What is wrong with my 1971 Chevy 350 engine?

i dunna

go for the timing. that engine was built to run on premium gas. also a cam with a tad more duration will probably do the trick. good luck…you have a potential powerhouse there!

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.

I’ll bet you don’t drive the car alot, and because of that your fuel system will give you fits because todays fuel is not specifically designed for carbs. The stuff dries up and leaves some funk in carb, lines, and tank.

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.

that carb is way too big for that engine should run a 650 at the most unless you have tons of high performance goodies in it. sounds like timing is your issue or distributer may also be bad . is it still a points style ignition or upgraded to hei? if points change them and reset timing

i own a repair shop,and you need to check the free play in the timing chain and gears,,Chevy’s are real bad for wearing out chains and gears ,i do a lot of them at my shop,you can check it by removing the cap,and rotating the engine by hand,,and watch the rotor button to see how long it takes it to move this will help determine how much play you have in the chain,if it has over 80 thousand on it,,its time to change it ,they get real worn at that mileage,,but its a timing problem with it,,good luck hope this help,s.,have a good x-mas.

You’re carb is WAY too big.Try a quadrajet or a smaller holley.

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.

I do belive your timeing is off. are you have missed placed plug wires meaning you have pluged them into the wrong place and crosswired them.

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.

zskip62 gave you the correct answer so change the carb.
And give him 10 points!

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.

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.

Disconnect, and plug the vacuum advance.
If engine performance changes, it’s timing.
If not, it’s carburation, or mechanical.

your problem is simple, have the crossover pipe put in.what is happening is your engine wont run right because it cant breathe correctly. essentially what ya have are 2, 4 cylinder engines. i think youd be amazed how much better itll run with the crossover pipe installed. what the crossover pipe does is allow for a scavenging effect in the exhaust system.ask any shop that makes exhaust systems.they’ll tell ya u need it

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.

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.