34’ Running Rich - Holley 94/Vacuum Leak

[Flathead Fever]

Originally Posted by flatjack9

The vacuum brake operates just opposite of what you described. At idle and high vacuum, the brake is retracted allowing normal advance. At low vacuum, the brake piston contacts the breaker plate causing the spark to retard around 4 - 5 deg.

Now that I'm thinking about it, instead of writing with my brain in neutral, that does make more since. You want the vacuum to retard under a load to prevent detonation. Less vacuum less advance.

I'm sorry guys, I don't like to give bad advice and I appreciate being corrected if I'm wrong. Its hard enough to fix this stuff with the correct information. I don't want to sabotage anybody.

I have an MRI scheduled for my brain and back so I will least find out if there is a brain in there? I've decided I have cloth covered wiring in my nervous system and its starting short out. I'm screwing up a lot lately and I'm only 57. Something is not right?

Thanks.

[scicala]

"The power valve should be closed at idle. But if the vacuum is at 2.5 inches the power valve will be open, a bunch, and dumping fuel into the engine. If it is, you should be able to look down the carb and see the fuel going in there. If the vacuum dropped to 2.5 inches at idle that would be a big air leak, you would probably be able to here it. The timing could be off but I doubt that changed by itself. The cam gear could be off. I'm not sure it could jump a worn tooth and not destroy the gear some more and then totally quit running? You could try checking the timing and see if everything is still where it should be. If the diaphragm on the power valve ruptured it would be the same as if the valve was open, fuel would be flowing through it."




I disagree with a couple of things here (in red). First one..., 2.5" of vacuum stated is venture vacuum/suction and not the manifold vacuum the power valve sees.


Second one..., If the power valve diaphragm is ruptured it will flow a lot more fuel into the intake manifold, that if it were just open. When the power valve opens it lets extra fuel go thru two metered hole into the main wells. If the valve is ruptured it dumps a lot more fuel through a vacuum hole in the carb base.


Sal

[dbtenner]

[UPDATE]: The issue has been fixed. Apparently I had a vacuum leak at the carb base to intake manifold. One of the “feet” had cracked which is why my vacuum pressure was fluctuating 15-17inHg. I’m glad I had a spare base to reconnect the carb to and the intake manifold is now 21inHg steady Here is a pic of the culprit. IMG_3475.jpg

The reason why it cracked is simple. One of the studs from the intake manifold that the carb sits on is a little crooked kind of like when you yank an electrical plug from the wall at an angle. So when I cranked down on the stud to install the base to manifold, it must have started to split the base foot.



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[Tinker]

Smart one. Nice work!

[tubman]

Dang! I've never seen that before. Thanks for getting back to us with the results; too many times we are left hanging.

[dbtenner]

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Yeah it was an odd anomaly. Do y’all know how to remove the butterfly flaps from the base? They look riveted.


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[tubman]

They are actually special (usually brass) screws with an indentation on the tip, which allows the screw to be "mushroomed" after they and the throttle plates (I hope that's what you mean by "butterfly flaps) are installed. To remove them, I use a small drill bit to carefully remove the "mushroomed" end of the screw. Remove the base from the body of the carburetor and use a proper size screwdriver to carefully remove the screws. I emphasize this, because if you strip a screw head, you will have a problem. To assemble everything, you should ideally get new screws and "re-mushroom" them. In a pinch, I have reused the screws with red Loctite and "re-mushroomed" them as well as I could with a center-punch. I have never had a problem, but new screws are much preferred.

[scicala]

Yes, some of those carbs had rivets holding the throttle plates to the shaft. Just carefully drill off the peened end of the rivet, and knock them out. The hole where the rivets were are the correct size to just tap threads the same size as the screws were. (I think it's a 6-32 thread, but not positive).
And please don't use Loctite when assembling the screws. Makes it very difficult to service at a later date. When is the last time you saw one come out or get loose after being tightened ? A little peening on the end is all that's necessary.


Sal

[tubman]

I have never seen one with rivets. Were they used in a specific time period or make/model? I have never had a problem removing "Loc-Tited' screws; a little heat and out they came. Also, I said "in a pinch"; not recommended. I'd rather risk a carburetor than an entire engine.

[RKS.PA]

db, nice piece of "trouble shooting" work and congratulations on locating your problem. Please let us know how she runs once you get everything back together. And......careful when you bolt down that "rebuilt" carburetor!!!

[scicala]

Originally Posted by tubman

I have never seen one with rivets. Were they used in a specific time period or make/model? I have never had a problem removing "Loc-Tited' screws; a little heat and out they came. Also, I said "in a pinch"; not recommended. I'd rather risk a carburetor than an entire engine.

I guess it's a matter of preference for each person. Have never heard of an engine damaged buy a properly tightened and peened screw coming out. I've rebuilt a few that had Loctite on the screws and even with heat it was tough and risked snapping the screw off.


Sal