory 3.08 rear axle for a 2.56 freeway gear one.

I don't want to be a stick in the mud, but there is also the possibility that somebody put a Chevy style camshaft in your Pontiac engine (because they are a Chevy enthusiast or used a machine shop well versed in Chevy engines). Pontiac V8's were designed for lower red line RPM's and low end torque. Chevy engine cams lean toward higher RPM's and a torque curve higher in the RPM band.

You could also have a totally mismatched aftermarket intake manifold that is designed for higher RPM's. That will make your Pontiac motor pretty weak lower in the RPM band.

There is the possibility that the person you bought the engine from was getting rid of it because it acted as you describe.

What is your timing set at? Timing can make a huge difference in how the car runs.

Wow, I would think if your engine had any punch at all that a 400 would smoke the rear skins off!
To help get you started with IDing the rear axle ratio, you can do the following if you cannot locate the code, or want to check to make sure the innards have not been changed out:
Transmission in neutral, brake off, front wheels blocked, jack rear up by a jack under the differential.
Place your jack stands under it.
First spin one of the tires around, probably the other rear tire will be turning in the same direction. This will indicate you have a safety-trac (posi) rear axle.
Spin a tire around until you have the rear U-Joint in a position where you can mark one cap. If you are turning the drivers side wheel, maybe have the U-Joint positioned to where a cap is straight down at the 6 oclock position. Mark the cap so that you can watch it as you spin the tire. Mark the tire so that you can spin the tire one complete revolution, probably straight down to the 6 oclock position also.

What you want to do is spin the tire one complete revolution and watch and count the U-Joint revolutions. If the tire turns once and the U-Joint spins twice you would have a 2.00 rear axle, 3 complete spins would be a 3.00 rear axle. It will not be exact, but you will be able to tell the difference between a 2.56 and a 3.23, or a 3.23 and a 4.33.
Cal

Re-reading your first post, do you have any clutch slippage?
Cal

It sounds like you are running out of camshaft at 2000 rpm if the acceleration dies of at that level. Can't see a camshaft in a Pontiac doing that though unless the lobes are down. That tranny must have pretty close gears to only have a 1000 rpm drop between first and second , second and third. I wouldn't be afraid to shift at 4000-45000 and see what your performance is there. You may be able to accelerate through the bog. When you had the camshaft out did you notice any stampings on the snout of the shaft? A letter stamped there could tell you what the camshaft is.

IF you are getting a good acceleration from stand still and then bogging at 2000 I wouldn't think it's due to the rear gear. If you had a low ratio gear it would bog right from the start and pick up a bit as speed increased. If you had a really high ratio gear you would get good initial acceleration right up to the point your engine stopped making more HP and your rpm would increase rapidly. Definitely check the ratio though as Cal says. You could spin the rear wheel 10 times while counting driveshaft revolutions to get a more accurate reading.

You could check the distributor's mechanical advance weights to be sure they are free, maybe they are not moving out as speed increases thus not giving you more advance with more speed. Also check the initial timing and the vacuum advance. And definitely make sure the secondaries are opening. Did you have that carb apart to clean and/or rebuild when you did the engine?

Only other thing that jumps out at me is fuel delivery, maybe running out of fuel when you go through 2000 rpm. Filter, pump, hose collapsing?

This may be a simple tuning issue. What is the timing set at? And, what distributor is installed. There are several factors that can affect acceleration.
If the distributor is curved for an automatic trans [or heavier car] it won't advance the timing correctly. Do you have a good timing light?
Does the car ping under load? If not, move the timing up a degree or two and test drive.

Does the rpm shoot up, but the car doesn't accelerate? This may be a clutch that's not fully engaging. I've seen this before, and it was found the wrong pressure plate was installed. The car moved but didn't accelerate very well - it was fine at high speed.

good luck..

>This may be a simple tuning issue<

Finally! Forget what the timing is set at. Advance the timing about 3’ to 5’ and see what happens.

