What kind of Manifold Vacuum Pressure should I have?

[Sluggo]

Just rebuilt & installed the engine. Did the intial break in but not the driving break in. Haven't gotten past the fine tuning. I think I have a big vacuum leak. :smt093 Manifold Vacuum is at about -7 PSI. What should it be? I read somewhere that it should be around -14 but I can't remember. And I always prefer more than one opinion. I replaced the crosspipe and that's not it nor is it the connectors to the manifold. I'm gonna replace the manifold to head gaskets. I made them from that make your own gasket material and it was very thin. I can hear a pulsating hiss on the right side of the engine by the back of the carb. I thought it was from the carb to manifold (same crappy thin gasket material) but replaced those and it's the same.

Any advice on where to get thick manifold gaskets (not the spacers) for a Weber 44 Intake?

This is what I get for manufacturing a crosspipe (with built in gauge), fuel pump relay, solid brake vacuum line, fuel line & breather system at the same time as I built the engine. Too much stuff to work out at once. At least the fuel delivery system (- the relay), crosspipe & breather system work well. I'll add the brake vacuum lines to that list when I rule them out as a possible leak.

[dingo]

optimally 18-20 psi...or maybe as low as 15psi. maybe itll get up there when your rewbuilt parts settle in to their groove. What is the needle doing ? steady ? fluctuating ? cyclical ? etc etc The vacuum needle tells the tale

[bottomend]

You can get thick gasket material from our "friend". He'll sell you how ever much you need and it'll be thick and cheep.

Hissing is definatly a leak. Good idea not to drive like that. I'm not sure if it'll mess anything uyp but too much air in the intakes could change your mixture and make it run lean which = heat.

Can you adjust some setting here on the board so I stay logged in? Every time I leave and then return I have to redo my pass word ect... thanks.

[Sluggo]

optimally 18-20 psi...or maybe as low as 15psi. maybe itll get up there when your rewbuilt parts settle in to their groove. What is the needle doing ? steady ? fluctuating ? cyclical ? etc etc The vacuum needle tells the tale

Needle is rapidly bouncing between 6.5 & 7.5 in synch with the hiss.

[Sluggo]

You can get thick gasket material from our "friend". He'll sell you how ever much you need and it'll be thick and cheep.

Hissing is definatly a leak. Good idea not to drive like that. I'm not sure if it'll mess anything uyp but too much air in the intakes could change your mixture and make it run lean which = heat.

Can you adjust some setting here on the board so I stay logged in? Every time I leave and then return I have to redo my pass word ect... thanks.

Thanks! I'll go see him.

I'm trying to figure this one out. The only two bugs that have really been plaguing me are the "keep me logged in" feature and making pics from the gallery visible in posts. I can only get it to work in some forums.

[dingo]

Needle is rapidly bouncing between 6.5 & 7.5 in synch with the hiss

.

Assuming correct vacuum source, timing and rpm...thats wayyy low....must be a large vacuum leak or maybe just very early stages of braking in..but thats the boundary of my knowledge

this is a good starter on reading vacuum guage:

http://www.centuryperformance.com/vacuum.asp

[Sluggo]

Needle is rapidly bouncing between 6.5 & 7.5 in synch with the hiss

.

Assuming correct vacuum source, timing and rpm...thats wayyy low....must be a large vacuum leak or maybe just very early stages of braking in..but thats the boundary of my knowledge

this is a good starter on reading vacuum guage:

http://www.centuryperformance.com/vacuum.asp

Thanks! Great info. I added it to the Info Links Forum.

[bottomend]

I own a MightyVac. It's somewhere in your workshop. Feel free to use it if you can find it. The instruction book that came with it shows different kinds of vacuum and the causes/effects of the readings.

Also, did you match port the manifolds to the heads? Did you surface them so they are flat?

[Sluggo]

did you match port the manifolds to the heads?

Not to sound stupid, but I didn't even know that needed to be done. Are you talking about the bottom openings (ports) being the same size as the intakes in the heads?

[DurocShark]

Match porting is where you make the openings perfectly line up. Some prussian blue dye on the manifold, bolt it in place, wait a few minutes, then unbolt and remove. You'll see where the differences are. It's a good idea to do it again with the dye on the head side instead to be sure to catch any variances. You also need to cut the gasket to match just as perfectly.

