Got questions? We’ve got the answers—every Monday when the Summit Racing tech department tackles your automotive-related conundrums. This week, we’re talking vacuum leaks, torque converter stall speed, and setting the proper valve lash.        

From: S. Arikian • Montauk, NY 

Q: I have a 1970 Challenger with a 440. The short block was internally balanced and stroked to a 496. I’m running stock rockers, an Edelbrock RPM intake, a Holley StreetAvenger 870 cfm carb, and a few other performance parts. It’s putting out around 550 hp.

The problem is that at 4,000 rpm the engine produces a screaming whistle! Have you ever heard of this before? What’s the problem?

A: It sounds like you’ve got a vacuum leak in the carburetor or between the cylinder head and intake manifold. Next time you fire up the engine, spray some carb cleaner around those areas. You’ll know when you’ve found the leak because the whistling will let up.

Also, try tightening everything down to spec. If that doesn’t help, you may need to replace a gasket or rebuild your carb. If that’s the case, be sure to visit SummitRacing.com to find handy tools, gaskets, and carburetor rebuild kits to get your Challenger running right!

From: Neil Borror • Riverside, CA

Q: I’m considering a new torque converter but I’m not quite sure which stall speed best fits my needs. I have a posi rear end with 3.73 gears and a pretty stout engine setup with a mild camshaft. But the stock converter in the TH-350 just won’t get the power to the ground! If an engine has to reach stall rpm levels before moving from a dead stop, wouldn’t an excessive stall speed be very inconvenient in anything street-driven? I don’t want to have to floor it just to move at every stoplight. What stall speed do you suggest?

A: Choosing a torque converter can be tricky. For instance, a converter with too low of a stall speed can hinder your performance. If the engine makes power in the mid to upper rpm range it will have to slog through the lower rpms first. But with a higher stall converter, you’ll bypass the lower rpms and jump right to where the power is!

On the other hand, running a converter with too high of a stall speed will cause you to use too much of the good power before the vehicle even starts to move. Too large of a converter can also build excessive heat from slippage and damage your transmission. You’re right in your assumption that a converter with excessive stall speed would be tougher to drive on the street.

Here are some basic guidelines for choosing the right converter. In most street applications with 3.73 gears and a mild cam, around 2,200 rpm stall speed is ideal. With bone stock applications a 1,400-1,600 rpm stall speed is just right.

From: Kenneth Edwards • Monticello, KY

Q: I have a Chevy 400 small block with flat top pistons, double hump heads, and several Summit Racing parts, including roller rockers and a hydraulic cam and lifter kit. It also has an MSD 6A ignition box, a Holley 750 carburetor, and a Holley dual plane intake manifold. What is the best way to set the valve lash on this part combination?

A: When adjusting your valves on a hydraulic camshaft engine, you need to make sure the lifter is on the base circle of the cam. After that, you should tighten down the rocker arm until all of the up and down pushrod free play is gone. This is known as zero lash. Then continue to tighten the rocker an extra half to three-quarters of a turn. Repeat these steps for every valve. The main goal is to make sure the valve that is being adjusted is on the base circle of the camshaft.