What is wrong with my 1971 Chevy 350 engine?

Question

I have a 1971 Chevy Impala with a 350. The exhaust manifold has been replaced with headers and a dual exhaust system (no crossover pipe), and the original carb has been replaced with an Edelbrock 750cfm 4bbl. It has plugs and wires that are about 3 years old.

The engine runs poorly and does not produce very much power. When I try to accelerate from a stop it bogs down and nearly stalls unless I give it lots of gas. Often it will backfire through the carb shortly after it has been started, the air cleaner is black with soot. After this happens it runs a little better for a while.

I've tried adjusting the accelerator pump on the carb to provide more or less fuel, this does not appear to help. I've adjusted the set screws on the front of the carb which also does not seem to help. I've adjusted the timing in the past, but have not checked it recently. I've checked that the plug wires all go to the right cylinders and cleaned the plugs.

I forgot to mention that the intake has been changed from the stock to an Edelbrock RPM Performer.

I just tried starting it up to check the timing but it's 34F out there right now and it won't fire up even with some starting fluid. I also have a '66 impala that gave me fits setting the timing, the mark would jump all around when I tried to set it. After I pulled the engine I found that the timing chain was quite lose. The mark on this engine doesn't jump around like that, and when I rev the engine up I can see it advance fairly smoothly.

The advance on the distributor is connected to the 'ported vaccum advance' on the carb, not the 'full time advance' (I've got that one connected to the heater switch, heh).

I have an old quadrajet carb and adapter plate for it, but it runs even worse on that, I'm not convinced that it works properly (may be missing some linkage rods, I don't know for sure).

Answer 1

ok,try this.its a 71,so it probably got points.get some blue streak points,condenser,rotor and cap,a set of plugs and wires.also check to see where the vaccum advance is hooked up to(it should be hooked up on the carb.on the ported side)if its hooked up straight to the intake,then its advanced all the time,what you dont want.also put a timing light on it,and get a new air filter.whats the matter,i figure someone hooked up the advance to the intake,so when you need the power,its not there,because it cant advance anymore,because its advanced as far as it can.hope this helps you.

Answer 2

Put in a fresh set of plugs and check the timing (vacume advance plugged). Sounds like a timing issue to me. Also, an older V8 needs new plugs at a minimum of once a year. If you have done any mods to the engine I would change them every 6 months depending on how often you drive it.

Also, screw in the two small adjustment screws all the way on the carb, then back them out a 1/4-1/2 turn. That should be good to get the car running and you can fine tune the carb after it starts.

Answer 3

71 Impalas are low compression pollution controlled engines with low horsepower as well. If it had a 2 barrel originally then only has 145 horsepower stock, the higher version of the engine in the Impala was only 190 horsepower. You have way too much carburetor on that car for that much horsepower. To use a 750 cfm on that engine you would want to change the cam, heads and maybe the pistons to give you more compression and better flow. For instance the Corvette in that year only shows 250 horse stock according to Chiltons. One other thing to check in addition to many of the other great suggestions on timing, etc. is that the camshafts on Chevy v-8's are soft as heck and wear easily. You can see this with the rocker covers off and the engine running as one or more rocker arms will be moving slightly compared with the rest. I am still voting though with way the heck too much carb for the engine. It has no way to handle all that fuel. The cars of all makes in those early 70's of American manufacture had drastically reduced compression and restrictive heads all attempting to reduce emissions. In fact the reason that Chevy came out with the 400 which was also offered in those same years was for a bit more torque to make up for the drastically reduced horsepower because of emissions. It had more torque than the Vette that year, but only 150 horsepower itself. Crazy years for automobiles huh??? :-) Try a much smaller carb and see if that doesn't fix the problem or address the internal engine issues to increase compression and flow.

Answer 4

1971 Chevy

Answer 5

Primary metering rods in carb aren't working right. Do"shotgun tuneup"(replace plugs,wires,cap,rotor) and rebuild carb. Backfire can be lean condition. Engine is running on secondary rods so it will deliver power at high throttle. Get Edelbrock Carb Owner's Manual and Calibration kit for your series carb. That Edelbrock will run ANY V8 if it's set right (I'm using a 1406 on a V6.It works great!) It's one of the most versatile carbs you can buy.

Answer 6

Sneezing (backfiring thru the carb) is almost always a sign of timing issues. This is caused by having a cylinder fire before the intake valve closes. The difference could be mnimal yet have a huge effect on engine performance.Also, timing chain stretch could account for the symptoms as well since the stretched chain can alter timing.

Answer 7

1971 Chevy Impala

Answer 8

First choice- way too rich on fuel to air mixture. Which means adjusting the mix screws properly or changing the jets in the carb. It is surprising considering that more opened exhaust system should allow airflow like crazy.

Second choice- possible wrong intake and carb setup for the engine.

Third choice- low compression [worn out cylinders], low compression ratios 8 to 1 or less, wrong or worn out camshaft, distributor problems.

Answer 9

Sounds like a couple of the responses to your question came close to helping you identify a timing problem, simply check with a good quality timing light, if the line on the harmonic balancer and the timing pointer move around a lot or jump around a lot, it's a sure sign of a worn timing chain and gears, be sure to replace both gears and chain, dual roller is the best type or replacement type. GOOD LUCK.

Answer 10

Seems to me like you are running on the rich side. Aslo, you mention that you cleaned the plugs...when was the last time you did a tune up and replaced the plugs?

You are concentrating on the carb when it seems to me like you need to concentrate of general maintenance.

A '71 engine with even updated ignition is still going to need regular (read yearly) tune ups and maintenance. 3 years for a set of plugs is about 2 years too long.