View Full Version : 1978 Pontiac TRans Am 400 motor trouble shot

Joshua VanMeter
01-18-2003, 04:50 PM
My father and I have been restoring a 1978 Pontiac T/A. When we
bought this car the motor had been rebuilt, but it was'nt running
right . So my father and I decided to tear it down to see what
we realy had.
This is what we have it is a Pontaic 400 motor with a Competition
cam. Here are the specs. Gross valve lift = intake .454
exhaust .454
duaration .006 tappet lift intake .268
exhaust .268
Valve timing open = intake 28 btdc
exhaust 68 bbdc
closed = intake 60 abdc
exhaust 20 atdc
Duaration at .050 = intake 218 exhaust 218
Lobe lift = " .3026 " .3026
Lobe separation " 110
Part # 51-232-3

We also have the original Quad Jet carb. that has been rebuilt.
Heres the problem.
The engin runs, but the rpm needs to be set around 1200 rpm
to keep the engin running when you put the car in gear (it will stall and die.) Need help to find the problem, we went threw the
motor and put every thing new in it except the cam & carb.
We had the motor hot tanked & head reworked etc.
We still have the "Same problem as we did before we rebuilt it"
We think its in the carberator but we're not sure.
Can someone Help Trouble Shot THIS PROBLEM?

01-18-2003, 08:38 PM
Unless you want that "tha thump.....tha thump" sound and engine vibration, install a standard cam.

01-18-2003, 11:00 PM
i second that-the cam is probably the culprit here. this sounds like the high energy comp cams 268 extreme energy. these cams don't provide much vacuum and sometimes require a vacuum pump for your vacuum accesories. you would probably be better off with a crane 266. the 400 is a good engine and has plenty of torque stock.

Joshua VanMeter
01-19-2003, 01:56 PM
If my problem is the cam, will the valve springs need to be replaced to stock size or can we still use the ones that were used with the high lift cam? Would we need to purchase new hydrolic lifters and push rods to make sure the cam is broken in correctly? Also is the stock Quad Jet carb. a good carb. to leave on or should we look for a different carb. We've heard some Quad Jets are good and some aren't.

01-19-2003, 02:03 PM
Could you post the vacuum signal while it is at idle? I would have a better idea of the problem if I, and the other boy's on here, had those specs.
Even with the bumpier cam it should idle at a lower rpm than 1200.If you and your dad reassembled this engine,I'm sure you must have timed the valve train at least straight up.I think that if you have a problem with the carb then either the low or(and) intermediate ports are at fault or even check the secondary trottle plates and the spring tension that keeps the top flap shut and that it is not falling open at idle.
Without more info we could just ramble on so post some more operating qualities and specs please and see what happens.

01-19-2003, 02:28 PM
Gettin some good info here........

The cam you have have the same specs as the Comp Cam 51-233-4. This cam isn't much different that the GM OE cam 066 for Pontiac. This isn't a real radical cam seeing how I have the Lunati equivilant to the OE 744 cam or Ram Air III cam. Mine has the 110 LSA instead of the 111.2. I pulled out the 066 because is wasn't lumpy enough. I even have the OE Melling Ram Air III cam unused in the box because it wasn't lumpy enough.

The Pontiac heads will take up to .470 lift without changing valve part geometry. If you plan on a high rpm use a stiffer spring set will be your best bet.

If the engine has already been run with that cam in it then it should already be broken in. Only takes about 20 min at 2500 rpm. No special parts are needed to break in the cam correctly. Just an oil change afterward.

The no low idle problem may be vacuum leak, ignition timing or the carb. The engine should pull between 12 and 15 inches of mercury on a vacuum gauge. My lumpy cam pulls 12 with a vacuum advance on it uping the advance to 18 deg at idle (1,000 rpm) with 10 deg static with a 9.0:1 compression ratio. I'm telling you this to try to give you a reference point. My 78 T/A is quite a bit different so we can't compare apples to apples. Stick with this, the 400 Pontiac motor is a good building platform.

The Q-Jet is a good carb, easy to rebuild and tune. The 750 CFM version is a good size too.

Joshua VanMeter
01-19-2003, 03:27 PM
We have an old style engine vacuum fuel pump tester guage for checking but, we're not sure how to use it, would this help us in any way if so, how exactly would I do it. With the engine running to low at idol (about 750 rpm) when you put it in gear the engine rpm drops to around 500 rpm and dies while it's in drive. Is this due to a lack of vacuum? Thats why we have to keep it running at about 1200 rpm at idol just to keep it running in gear.

01-19-2003, 04:14 PM
That type of vac. gauge is a good one.....just make sure it is fairly accurate. The needle should rest at zero and move evenly when you apply vacuum to it.

Hook it to MANIFOLD vacuum. A port at the base of the carb or a supply right off the manifold runner.

At any good idle setting so that the engine stays running but as low as it will go. Get a reading it should be close to 12 inches. If it is real low and steady then look for a retarded timing setting.

If it fluctuates up and down slowly but steadily then you have a vacuum leak or the carb is not balanced. Unplug the vacuum advance and see if the vacuum changes, if it doesn't your vac. advance is not working.

If the needle bounces rapidly then you have a mechanical problem. Stuck or burnt valve, broken valve spring, etc.......

Do this and get back to us.....with details........

Joshua VanMeter
01-20-2003, 08:17 PM
After letting the car warm up we got it to idle at 750 rpm. and took a vacuum reading from a manifold vacuum port, it was a steady reading of 15-16 inches.
Then we took the vacuum advance off and vacuum did not change.
Then we took the timing light and checked timing mark with the timing advance on and reved the motor and the timing marke advanced correctly.
Then we unplugged the advance and reved motor didnt advance as far or as quickly.
Moved rpm up to about 850 and it did advance the vacuum up to a steady 16-17 inches.
We did put the car in gear while at 750 rpm and it dropped to 600 rpm and continued running.

01-20-2003, 09:30 PM
did you do anything before starting/warming it up or was the engine as you stated before posting? meaning maybe you didn't let it warm up before?

01-22-2003, 12:33 AM
Actually it doesn't sound too much out of line for a cam with a short LSA (111) and a little more duration.

A cam like this will normally need the idle to be increased some. 800 to 850 is not out of line for this type of cam. My SS idles at 900 with the factory fat cam in it.

My Pro Street T/A idles at 1000 with a 301/313 duration and a 110 LSA.....The lobe separation angle (LSA) creates an overlap where both valves are open for a short time....this reduces the vacuum availability which will naturally call for an increase in base idle settings.

It does sound like your vacuum advance is not operating properly. You should have noticed a sizeable drop in vacuum and idle speed when you pulled it off.

The increase in advanve you see when revving the engine is from the mechanical advance doing it's thing.

To check the vacuum advanvce you should check the base setting say at what the factory setting is 12 deg. Vacuum advance should take it up to around 22 deg. When the throttle is slightly opened it should drop to the static setting then rise slowly.

Balance the carb correctly with the air/fuel mixture screws and you should be good to go.......