Ben1

Recently bought a 1978, previous owner claimed it had a knock in the motor, when I went and looked at the car he started it up and I realized the knocking was not from the motor, but more than likely it was the flex plate or torque converter. I bought the car right on the spot and for the low price he wanted, took the car to a mechanic and he confirmed what i thought, he said that it is either the torque converter or the flex plate. The car is scheduled to go into the shop to fix the problem on the 25th. It has a rough idle, almost like it is missing on a cylinder or two. It drives okay but it's gutless, doesn't really get up and go at all. My question is, could the converter/flex plate issue cause the rough idle and poor performance? It also idles high, after warmed and at running temp, it idles around 900 to 1100 rpm, but once in gear, it'll drop to 650 to 750 in drive while holding down the brakes. Any ideas????

thanks

JayRay

The RPM dropping under load (in gear) is normal. For the rough idle it will either be ignition related or carb related. Have you changed the spark plugs and plug wires since you got it?

Ben1

Plugs no but the wires are already done. Also, it runs rich, very rich.

san

If its a quadrajet its likely a leaky fuel bowl. They are well know to have this issue, and it makes them run like crap at idle and its keeps it rich at all times...which makes them loose alot of power overall. A good way to know is let it sit overnight, in the morning pop rhe hood, remove breather, and see if you can get more than 4 squirts from the accelerator pump without getting any air. If you get one squirt then air, its leaking. Also makes them have to whirl over a hundred times when cold before it starts. Someone may have played with idle screw to help it stay running.For now you can back the air screws out a couple turns to keep it a little more lean at idle, amd it should idle alot faster then, then tuen idle back down, but there is no fix for hard to start other than a rebuild of carb. Also...torque converters can knock like a mother...seen it a couple times...flex plate should also make a scrape sound if its that.

Ben1

It's not the original carb, I'm not sure what style it is, all I know is that it is a holley. Not sure of size but it is a 4 bbl

HeadsU.P.

First order of business would be to watch some videos about Holleys at Summit Racing web site. Then you can start making adjustments such as float level, idle mix screws and maybe secondary throttle adj. You can bet that Ethanol likely ate some parts but gaskets, needle seats & valves, floats are cheap. Use nothing but Holley parts, please.

Ben1

I start watching down videos tonight and get a better understand on how it works

HeadsU.P.

If you buy gaskets: bowl gasket or metering block, get the blue ones. They won't stick to the main body and can be used over & over.

san

I wouldnt worry too much about carb...easy fix. Just check all the bolts that hold it down to manifold...they are likely loose. You can watch videos on youtube about tuning and settings. Likely needs a good cleaning and rebuild. I would focus on the knocking first....then its time to super tune!!!

Ben1

Knocking will be fixed at the end of this month. Hopefully the bill isn't too steep. I'm just glad it wasn't the motor knocking. I'm going to be out of town this weekend, going to visit family and I unfortunately have to leave the vette at home. When I get back I'll start looking Into loose bolts, damaged gaskets and things like that. Thanks everyone!!!

z28cp

I had a Camaro with a cracked flexplate. Knocked like a mother, idled a little rough. But it didn't affect the power any.

san

Had a 78 z here..cracked plate and had to drive the mother for like a hundred miles....it knocked lile a mother too...but at first couldnt here it when idleing , only with a shot of gas.....after ride....it was scraping at idle....knocked like a mother still with a shot!!!! Hahahaha good catch about idle.....im trying to remeber exactly how it ran in both instances...i believe torque converter came and went.....it was in a chevette....HAHAHA same torque converter , smaller bolt pattern.

san

Damn i was told the same thing 25 years ago when my buddy bought a chevette that knocked....swapt it out amd it worked!!! If i heard it today i would still think its the motor....had a 351W smallblock ford in a 80 ranger that let go of a piston.....sounded EXACTLY the same!!!! Hope that 4 bolt hasnt suffered the same fate....is it an l-82? I talked with an engine builder about those blocks just the other day....he is retired now and knows them well...lol

Pink connecting rods.....HAHAHA!!!