vacuum secondarys
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Junior User
Joined: Oct 2004
Posts: 54
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From: so cal
vacuum secondarys
well i just got a new carb for my 87 f150 5.8l truck and i was looking at it and it has vacuum secondarys. so for some reason i got it in my mind that my secondarys werent opening. so we put the new carb on and my dad leaned over the truck and looked into the carb and reved the engine to full throttle a few times and they didnt even budge. now this is supposed to be the stock version 650 holley but what i was wonderin is...
A: can vacuum secondarys open without the filter/carb top on it.
B: i found a vacumne tub going into the area of the filter/carb top area and it doesnt have a tube going to it i pulled the rubber thingy off but when i looked to see if the secondarys were opening the plug was on this tube, i was wonderin if that might have something to do with it since i guess it would get vacuum from the engine just by the fact its inside the intake. unless my thinking is way off.
oh and btw yes the engine was on when i tested it and any help on this would be great. thx
A: can vacuum secondarys open without the filter/carb top on it.
B: i found a vacumne tub going into the area of the filter/carb top area and it doesnt have a tube going to it i pulled the rubber thingy off but when i looked to see if the secondarys were opening the plug was on this tube, i was wonderin if that might have something to do with it since i guess it would get vacuum from the engine just by the fact its inside the intake. unless my thinking is way off.
oh and btw yes the engine was on when i tested it and any help on this would be great. thx
Senior User
Joined: Sep 2004
Posts: 484
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From: SC lowcountry
Some thoughts - vacuum secondaries do not normally open from just opening the throttle with the vehicle not moving. The vacuum for the secondary diaphragm comes from load sensed port in throttle body and you can't normally get enough to tip them in sitting still.
The air cleaner should not affect secondaries cut in so you could tell by watching.
The vacuum port that had the cap should probably remain capped for dirt etc; it may see some vacuum under certain operating conditions but should not affect secondary operation, that is internal to throttle body.
The air cleaner should not affect secondaries cut in so you could tell by watching.
The vacuum port that had the cap should probably remain capped for dirt etc; it may see some vacuum under certain operating conditions but should not affect secondary operation, that is internal to throttle body.