But, honestly, for a mostly stock engine? Meh. I wouldn't worry about it. The improvement wouldn't even be noticable.

Machining the intake surface, however, *is* a good idea. That makes sure it's perfectly flat and square to the bolts. Ideally you want to do this to the head as well so that one side isn't wavy while the other is straight.

Thick gasket material will work wonders to seal uneven surfaces.

[Amskeptic]

did you match port the manifolds to the heads?

Not to sound stupid, but I didn't even know that needed to be done. Are you talking about the bottom openings (ports) being the same size as the intakes in the heads?

Give your engine time to break in over the next 200 miles. Do not assume a vacuum leak yet, particularly if it responds well to your mixture adjustments. If your idle goes up when you remove a small vacuum hose or pump the brakes lightly with your booster hooked up, mixture is sufficient. Do not waste time tracking this stuff down on a fresh rebuild. I can hear too much idling from here. Drive it. Let those intake valves seat. Let the rings seat. Keep oil pressure and flow up to rinse out all the metal particles that are being shed. If it drives well, keep driving. Only monitor the temperatures and exhaust pulse, for cool enough and smooth enough.
Dancing low needle can be tight valve guides and weak valve closing that will only improve.
Colin
(if it is running rough, however, track it down)

[Sluggo]

Give your engine time to break in over the next 200 miles. Do not assume a vacuum leak yet, particularly if it responds well to your mixture adjustments. If your idle goes up when you remove a small vacuum hose or pump the brakes lightly with your booster hooked up, mixture is sufficient. Do not waste time tracking this stuff down on a fresh rebuild. I can hear too much idling from here. Drive it. Let those intake valves seat. Let the rings seat. Keep oil pressure and flow up to rinse out all the metal particles that are being shed. If it drives well, keep driving. Only monitor the temperatures and exhaust pulse, for cool enough and smooth enough.
Dancing low needle can be tight valve guides and weak valve closing that will only improve.
Colin
(if it is running rough, however, track it down)

It's definitely a vacuum leak. On the rear barrel (#1) of the right manifold. Mixture screw barely makes a difference. It got worse (the hiss) as it went. Actually ran pretty good when I gave it some throttle. But idle is very rough and the pulsating hiss is very evident.

I'm gonna check the position of the ports and file down the head mating surfaces of the manifolds. I'm assuming I would do this the same way we did the exhaust manifold when you were here last. And then add some thick gasket material. If it runs well enough then, I'll dial in the carbs, set the timing and finish the break in.

Speaking of idling. Since I live in Hollywood and spend a lot of my drive time idling, should I rev it slightly while sitting? Just for the first few hundred miles?

[Amskeptic]

But idle is very rough and the pulsating hiss is very evident.
Since I live in Hollywood and spend a lot of my drive time idling, should I rev it slightly while sitting? Just for the first few hundred miles?

It is important at the very beginning and loses importance as the miles accumulate. I would not worry about it after the first hour or two of runtime unless you have a sig alert shutdown. I would shut the engine off if it looks like you are standing for more than a couple of minutes.
Colin

[Sluggo]

I own a MightyVac. It's somewhere in your workshop. Feel free to use it if you can find it. The instruction book that came with it shows different kinds of vacuum and the causes/effects of the readings.

Also, did you match port the manifolds to the heads? Did you surface them so they are flat?

The ports match. They are flat. I got the gasket material from "our friend". 4 times a thick as the Autobone crap I got. He's also fixing the manifold vacuum ports (the right sid eones that the PO put in crooked). That way I can hook up the new vacuum crosspipe I manufactured with built in guage. He said he was pretty impressed with it. I'm supposed to pick up the manifold today. If I have time, I'll put it in this evening. Otherwise tomorrow. I'll post the results. When I get this all broken in I'll make a web page detailing the rebuild and mods.

[Sluggo]

I should also add that I'm putting 14 ft lbs on the manifold to head nuts. Is that enough? I couldn't find the torque in any of the manuals but I got 14 from someone on Samba.