Running Rough

[bobcostas]

Hey Dudes,
I am so close to a daily driver i can smell it... and it smells good.

I have a 400 small block with a 4-barrel Edelbrock that i put on recently.
Now, when the motor warms up (after ~10 or 15 minutes of driving) the engine gets louder. It seems can hear the exhaust. It just runs rougher than when it was cooler. A mechanic took a look at it when I was getting the new exhaust on, and said the timing wasn't advancing as the RPMs increased. could this be causing the roughness?

thanks guys. I'm gonna post some pictures when i get 'er all done.

[geezer#99]

Use your timing light to check if the advance is working. Plug off the vacuum hose to the distributor and while slowly reving up the motor watch the timing mark on the balancer. You should see it advance. Hook up your vacuum to the dizzy and it should advance even more. Don't be concerned about how much. Just that it does advance.
Also check for a vacuum leak for ruff runnin'. New carb installed!!! Possible base gasket leak or cap off a vacuum port.

[bobcostas]

It seems a bit odd to me that it only gets rough when warm. After a few minutes of sitting, it'll start right up and sound fine. Then 10 minutes later it sounds like i can hear each cylinder firing.

Im pretty certain there aren't any leaks at the carb. ill check how the timing advances in a few min.

[markeb01]

Did the noise and roughness just start to occur since you installed the Edelbrock carb? Was the truck running with a different carburetor previously, or is this a fresh build and wasn’t running at all before?

[bobcostas]

ITs been running on and off. I've just been replacing bits and pieces. but the carb i put on last week to replace a Holley POS. The roughness is not unique to the carb however. It started this nonsense after i put the motor back in with the new exhaust and 4-barrel set up.

[bobcostas]

Geezer, it seems to be advancing just fine.

[markeb01]

Originally Posted by bobcostas

ITs been running on and off. I've just been replacing bits and pieces. but the carb i put on last week to replace a Holley POS. The roughness is not unique to the carb however. It started this nonsense after i put the motor back in with the new exhaust and 4-barrel set up.

If that’s the case, I’d start looking for an intake manifold leak or poor functioning of the exhaust riser just beyond the passenger side exhaust manifold. It would really help if you could hook up a vacuum gauge. This will help identify if you do or don’t have any vacuum leaks, and will also indicate if there are any problems with the valves as well.

There are any number of web links that explain what the gauge readings mean. Here’s a typical example:

http://www.classictruckshop.com/club...ts/vac/uum.htm

If you also had recent exhaust work, make sure the heat riser is functioning properly. If it won’t open completely the right side of the engine will overheat, and eventually the intake manifold will get so hot it will boil the gas in the carburetor, which will certainly make your engine run rough. The valve itself could be old, tired and sticky, and not opening completely. I’ve also experienced a problem where the installer built the system incorrectly, and the weight on the heat riser collided with another exhaust component, blocking it partially shut. Eventually the engine overheated enough to blow the head gasket.

You might also want to check the spark plugs to make sure they are still serviceable. If the old carb was dumping fuel, they could be fouled, contributing to the rough running. I agree with Geezer on checking the advance with a timing light. You need to know if both the centrifugal and vacuum advances are working. First disconnect and plug the vacuum line going to the distributor. With the timing light connected rev the engine and see if the timing advances on the crank. If it doesn’t, or is erratic, there’s a problem with the weights or springs in the distributor. Then with engine idling, connect a hose from full manifold vacuum to the advance and see if it works as well. If there’s no change at the crank the advance mechanism is dead. These are easily replaced, but usually requires pulling the distributor.