# 73 firebird rev bumps **URL:** \ **Category:** Maintenance/Repairs\ **Tags:** pontiac, firebird\ **Created:** [August 29, 2020, 7:31pm UTC](https://community.cartalk.com/t/73-firebird-rev-bumps/169331 "2020-08-29T19:31:00Z")\ **Posts on this page:** 7\ **Page:** 2
**Author:** ![nogains](https://avatars.discourse-cdn.com/v4/letter/n/ecb155/32.png) [@nogains](https://community.cartalk.com/u/nogains)\ **Post date:** [August 31, 2020, 10:52pm UTC](https://community.cartalk.com/t/73-firebird-rev-bumps/169331/21 "2020-08-31T22:52:55Z")
Alright, I did a vacuum test. When tested from the manifold I’m getting 10" and when revving the engine the needle moves up a smidge. When attached to the carb i’m getting 12" and its the same barely any movement when revving. I’m suspecting the intake manifold. Also just realized that the previous owner attached the aftermarket intake manifold with what looks like header bolts. ---
**Author:** ![Mustangman](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/mustangman/32/44130_2.png) [@Mustangman](https://community.cartalk.com/u/Mustangman)\ **Post date:** [August 31, 2020, 11:20pm UTC](https://community.cartalk.com/t/73-firebird-rev-bumps/169331/22 "2020-08-31T23:20:11Z")
10 inches is a vacuum leak, especially if the cam in that TBI engine is stock. Should be more like 18 inches. ---
**Author:** ![NYBo](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/nybo/32/17281_2.png) [@NYBo](https://community.cartalk.com/u/NYBo)\ **Post date:** [August 31, 2020, 11:22pm UTC](https://community.cartalk.com/t/73-firebird-rev-bumps/169331/23 "2020-08-31T23:22:06Z")
Are you sure you hooked up to manifold vacuum? The dial should **drop** when you rev the engine, not go up. ---
**Author:** ![nogains](https://avatars.discourse-cdn.com/v4/letter/n/ecb155/32.png) [@nogains](https://community.cartalk.com/u/nogains)\ **Post date:** [August 31, 2020, 11:44pm UTC](https://community.cartalk.com/t/73-firebird-rev-bumps/169331/24 "2020-08-31T23:44:20Z")
Yes, both the manifold and one of the ports on the carb. Both did the same, but manifold was at 10" and carb was reading 12". Dial moved a smidge up, barely moved but moved just a tiny bit. I thought that was weird. ---
**Author:** ![Nevada\_545](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/nevada_545/32/41232_2.png) [@Nevada\_545](https://community.cartalk.com/u/Nevada_545)\ **Post date:** [August 31, 2020, 11:47pm UTC](https://community.cartalk.com/t/73-firebird-rev-bumps/169331/25 "2020-08-31T23:47:38Z")
> [@nogains](#): > > It stays strong at 1k and it will bounce to about 1.5k 2k If the engine won’t idle at less than 1000 RPM and you only have 10 inches of vacuum this engine must have a performance cam. I suspect that the metering rods in the carburetor are going up and down causing the lean surging. The carburetor needs to be tuned to the engine, larger jets, thinner metering rods and metering rod springs that match the vacuum of the engine. ---
**Author:** ![ok4450](https://avatars.discourse-cdn.com/v4/letter/o/e9bcb4/32.png) [@ok4450](https://community.cartalk.com/u/ok4450)\ **Post date:** [September 1, 2020, 2:52am UTC](https://community.cartalk.com/t/73-firebird-rev-bumps/169331/26 "2020-09-01T02:52:13Z")
Ten or 12; both numbers are horrible. If the gauge is connected correctly the gauge needle should drop to zero when you blip the throttle; not move up. You are double sure that you are connecting to manifold vacuum; not ported vacuum which means above the throttle plate? It might not be a bad idea to run a compression check on at least a couple of cylinders just to make sure you’re not dealing with a worn out engine. If a few easy to get to cylinders show poor compression then check all of the rest of them. The last thing you need is to wrestle a worn out engine so weed that factor out at the start. ---
**Author:** ![nogains](https://avatars.discourse-cdn.com/v4/letter/n/ecb155/32.png) [@nogains](https://community.cartalk.com/u/nogains)\ **Post date:** [September 2, 2020, 10:35pm UTC](https://community.cartalk.com/t/73-firebird-rev-bumps/169331/27 "2020-09-02T22:35:35Z")
Ran compression, everything looks good? Highest is 185, lowest was 175, majority are 180. Replaced the spark plugs. Looks like it’s running over gassed. Readjusted timing, and it’s running better, but I’m positive it’s some sort of vacuum leak. Got a kit to replace all the gaskets and seals. [Previous page](https://community.cartalk.com/t/73-firebird-rev-bumps/169331.md?page=1)