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56 Merc w/312 Vacuum at 15hg after a rebuild

Posted By TheWitz Last Month
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TheWitz
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This engine was rebuilt at a local shop. They went .030 over on the pistons and the heads were shaved. The power brakes were working fine before the rebuild, now they're not.
The vacuum reading is 15 inches and the shop is telling me that because of the 0.030 over and the shaved heads, that I will not have the 20 inches needed for the brakes. He's suggesting I install a vacuum booster pump. I wouldn't think 30 thousandths oversize and head resurfacing would result in a substantial vacuum loss.
Is vacuum loss common after the changes that were made in this case?
Anyone know how much vacuum is required to operate the power brake booster?
I know that timing, valve adjustment, and carb settings can play a major role in vacuum. The shop believes they have it right.
Your input is appreciated. Thanks.

Cliff
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Not true, the shave and over bore has nothing to do with it, check your valve adjustment, check the timing, verify that the marks on the damper are correct (not a slipped ring), adjust the carburetor, I would first advance the distributor about 3/8 on an inch and then test the vacuum, if it comes up start looking at the timing (good chance that's the problem), if all else fails run a compression check.

Good Luck      
Joe-JDC
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If anything, the slight increase in compression should help with vacuum.  Joe-JDC

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You don't say?  Is the Cam stock, or close?. High performance Cams rob vacuum..  


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check the timing, verify that the marks on the damper are correct (not a slipped ring), adjust the carburetor, I would first advance the distributor about 3/8 on an inch and then test the vacuum, if it comes up start looking at the timing

This is true, but for diagnostic or testing purposes the manifold vacuum should be measured at factory ignition specs, probably around 6 deg.

A few different factors will tend to lower manifold vacuum. A hotter cam will not have nearly as much, late or retarded ignition timing, elevation or altitude plays a role, too tight on the valve lash, and late or retarded valve timing. The latter will be compensated for somewhat by advancing the ignition timing, so be sure to check vacuum at factory timing specs.

Some judgment is necessary when making a determination, but generally speaking 15" manifold vacuum in a stock-ish is not what we want to see.
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I’ll suggest performing a cranking compression test on the engine to first determine if the compression numbers are within spec.  If they are down, then it’s possible that the pistons are sitting too far down in the hole and/or the camshaft is not degreed in properly.  If the camshaft was not degreed in when the engine was rebuilt, then it’s going to be pot luck on it being in the right position.
 
It was not mentioned what camshaft is now in the engine but any upgrades in that area will have the vacuum levels lower.  Even at 15” Hg of manifold vacuum, I would think that you should still have sufficient vacuum for the power brakes.  Loosening up the valve lash will help to increase the manifold vacuum at idle.  Has the valve lash been rechecked since the engine was initially rebuilt?
 


Lorena, Texas (South of Waco)


Joe-JDC
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No mention of changing camshaft, so if all else is the same, increase in compression would help vacuum.  The only negative would be the pistons are down the hole more that the stock ones were.   Joe-JDC

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TheWitz
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Thanks to all for your insights.

The cam is stock, the valves have been adjusted by the shop a couple of times.
The distributor is from a 57 Tbird so it only has the one vacuum line to the dashpot for vacuum advance.
The carb is the Carter WCFB.
The shop did a compression test but they did not give me the numbers.
I've blocked off the vacuum line to the wipers and heater controls, but that made no difference in the Hg reading.
Sprayed around the intake and carb looking for vacuum leaks and found none.
Applied a hand vacuum pump to the brake booster and it very slowly leaks down. Took the booster out of the mix and still get15Hg.

So based on the feedback I've received so far. The low vacuum is related to tuning and not because of the oversize pistons and head resurfacing.




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Where are you at, altitude wise? If it's in your profile I couldn't find it in the mobile site.

A correction factor is applied for manifold vacuum measurement at anything above sea level. Subtract approx. 1" per thousand feet elevation ASL. This is partly the reason why there is a 3" to 4" spread for "normal" on the gauges.

An engine that "should" ordinarily pull 20" manifold vacuum is most definitely not tuned correctly if it only manages 17", even though the gauge seems to indicate in the green. A mechanic's vacuum gauge can tell quite a lot about engine condition and tuning, but like I said they take some experience and judgment to know exactly what it's actually telling you. My money is on late valve timing, simply because I've seen it cause this, so what else could it be? /s

Seriously though, a quick check of the #1 and #6 valves at TDC and at overlap with a straight edge, and observing where the timing pointer falls in relation to the "0" mark on the balancer will tell you if the camshaft is generally in an advanced or retarded position. This isn't nearly as precise as degreeing a camshaft but it is a quick and easy method that would eliminate the camshaft & timing set installation as a source of the Unpleasantness.

A mistuned carburetor - the idle mixture setting adjustment is good for maybe a full point or so swing or improvement, depending.
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Was the distributor installed at the same time as the overhaul?  Maybe the timing at idle is wrong because of different vacuum requirements of the carb vs the distributor.  I can't recall if the '57 Bird had full manifold vacuum to the distributor, or if the '56 Merc provided vacuum at idle.  The Merc had dual chamber vacuum advance pots, so whiscruber carb port is used for the distributor could be a factor.

John - "The Hoosier Hurricane"
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