>It sounds mean and tough<

At an OC MD cruise week, a passenger said to his driver that my car “sounds like a pussy.” To the raiii clone 9-owner who may be a member here: What do you think about that pussy sounding engine? Anyway, if the timing is late, it allows lots of fire to exit the exhaust ports, and mixed with the fire is a bunch of noise because the bang is happening after the fuel leaves the cylinder.

>just the nature of the car with low acceleration<

Same engine I have in my car, 350 slush box, 3:36 screw. A quarter throttle leaves a pair of 10’ slabs, and it asswalks for quite a while if I shift to second while the engine is low in the powerband. Idle to wot; the car sits there and white smokes the tires,. Yes, there is a significant learning curve on the throttle to get hooked up.

Gang I figured out the problem after talking with other Pontiac sites and this one. I just took everything apart on the carb. I was suggested to look to make sure the secondaries were opening. I had no idea what the hell that meant as I am a newbie at all of this???? I started with the throttle cable and found that it was not pulling the throttle totally open. The one I bought from summit had extra aluminum on it and I had to shorten the length. Once shortened it would open the throttle and secondary valves all the way manually. Then I noticed on the carter carburetor there was a part of the choke that was getting in the way of the secondaries opening as well so I had to shave some of the metal so that it wouldn't catch. After doing all of that (a whole hour😂) after spending two weeks trying other things. The bird flies out of the stop position and squeals the tires like I just had stole something👍. I wish it could be all the time I spent figuring this out. I want to thank everyone who posted for your help. I'll post the throttle cable bought for reference if anyone else has any problems like me. Sometimes it's something just simple, but if you don't know you don't know!!!!!

http://m.summitracing.com/parts/OER-9792222?ibanner=MobileSwitchYes

Okay after seeing this its for a 2 barrel carb 😂😩

Don't sweat it. You're not the first with that problem, you most certainly won't be the last.

Excellent news.

Glad you got it figured out; and thanks for posting the solution!
Cal

Thanks for the update man... Knew it had to be in linkage... Always start small and go big with those type of issues... Enjoy the ride

Beat me to this one, Goose. I had the EXACT same problem, and it drove me absolutely nuts. $4.39 to fix the problem. Awesome call!

after waying all the options regarding putting an ls engine or an original Pontiac engine into my bird I am sold on old school builds. Once I completed the rebuild and feel what I got, I can't imagine going the other way. It is definitely a preference but I would steer anyone who asks in the direction of going original. The feel of that engine and smell of gas and oil is worth it. These cars need that. That's what made them great then and now. Go buy a new car if you need that feel of a new car and less trouble. It can get frustrating sometimes with old builds but the outcome makes all the difference. Enjoy your ride and pontiacs Rock!!!!!!

Flashyou where in AZ ?

North Glendale.

We just started a similar thread on throttle response. Do you have a first Gen 8cyl Firebird? Found your answer interesting....are you referring to the plastic bushing/retainer 403929 shown here:
https://www.ebay.com/itm/NOS-1966-1979-Chevelle-Camaro-Firebird-Toronado-accel-cable-retainer-403929/323050967801?_trkparms=aid%3D111001%26algo%3DREC.SEED%26ao%3D1%26asc%3D20160908105057%26meid%3D5ab0023f8d2c4ab89db13898ad68fcb8%26pid%3D100675%26rk%3D6%26rkt%3D15%26sd%3D351434061297%26itm%3D323050967801&_trksid=p2481888.c100675.m4236&_trkparms=pageci%3Ab03c351c-aecb-11e8-8433-74dbd180db3d%7Cparentrq%3A9b132a8b1650aa15b389df37fff8bdda%7Ciid%3A1

My reason for asking, a few of us did a quick research on this retainer and found it was only used on 6cyl and 70-72 years. If you could give an update/confirmation would be appreciated.

I have read somewhere that the 68 ohc 6 and v8 4bbl took the same cable. I have a mickey mouse setup on my 69 (2bbl cable on a 4bbl and it does not have the required travel for the 77 q-jet that is on there)