[Bermudagto]

I just got my newly built 428 installed after its dyno run up. Idles Nicely in car. When I go to connect the vacuum hose from rear of intake to brake booster, the engine takes a major dump. Barely runs! What’s going on? Thanks!

[johnta1]

If the vacuum line is blocked off when not connected to the brake booster, I'd say that possibly the brake booster is leaking or hose is leaking.
is there any noise from the booster or hose?

[JLMounce]

Have to agree with John on this one. Assuming you are blocking off the booster line and the engine runs fine, then it runs like crap once you've connected the line to the booster, this indicates a leaking booster.

[Bermudagto]

Ok! The engine was running fine with the vacuum port at rear of intake uncapped (and no connection to booster). I had lack of braking on first drive so I realized the brake booster hose was disconnected. I connected it and that’s when motor took a dump.

[Bermudagto]

The brake booster hole does in fact make a hissing noise once disconnected.

[johnta1]

WOW! Usually it runs like crap when there is a huge vacuum leak like that hose would have.

I'd say check timing with it disconnected then see what it is with the brake booster hooked up.
The carb might be way off from the extra air going in also.

[65 Lamnas]

Originally Posted by Bermudagto

Ok! The engine was running fine with the vacuum port at rear of intake uncapped (and no connection to booster). I had lack of braking on first drive so I realized the brake booster hose was disconnected. I connected it and that’s when motor took a dump.

If I’m reading this correctly and understanding what you’re saying, then the engine “runs fine” with a huge vacuum leak, which might indicate a rich condition.

Take the brake booster out of the equation by capping the vacuum port for the booster. Figure out if it runs better with the huge vacuum leak or without.

[Bermudagto]

When I cover the rear intake inlet (that I would expect connect to the brake booster) with my thumb, the car bogs down very significantly.

[SRGN]

It's relying on the vacuum leak to idle. With the port blocked, adjust the carburetor mixture screws and throttle blade opening to achieve the best idle. Adjust timing to spec. Make sure you have the right PCV valve and it's plumbed properly, most will flow a little bit at idle vacuum and the carb is likely tuned for that.

[Bermudagto]

That seems to make sense! So when I block the intake hole (or connect the brake booster hose to it), the car goes rich and stumbles? So my plan is to reconnect the brake booster hose, then lean out the mixture on both fuel inlets of this double feed carb. I don't quite know how to adjust the throttle blade opening unless you just mean the throttle screw at the linkage. Thank you. I will report back when able.

[SRGN]

Yes, mixture screws to lean it out, the idle screw to set the throttle opening. It wants more air, and gets it with the open vacuum port. Once you set the throttle plates open a little with the idle screw and adjust the mixture screws, should run a lot better at idle. Then you can get the idle adjusted and set the timing where it needs to br. 34 degrees total, all in by 3000 RPM with the vacuum advance disconnected is a good starting point.

[Bermudagto]

Ok that's a plan. I'll get all the vacuum lines hooked up and study up on mixture adjusting. Thanks again

[JLMounce]

I would recommend starting from an out of box baseline here. It seems as though the carb may have had some tuning done to it while the engine had a severe vacuum leak. It looks like you've got some type of holley style carb on there. If you have the carb documentation laying around, read up on it's baseline calibration. It may typically have you start by setting the idle speed adjustments at a specific spot and then depending on how many idle circuit screws it has, also setting them in a specific spot. Something like, 3 turns out from fully seated.

Set everything back to how the carb would have been delivered. Additionally, check the base timing without vacuum advance plugged in and make sure it's within a good spec for your engine. Likely somewhere between 8-14 degrees initial.

Then go about setting your mixtures from that baseline. Right now, for the car to be running somewhat well with that large vacuum leak, it's likely that your throttle butterflies are essentially completely closed. You may find that to get the car to run you end up opening those butterflies enough that you get into the idle transfer circuit. As this begins to occur, you typically will gain a lean tip-in stumble. That "might" be the next thing you encounter while doing this.