A friend recently did a motor swap, and changed carbs at the same time.

The old engines carb was in bad shape, so he replaced it with a different unit on the new motor.

Now there is a whistling coming from the carb, he can find no obvious vacuum leaks. And while the motor does run, the truck is really low on power.

The motor was a good one, used for several years in another truck.

Any ideas what the cause of the whistle might be?

Did he change the timing chain.

It's a vacuum leak.
Many carbs have a large port in the back for feeding vacuum to a brake booster. Some of them use a pipe plug to seal it. That's a common area for leaks.

What kind of carb is it?

Try this:

1)Run the engine at idle until warm.
2)Open up a propane torch, but DON'T light it.
3)Aim/hold the torch towards the area where you think the whistling may be coming from.
4) Since the leak is a vacuum leak, the engine's RPM will increase as the propane gets sucked in and
5) Viola
You've located your leak.

Use a couple feet of old heater (or garden) hose, One end close to your "good" ear, and wave the other end around suspected problem area. Very obvious. Can hear vacuum leaks, mechanical noise, various clicks and bangs, etc. Better than a stethoscope, and cheaper, too.

Stock or after market intake????

What type of carb????

Probably not the case, but I had a fuel injected car that whistled and had no power when the catalytic converter element came loose and plugged the exhaust.

If he does have a Cat, bang on it and see if it rattles from the pieces bouncing around.

Sounds like a situation I once had. My whistling sound went away as idle was increased and the sound increased as idle was decreased. I searched all over for vacuum leaks, but found nothing. The problem ended up being the throttle bushing.
Jim

I once had a whistling vaccum leak on my LRE. I could not find the issue after checking everywhere, but it got louder and louder. This was confusing as I had recently put on a an M1 intake and new carb. Took it off and the black intake gaskets from ma mopar looked like potato chip crumbs. Replaced the gaskets and truck idled 900 higher. Tragic story, truck got totaled a couple days later.

On a buddy's car (440 with M1 intake) and a Holley carb it had a vacuum leak at the rear of the carb because he used one of the thin paper gaskets.
With that particular intake when using a Holley carb it exposed a vacuum passage to the outside at the rear center of the carb.
The manifold had a cast pedestal for one of the rear mounting bolts and a very thin flange around the throttle bores.
He had problems for months. He would pull the carb, install a new thin gasket and it would run okay for a short while.
Then it would start running bad, backfire and really start running bad and start whistling.
It was at the point that he needed someone else s opinion on it.
I helped him pull the carb started looking at it.
The gasket looked funny, like it had been blown down.
I turned over the carb, saw the large vacuum passage (pcv) and figured out that nothing was supporting it on the intake (since that area was open on the intake).

The fix was use either one of the 1/4 inch thick gaskets or 2 thin gaskets with a metal gasket/carb spacer in between them. He chose the latter option.

It cured the vacuum leak problem permanently.

yep, sounds like the same issue.

As it was mentioned, if you are running an aluminum spreadbore eddy intake and a 600 eddy carb, you need this adapter kit, from edelbrock:




http://www.summitracing.com/parts/EDL-2732/

Intake is a stock 340 cast intake, square bore. so its not the spread bore issue.